PGDEM-01
UNIT-IIA Ecological Principles, Biosphere And Its Organisational Levels
30
Dr. Asha Gupta
UNIT-IIB COMMUNITY
23
Dr. Asha Gupta
UNIT-IIC Eco-System, Its Structural And Functional Aspects, Energy
44
Flow,Biogeochemical Cycles And Regulation Of Ecosystems
DR. ASHA GUPTA
UNIT-IIIA Human Activities & Environmental Degradation
20
Prof. Anubha Kaushik
UNIT-IIIB Anthropogenic Impacts – Land Degradation,
19
Deforestation And Eutrophication
Prof. Anubha Kaushik
UNIT-IVA Human Evolution And Population Trends
17
Prof. Anubha Kaushik
UNIT-IVB Human Population Growth And Its Impact
17
Prof. Anubha Kaushik
UNIT – I
PGDEM-01
ENVIRONMENT AND ITS COMPONENTS-I
Dr. Krishan Kumar
STRUCTURE
1.0
Objectives
1.1
Biotic and Abiotic Components
1.2
The Atmosphere
1.3
1.2.1
Evolution of Atmosphere
1.2.2
Composition of the Atmosphere
1.2.3
Vertical Distribution of Gases in the Atmosphere
1.2.4
Thermal Structure of the Atmosphere
The Lithosphere
1.3.1
Soil
1.3.2
Soil Characteristics
1.3.3
Soil Biota
1.3.4
Soil Types
1.4
Summary
1.5
Key Words
1.6
Review Questions
1.7
Suggested readings
1.0
Objectives
In this chapter, we shall develop a conceptual understanding of the following:
•
What is environmental science?
•
What are different components of the environment?
•
Atmosphere – its evolution and present day composition.
•
How are various gases distributed vertically in the atmosphere?
•
The thermal profile of atmosphere.
•
What is soil and what are its characteristics?
•
What are major soil types?
1.1
Biotic and Abiotic Components
Literally speaking, environment means surroundings. Environmental science can, then,
simply be defined as the study of surroundings. Our surroundings are comprised of two
different components, namely, the physical environment and the biological environment.
The physical environment is composed of the lithosphere, the hydrosphere and the
atmosphere. The biological environment is composed of the plant and animal life. In
terms of the above components, environmental science can be defined as the study of
relationship between the biological environment and the physical environment. To study
this relationship, one needs to take the help of various tools in the form of knowledge of
various disciplines such as botany, zoology, chemistry, physics, mathematics, geology,
atmospheric science, hydrology, oceanography etc. Environmental science, therefore, is a
multidisciplinary field in which all the subjects mentioned above play an equally
significant role. The aim of environmental science is to interconnect the knowledge from
various science disciplines in order to solve the environmental problems.
An environmentalist is essentially identified by his approach towards investigating an
environmental problem. For instance, a decision about building a dam in a region is to be
taken on the basis of likely impacts of the dam on the surrounding environment. A
botanist may be concerned more about the damage that may occur to the flora of the
region. Similarly, a zoologist may be concerned more about the likely damage to the
fauna of the region and a geologist may be concerned more about the damage to the
stability of the rocks in the region. An environmentalist on the other hand will draw
knowledge from different disciplines and will take into consideration the likely impacts
on all the components of environment viz. air, water, soil, plants and animals. This is
what differentiates an environmentalist from a person with bias towards a particular
discipline.
The subject of environmental science has emerged essentially due to the need for
producing manpower with the above mentioned approach towards
environmental
problems. However, to have a better grasp of environmental issues, it is important for us
to have an understanding of the different components of our environment. In the present
unit, we shall acquaint ourselves with some of the basic concepts required for the
understanding of our physical environment.
1.2
The Atmosphere
Atmosphere is a thin envelope of gases surrounds our planet earth. It is impossible for us
to visualize about life without the existence of atmosphere on our planet. Atmosphere
provides us oxygen and carbon dioxide required for two fundamental biological
processes -respiration and photosynthesis. To make our surroundings more hospitable,
atmosphere plays another important role. It redistributes energy and water on our planet
through various weather processes. So, it is important for us to know the composition and
structure of the atmosphere and how it evolved to its present composition.
1.2.1– Evolution of Atmosphere
It is generally believed that the solar system condensed out of an interstellar cloud of gas and dust or what
we call as the “solar nebula” about 4.6 billion years ago. One of the products of this process was our planet
“earth” which developed due to the accretion of celestial material during millions of years. As the accretion
of celestial matter occurred, earth started acquiring an atmosphere by capturing gaseous molecules which
entered its gravitational field. The primitive atmosphere of the earth consisted mainly of hydrogen, helium,
water, neon, ammonia, methane and argon (Table-1.1).
Table- 1.1 Primitive atmosphere of the earth
Gas
Percent by weight
Hydrogen
63.5
Helium
34.9
Water Vapor
0.6
Neon
0.34
Ammonia
0.26
Methane
0.11
Argon
0.15
This initial gaseous envelope was swept away to the space by the solar winds* quite early
in earth’s history. However, due to the process of accretion, impact of falling meteorites
on the surface and decay of radioactive elements, earth began to heat up. This led to
immense volcanic activity on earth. As a result, large volumes of gases locked in the
accreting material were released to the atmosphere by the process called as
degassing/outgasssing. The gases released during the process of degassing included
nitrogen, carbon dioxide, methane, water vapors, and gases containing sulfur. This started
transforming the early atmosphere of earth which contained mainly hydrogen and helium.
Once the earth acquired its present mass, it started cooling. This happened about 3.7
million years ago. During the process of cooling, the gaseous water in the atmosphere
began to condense and clouds were formed. As a result, torrential rains occurred during
this period and oceans were formed. Early life forms are believed to have existed on earth
about 3 billion years ago (called as the Archeozoic era). It is believed that the early life
forms on earth were single cell organisms which carried out their metabolic activities
without free oxygen. The only source of free oxygen on earth at this time was the photodissociation of water present in the upper atmosphere. The first oxygen utilizing cells
must have made use of the free oxygen produced in this manner. Gradually, some
unicellular organisms must have developed the ability to utilize carbon dioxide and water
to build hydrocarbons in their structures and release oxygen in the atmosphere through a
process called photosynthesis. These primitive green plants produced the free oxygen
necessary for supporting higher life forms. As the plant life evolved on earth, free oxygen
began to accumulate in the atmosphere. By about 600 millions years ago, free oxygen
levels had reached to 1 percent of their present level. About 400 million years ago, a great
expansion of green plants occurred thus taking the oxygen levels to 10 times their present
level. However, most of this oxygen was utilized by the decaying vegetation about 300
million years ago. Oxygen production by the green plants again increased the oxygen
level and brought it to its present level by about 200 million years ago. The concentration
of oxygen slowly decreased in the next 100 million years but again reached to the present
level by about 65 million years ago. Ever since then, oxygen concentration in the
atmosphere has been relatively stable.
1.2.2 Composition of the Atmosphere
The composition of our atmosphere is not constant. It, rather, varies both spatially and
temporally. However, if we consider only the lower 100 Km of the atmosphere and
ignore the presence of suspended particles and water vapor, the composition of the
atmosphere can broadly be considered to be stable and uniform all over the earth. The
two most dominant gases present in the earth’s atmosphere are nitrogen and oxygen
which together constitute about 99 percent of the atmosphere. It is not very difficult to
appreciate the importance of oxygen since all living organisms on earth depend on it for
respiration. Nitrogen, being one of the elements that constitute proteins ( though only a
few plant species are able to fix nitrogen in proteins directly from the atmosphere), is also
a very important component of the atmosphere There are a number of other gases which
are present in minute quantities in the atmosphere ( Table-1). Some of these gases,
though present in very small quantities, are nonthless very important. Without the
presence of carbondioxide, the earth would have been a much cooler ( and hence less
habitable) planet than what it is today. Ozone plays a very significant role in preventing
the harmful ultraviolet radiations from sun to reach the earth’s surface. Water vapor is
present in the atmosphere in variable amounts almost all of which is present in the lower
10-15 Km. Its presence in the atmosphere is significant since it governs the phenomena
of cloud formation and precipitation. In addition, water vapors in the atmosphere play a
very crucial role in the overall radiation budget of the earth atmospheric system. The
amount of suspended dust and other aeosol particles in the atmosphere is also significant
since it determines the proportion of incident solar radiation that is reflected back to
space directly by the atmosphere. Further, there are a number of constiuents that are
present in trace amounts in the atmosphere. Many of these constituents (e.g. the oxides of
nitrogen and sulphur, the chlorofluorocarbons ) owe their presence in the atmosphere
mainly to anthropogenic activities. Some of these gases are known to have the potential
to change our environment significantly.
TABLE –1.2 Composition of Earth’s Atmosphere (Dry Air)
Constituent
Percent by Volume
Concentration in ppm
Nitrogen (N2)
Oxygen (O2)
Argon (A)
Carbondioxide (CO2)
Neon (Ne)
Helium (He)
Methane (CH4)
Krypton (Kr)
Hydrogen (H2)
78.084
20.946
0.934
0.034
0.00182
0.000524
0.00015
0.000114
0.00005
780840
209460
9340
340
18.2
5.24
1.5
1.14
0.5
1.2.3 THE VERTICAL DISTRIBUTION OF GASES IN THE ATMOSPHERE
Due to the earth’s gravitational force of attraction, each of the gases present in the
atmosphere exerts a pressure on the earth’s surface. The pressure exerted by the mixture
of these gases on the earth’s surface is known as the atmospheric pressure. Its average
value at sea level is approximately 105 Pa. As one moves to an altitude higher than the
sea level , one experiences a decrease in the atmospheric pressure since the mass of the
atmosphere above that altitude also decreases. As one goes up, the atmosphere becomes
thinner and thinner. The density of each gaseous constituent decreases almost
exponentially with height in the atmosphere. However, The rate at which the density
dereases with height is different for different gases. The density of heavier gases
decreases much more sharply with height in comparison to that of the lighter gases. There
are two main processes which govern the vertical distribution of these gases in the
atmosphere. These are (a) the procees of molecular diffusion and (b) the process of eddy
diffusion or in other words the mixing due to turbulent fluid motions.
The process of molecular diffusion favours an atmosphere in which the mean molecular
weight of the mixture of gases decreases with height i.e. an atmosphere where heavier
gases are abundant at lower altitudes and the lighter gases become the dominant species
at the higher altitudes. The effectiveness of this process is however affected by the mean
free path between collisions of gas molecules. Higher is the mean free path of the gas
molecules more effective is the process of molecular diffusion. In the lower atmosphere,
there is so much crowding of gas molecules that the gas molecules keep on colliding
with each other at a very fast rate. The mean mean free path between collisions in the
lower atmosphere is, therefore, quite short. As result molecular diffusion is not a very
effective process in the lower atmosphere. The upper atmosphere, on the other hand, is
thin or rarified. The number of collisions here are, therefore, quite less and hence the
mean free path between collisions is quite significant at such heights. Consequently, the
process of molecular diffusion is more effective in the upper atmosphere.
Contrary to the process of molecular diffusion, the process of eddy diffusion tends to
produce an atmosphere which is homogeneous in gaseous composition i.e. it does not
differentiate between the lighter and heavier gaseous constiuents of the atmosphere.
Eddies are parcels of air that are produced mainly because of two reasons : (a) heating up
of the earth’s surface and (b) the roughness of earth’s surface. These eddies can move
from one place to the other without altering their properties i.e. they conserve their
properties during their movements. The effect of the surface of earth remains significant
only in the lower atmosphere and hence eddy diffusion is a dominant process in the lower
atmosphere only.
The two processes discussed above, compete with each other in opposite direction.
Whereas the process of molecular diffusion favours a stratified atmosphere on the basis
of molecular weights of the gases, the process of eddy diffusion favours an atmosphere
which is homogeneous in composition regardless of the molecular weights of the gases.
In the lower, the mean free path between collisions is so short that the time required by
the process of molecular diffusion to vertically separate the lighter and heavier gaseous
constituents is much more in comparison to the time required by the process of eddy
diffusion to mix them uniformly. The process of eddy diffusion dominates the process of
molecular diffusion upto a height of about 100 Kms. At this height, the two processes are
almost equally effective. Above this height, the process of molecular diffusion gradually
starts domonating the process of eddy diffusion. The layer of atmosphere below a height
of about 100 Km. which is uniform in composition is called the homosphere, the layer
above which is more stratified in nature is called the heterosphere and the region of
transition between the two layers is known as the turbopause. The height of turbopause
is not rigid or fixed. It may vary from place to place and time to time depending upon the
conditions of thermal structure of the atmosphere and the surface roughness of earth.
The composition of the atmosphere is also affected by the incoming solar radiation at the
upper levels. Most of the ozone is present in the atmosphere between the heights 15 to 50
Km. This is primarily because most of the ozone in the atmosphere is produced in this
region with the help of certain photochemical reactions. Above a height of about 120
Km., the solar radiation is so energetic that most of the oxygen exists in its atomic
form.Further higher at a height of about 500 Km, atomic oxygen becomes the dominant
species in the atmosphere since by this height, the concentration of another heavier and
major constituent the diatomic nitrogen declines to very low levels. Also present at this
height are the traces of lighter gases helium and hydrogen. Around a height of about 1000
Km, helium and hydrogen are the dominant species present in the atmosphere.
It is pertinent here to mention that the molecular collisions are so infrequent at a height
above 500 Km that the mean free path between the collisions is quite long. Individual gas
molecules may travel distances upto several hundred Kms without any collision. A very
minute fraction of these molecules may possess the necessary velocity to take them out of
the influence of earth’s gravitational field. In other words, a number of molecules at
heights above 500 Km may have velocities greater than the escape velocity. Escape
velocity of an object at a given height is the velocity possessed by the object when the
kinetic energy of the object becomes equivalent to the potential energy required to lift the
object out of the earth’s gravitational field. It can be shown on the basis of kinetic theory
of gases that the probability of escape of lighter gas molecules is higher than that for the
heavier gas molecules. This explains the relative absence of free hydrogen in the earth’s
atmosphere despite its continuous production by the dissociation of water.
It would be worthwhile here to note that there is also a minute fraction of charged
particles in the atmosphere in addition to the constituents discussed above. These charged
particles are produced in the upper atmosphere by the highly energetic component of the
incoming solar radiation and the cosmic rays emanating from sources other than sun.
During the process of ionization, the high energy incident radiation turns the gas
molecules and atoms into positively charged ions, thus, setting the electrons free in the
atmosphere. The region of atmosphere in which these charged particles are present is
known as the ionosphere. Although the ionosphere extends from an altitude of about 50
Kms till the upward limits of the earth’s atmosphere, the majority of ionized particles
exist in the altitude range 80 to 400 Kilometers. The concentration of ions below a height
of about 80 Km is low because most of the high energy short wave radiations are already
absorbed by the time the incident radiations reach these lower altitudes. The
concentration of ions above a height of about 400 Km is low because the density of air is
very low at such great heights and relatively very few gas molecules are available for
ionization. Depending upon the variations in ion density with height, the ionosphere can
be divided into three different layers. These layers, however, undergo a diurnal change in
their strength since the production of these ions is mainly dependent on the solar radiation
directly incident upon the gas molecules. It must be commented here that the existence of
ionosphere has not to do much with our daily weather. Nonthless, it is a very important
layer of atmosphere since the transmission of radio waves necessary for our radio
broadcasts is dependent on the existence of this layer only.
1.2.4 THERMAL STRUCTURE OF ATMOSPHERE
The atmosphere can be divided into four different layers on the basis of vertical profile of
temperature within it. The rate of change of temperature with height is called the lapse
rate. Mathematically, we can denote it by dT/dZ, where dT represents the change in
temperature and dZ represents the change in height. The value of
dT/dZ becomes
negative, positive or equal to zero according as the temperature decreases, increases or
remains constant with height.
The lowest layer of the atmosphere on the basis of its thermal structure is called the
troposphere. Within the troposphere, the temperature decreases with increasing height,
i.e. dT/dZ is negative in the troposphere. On an average, the temperature decreases at a
rate of 6.5°K per kilometer within this layer. This is also known as the environmental
lapse rate (ELR). In reality, however, ELR varies greatly from one place to the other in
the atmosphere. The height of the troposphere is different at different parts on earth. It
varies from an average height of 8 kilometers over the poles to an average height of 18
kilometers over the equator. The word average is used here to signify that the height of
troposphere is not fixed in the atmosphere. It may change from place to place and from
time to time. The greater height of troposphere over the equator is mainly because of the
greater extent of vertical mixing induced by a net heat surplus over the equatorial region.
The polar regions, on the other hand, are deficient in heat content and hence the vertical
mixing over the poles is quite limited. Troposphere is a very special layer of the
atmosphere. All weather related phenomena such as cloud formation, precipitation,
thunderstorms take place in the troposphere. Almost all of the moisture content of the
atmosphere is present in this layer only. The atmosphere lying above this layer is
relatively very dry. The troposphere is an unstable layer of the atmosphere characterised
by strong vertical mixing which is induced mainly by the roughness of earth’s surface
and its heating by the incident solar radiation.
Above the trposphere, lies the stratosphere which is different from tropsphere in many
ways. Unlike troposphere, the temperature increases with height in the stratosphere, i.e.
dT/dZ is positive in this layer of the atmosphere.The zone of transition between the
troposphere and the stratosphere is called the tropopause. In this zone the temperature
remains constant with height, i.e. dT/dZ is zero in this zone. As the name indicates, the
stratosphere has got a layered (strata means layer) structure.This is because the prevailing
positive vertical gradient of temperature with height makes it a very stable layer of the
atmosphere. The stratosphere, thus, is characterised by very little mixing in the vertical
direction. Due to this reason, the stratosphere acts like a reservoir for aerosol particles
and certain species of air pollutants that may enter this layer. Any aersol particles or air
pollutants that enter this layer may remain trapped here for a very long period of time.
Stratosphere is also different from the troposphere because of the higher concentration of
ozone present in it. The maximum concentrations of ozone in the stratosphere exist
approximately in the altitude range 15-30 kilometers. Because ozone absorbs the harmful
ultraviolet radiation from sun and prevents it from reaching the earth’s surface, the ozone
rich zone of the stratosphere is commonly called the ozone layer. This, however, noway
means that the concentration of ozone is maximum in this zone among all the
atmospheric constituents. The existence of ozone layer in the stratosphere is very crucial
for the safety and health of living organisms on earth. Unfortunately, it has been observed
in the past few decades that the concentration of ozone in this ozone rich layer of
atmosphere has started declining particularly over the polar regions. It is now established
that the chloroflurocarbons used in aerosol sprays, refrigerators, rockets etc. are mainly
responsible for this sudden decline in the ozone concentrations in the polar stratosphere.
The stratosphere extends to height of about 50 kilometers. The layer above the
stratosphere is called the mesosphere. Like the troposphere, the temperature decreases
with height in the mesosphere, i.e. dT/dZ is negative in this layer of the atmosphere. The
zone of transition between stratosphere and mesophere where the temperature remains
constant with height, is known as the stratopause. The mesosphere extends to a height of
about 80 kilometers with temperatures approaching to a level of about -90°C when
mesopause occurs. Above the mesosphere, lies the thermosphere. In the thermosphere,
the temperature again starts increasing with height, i.e. dT/dZ again becomes positive in
this region. The thermosphere extends upwards to a height of several hundred kilometers
till the outermost limits of the earth’s atmosphere. The temperatures in the thermosphere
may range from 500°K to 2000°K. Above a height of about 500 kilometers, the
molecules collide so infrequently that they practically move independently of each other.
Since temperature is expressed in terms of the average speed at which the molecules
move, it becomes very difficult to define temperature under these conditions.
Fig. 1.1 Vertical thermal profile of the atmosphere
1.3
Lithosphere
The outer part of earth as we can see in the quarries, cuttings along the roads,
mines or borings is the earth’s crust. This most significant outer crust of the earth is part
of Lithosphere. The word ‘Lithosphere’ means a sphere of rock, whether hard as granite
or soft as clay, gravel or sandstone. Down to a depth of 16 km. from the surface of land,
95 percent of the earth materials found in the earth’s crust consist of rocks. Rocks are
made of individual substances, called minerals found mostly in a solid state. Each
mineral usually contains two or more simple substances called elements of which whole
earth is made. These on the basis of the mode of formation are of three major types. One
such type may be distinguished from the other not only on the basis of its origin but also
with reference to mineral contents & arrangements of mineral crystals. They are Igneous
Rocks, Sedimentary Rocks and Metamorphic Rocks, which are inter-convertible into one
another under a set of physical conditions.
1.3.1
Soil
Soil is a mixture of many solids, liquids and gaseous substances. It forms the topmost
layer of earth’s crust. It has both the living and non-living matter like mineral particles,
decaying plant remains and insects living together with countless bacteria on its organic
matter. In addition soil holds water and air in its pore. The combination of all these
nutrients in the body of soil provide the plants food for their growth. When the plants
die, the nutrients absorbed by them again become part of the soil system as they are
decomposed by soil biota into simpler substances.
Soil is formed through the interactions among the weathering of underlying rock,
the climate, plants and the activities of millions of insects and earthworms etc. All these
physical, chemical and the biological activities build up the soil layer over a long period
of time. As a result of various processes (i.e. physical, chemical and biological) occurring
within the soil of a particular place, a mature soil develops a definite soil structure which
is characteristic of the environmental conditions of that place. Starting from the
underlying rock which supplies the parent material for the formation of soil, a series of
layers are developed. Each layer is called a soil horizon and the sequence of horizons is
termed as the soil profile. Horizons within a profile can be distinguished visually. They
have different physical and chemical properties depending on the soil forming processes,
which give rise to them. There are basically five main types of horizons in soils. These
are denoted as the O, A, B, C and R horizons (fig. 1.2 )
O-Group is organic horizons form the above mineral matrix and is composed of
fresh and partly decomposed organic matter. Commonly occur in forested areas,
generally absent in grassland regions.
O1: Organic horizon wherein the original forms of plant and animals
residues can be recognized by naked eye.
O2: Here the original plant and animal forms cannot be so distinguished.
A-horizon (ELUVIAL): this is mineral horizon, which lies at or near the surface
and is recognized as zones of maximum leaching or eluviation.
A1: topmost mineral horizon consists of strong mixture of humified
organic matter, which imparts a darker color than that of other lower horizons.
A2: horizon of maximum eluviation of clay, iron, and aluminum oxides
and a corresponding concentration of resistant minerals e.g. quartz, in the sand
and quartz sizes. Lighter in color than A1.
A3: Transition layer between A and B with properties more nearly similar
to those of A1 and A2 than that of the underlying B.
B-horizons( ILLUVIAL) are areas below, and often derived from, the A layers
where maximum accumulation of iron and aluminum oxide and silicate clays occurs.
Enrichment can occur by other than down word movement, for example alteration of the
original rock material. The B Horizons are sometimes referred to as subsoil.
B1: transition layer between A and B with properties more nearly like B
than A. Sometimes absent.
B2: zone of maximum accumulation of clays and hydrous oxides.
Maximum development of blocky or prismatic structure or both.
B3: transition between B and C with properties more like B than C.
C-horizons are mineral layers beneath the B-horizon. These layers lack any of the
characteristics of the horizons above them and may or may not be derived from the
material from which A and B-horizons were formed. C layers are less weathered and are
usually formed under different conditions than the horizon above them.
R-horizon represents bedrock from which other horizons may or may not have
been derived (i.e. soils may develop from drift material overlying the bedrock).
Fig.1.2 The Soil Profile
1.3.2
Soil characteristics:
Soils are composed of five major components: mineral materials, organic matter,
water, air, and soil organisms. The composition and proportion of these components
together with the environmental conditions determine the characteristics of soil. Here we
shall briefly discuss a number of important soil characteristics.
1. Soil texture
Soil texture is concerned with the size of mineral particles. Specifically, it refers
to the relative proportion of particles of various sizes in a given soil. For this purpose, soil
particles are classified into different size (diameter in mm) categories, :
Name of the particle
Diameter range(mm)
Clay
Less than 0.002
Silt
0.002-0.02
Fine sand
0.02-0.20
Coarse sand
0.20-2.0
Stones and Gravel
Above 2.0
Soil texture directly influences the soil-water relationships, aeration and
root penetration through its relationship with inter-particle pore space.
Indirectly it also affects the nutritional status of soil. Sandy soils are
nutrient-deficient due to high porosity.
2. Bulk Density :
This is a method of expressing soil weight. It is defined as the mass of a unit
volume of dry soil. The volume here means the space occupied by both the solid
particles and soil pores. This is different from particle density (mass of solid
particles occupying unit volume space) in which volume means only the space
occupied by solid particles. In other words, bulk density is determined by the
quantity of pore spaces and that of soil solids. Therefore, soils that are loose and
porous have low bulk densities while those that are compact have high bulk
densities. Since, the particles of sandy soils lie in close contact, such soils have
high bulk densities. On the other hand, particles of fine textured soils such as
clays or clay loams do not rest so close together because of their granulated
structure. Granulation encourages a fluffy, porous condition, thus lowering the
bulk density.
3. Pore Space:
This is the portion of soil which is occupied by air and water. The pore space is
determined mainly by the arrangement of solid particles. If they lie close together
such as in sands or compact subsoils, the total porosity is low. However, if the
particles are arranged in porous aggregates such as in the case of medium textured
soils high in organic matter, the pore space per unit volume will be high. Poer
space, space occupied by solid particles, bulk density and particle density are
related to each other through simple formulae.
% solid space = (bulk density/particle density) x 100
% pore space + % solid space = 100
4. Soil air.
Pore spaces in the soil are filled either with water or with air. The amount of soil
air present is largely determined by the amount of soil water present. Air simply
moves in those pores where water is not present. The composition of soil air
differs from that of the atmosphere in many ways. First, soil air is not a
continuous medium but is located in the maze of soil pores separated by soil
solids. In isolated locations, reactions involving the gases may greatly alter the
composition of soil air. Hence soil air composition may not be uniform
everywhere. Further, moisture is present in soil air generally in higher
concentrations than the atmosphere. The concentration of CO2 is usually much
higher and that of oxygen lower than that found in the atmosphere. For aerobic
soils, the nitrogen + argon content remains close to that of the external
atmosphere (79% v/v) while O2 and CO2 vary in complementary proportions to
make up the remaining 21%. Anaerobic soils such as water logged soils are
highly deficient in oxygen.
5. Soil moisture
Water present in the pore spaces of soil is a very critical factor for the growth of
plants. .Not only does it fulfill the moisture requirements of the plants but it also
provides nutrients to them.
Main source of water in the soil is rainfall. After the rainfall, some of the water
percolates into the ground and fills the pore spaces. The amount of water held in the
pore spaces depends upon the soil texture and structure. Water held in the pores
dissolves in it small but significant quantities of salts, thus forming the soil solution
which supplies the nutrients to plants. Thus, there occurs an exchange of nutrients
between the soil solids and the soil solution and between the soil solution and the
plants. However, all the water present in soil is not available to plants. Soil water
may be of the following types
i.
Gravitational Water- Water present in excess of the field capacity of
soil is called the gravitational water. As the name suggests, this water
moves downwards readily under the force of gravity. This water is of
little use to the plants as it occupies large pore spaces thereby reducing
the amount of soil air.
ii.
Capillary Water- When all the gravitational water has drained from a
given layer of soil, it is said to be at its field capacity. Water remaining
in the soil layer is then mainly held by the capillary forces in the
smaller soil channels. This is the water that is readily available for
plants. Water absorbed on soil particles and freezing when super
cooled to –4.0°C, called capillary absorbed water. Readily available to
the plants.
iii.
Hygroscopic Water – This is the water that is present as a very thin
film covering the soil particles and is held by tensions as great as 31
atmospheres. This means that great amount of power is required to
detach this water from the soil particles. For this reason, it is
essentially unavailable to plants. However, it may still support some
microbial activity in the soils containing only the hygroscopic water.
Fig. 1.3 - Soil air & water
6. Soil Temperature:
The thermal status of the upper layers of the soil depends upon a number
of factors such as the amount of solar radiation falling on the surface, soil
color, soil moisture, soil composition, type of litter present on the soil
surface etc. Soil is essentially a bad conductor of heat. Thus, the heat
propagated to the lower layers is considerably reduced and there is a time
lag between temperature changes at the surface and those in the lower
layers. In other words, the deeper layers of soil are out of phase with the
upper layers with respect to fluctuations in soil temperature. Such phase
differences in the soils may be observed both diurnally and seasonally.
The temperature regime of soil is very crucial not only for the plants but
for the soil flora and fauna as well.
7.
Light:
The amount of light reaching the soil surface depends upon the characteristics of
the canopy and the sun angle. But the light penetration into the soil is variable and
modulated by the presence of burrowing animals and the degree of soil fracture.
8. Soil pH
Whether the soil is acidic or basic in nature is determined by the soil pH (negative
logarithm of hydrogen ion activity). Soils show a great degree of variation in their
pH values ranging from as low as 3.0 to as high as 10. Soils are generally acidic
in regions where precipitation is high enough to leach significant amounts of
exchangeable bases from the surface layers of soils. Alkaline soils, on the other
hand, are characteristic of most arid and semi-arid regions. Soil pH greatly affects
the availability of phosphorus to plants as the kind of phosphate ion present varies
with variation in pH of the soil solution. Soil pH also influences the presence of
soil organisms. It is known that bacteria and actinomycetes function better in
mineral soils at intermediate and higher pH values than in low pH soils. Fungi, on
the other hand, exhibit a great deal of versatility and can flourish satisfactorily in
a both acidic as well as alkaline soils.
1.3.3 Soil Biota
The organic matter in the soil supports a complex microflora and fauna and a
complex biota of higher organisms.
(a) Microflora:
Soil supports a rich microflora that helps in channeling the energy through
the detritus food chain and plays an important role in the biogeochemical
cycling of the nutrients in the ecosystem. These exhibit a series of mutual
inter-relationship. Various species of fungi and algae live together in the
soil. This lichen association helps considerably in the decomposition
processes occurring in the soil. Then there are mycorrhizal associations in
which an enveloping sheath of fungus, mycelium surrounds short thick
rootlets of flowering plants. Numerous symbiotic bacteria inhabit the
different soil strata, decomposing minerals and releasing nutrioent
elements. Symbiotic nitrogen fixing bacteria such as
Rhizobium
leguminacearum occur in the root nodules of leguminous plants. They fix
nitrogen in the form of ammonia. Hetrosporic bacteria such as bacillus
mycoides, B.Ramosus and B. Vulgarus utilize nitrogen rich organic
substrate to release ammonia. Soil bacteria such as nitosomonas sp.
Convert ammonia to nitrites while nitrobacter converts nitrites to nitrates.
Denitifying bacteria such as Pseudomonas sp., bacterium denitificans and
various fungi convert nitates and nitrites to free nitrogen. Dissmilatory
sulphate reducing bacteria convert sulphates to sulphides while
chemoautotropic sulphide oxidizing bacteria convert sulphides to elemetal
sulphur. All these microflora in their function of decomposing organic
materials to nutrient elements are releasing energy for their own food
production.
(b) Soil Fauna :
A soil animal is one that spends at least a part of its life cycle in the soil.
On the basis of size, soil animals are divided into following categories:
(i)
Micro-fauna: Body size within the range of 20-200microns
and include protozoans, rotifers, larvae, small nematodes
and tics and mites.
(ii)
Meso-fauna : Body size within the range of 200microns to
1 cm., consisting of nematodes, mites, crustaceans,
millipedes, insects, spiders, snails etc.
(iii)
Macro-fauna: Body size greater than I cm., consisting of
centipedes, earthworms, molluscs, toads, lizards, snakes,
moles and other relatively large animals.
1.3.4
Types Of Soils:
On the basis of climate and vegetation soil can be classified into the following groups:
1. Podzolic soils: in humid temperate climate, under forest vegetation. The A2
horizon is well developed, for there is sustained leaching. Soils are more or less
acidic and moderately fertile. They develop under coniferous forests and have
more type of humus. Grey brown and brown podzolic soils are found under hard
wood forests and have a mull humus.
2. Latosolic soils: develop in humid tropical or semitropical-forested regions.
Humus is quickly oxidized by the action of microorganisms and hence does not
accumulate. The soil fauna is very low. Chemical weathering of the parent
material is relatively low, so leeching is extensive. In early stages of its formation,
the soil is neutral or slightly alkaline, but as leaching continues it becomes acidic.
The soil has thin organic layer (A0&A1 horizons) on reddish leached soil (A2 ---horizons) that extends to the great depth below the surface.
3. Chernozemic soils: occur in humid to semiarid temperate climates under grass
vegetation. The grass on dying return considerable organic matter to the soil. The
soil is more basic and less acidic. Where there is less rainfall, calcium salts may
accumulate to form a hardpan. E.g. prairie soils in temperate climates are among
the most fertile soils of the world, but fertility deceases in the tropical and desert
climates.
4. Desertic soils: are characteristics of the arid climates. The amount of water
percolating through the soil is insufficient to cause strong leaching of carbonates
and concentration of calcium near the surface are held by colloidal particles in a
high state of base requirement. The surface soil is brownish gray, and grades
quickly into the calcium carbonate horizon, which form a hardpan. The profile is
very poorly developed. Wind erosion removes the finer particles, leaving the
coarser materials to form a hard pavement. Scarcity of water restricts the growth
of vegetation, so that such soil have low organic material, lack nitrogen and have
a slightly neutral to slightly alkaline pH. The soils are lacking in the nitrogen and
are infertile.
5. Mountain and mountain valley soils: vary from shallow layer or eroding rocks
to deep organic soils of valleys and swampy areas.
6. Tundra soils: occur in cold areas where the substratum remains continuously and
the vegetation of lichens, mosses, herbs and shrubs make a party soil surface
layer.
7. Alluvial soils: lack a well-developed profile and are formed as a result of
deposition by the rivers and streams. They are very fertile and support a luxuriant
growth of vegetation.
8. Saline soils: are found in the dry climates where rapid evaporation of water
results in surface deposition and accumulation of salts leached from surrounding
areas.
1.4
Summary
Environment means surroundings. Environmental Science, therefore, is the study
of our surroundings. Environment consists of the biotic (or living) and the abiotic (or
nonliving) components. There are three major abiotic components – the atmosphere, the
lithosphere and the hydrosphere. Atmosphere is composed of predominantly nitrogen and
oxygen. Other constituents, though present in very small quantities, are nevertheless quite
significant for the survival of life on earth. In the lower atmosphere, the composition of
atmosphere is rather uniform (called the homosphere) while in the upper atmosphere,
heavier gases occur at the lower altitudes and the lighter gases occur at the higher
altitudes (called the hetrosphere). Thermally, atmosphere can be divided into four major
layers – the troposphere, the stratosphere, the mesosphere and the thermosphere.
Lithosphere is the solid outer part of the earth and consists of soil and rocks. Soil is a
mixture of many solids, liquids and gaseous substances. It forms the topmost layer of
earth’s crust. It has both the living and non-living matter like mineral particles, decaying
plant remains, air, water and insects living together with countless bacteria on its organic
matter. Soils may be classified into different types on the basis of climate and vegetation
of a place.
1.5
Key words
Environmental Science: Study of the surroundings.
Atmosphere: Thin layer of gases surrounding the earth.
Homosphere: Lower layer of the atmosphere which is uniform in composition.
Troposphere: Lowest layer of the atmosphere in which temperature decreases
with altitude and where all weather phenomena occur.
Lithosphere: The solid outermost shell of the earth consisting of soil and hard
rock.
Soil: Topmost layer of the earth’s crust, made up of a mixture solid, liquid and
gaseous substances, derived from living or non-living matter.
1.6 Review Questions
1. Briefly discuss the components that constitute our environment
2. How does the present day atmosphere differ from the primitive atmosphere of
earth?
3. The lowest 100km of the atmosphere are rather uniform in composition. Why?
4. Explain the thermal profile of atmosphere with the help of a suitable diagram.
5. Fill in the blanks:
iv.
In the early atmosphere on earth, ………….. gas had maximum
concentration.
v.
Atmosphere above 100km is called ………….. because of its nonuniform composition.
vi.
Most of the water vapor in the atmosphere is present in the layer
……………
vii.
Maximum ozone concentration is found in the layer ……….
viii.
Height of troposphere is maximum over ………….
ix.
The stable layer above troposphere is called …………….
6. What is soil? Explain its layered structure with the help of a suitable diagram.
7. Define the following:
a. Bulk Density
b. Particle Density
c. Pore space
8. What is soil texture? How can the soils be classified on the basis of soil texture?
9. What are different kinds of soil water?
10. How does the soil air differ from atmosphere in its composition?
11. What are different types of soils?
1.7
Suggested readings
1.
Lutgens, F. K. and Tarbuck, E. J. (2007), “ The Atmosphere: An
Introduction to Meteorology” – Prentice Hall Inc.
2.
Barry, R. and Chorley, R. (2003), “ Atmosphere, Weather and Climate” –
Routledge
3.
Wild, A. (1996), “ Soils and the Environment – An Introduction” –
Cmbridge University Press.
4.
Brady, N.C. and Weil, R.R. (2002), “ The Nature and Properties of Soil” –
Prentice Hall Inc.
UNIT – I
PGDEM-01
ENVIRONMENT AND ITS COMPONENTS-II
Dr. Krishan Kumar
STRUCTURE
2.0
Objectives
2.1
The Hydrosphere
2.2
Classification of Aquatic Systems
2.2.1
2.2.2
Freshwater Systems
2.2.1.1
Lentic Systems
2.2.1.2
Lotic Systems
Marine Systems
2.2.2.1
The Ocean Relief Features
2.2.2.2
The Ocean Deposits
2.2.2.3
Zonation of Marine Water
2.2.2.4
Properties of Marine Water
2.3
Summary
2.4
Key Words
2.5
Review Questions
2.6
Suggested readings
2.0
Objectives
In the present chapter, we shall learn the following aspects of the hydrosphere:
• Significance of hydrosphere
• Types of freshwater systems and their properties
• Marine systems and their properties
2.1
The Hydrosphere
Hydrosphere refers to water on our planet. About 70 percent of the earth’s surface is
covered with water. Water is an essential component of our life. It is required by plants
and animals alike in their metabolic processes. What makes water so important and useful
to us are its unique properties. Its boiling point (100°C) and melting point (0°C) are such
that it can exist in all the three states of matter i.e. solid, liquid and gas. The chemical
structure of water makes it a very good solvent that is able to dissolve many substances in
it. Further, water attains maximum density at a temperature of 4°C. This means that ice or
water at 0°C are lighter than water at 4°C. The direct implication of this fact is that water
in various water bodies such as oceans, lakes, ponds, rivers etc. always freezes from top
to bottom. This is a highly favorable scenario particularly for those water bodies which
serve as a habitat to various kinds of species.
Almost all of the water present on our planet is found in oceans and seas. We call this as
marine water. In comparison, water present in the water bodies found on land (i.e. lakes,
ponds, rivers, aquifers etc.) is only a minute fraction of the water present in the oceans.
We call this as freshwater. Human beings and several other species found on land are
dependent upon this freshwater for their survival. The terrestrial organisms would have
found this amount of freshwater quite insufficient for their survival had not there been a
mechanism to replenish it. Fortunately for us, there is a natural pump that continuously
purifies the water present in the oceans and redistributes it over the entire planet without
any cost. This pump is none other than the sun which provides the energy to drive this
pump. Water in the oceans and other water bodies on earth continuously gets evaporated
into the atmosphere. A significant amount of water is also lost to the atmosphere through
transpiration by plant leaves. Water vapors present in the atmosphere condense to form
clouds containing pure water. Clouds blown by the wind systems redistribute this water
through precipitation in the form of rainfall / snowfall. Upon precipitation, water is either
absorbed by the soil or runs off the surface in the form of water channels, streams and
rivers to ultimately join the oceans. The water absorbed by the soil is available for plants
and a large part of it seeps into the sub-surface to replenish the groundwater aquifers.
Water precipitated in the form of snowfall either melts or contributes to the glaciers
present in the high altitude and latitude regions of the world. Thus water is continuously
cycled between its three phases. This is called the hydrological cycle (fig. 2.1).
Fig. 2.1 – The hydrological cycle
2.2
Classification of Aquatic Systems
Aquatic systems on earth can be classified broadly into two categories:
•
Freshwater systems
•
Marine systems
The freshwater systems can further be subdivided into two categories :
•
Lentic (standing water systems)
•
Lotic (running water systems)
In the coming sections, we shall briefly discuss the characteristics of each of the
above mentioned systems.
2.2.1
Freshwater Systems
The freshwater systems refer to the inland waters such as lakes, ponds, swamps
and rivers. Depending upon whether water is standing or flowing, freshwater
water systems may be classified into lentic and lotic systems.
2.2.1.1 Lentic Systems
This refers to non-flowing or standing water generally found in lakes, ponds,
ditches and swamps. Expressed sequentially, we also call it as a lake-pond-swamp
system. This indicates a successional pattern whereby a lake changes through
geologic time with the accumulation of inorganic and organic bottom deposits, silt
etc. into a swamp which finally turns into a terrestrial habitat.
Physical Properties of Lakes:
(i)
Light penetration: Intensity of light penetrating the lake waters is
reduced both by absorption and scattering. On the basis of light
penetration, the lake waters can be divided into three different zones :
Littoral Zone: It is the illuminated or the lighted zone in where
light easily penetrates. It may be defined into marginal, shallow
and deeper zones. The marginal zones (bank) contain amphibious
plants, animals, certain larvae etc. The shallow zone i.e. up to 3
meters depth contains partially submerged plants like Lotus, Lilly,
Hyacinth etc. Their roots lie in mud while leaves float. Animals in
shallow
zones
are
zooplanktons,
protozoa
and
certain
invertebrates. The deeper zone contains submerged plants like
Hydrilla, Trapa, Vallisneria etc. Animals are zooplanktons, worm,
Leech, Vertebrates like fishes, frogs, lizards etc.
Limentic Zone: It is up to 8 meters in depth. Here intensity of light
is very low therefore heavy vegetation is not present. Certain free
floating algae like Oscillatoria, Spyrogyne and Stergeocoloneum
are found. Phyto and Zoo planktons are present. Animals are
fishes, arthropods etc.
Profundal Zone: It is the dark zone below 8 meters. It is devoid of
vegetation. However, it is rich in decomposers like bacteria and
fungi and the decomposed/under decomposed organic matter of
dead bodies of plants and animals.
(ii)
Colour: The colour of fresh water bodies is derived from the constituents
present in it. This includes the bottom and suspended materials, plants and
animals, and also the chemical composition of water. Blue colour of pure
water is derived from the dominant scattering of the blue light by the
water molecules. Iron gives water yellow/brownish hue. Green colour is
usually associated with the presence plant material containing chlorophyll.
(iii)
Turbidity: The turbidity of lake waters may be due to the suspended
matter such as silt, clay or even planktons. Since turbidity greatly restricts
the penetration of light, it is an important limiting factor in the
productivity of lakes and ponds. Further, the suspended particles often
adsorb the plant nutrients and make them unavailable for uptake by
planktons. However, turbidity arising mainly due to the plankton
population is indicative of high fertility levels of a water body. Turbidity
may be temporary or perennial depending upon the nature of processes
causing turbidity.
(iv)
Temperature: One of the most important factors affecting various aquatic
processes is temperature. Since, aquatic organisms are thermally very
sensitive, the thermal structure of a standing water body mainly
determines the number and type of species found in it. In lakes of
significant depth, a striking seasonality is observed in their thermal
structure (particularly for those located in the temperate regions).
To understand the seasonal changes occurring in its thermal structure,
consider a typical lake situated in the temperate region. At the time of
onset of spring season, only the surface of the lake is frozen. The
temperature of water just below the surface is close to the freezing point.
As one moves down to deeper and deeper layers of water, the temperature
gradually increases. A condition exists such that the colder and lighter
water occurs on top of the warmer and heavier water.
However, as the spring season progresses, the surface water melts and
gradually the temperature of surface water rises to 4 C. This makes the
water in the top layer heavier than that in layers below. So, an overturning
of lake water occurs, thus mixing the lighter and heavier water. The
mixing process continues until the whole lake attains a uniform
temperature and density.
As the spring passes and the summer season begins, the temperature of
surface waters in the lake continue to increase beyond 4C. Thus the
surface water becomes lighter and lighter as the summer attains its peak.
Thus, a thermal stratification comes into existence with lighter and
warmer water at the top and heavier and colder water at the bottom. The
water body at this stage may be divided into three layers: An upper layer
called the epilimnion consisting of a uniformly warm, circulating and
fairly turbulent water. A deep, cold and relatively undisturbed layer called
the hypolimnion and a transition layer called the thermocline, which
separates the warm epilimnion with the cold hypolimnion. Thermocline is
a layer that is marked with very rapid decrease in temperature with depth.
As the summer season ends and the fall/autumn starts, surface
temperatures in the lake begin to decline. As the surface water cools, it
becomes heavier than the water below it and sinks to the bottom. Thus,
again a process of vertical mixing is initiated which continues till the lake
water is of same temperature and density.
Finally, as the winter starts, the surface water becomes still colder
approaching 0°C, thus becoming lighter than the water below it. Thus,
again a thermal stratification takes place in the lake leading to colder
waters at the top and warmer waters near the bottom.
Fig. 2.2 – Thermal profile of a lake
Chemical Properties of Lakes:
1. Oxygen: A very good indicator of the health of a water body is the amount
of dissolved oxygen present in it. Greater the amount of dissolved oxygen
better is the quality of water in a lake or a pond. The solubility of oxygen
in a water body decreases with increase in temperature. The concentration
profile of oxygen at various depths in a lake depends upon the presence or
absence of thermocline, the amount of vegetation, and the organic nature
of the bottom. Lakes rich plant life exhibit a marked diurnal pattern in the
dissolved oxygen concentration. The upper layer of the lake where
photosynthesis predominates due to the presence of light may become
supersaturated with dissolved oxygen during the day time. On the other
hand the lower layers where light availability is negligible, may be a zone
of oxygen deficit. During the night, when there is no photosynthetic
activity, oxygen consumption due to respiration and decomposition
continues. In eutrophicated lakes, early mornings just before the sunrise
may be periods of great scarcity of dissolved oxygen. These are the critical
periods when the lake may choke due to lack of dissolved oxygen.
2. Carbon dioxide: Amount of carbon dioxide present in water is quite
crucial for the aquatic organisms. Carbon dioxide is readily soluble in
water but since its atmospheric concentration is quite low (about 0.3 cc per
liter), water will hold only about 0.5 cc per liter or about 0.2 cc per liter of
CO2 at equilibrium. However, CO2 is stored in significantly large amounts
in the bound form i.e. carbonates and bicarbonates. Other than the
exchange through the air-water interface, there are a number of other
sources of carbon dioxide viz. respiration by the aquatic organisms,
bacterial decomposition of organic matter, bound forms of carbon dioxide.
Whereas respiration adds to the CO2 supply of the water, photosynthesis
tends to reduce it. In poorly buffered waters, a diurnal pulse (reciprocal to
the dissolved oxygen pulse) in the carbon dioxide concentration is
observed.
3. pH: The pH of ponds and lakes varies diurnally, being most alkaline in the
mid afternoon and most acidic just after the daybreak. PH of most
unpolluted lakes is found between 6.0 and 9.0.
4. Dissolved solids: Lake waters vary in dissolved solids from 15 to 350
ppm even though in extreme cases the dissolved solids concentration may
go up to 10,000 ppm. These dissolved salts contain the nutrients so
important for the growth of aquatic organisms. Silicates provide silicon to
diatoms and sponges, nitrites and nitrates provide nitrogen for protein
synthesis and salts of calcium, magnesium, iron, copper, sodium,
potassium, manganese etc. provide nutrients essential for the growth of
plants and animals.
Biological Properties of Lakes:
The lake biota consists of primary producers and the consumers. The primary producers
mean the photosynthetic communities which can be divided into macro and micro
(phytoplankton ) vegetation. Consumers include the zooplankton, nekton, neuston,
benthos and periphyton all of which occupy different functional and spatial niche in
relation to their food habit and trophic level.
2.2.1.2 Lotic Systems:
These refer to waters that move in a definite direction in the form of a stream. When
rainwater falls, a part of it infiltrates into the ground while the rest of it runs off the
surface in the form of narrow channels of water which deepen with each succeeding
shower. The stream is at first a temporary one, dependent for its water flow on the
rainwater. But as the channel is cut below the level of ground water table, the stream
becomes permanently fed by the seepage of ground water. Depending upon the size of a
running water system it may be classified as a brook, creek or a river. In fact, brookcreek-river series represents a sequence of changes indicating the origin of a river from a
mere rivulet. The basic force behind the running water is the gradient in slope. Greater is
the gradient in slope more will be the speed of running water. As the water runs down a
slope, it erodes the land that comes in its contact. The ultimate fate of a lotic series, which
is constantly degrading the land, is the reduction of its bed to the base level (where we
say that the slope is zero). In most cases, by base level we mean the sea level. The
evolution of a river may be divided into three different phases: youth, maturity and old
age. In its youth, a river is very swift as it descends the swift slopes. The main erosive
action of the river is in the vertical direction which gives rise to deep, narrow V-shaped
valleys. A river is characterized by gorges, rapids, waterfalls and river captures along its
course in the youth phase. There are vary few tributaries in its way during the youth
phase. In the maturity stage, lateral erosion also becomes significant. This widens the
banks of V-shaped valley. The volume of water increases with the merger of many
tributaries and this increases river’s load. Velocity of flowing water in maturity phase is
sufficient enough to transport most of its load though some deposition may also take
place. A river is characterized by meanders, river cliffs and interlocking spurs along its
course in the maturity phase. In the old age, the river moves downstream across a broad
and level plain. Its load is quite high due to the debris it brought down from the upper
course. There is almost no erosive action in the vertical direction, though laterally the
river still goes on eroding its bank. River’s main work in this phase is mainly deposition
of the sediments along its bed and formation of extensive flood plains. While the coarse
sediments are dropped, fine silt is carried down towards the river mouth. Large scale
deposition near the river mouth may split the river into several streams linked in an
intricate manner. A river is characterized by the formation of extensive flood plains, oxbow lakes and delta in the old age.
Fig. 2.3 The longitudinal and the cross-sectional profiles of a river
2.2.2
Marine System
Oceans of the world contain about 97 percent of the total of amount of water on the earth.
About 71 percent of the surface of earth is covered with the oceans. Marine water is
basically heterogeneous in nature. The temperature, density, salinity, dissolved organic
and inorganic matter, dissolved gases and other physico-chemical parameters of the
oceans vary both spatially and temporally. In fact, temperature and salinity are two
important parameters that affect the density of water. Temperature and salinity gradients,
therefore, play an important role in the movement of large masses of marine water. The
circulation pattern of marine water plays an important role in the occurrence and
distribution of nutrients, which in turn, affects the location of areas of concentration of
marine biota. In the coming sections, we shall discuss some of the important aspects of
marine ecosystem.
Table – Distribution of water in different reservoirs of earth
Reservoir
Volume (km3)
Oceans
1,322,000,000
Polar ice caps and all glaciers
29,200,000
Exchangeable ground water
24,000,000
Freshwater Lakes
125,000
Saline lakes and inland seas
104,000
Soil and subsoil water
65,000
Atmospheric vapor
14,000
Rivers and Streams
1,200
2.1.2.1 The Ocean Relief Features:
The ocean relief can be subdivided into four categories namely, the continental shelf, the
continental slope, the deep-sea plain and the ocean deeps.
1. The continental shelf: It is a gently sloping shallow platform extending from the
coast towards the open sea. The seaward edge of the continental shelf is about
150-200m deep. Width of the continental shelf is highly variable. Places where
the coasts are mountainous, continental shelf may be completely absent. On the
other hand, broad low land coasts like those of Arctic Siberia, continental shelves
as wide as 750 miles have been recorded. The shallowness of continental shelves
allows sunlight to penetrate through the water. This encourages the growth of
planktons on which millions of surface and bottom feeding fishes depend for
their survival. Continental shelves are, therefore, the richest fishing grounds in
the world. In addition, they are also the sites having potential for mining. About
20 percent of the world production of petroleum and gas comes from continental
shelves.
2. The Continental Slope: At the edge of the continental shelf, the seaward slope is
becomes quite abrupt. The steep slope descends to a depth of about 3600m from
the mean sea level. The continental slope joins the continental shelf with deep sea
plain. It is believed that the continental blocks end at the site of continental slope.
3. The deep sea plain: This is the undulating plain found at a depth of 3000m to
6000m. The deep sea plains are present in all the major oceans of the world. They
occupy about 40 percent of the ocean floor. These are also called the abyssal
plains or abyssal floors.
4. The ocean deeps: These are long, narrow, steep sided trenches in the ocean floor
as deep as 5500m. Their bottoms go far below the average depth of the oceans
which is about 4km. They lie along the fringes of the deep sea plains. The
greatest known deep sea trench is the Mariana trench near Guam islands in the
Pacific Ocean. This trench is about 11 km deep. The other notable ocean deeps
are the Mindanao Deep (10.7km), Tonga Trench (9.5km) and the Japanese
Trench (8.5 km) all in the Pacific Ocean.
Fig. 2.4 The Ocean Relief Features
2.1.2.2 The Ocean Deposits:
Different kinds of sediments that deposit on the ocean floor may be classified into muds,
oozes and clays.
1. The muds: These are the sediments that are basically derived from land and
deposit mainly on the continental shelves. The chemical composition of muds
impart them colours and according muds may be referred to as blue, green or red
muds.
2. The oozes: These are fine textured deposits derived from the oceans. Oozes
contain the remains of marine organisms with calcareous or siliceous parts.
3. The clays: These occur mainly as red clays in the deeper parts of the oceans and
are believed to be derived from volcanic eruptions.
2.1.2.3 Zonation of Marine Water:
Marine habitat may be classified into two divisions Pelagic and Benthic.
Pelagic division: This refers to the whole body of water forming the ocean. Pelagic
division may be classified into different zones either horizontally or vertically.
(A) Horizontal Zonation -Horizontally, ocean can be divided into coastal and shallow
Neretic Zone and deeper Oceanic Zone.
•
Neretic Zone – It starts from coastal margin up to the depth of 180m
generally. It is further divided into
(a) High Tidal Zones
(b) Inter Tidal Zones
(c) Low Tidal Zones
The high tidal zone remains exposed except few days of the months. Intertidal zone remains exposed in low tides and covers with water during high
tide. Low tidal area generally forms water covered oceanic floor. Neretic zone
is the most productive area. Water of this area contains more oxygen, carbon
dioxide and sunlight as compared to oceanic zone.
•
Oceanic Zone – The deep sea is called oceanic zone. In this area vegetation is
less.
(b)
Vertical Zonation
Vertically marine habitat is divided into following major zones i.e.
(i)
Epipelagic zone: This extends from the surface to a depth of 200m.
This is the illuminated zone where sunlight can enter. The whole
neretic zone lies in this area. Vegetation is thick and productivity is
high. Diurnal and seasonal changes in illumination distinguish it from
the mesopelagic zone.
(ii)
Mesoplagic Zone: It extends from a depth of about 200m to 1000m.
Very little sunlight (insufficient for photosynthesis) is able to penetrate
this zone. Temperature gradient is more gradual and does not show
much seasonal variation.
(iii)
Bathypelagic Zone: It is the aphotic or darker zone extending from a
depth of 1000m to 4000m. Temperature in this zone is low and water
pressure very high.
(iv)
Abyssopelagic Zone: It is the deepest part of oceanic zone which
extends beyond 4000m in depth. Due to absence of light, it is devoid
of vegetation. However, it is rich in organic matter, shells,
decomposers and scavenger fishes with large mouth.
(v)
Hadal Zone: The deep ocean trenches form what is called as the hadal
zone. In the deeper zones such as the hadal zone or the abyssopelagic
zone, the only source of light is the bioluminescence generated by
some of the marine organisms.
Fig. 2.5 The bathymetric zones of oceans
2.1.2.4 Properties of Marine Water
1. Temperature`
Temperature is an important property of marine water. It is a factor that affects
the movement of large masses of water as well as the type and distribution of
marine organisms. Temperature of ocean water varies spatially and temporally
both at the surface and at great depths. The temperature of ocean water at a given
place is affected by many factors such as the intensity and daily duration of solar
radiation, salinity and density of water, rate of evaporation, albedo of sea surface,
location and shape of the sea etc. However, due to the high specific heat and
constant churning of water, the annual range of temperature in any part of the
ocean is much less in comparison to land masses. Spatially, sea surface
temperatures range from –2°C in polar regions to about 25-30°C.Seasonal
variation in surface temperatures is small in low and high latitudes but is
somewhat greater in mid latitude seas. A seasonal variation of 8-10°C in the
temperate zones is not unusual. In the tropical oceans, warm surface water is
separated from the cold and dense water at the bottom by a permanent
thermocline at a depth of about 100-150m. In temperate waters, a temporary
thermocline develops at a depth of 15-40m during the summer. This layer,
however, disappears as the water cools in the winter and convectional mixing
occurs up to several hundred meters. Temperatures in the polar region vary very
little with depth due to continuous mixing of water initiated by cooling and
sinking of surface layers.
2. Salinity
Sea water contains a number of dissolved salts which give it a brackish taste.
Concentration of dissolved salts in water is represented by the salinity of water. It
is expressed as the number of grams of dissolved salts in 1000 grams of sea water.
The average salinity of marine water is 35 per thousand which means that on an
average 35 grams of dissolved salts are present in one kilogram of sea water.
Major salts present in sea water are sodium chloride (77.7%), magnesium chloride
(10.9), magnesium sulphate (4.7%), calcium sulphate (3.6%) and potassium
sulphate (2.5%). Though the relative concentrations of major elements in sea
water is same, the total amount of dissolved salts i.e. the salinity varies spatially.
Salinity of sea water is mainly determined by the difference between evaporation
and precipitation. To some extent, it is also affected by stream run off, freezing
and melting of ice, winds and the movement of ocean water.
In general, salinity is found to be the highest in ocean waters of the tropics. This is
because of the high rate of evaporation in the tropics due to clear skies, high
temperatures and presence of trade winds. Salinity decreases as one moves
towards equator or polewards from the tropics. In the equatorial region, salinity is
low since the difference between evaporation and precipitation is less owing to
heavy rainfall, high relative humidity, cloudiness and calm air. In the polar region,
there is very little evaporation. At the same time, melting of ice releases
freshwater, thus, decreasing the salinity in the range 20-30 per thousand.
3. Dissolved Gases :
Ocean water contains many dissolved gases such as nitrogen, oxygen and carbon
dioxide. The solubility of gases in the sea water is determined by the temperature
of water and the partial pressure of gases in the atmosphere. Exchange of gases
occurs across the air-sea interface, so that the surface water approaches saturation.
Subsequent mixing with deeper waters distributes these gases throughout the
ocean.
Nitrogen is the most abundant gas with concentrations ranging from 8-15 ml/liter.
Dissolved nitrogen is utilized by the nitrogen-fixing bacteria and the blue-green
algae that are found abundantly in the tropical and the subtropical waters.
The dissolved oxygen content of marine water varies from 2-8 ml/l depending on
water temperature and the biotic activity. The dissolved oxygen concentration
increases as one moves polewards from the tropical waters. Vertically, dissolved
oxygen concentrations are high at the ocean surface due to exchange of gases at
the atmosphere-water interface and the photosynthetic activity of phytoplankton,
floating in surface water.
Minimum oxygen concentrations are found at
intermediate depths where it is removed by animal respiration and the bacterial
oxidation of falling organic debris.
Carbon dioxide dissolved in sea water reacts with water to form carbonic acid
which dissociates and equilibrates as bicarbonate (HCO3-) and carbonate (CO32-)
ions.
CO2(g) ↔ CO2(l) ↔ H2CO3 ↔ HCO3- + H+ ↔ CO32- + 2H+
The above set of reversible reactions plays a very important role in controlling the
pH of marine waters. As CO2 is consumed in photosynthesis or dissolves in from
the air, pH should change since carbonic acid is either removed or added.
However, the change in pH is reduced or buffered by the huge reservoirs of
carbonates and bicarbonates of alkaline metals such as sodium, calcium and
potassium.
2.3
Summary
Hydrosphere refers to water on our planet. About 70 percent of the earth’s surface
is covered with water. It is required by all plants and animals for their metabolic
processes. It can exist in all the three states i.e. solid, liquid and gas on our planet
and is continuously cycled between different components (biotic and abiotic) of
our environment, a process called as the hydrological cycle. Aquatic systems may
broadly be classified into two categories – (i) freshwater systems and (ii) marine
systems. Freshwater systems may further be classified into running water (lotic)
systems and standing water (lentic) systems. Marine systems (oceans) are the
largest reservoir of water on earth. Freshwater systems, in comparison, contain
only a very small fraction of the total water present on our planet. Lakes are a
major class of freshwater systems. The physico-chemical and biological
properties of lakes govern the quality of freshwater present in them. Another
major class of freshwater systems is the rivers and streams. The properties of
running water in a river change as water flows from the upper course, through the
middle course up to the lower course till the river finally merges with the sea.
Marine water systems may be classified into different regions on the basis of
ocean relief features or into different zones on the basis of water depth.
Temperature and salinity are two of the most important properties of marine
water.
2.4
Key Words
Hydrosphere – refers to the water present on our planet.
Hydrological Cycle – Continuous cycling of water in different components of the
environment.
Lentic System - refers to non-flowing or standing water generally found in lakes,
ponds, ditches and swamps.
Thermocline – A layer of water that is marked with very rapid decrease in
temperature with depth.
Lotic System - refers to water that moves in a definite direction in the form of a
stream
Marine Systems – refers to the highly saline water contained in sea and ocean
basins of the world.
2.5
Review Questions
1.
What are lentic systems? What are different zones in which a lake can be divided
on the basis of light penetration?
2.
Explain the seasonal changes that take place in the thermal structure of a lake?
3.
Give a brief description of chemical properties of lakes.
4.
What are different stages in the course of a river?
5.
Explain the relief features of ocean bottom with the help of a diagram.
6.
What are different kinds of zones in marine water?
7.
What is salinity? How does it vary spatially in the oceans?
2.6
Suggested Readings
1.
Desonie, D. (2007), “Hydrosphere (Our Fragile Planet)”, Chelsea House
Publication.
2.
Smithson, P., Briggs, D., Atkinson, K. and Kenneth, A. (2004), “Fundamentals of
the Physical Environment”, Routledge ,an imprint of Taylor & Francis Books Ltd
3.
Leong, G.C. (1996), “Certificate Physical and Human Geography”, Oxford
University Press.
UNIT-II
PGDEM-01
ECOLOGICAL PRINCIPLES, BIOSPHERE AND ITS
ORGANISATIONAL LEVELS
Dr. Asha Gupta
STRUCTURE
1.0
Objectives
1.1
Introduction
1.2
Organisation
1.3
Definition of ecology
1.4
Terms used in ecology
1.5
Basic concept of ecology
1.6
Approaches to ecology- its main subdivision
1.7
Population ecology
1.7.1 Characteristics of a population
1.7.1.1 Population size and density
1.7.1.2 Dispersion
1.7.1.3 Natality (Birth Rate)
1.7.1.4 Mortality (Death Rate)
1.7.1.5 Age Distribution
1.7.1.6 Biotic Potential
1.7.1.7 Population Growth Forms
1.7.1.8 Population Fluctuations
1.7.1.9 Population Dispersal
1.7.2 Population dynamics
1.7.2.1 Theory of Population growth
1.7.2.2 Growth of Laboratory and Field Populations
1.7.2.3 Modifications of Logistic theory
1.7.3 Regulation of population density
1
1.7.3.1 Nature of factors that influence population
density
1.7.3.2 Key Factor Analysis
1.7.3.3 Self-regulation of Populations
1.7.3.4 Immigration, Emigration and Population
Dynamics
1.7.4 Population interactions
1.8
Summary
1.9
Keywords
1.10 Self assessment questions
1.11 Suggested readings
1.0
OBJECTIVES
•
To introduce concept of organisation.
•
To understand basics of ecology and various terms used in it.
•
To understand various characteristics of population and
various theories of population growth.
•
To introduce regulation of population density and various
interactions taking place among themselves.
1.1
INTRODUCTION
The portion of the earth where life exists is known as biosphere. It
ranges about 22.5 kms from the highest mountains to the deepest
oceans. The thin life supporting belt of the earth is known as biosphere.
It has three components as under:
Atmosphere:
Gaseous envelope around the earth’s surface is
known as Atmosphere.
Lithosphere:
Solid portion of earth’s crust is known as
Lithosphere.
2
Hydrosphere:
Aquatic
portion
of
biosphere
is
known
as
hydrosphere.
1.2
ORGANISATION
A
specific
pattern
of
interrelationships
between
various
components of a system at a particular level is known as organisation.
Important aspects of an organization are aggregation, interaction
and equilibrium.
Aggregation:
Act of collection or getting together to form the
whole unit or the organism.
Interaction:
Relationships
between
the
members
of
participants involved in the aggregate.
Equilibrium:
Aggregation
and
interaction
results
in
the
balance or stability in the biological systems e.g.
Hydrogen
and
oxygen
atom
aggregate
and
interact in the ratio of 2: 1 to form a more
stabilized structure of water (H2O) molecule.
They can also aggregate and interact in a
different ratio to form a molecule of H2O.
Biosphere
↑
Biomes
↑
Eco-system
↑
Community
↑
Population
3
↑
Individual
↑
Organ system
↑
Organs
↑
Tissues
↑
Cells
↑
Sub cellular components (Organelles)
↑
Molecules and compounds
↑
Atom
Living world
Non-living world
ORGANISATION IN LIVING AND NON-LIVING WORLD
From the above figure, it is clear that atom is the lowest level of
organization in the living world and non-living world while biosphere is
the highest level of organization in the living world.
1.3
DEFINITION OF ECOLOGY
Study
of
interrelationships
and
interdependence
organisms and their environment is known as ecology.
Ecology is made up of two words:
Ecology =
Oikos
+
Logos
House-hold or
Study
home.
4
between
i.e. study of organisms at home is ecology, in the simplest terms.
However, ecology has been defined in various ways by different
ecologists:
1961 Andrewartha
-
Ecology is the scientific study of the
distribution and abundance of animals
1967 R. Mishra
-
Study of interactions of form, function and
factors is ecology.
1971 Odum
-
Study of structure and function of nature
may be defined as ecology.
1985 Krebs
-
The Scientific study of the interactions that
determine the distribution and abundance
of organisms is known as ecology.
1988 Pianka
-
Ecology is the study of the relationships
between organisms and the totality of the
physical and biological factors affecting
them or influenced by them.
The wider definition of ecology is the study of structure and
function of nature. Structure includes the distribution and abundance of
organisms
as
influenced
by
the
biotic
and
abiotic
elements
of
environment and function includes how population grows and interact
including competition, predation, parasitism, mutualism and transfer of
energy and nutrients.
1.4
TERMS USED IN ECOLOGY
Species: A uniform interbreeding population spread over time and
space represents species. For maintenance of uniformity in structure,
function, reproduction, growth and development, it preserves its own
5
genetic stock i.e. a group of similar organisms which can interbreed
among themselves and which are reproductively isolated from others is
species.
Population: A group of similar organisms or members of a species
living in a specific area at a specific time constitutes population.
Community: A group of species (similar or dissimilar) living
together under more or less similar environmental conditions. So
community is aggregation of individuals of different species under more
or less similar environmental conditions.
Vegetation: Collective and continuous growth of plants in space is
called vegetation. Thus vegetation is actually the totality of plant growth
including large or small populations of each species intermixed in a
region or vegetation is the sum total of plant population covering a
region.
Flora: Species composition of the region irrespective of the
numerical strength of each species.
Factor: Any internal force, substance or condition that affects the
organism in any way is known as factor.
Environment: The sum of all factors affecting the organisms in
any way constitute the environment.
Habitat: The place where an organism lives.
Ecad: Ecad of a plant species is a population of individual~ which
although belong to same genetic stock but differ markedly in vegetation
characters such as size, shape, number of leaves, stems etc.
Eco-types: A population of individuals of a species which are
genetically different.
6
Ecotone: A zone of transition (presenting a situation of special
ecological intersect) between two different types of communities is known
as ecotone.
Life form: Sum of the adaptation of the plant to climate.
Biological spectrum: The percentage distribution of species among
various life forms of a flora is called biological spectrum of that place.
Ecological Succession: Ecological succession is a natural process
by which different groups or communities colonise the same area over a
period of time in a definite sequence.
Secondary Succession: Which starts from a primitive substratum
without any previous living matter.
Primary Succession: Succession which starts from previously
built substratum where living matter already exists.
Autogenic Succession: Replacement of the existing community as
a result of its interaction with the environment is known as autogenic
succession.
Allogenic Succession: Replacement of the existing community due
to the influence of any external force, condition is known as allogenic
succession. Eco-system: In a given area, the biotic assemblage of all the
organisms, plants as well as animals (i.e. communities) interacts with its
physical environment in such a manner that there is flow of energy
leading to clearly defined trophic structure, biotic diversity and material
cycles within a system known as ecosystem i.e. the interacting biotic and
abiotic components in an area which constitute eco-system.
Ecological niche: Ecological niche of an organism includes the
physical space occupied by it, its functional role in the community i.e.
7
trophic
position
and
its
position
in
environmental
gradients
of
temperature, moisture, pH etc and condition of existence.
Ecological equivalents: Organisms that occupy the same or
similar ecological niches in different geographic regions are known as
ecological equivalents.
1.5
BASIC CONCEPT OF ECOLOGY
1.
Living organisms and environment are mutually reactive
affecting each other in various ways.
2.
Environment is a complex of several inter-related factors and
is dynamic, works as a sieve selecting organisms for growth.
3.
A species puts every effort to maintain its uniformity in
structure, function, reproduction, growth and development
by preservation of its genetic pool.
4.
Not only the environment but also organisms modify the
environment (according to its needs) as a result of their
growth, dispersal, reproduction, death and decay etc.
5.
Clements and Shelford (1939) gave the concept of biome
where in all plants and animals are related to each other by
their coactions and reaction on the environment. According
to their view, under similar climatic conditions, there may
simultaneously develop more than one community, some
reaching the climax stage, others under different stages of
succession.
So the complex of several communities in any area, represented by
an assemblage of different kinds of plants, animals etc sharing a common
8
habitat is called biome i.e. the biome is the aggregation of different
communities in an area.
The above account about basic concept of ecology is only upon
structural basis. However, with the introduction of eco-system concept in
ecology, functional aspect along with structural concept is also
emphasized.
Tansley gave the concept of eco-system. With this new concept in
modern ecology, following are the basic concepts:
1.
When both, biotic and abiotic components are considered,
the basic structural and functional units of nature are ecosystems.
2.
There exist varying degrees of positive or negative or even
neutral interactions among organisms at both interspecific
and intraspecific levels within the members of same species.
Interspecific:
within
different
populations;
intraspecific:
within the members of same species.
3.
Energetics of eco-system is also involved in ecology as energy
is
the
driving
force
of
this
system.
Energy
flow
is
nutrients
in
an
unidirectional and non-cyclic.
4.
Biogeochemical
cycles-
Movement
of
ecosystem is always cyclic.
5.
Successful growth of organisms is governed by limiting
factors.
6.
Under natural conditions, different kinds of populations
undergo succession.
9
1.6
APPROACHES TO ECOLOGY- ITS MAIN SUBDIVISION
There are three main approaches:
1.
Based on taxonomic affinities
Plant Ecology
Animal Ecology
Study of interrelationships
of plants with their
environmental
2.
Study of inter-relationships of
animals with their environment.
Based on habitat- Habitat ecology
Study of habitats and their effects upon the organisms.
3.
Based on levels of organisation
a)
Population ecology: Study of interactions between individuals
of same species.
b)
Community
ecology:
Study
of
interactions
between
individuals of different species.
c)
Biome
ecology:
Study
communities of a biome.
10
of
interaction
between
different
d)
Eco-system ecology: Study of interactions between the biotic
and abiotic components of an eco-system.
Specialised fields of ecology
Freshwater ecology:
Study of interactions between freshwater
organisms.
Marine ecology:
Study
of
interactions
between
marine
organisms
Zoogeography:
Geographic distribution of animals.
Phytogeography:
Geographic distribution of plants.
Statistical ecology:
Statistical studies on population, sampling
techniques and community problem.
Estuarine ecology:
Study of interactions between estuarine
organisms.
Terrestrial ecology:
Study of interactions between terrestrial
(land) organisms.
1.7
POPULATION ECOLOGY
It is the study of individuals of the same species where the
processes such as aggregation, interdependencies between individuals
etc and various factors governing such processes are emphasized. It
plays an important role in a number of socio-economic problems both at
national as well as international level.
Population may be defined as a group of individuals of the same
species living in a specific area at a specific time.
11
1.7.1 Characteristics of a population
A population is characterized by several characters such as
discussed below:
1.7.1.1 Population size and density
Population size is expressed as the number of individuals in a
population. While population density is the size of a population in
relation to some unit of space i.e. population density is the number of
individuals
or
population
biomass
per
unit
area
or
volume
of
environment. Larger organisms such as trees may be expressed as 500
trees per hectare whereas smaller organisms such as fungi as 2 million
cells per cubic meter of soil.
Since the patterns of dispersion of organisms in nature are
different therefore it is important to differentiate between different types
of density. Population density is of two types:
(i) Crude Density
(i)
(ii) Specific or ecological density
Crude Density: It is the population density (number of
biomass) per unit total space.
(ii)
Specific or ecological or economic density: Population
density (number or biomass) per unit of habitat space i.e.
available area or volume actually colonized by population.
e.g. Individuals of plant species like Cassia tora are found
more in shady places in comparison to exposed areas. So
crude density of this plant will be total sum of the density of
Cassia tora in shady and exposed areas while ecological or
specific or economic density will be the density of Cassia tora
in shady places.
12
Concepts of growth rate
Since population is a changing entity, we are interested not only in
its composition and size but also in nature of its change. Population
density varies from place to place and from time to time. Such variations
in population density are brought about either by the birth of young ones
or the death of older individuals or the immigration or emigration of
individuals from one place to another place. This rate of change in
population density is obtained by dividing the change by the period of
time elapsed during the change.
So the growth rate of a population means the number of organisms
added to population per time
Growth rate=
Where,
ΔN
Δt
ΔN- Change in number of organisms
Δt- Change in time
Specific growth rate=
ΔN
= Average rate of change in the number
NΔt
of organisms per time, per organism.
N- Initial number of organisms
Percent growth rate =
ΔN
× 100
NΔt
1.7.1.2 Dispersion
Spatial distribution of individuals in a population relative to one
another is known as dispersion. Dispersion is of three types:
a)
Regular Dispersion: Here the individuals are more or less
equally spaced from one another. This occurs rare in nature
but common in managed systems such as cropland.
13
b)
Random Dispersion: Here the individuals are randomly
distributed in relation to each other. This is also relatively
rare in nature.
c)
Clumped Dispersion: In this, the individuals are aggregated
to form clumps i.e. found in groups. This type of dispersion
results from social aggregations such as family groups or
may be due to certain patches of the environment more
favourable for the population concerned.
1.7.1.3 Natality (Birth Rate)
It broadly covers the production of new individuals of any
organism. These new individuals are born, hatched, germinated or arise
by division etc. In human population, however, the natality rate is
equivalent to the birth rate.
Natality is the number of springs produced per female per unit
time. Natality is of two types:
a)
Maximum
(Absolute
or
Potential
or
Physiological)
Natality: It is the theoretical maximum production of new
individuals under ideal conditions. It is constant for a given
population. Also known as fecundity rate.
b)
Ecological or Realised Natality: Population increase under
actual, existing conditions is known as ecological or realized
natality. Here all possible existing environmental factors are
taken into consideration. It is also known as fertility rate.
ΔNn
= Absolute natality rate
Δt
ΔNn
= Specific natality rate (Natality rate per population)
NΔt
14
Where,
N = initial number of organisms
n = new individuals in population
t = time
1.7.1.4 Mortality (Death Rate)
Death of individuals in a population is referred as death rate.
Mortality is also of two types:
(a) Minimum mortality; (b) Ecological or realized mortality
a)
Minimum Mortality: It is also known as specific or potential
mortality. It is the theoretical minimum loss under ideal
conditions and is constant for a population. Individuals die
due to their old age only.
b)
Ecological or Realised mortality: It is the actual loss of
individuals under actual, existing environmental conditions.
Not constant for a population and varies with population and
environmental conditions. Since ideal conditions seldom
exist hence ecological mortality is always more than
minimum mortality.
Birth: Death ratio is known as vital index
Birth
× 100 = vital index
Death
Survivorship Curves
For a population rather than the death of individuals, their survival
is more important. Survival rates are generally expressed by survivorship
curves. The survivorship curves are of three types (Fig. 1.2):
15
FIG. 1.2: DIFFERENT TYPES OF SURVIVORSHIP CURVES PLOTTED ON
THE BASIS OF SURVIVORS AND AGE
i)
Highly Convex: It is characteristics of the species in the
population where mortality rate is low until near the end of
life span. Thus such species tend to live throughout their life
span with low mortality. e.g. Species of large animals such as
deer, mountain sheep and man exhibit such type of curves.
These species have more parental care.
ii)
Highly Concave: It is characteristic of species where
mortality rate is high during the young stages e.g. Oysters,
oak-trees and shell fish show this type of survivorship.
iii)
Diagonal Curves: If age specific survival is nearly constant
then such type of curves are shown. Diagonal curve is very
rare in nature. Small deviations to this diagonal curve exist
in nature e.g. slightly sigmoid or concave curve (As in figure
1.2) is characteristic of many birds, mice and rabbits. In
these organisms, mortality rate is high in the young stages
16
but lower and more nearly constant in the adult. In some
halometabolus
insects
(insects
with
complete
metamorphosis) such as butterflies, stair-case type of
survivorship curve is observed where survival differs at every
successive life history stages i.e. during transformation of egg
to larva to pupa, and finally to adult.
1.7.1.5 Age Distribution
In a population, individuals of different age groups are present. The
proportion of individuals in each group is known as age structure of that
population.
Age distribution is important as it influences both natality and
mortality of the population. Ratio of various age groups in a population
determines the current reproductive status of that population. There are
3 major ecological ages in any population; Pre-reproductive, Reproductive
and Post-reproductive. The relative duration of these three ages varies. In
man, the length of three ages is almost equal.
Age Pyramids
Model representing geometrically the proportion of different age
groups in the population of any organism is called age pyramid. These
are of the following three types:
FIG. 1.3: HYPOTHETICAL DIAGRAM SHOWING DIFFERENT TYPES OF
AGE PYRAMIDS
17
a)
A Pyramid with Broad base: It indicates a high percentage
of young individuals. In rapidly growing population, birth
rate is high and population may be growing exponentially as
in yeast and house flies etc. Each successive generation will
be more numerous than preceding one (Fig.1.3).
b)
Bell shaped polygon: It indicates moderate proportion of
young to old. Rate of growth becomes slow and stable i.e.
pre-reproductive and reproductive age-groups becomes more
or less equal in size and post reproductive age group
remaining as the smallest.
c)
Urn shaped: In indicates a low percentage of young
individuals. Here birth rate is reduced resulting in decrease
in no. of individuals in pre-reproductive, reproductive and
finally in post reproductive age group of a population. It
represents the declining population.
1.7.1.6 Biotic Potential
Each individual has the inherent power to grow. When the
environment is unlimited, specific growth rate becomes constant. The
value of growth rate under these favourable conditions is maximal and is
characteristic of a population. This is designated by ‘r’ and is known as
intrinsic growth rate or biotic potential. r is calculated as:
r=
Instantaneous specific
–
natality rate
Instantaneous specific
mortality rate
When a stable and stationary population exists under unlimited
environment, then the value of r is maximum and it is known as biotic
potential or r max or intrinsic rate of natural increase.
18
The term ‘biotic potential’ was introduced by Chapman and defined
as the inherent power of an organism to reproduce, to survive i.e. to
increase in numbers. It is sort of the algebraic sum of the number of
young ones produced at each reproduction, the number of reproduction
in a given period of time, the sex ratio and their general ability to survive
under given physical conditions.
1.7.1.7 Population Growth Forms
The characteristic pattern of increase of a population is known as
population growth forms. They represent interaction of biotic potential
and environmental resistance. There are two types of population growth
forms depending upon the types of growth curves:
a)
Sigmoid or S shaped growth form
b)
J shaped growth form
a)
Sigmoid or S shaped growth form: As shown in figure
(Fig. l.4), in this type of growth form, the population
increases slowly at first positive acceleration phase, then
more rapidly approaching a logarithmic phase, but slows
down gradually as the environmental resistance increases
negative acceleration phase until a more or less equilibrium
level is reached and maintained.
FIG. 1.4: TWO TYPES (S SHAPED & J SHAPED) GROWTH CURVES
19
The upper level, beyond which no major increase can occur, is the
upper asymptote of the sigmoid curve.
FIG. 1.5: THEORETICAL GROWTH CURVE SHOWING DIFFERENT
PHASES OF GROWTH IN A POPULATION
Upper asymptote is known as the carrying capacity of a population
which is defined as the maximum number of individuals that can be
supported in a given habitat.
b)
J Shaped growth form: As shown in figure (1.4) here the
population density increases rapidly in exponential fashion
and then stops abruptly due to environmental resistance,
followed by a crash.
1.7.1.8 Population Fluctuations
The size of a population is always under constant state of change.
After reaching the carrying capacity, the population density tends to
fluctuate below or above this level. These fluctuations may be due to
changes in the physical environment or due to interactions within the
population or both or due to interactions between closely related
populations. In nature there occur two types of fluctuations.
20
a)
Seasonal fluctuation: These are due to changes in life
history and seasonal changes. These are irregular e.g.
varying density of mosquitoes and house flies etc.
b)
Annual Fluctuation: These are-
i)
Primarily
by
annual
differences
in
physical
environment or extrinsic factors
ii)
By population dynamics or intrinsic factors
These fluctuations are cyclic and regular. The fluctuations can be
violent (inherent in J shaped forms) or damped (characteristic of Sshaped forms). These cyclic fluctuations are also known as oscillations
which are violent annually in communities with low species diversity.
Plant and animal trophic relationships may bring about oscillations
e.g. seed production in conifers is often cyclic. Seed eating birds and
other animals exhibit oscillations in their population density in relation
to these cycles.
Violent oscillations associated with exponential growth forms have
been observed in foliage insects in European forests and migratory
locusts etc.
1.7.1.9 Population Dispersal
Movement of individuals in or out of the population is known as
population dispersal. This plays an important role in the geographic
distribution of plants and animals to areas that were not previously
occupied by them. It may take place in search of food, to avoid predators,
preventing over-crowding due to wind, water, light or temperature
conditions, breeding behavior or due to some physical reasons. It occurs
in three ways:
21
a)
Emigration: Emigration involves the outward movement of
an organism from one place or country to another for
permanent residence.
b)
Immigration: Inward movement of an organism from one
place or country into a country or place e.g. Texas armadillo
during favourable conditions extends the border of its range
from Texas to Mississippi.
c)
Migration: Migration involves mass two way movement of
entire population, where the organisms return again to the
area from which they had moved. Such movements generally
take place during unfavourable conditions in the original
area to other areas where the conditions are favourable.
These are seasonal or periodical.
Migratory movements are common among mammals, bird, reptiles,
fishes and some insects. Among insects, migration is common in locusts,
butterflies, aphids and some coleopterans and Hemipterans.
Fishes like Salmons and eels also take long migratory route.
Migration is also common among birds e.g. Hirunda rustica- a migratory
bird, found in England in summer, whole of Europe and North-West
Africa whereas during winter it is found in Central and South Africa and
in India.
1.7.2 Population dynamics
Besides the characteristics, we have to study change in population
also. Populations exhibit specific growth forms which represent the
interaction of biotic potential and environmental resistance.
There are three approaches for the study of population dynamics:
22
a) Mathematical Models
b) Laboratory Studies
c) Field Studies
Theoretical
Simulation
Derive equations that may
describe population changes
and the behaviour of such
models can be tested by
comparing these with real
population
Use detailed data from real
populations and try to predict
future behaviour of these
populations under specific
environmental conditions.
Models are of use if they explain facets of population dynamics of
real populations. Their predictions are often tested experimentally by
using laboratory population. Mathematical and laboratory studies are of
value only if they help to explain the behaviour of natural populations.
Field studies are made, though these are complex.
1.7.2.1 Theory of Population growth
A model of geometric increase that assumes that there is no
environmental constraint on population growth may be assumed e.g.
Binary fission in protozoa where each individual divides into two at each
reproduction. To determine number (Nt) present in any generation where
there is doubling of population the following equation is used:
Nt = N0 (2t)
N0- Number present in the beginning.
The general equation for exponential growth is expressed as:
Nt = N0 (ert)
Where e is universal constant, the base of natural logarithm having
value 2.78; r is constant for a population under specific set of
environmental conditions and called as intrinsic rate of natural increase.
23
Logistic equation devised by Verhulst in 1838 to describe
population growth with an upper limit. Suppose environment has a
carrying capacity (K) for a particular population then logistic equation
assumes that the intrinsic rate of natural increase (r) is progressively
reduced as population size increases towards carrying capacity.
This equation simply assumes that r is reduced in relation to
proportion of remaining resources. Equation is derived from exponential
equation such that
⎡ (K − N )t ⎤
Nt = N0et ⎢
⎥ – Logistic growth equation
⎣ K ⎦
Logistic curve is S shaped on arithmetic co-ordinates and always
less steep than its exponential equivalent.
1.7.2.2 Growth of Laboratory and Field Populations
Exponential growth occurs only when there is no environmental
resistance which is possible only in laboratory culture if excess
organisms are removed to avoid over-crowding. However in nature such
type of growth experiences catastrophic decline as environment never
remains static it goes on changing e.g. insect population.
In laboratory, logistic growth has been observed in bacteria, yeast,
algae and protozoa. But these populations fluctuate around a carrying
capacity due to certain reasons such as competition, loss in female
fecundity, increased egg mortality etc at high density.
1.7.2.3 Modifications of Logistic theory
The following assumptions are considered in logistic theory:i)
Constant Environment do not change.
24
ii)
No time lags in response of population to changes in its
density.
iii)
Individuals have identical ecological characteristics in terms
of their response to density, regardless of age or sex.
iv)
Continuous growth.
v)
Age structure does not change in successive generation.
These assumptions unlikely to be met in lab and never obtained in
field, so logistic theory is modified to meet lab and field condition as
under:
i)
Inclusion of time lag.
ii)
Fluctuations in the carrying capacity.
iii)
There may be chance events for survival and reproduction of
individuals as well as for their death due to landslide etc.
iv)
Discontinuous growth of some populations which grow in
discrete steps with little or no overlap between successive
generation.
Keeping above account into view, one can say that mathematical
models are of use as:
i)
As theoretical abstractions, they set the crude limits to
possible pattern of population growth.
ii)
As models become more refined, they more closely fit the
behavior of real populations.
iii)
From above two it is clear that population models can be
applied to real practical problems as human population
growth and the management of plant and animal population.
1.7.3 Regulation of population density
The logistic equation and its derivatives assume that a population
will level off at its carrying capacity and the population fluctuates around
this level. The significance of population density has been a subject of
25
controversy among ecologists for last more than 50-years. Krebs (1985)
presented an account on factors which regulates population density:-
1.7.3.1 Nature of factors that influence population density
Population density can be increased either by natality or immigration
and decreased by mortality or emigration. These factors may be:
Both density dependent and density independent factors interact in
a population to determine observed densities and play an important role
in population dynamics.
1.7.3.2 Key Factor Analysis
Method to analyse mortality factors, to find out which may be
regulatory. The K values of each mortality factor together with total K
(sum of all factors) are plotted for successive generations. K factor that
most closely follows the pattern of K is called key factor e.g. in insect
population, loss of adults through migration or death is the key factor.
1.7.3.3 Self-regulation of Populations
Intraspecific
density
dependent
interactions
regulate
many
populations in the laboratory and in that sense, they are self regulatory
e.g. Accumulation of waste products may depress population growth.
26
1.7.3.4 Immigration, Emigration and Population Dynamics
Besides natality and mortality, immigration and emigration also
affect population density. These both are the features of dispersal.
Whereby individuals (singly or in groups) move from the population and
die if no suitable environment is found, establish a new population or
join an existing one at a new locality.
1.7.4 Population interactions
Individuals in single species populations interact not only with the
members of their own species but also with the individuals of other
populations e.g. aphids suck juices from the plants. Population
interactions are of two types:
i)
Intraspecific: Which occurs among the members of the
same species in a population interact with others for the resources (food,
space etc.). Ultimate effect is the decreased contribution to next
generations.
ii)
Interspecific: Interspecific interactions occurs among the
individuals of different species in population. The effect of interaction on
both the population may be either positive or negative or neutral or one
species may be benefited while the other is harmed.
Type of Interactions
Response of populations
A
B
O
O
+
+
—
—
—
O
+
O
+
—
+
—
Neutral
Mutualism
Competition
Amensalism
Commensalism
Parasitism
Predation
27
The difference in parasitism and predation is that in the former,
parasite causes the slow killing of host and parasitizes the body of host
while in the latter, predator kills and consumes the prey.
1.8
SUMMARY
Life
supporting
belt
of
earth
is
biosphere
and
has
three
components namely lithosphere, hydrosphere and atmosphere. Atom is
the smallest level of organisation in living and non-living world while
biosphere is the highest level of organisation in living world. Study of
interaction
and
interdependence
between
organisms
and
their
environment is known as ecology. There are 3 approaches to ecology–
taxnomic affinities, habitat ecology and based on levels of organisation.
Population is a group of individuals of same species living in a specific
area at a specific time. Characteristics of population are well explained in
this chapter. Besides this, an attempt has been made to clear concept of
population dynamics, regulation of population density and various
interactions taking place among population.
1.9
KEYWORDS
Ecological balance: Refers to a rarely attained state in which all
ecosystem inputs are equal to the system outputs.
Ecology: Study of inter-relationships between living organisms and
their physical environment.
Ecotone: Any zone of transition between clearly demarcated
groups of organisms or communities.
Ecological succession: Process in which communities of plant and
animal species in a particular area are replaced over time by a series of
different and often more complex communities.
28
Environment: Refers to the combination of external conditions
that influence the life of individual organism.
Population: Group of individuals of same species living in a
specific area at a specific time.
Denudation: Lowering of earth’s surface by the process of
weathering, mass movement, erosion and transportation.
Interspecific competition: Members of two or more species trying
to use the same limited resources in an ecosystem.
Intraspecific competition: Two or individual organisms of a single
species trying to use the same limited resources in an ecosystem.
1.10 SELF ASSESSMENT QUESTIONS
1.
2.
Define following terms:
i)
Biosphere
ii)
Flora
iii)
Species
iv)
Vegetation
v)
Edge-effect
What do you mean by ecology? What are the basic concepts
of ecology?
3.
Discuss the characteristics of a population.
4.
What are different types of population density?
5.
Discuss the models of population growth.
6.
How is the population of a place regulated?
29
1.11 SUGGESTED READINGS
1.
Kormondy, E.J. (2001): Concept of Ecology, Prentice Hall of
India, New Delhi.
2.
Begon/harper/Townsend
(1996):
Ecology-
Individuals,
populations and communities. Blackwell Science, USA.
3.
Smith, R.L. (2001): Ecology and Field Biology, Harber Collins
College Publishers, USA.
4.
Krebs,
C.J.
(2001):
Ecology.
Harber
Collins
College
Publishers, USA.
5.
Chapman J.L. and Reiss, M.J. (2000): Ecology Principles and
Applications, Cambridge University Press, USA.
30
UNIT-II
COMMUNITY
PGDEM-01
Dr. Asha Gupta
STRUCTURE
2.0
Objectives
2.1
Introduction
2.2
Characteristics of a community
2.2.1 Species Diversity
2.2.1.1 Local Vs Regional Diversity
2.2.1.2 Global Diversity
2.2.1.3 Species Diversity Hypothesis
2.2.2 Growth form and structure
2.2.2.1 Composition
2.2.2.2 Structure
2.2.2.3 Origin and development
2.2.3 Dominance
2.2.4 Ecological Succession
2.2.4.1 Causes of Succession
2.2.4.2 Trends of Succession
2.2.4.3 Types of Succession
2.2.4.4 General process of succession
2.2.5 Trophic Structure
2.2.5.1 Edges
2.2.5.2 Ecotones
2.3
Methods of study of communities
2.4
Summary
2.5
Keywords
2.6
Self assessment questions
2.7
Suggested readings
1
2.0
2.1
OBJECTIVES
•
To understand concept of community
•
To expose to different characteristics of community
•
To understand ecological succession
•
To have an idea about methods of study of community
INTRODUCTION
A population consists of organisms of a particular species. But
when several populations share a common habitat and its resources,
they interact among themselves and develop into a community. So
community is a larger unit than population.
In fact- A community is a naturally occurring and interacting
assemblage of plants and animals living in the same environment and
fixing, utilizing and transferring energy in some manner.
OR
It is a group of several species (plants and animals) living together
with mutual tolerance and beneficial interactions in a natural area. In a
community, organisms share the same habitat growing in an uniform
environment. A forest, a grassland, a desert or a pond are natural
communities. (By definition, it must include only living entities of
environment). A biotic community has its own characteristics. Animal
populations sharing a common habitat and interacting among themselves
form an animal community and plant population of an area form a plant
community.
A biotic community includes all populations of living organisms of
a common habitat, ranging from a tract of forest to the whole of forest,
from a small pond to a large lake and so on.
A community may be autotrophic i.e. it includes photosynthetic
plants and gains its energy from sun.
2
Some communities such as springs and caves are heterotrophic i.e.
they depend upon others for the energy.
The range of environmental conditions which a taxon can tolerate
is called its ecological amplitude. The composition of biotic community in
any habitat is dependent upon the environmental conditions present in
that habitat and the ecological amplitude of populations. Therefore the
climate and other biotic and abiotic conditions of a habitat determine the
nature of community i.e. the nature of a community is determined by the
adaptations of its organisms to physical environment-soil, temperature,
moisture, light, nutrients and interactions among its organismscompetition, predation, parasitism and mutualism.
2.2
CHARACTERISTICS OF A COMMUNITY
Like a population, a -community has also its own characteristics
which are not shown by individual components but by the group as a
whole. These are as follows:
2.2.1 Species diversity.
2.2.2 Growth form and structure.
2.2.3 Dominance.
2.2.4 Succession.
2.2.5 Trophic Structure etc.
2.2.1 Species Diversity
Each community is made up of different organisms-plants, animals
and microbes etc. which differ taxonomically from each other.
Among these species few are abundant while a few are present in
large proportions. There are two characteristics of distribution of species
within a community:
3
Species richness: Representing the no. of species in a community
and
Species evenness: indicating the relative abundance of individuals
among the species in a community.
Species diversity increases as the number of species increases and
as the number of individuals in the total population is more evenly
distributed among them.
In order to quantify species diversity, for purposes of comparison, a
number of indices have been proposed.
The most widely used index is Shannon index ( H ) it includes both
richness and equitability.
This index measures diversity by the formula:
H=
s
∑ (p )(log p )
i =1
i
Where,
i
H = diversity index
S = No. of species
I = Species number (ith species)
Pi = Proportion of individuals of the total sample
belonging to ith species.
This index is a measure of uncertainty.
If the value of H is higher then, greater is the probability or
uncertainty that next individual chosen at random from a collection of
species containing N individuals will not belong to the same species as
previous one. If it is lower then, greater the probability that the next
individual encountered will belong to same species as previous one.
The
more
abundant
species
are
not
necessarily
the
most
influential. In communities having a wide range of sizes, an index may
4
lead us to underestimate the importance of fewer but larger individuals
and overestimate that of more common species. One of the distinctive
features of indices is the inability to distinguish between the abundant
and the important species
Diversity indices may be used to compare species diversity within a
community (α diversity), between communities or habitats (β diversity)
and among communities over a geographical area (γ diversity).
2.2.1.1 Local Vs Regional Diversity
Species present in a local area are exposed to specific sets of
conditions. The presence or absence of species in a local area is influenced
by availability of nutrients, moisture and food, disturbance and physical
conditions of the environment, all of which change over time and space. The
death of one or several large trees in a forest can open up the habitat for
others which include shrubby growth and associated wild life. The influence
of this disturbance is temporary and will decline as the vegetation grows.
Local diversity is influenced by regional diversity which is a
product of climatic history, historical accidents and geographical position
of dispersal barriers.
2.2.1.2 Global Diversity
It is influenced both by latitudinal and altitudinal gradients.
Species of nesting birds, mammals, lizards, fish and trees decrease
latitudinally from tropics to the arctic. Species diversity of marine life
increases from the continental shelf where food is abundant but the
environment is changeable to deep water, where food is less abundant
but environment is more stable. Mountain areas generally support more
species than flatlands because of topographic diversity.
5
2.2.1.3 Species Diversity Hypothesis
Several hypotheses have been proposed to explain species diversity.
Many of them are similar but not identical.
i)
Evolutionary time hypothesis: According to this, diversity
is related to the age of communities. Old communities hold a greater
diversity than young communities. Tropical communities are older and
evolve and diversify faster than temperate or arctic communities because
the environment is more constant and climatic catastrophes are less.
ii)
Ecological time theory: It is based on time required for a
species to disperse into unoccupied areas of suitable habitat. As many
species do not get enough time to move into temperate zones, so these
areas are unsaturated by the total number of species they support. Many
cannot move until the barriers are broken, others are moving out from
the tropics into temperate regions. For example, natural spread of cattle
into North America from Africa by way of South America and the
northward spread of the armadillo.
iii)
Spatial heterogeneity theory: According to it, the more
complex and heterogeneous the physical environment, the more complex
will be its flora and fauna. Greater the variation in topography, more
complex will be the vertical structure of vegetation and the more types of
habitats, the community contains the more kind of species it will hold. For
example, forests with marked vertical structure holds more species of birds.
iv)
Climatic stability theory: More stable the environment,
more species will be present. Through evolutionary time, tropics among
all regions of earth has probably remained the most constant and
relatively free from severe environmental condition that could have a
catastrophic effect on a population. Under tropical conditions, selection
favours organisms with narrow niches and specialized feeding habits. At
6
higher altitudes, where the climate is severe and unpredictable, selection
favours organisms with broad limits of tolerance for variations in the
physical environment and with more generalized food habits.
v)
Productivity theory: The level of diversity of a community is
determined by the amount of energy flowing through the food web. The
rate of energy flow is influenced by the limitations of ecosystem and by
the degree of stability of the environment. This theory proposes that the
more food produced, greater will be the diversity.
vi)
Stability-time hypothesis: Sanders combined environment
stability hypothesis and the time hypothesis into another one- stability
time hypothesis. According to this two contrasting types of communities
exist- physically controlled and biologically controlled.
In physically controlled communities, organisms are subjected to
physiological stress by fluctuating physical conditions. The organisms are
subjected
to
severe
physiological
stress
and
the
probability
of
reproductive success and survival are low. So as a result, diversity is low.
In biologically controlled communities- physical conditions are
relatively uniform over long periods of time and are not critical in
controlling the species. The environmental is more predictable, the
physiological tolerances are low and diversity is high. However, there is
no wholly physically or biologically controlled community.
vii)
Competition theory: In environment of high physical stress,
selection is controlled largely by the physical variables.
viii)
Predation theory: According to this, higher species diversity
exists in those communities in which predators reduces the number of
prey to a level where inter-specific competition among them is greatly
reduced, allowing the co-existence of a number of prey species.
7
All the above hypotheses are based on the assumption that the
communities exists at competitive equilibrium. The rate of change of
competitors is zero. But rarely the natural communities are at
equilibrium. Fluctuations in the physical environment, predation,
herbivory and all sorts of density dependent mortality keep natural
communities in a state of nonequilibrium.
ix)
Dynamic equilibrium hypothesis: It is based on the
differences in rates at which populations of competing species reach
competitive equilibrium.
The major determinant of diversity in non-equilibrium situations is
the population growth rates of competitors. Most communities fail to
achieve equilibrium because of fluctuating environment and periodic
reductions in population. In the absence of disturbances, an increase in
population growth of major competitors results in low diversity. Greatest
diversity occurs at intermediate levels of disturbance.
Species diversity can be related to structure of habitat, diversity of
micro habitat, nature of physical environment, climate and protection for
its adverse effects, competition, predation, food availability, time and
geographical barriers etc.
2.2.2 Growth form and structure
Each
community
has
its
own
composition,
structure
and
developmental history.
2.2.2.1 Composition
Communities may be large or small. Larger ones extend over areas
of several thousands of square kilometers as forests, others such as
deserts etc are comparatively smaller with dimensions in hundreds of
kilometers and still others such as meadows, rivers, ponds, rocky
8
plateaus etc occupying a more restricted area. Very small sized
communities are the groups of micro-organisms in such microhabitats,
as leaf surface, fallen log, litter, soil etc. The number of species and
population abundance in communities vary greatly.
2.2.2.2 Structure
Besides composition and dominance the communities exhibit a
structure or recognizable pattern in the spatial arrangement of their
members. Thus structurally a community may be divided horizontally as
well as vertically into sub-communities.
i)
Horizontal stratification: A community may be divided
horizontally into sub-communities which are the units of homogenous
life form and ecological relation. This horizontal division constitutes the
zonation in the community.
Horizontal stratification results from an array of environmental and
biological influences. Soil structure, soil fertility, moisture conditions and
aspect influence the micro distribution of plants. Patterns of light and
shade, shape the development of understory vegetation. Run off and small
variations in topography and microclimate produce well-defined patterns of
plant growth. Grazing animals also have an impact on spatial patterning of
vegetation. Vegetative or colonial reproduction produces distinctive clumps
or patches of certain plants in a homogenous environment. Allelopathic and
shading effects lead to the suppression of some plant species and to the
development and growth of others. A patchy environment in turn influences
the distribution of animal life across the landscape.
ii)
Vertical Stratification: Stratification of a community is
determined largely by the life form of plants-their size, branching and
leaves which in turn influences and is influenced by the vertical gradient
9
of light. The vertical structure of the plant community provides the
physical structure in which many forms of animal life are adapted to live.
A well developed forest eco-system consists of several layers of
vegetation each of which provides a habitat for animal life in the forest.
From top to bottom, these layers are the shrub layer, the herb or ground
layer and forest floor and then root layer and soil strata.
DIAGRAMMATIC REPRESENTATION OF A COMPLEX DECIDUOUS
FOREST COMMUNITY SHOWING STRATIFICATION
10
Canopy is the major site of primary production and has a
pronounced influence on the structure of rest of forest. If it is open, then
considerable sunlight reaches the lower layers and shrub and understory
tree layer are well developed and vice-versa.
The nature of herb and shrub layers depends upon soil conditions,
slope position, density of over story and aspect of slope, all of which can
vary from place to place throughout the forest. The final layer forest floor
depends on all these factors and in turn determines how the nutrients
will be recycled.
Similar is the case with other communities such as grassland and
aquatic communities.
Grassland communities have a herbaceous layer, then ground
layer and root layer.
The strata of aquatic communities are determined by light
penetration, temperature profile and oxygen profiles. e.g. A fresh water
lake consists of littoral zone, limnetic zone and then profundal zone.
A well stratified lake in summer contains a layer of epilimnion,
then metalimnion, hypolimnion and a layer of bottom mud.
DIAGRAMMATIC SKETCH SHOWING THREE MAJOR ZONES OF A
FRESHWATER BODY, AS LAKE
11
Each vertical layer in community is inhabited by its own more or
less characteristic organisms. Many highly mobile animals confine
themselves to only a few layers, particularly during the breeding season.
Occupants of the vertical strata may change during the day or season.
Such changes reflect daily and seasonal variations in humidity,
temperature, light, O2 content of water and other conditions or different
requirements of organisms for the completion of their life cycles.
2.2.2.3 Origin and development
Each community has its own origin and development and process
of formation or development of a community is known as ecological
succession. During this process; seeds and propagules of the species
reach in a barren area. This is known as migration. The seed or
propagules germinates into seedlings and develops into adults. But only
few of them survive due to non-suitability of environment. The surviving
adults are capable of successful growth and establish there. This process
of successful growth and seedling establishment is called ecesis. As a
result of migration and subsequent ecesis, species colonize the new area.
By this time, with the changing environment due to plants growth,
several other species of both plants and animals start colonizing the area
and sooner or later the area is colonized by a definite community. This
community constitutes the biotic environment of that particular area.
Species of plants and animals live together in a community under a
specific set of environmental conditions. But due to changing time and
environment, individuals in a community interact and establishes two
types of relationships between the organisms of a community and
environment as under:
i)
Inter-relation
between
organisms
themselves:
relations are chiefly in respect of food and space. These include:
12
Such
a)
Competition: In the process of rapid colonization, individuals
become aggregated at a place and there develop intra as well
as interspecific interactions for species and food. As a result
pioneer species change the environment while others perish.
b)
Stratification: Stratification is the result of interdependencies
of species e.g. lianas and epiphytes grow on other plants.
c)
Cohabitation: A number of chemicals are reported to be
secreted by vegetative organs of plants of a community which
modify the edaphic conditions. Similarly, chemicals secreted
by fungi, bacteria and actinomycetes in soil affect the root
system as well as shoots of higher plants.
ii)
Relationships between species and the environment:
Organisms and the environment interact with each other and bring about
several modifications in the environment. This is one of the chief reasons
that a particular community changes to another type sooner or later in a
particular area.
2.2.3 Dominance
In each community, there are diverse species All these species are
not equally important but there are only a few overlapping species which
by their bulk and growth modify the habitat and control the growth of
other species of community, thus forming a sort of characteristic nucleus
in the community. These species are called dominants. The dominants in
a community may be the most numerous, possess the highest biomass,
cover most of the space, make the largest contribution to energy flow or
mineral cycling or by some means control or influence the rest of the
community.
13
Generally in most of the communities, only a single species is
dominant and in such a case the community is named after the name of
dominant species e.g. red wood forest community.
The degree of dominance expressed by anyone species appears to
depend upon the position it occupies on a physical or chemical gradient.
A species becomes dominant because it can exploit a range of
environmental conditions more efficiently and adapt to a wider range of
ecological tolerances than associated species. For example, at one point
on a moisture gradient, species A & B may be dominants. But as the
conditions becomes dry, species B may assume a subdominant position
and its place may be taken by some other species. Nutrient enrichment,
too, can change the structure of community.
Dominance is measured either as relative abundance of a species,
comparing the numerical abundance of one species to the total
abundance of all species or as relative dominance- a ratio of basal area
occupied by one species to total basal area or as relative frequency.
Generally all these three measurements are combined to arrive at an
importance value for each species.
Abundance=
Total no. of individuals of species in all sampling units
No. of sampling units in which species occurred
Frequency (%) = No. of sampling units in which species occurred ×100
Total no. of sampling units studied
Density =
Total no. of individuals of species in all sampling units
Total no. of sampling units studied
Relative density =
Density of the species
× 100
Total density of all species
Relative frequency =
Frequency of the species
× 100
Total frequency of all species
14
Relative dominance =
Dominance of the species
× 100
Total dominance of species
Important Value Index = Relative density + Relative frequency
+ Relative dominance.
2.2.4 Ecological Succession
Each community has its own developmental history. It develops as
a result of some directional change in it with time. The occurrence of
relatively definite sequences of communities over a period of time in the
same area is known as ecological succession.
Or it may be defined as an orderly process which involves change
in species structure and community processes with time and results from
modification of the physical environment by the community and it
culminates in a stabilized eco-system which is known as climax.
2.2.4.1 Causes of Succession
There are three major causes of succession:
i)
Initial or initiating causes: These are climatic as well as
biotic. Climatic factors include erosion and deposits, wind, fire etc caused
by lightening or volcanic activity and biotic include various activities of
organisms. These causes produce bare areas or destroy the existing
populations in an area.
ii)
Ecesis (continuing) causes: These are the processes as
migration, ecesis, aggregation, competition reaction etc which cause
successive waves of populations as a result of changes chiefly in the
edaphic features of the area.
iii)
Stabilizing ca uses: These cause the stabilization of the
community. Climate of the area is the chief cause of stabilization.
15
2.2.4.2 Trends of Succession
It proceeds along four lines:i)
Continuous change in the kind of plants and animals.
ii)
An increase in the diversity of species
iii)
An increase in the organic matter and biomass supported by
the available energy flow.
iv)
Decrease in net community production or annual yield.
2.2.4.3 Types of Succession
Succession is of many types as under:
i)
Primary;
ii) Secondary;
iii) Autogenic;
iv)
Allogenic;
v) Autotrophic;
vi) Heterotrophic etc.
i)
Primary succession: It starts from primitive substratum.
The first group of plants establishing there are known as the pioneers,
primary community or primary colonizers.
ii)
Secondary succession: It starts from previously built
substratum with already existing living matter.
iii)
Autogenic
succession:
Vegetation
itself
modifies
the
environment by reacting with it and causes its own replacement.
iv)
Allogenic succession: Replacement of the vegetation is
caused largely by any other external condition but not by the existing
vegetation.
v)
Autotrophic succession: Here the early and continued
dominants are autotrophs. There is gradual increase in the organic
matter content supported by energy flow.
vi)
Heterotrophic
succession:
Here
early
dominants
heterotrophs. There is progressive decline in energy content.
16
are
There are so many kinds of succession depending upon the
environment where the process has begun such as:
a)
Hydrosere: Succession takes place in water such as in
ponds, lakes and stream.
b)
Xerosere: In xerophytic conditions, process of succession
takes place.
c)
Lihosere: Succession takes place on rocks.
d)
Halosere: Succession takes place in saline water or soil
e)
Psammosere: Succession takes place on sand.
DIAGRAM SHOWING DIFFERENT PLANT COMMUNITIES APPEARING AT
DIFFERENT STAGES OF A HYDROSERE ORIGINATING IN A POND
17
2.2.4.4 General process of succession
The whole process of a primary succession is completed through a
number of sequential steps, which follow one another. These steps in
sequence are as follows:
i)
Nudation: This is the development of a bare area without
any form of life due to several causes such as landslide, erosion,
deposition etc. The cause of nudation may be:
a)
Topographic: Due to soil erosion by gravity, water or wind,
the existing vegetation may disappear. Other causes may be
deposition of sand etc., landslide, volcanic activity and other
factors.
b)
Climatic: Glaciers, dry period, hails and storm, frost, fire etc
may also destroy the vegetation.
c)
Biotic:
Man
is
responsible
for
destruction
of
forests,
grasslands etc. for industry, agriculture, housing etc. Other
factors are disease epidemics due to fungi, viruses etc which
destroy the whole population.
ii)
Invasion: It is the successful establishment of a species in a
bare area. The process is completed in following 3 successive stages:
a)
Migration (dispersal): The seeds, spores or other propagules
of the species reach the bare area. This process is known as
migration, and is generally brought about by air, water etc.
b)
Ecesis (establishment): After reaching to new area, the
process of successful establishment of the species starts and
is known as ecesis.
18
c)
Aggregation: After ecesis, as a result of reproduction, the
individuals of the species increase in number and they come
close to each other. This process is known as aggregation.
iii)
Competition and Co-action: After aggregation of a large
number of individuals of the species at the limited place, there develops
competition mainly for space and nutrition. Individuals of a species affect
each other’s life in various ways and this is called co-action.
iv)
Reaction: This is the most important stage in succession.
The mechanism of modification of the environment through the influence
of living organisms on it is known as reaction. As a result of reactions,
changes take place in the environment and as a result it gets modified,
becoming unsuitable for existing community which sooner or later
replaced by another community. The whole sequence of communities
that replaces one another in the given area is called a sere and various
communities constituting the sere are known as seral communities, seral
stages or developmental stages.
v)
Stabilization (climax): Finally, there occurs a stage when
the final terminal community becomes more or less stabilized for a longer
period of time and it can maintain itself in equilibrium with the climate of
the area. This final community is not replaced and is known as climax
community and the stage as climax stage.
2.2.5 Trophic Structure
Nutritionally, each community, a group of species of autotrophic
plants as well as heterotrophic animals, exists as a self sufficient,
perfectly balanced assemblage of organisms.
19
2.2.5.1 Edges
The region where two or more different vegetational communities
meet is known as edges. Some edges result from abrupt changes in soil
type, topographic differences, geomorphic differences and microclimatic
changes. Edges may be inherent or induced. Inherent edges are naturally
produced while induced ages are produced by natural disturbances such
as fires, storms and floods or from human induced disturbances such as
livestock grazing, timber harvesting, land clearing and agriculture.
2.2.5.2 Ecotones
The region where two or more communities not only meet but also
intergrade (blend) is called ecotone. Ecotones arise from the blending of
two or more vegetational types. Because of species responses, the variety
and density of life are often greatest in and about edges and ecotones.
This is known as edge effect.
Edge effect is influenced by the amount of light available- its
length, width and degree of contrast between adjoining vegetational
communities.
2.3
METHODS OF STUDY OF COMMUNITIES
Various methods can be used for the study of communities such
as:
1.
Floristic Methods: Here the flora is studied by listing
various genera and species present in the community. These are actually
descriptive methods and give no idea of the composition, structure and
growth forms of the community.
2.
Physiogenomic Methods: In this method, various species of
the community are studied chiefly in terms of their growth forms, general
20
structure and spread etc. Out of various physiognomic methods,
Raunkiaer’s
life
form
method
is
more
popular
but
still
these
physiogenomic methods do not provide much detailed information on the
composition, structure, species diversity, growth, trends of succession
and other characteristics of the community.
3.
Phytosociological Methods: As discussed above, none of
the above two methods provide detailed information on the composition,
structure, species diversity, growth, trends of succession and other
characteristics of the community. So these phytosociological methods
were developed in USA, Britain and Southern Northern Europe.
In these methods 03 forms of sampling units viz. area, line and
point are used and the sampling unit is in the form of a quadrat (giving
square, rectangular or spherical area), a transect (as a line or belt
transect for study along a gradient) and a point.
These three quadrat methods are commonly used which involve
determining the minimum requisite size of quadrat to be laid down,
determining the minimum number of quadrates to be laid down and
recording of the species their listing and counting of individuals of each
species followed by computation of frequency, density, abundance,
dominance and IVI, so that a complete picture about the qualitative and
quantitative characteristics of the community are known.
2.4
SUMMARY
A population is an aggregation of individuals of same species in a
specific area at a specific time while community is aggregation of
individuals of different species. Various characteristics of community
such as species diversity, growth form and structure, dominance,
succession and trophic structure are discussed in detail in this chapter.
Species diversity is of various types and diversity indices are used for
21
estimation of species diversity in a community. Several hypotheses have
been proposed to explain species diversity. Many of them are similar but
not identical. Each community has its own composition, structure and
developmental history. Structurally a community may be divided
horizontally as well as vertically into sub-communities. Each community
has its own origin and development & process of formation or
development of a community is known as ecological succession.
Ecological succession occurs through nudation, invasion, competition
and co-action, reaction and then stabilization. It may be of various types
such as primary, secondary, autogenic, allogenic, autotrophic and
heterotrophic. A number of methods such as floristic, physiogenomic and
phytosociological can be used for study of community.
2.5
KEYWORDS
Community: A group fo interacting population in time and space.
Biodiversity: Variety and variability among living organisms.
Autotroph: Organisms capable of manufacturing their own food.
Heterotrophs: Organisms dependent upon another organisms for
their food.
Climax community: Final phase of primary succession within
which a fully developed and mature ecosystem attains equilibrium with
environment.
Dominant: Any plant or animal species that competes most
successfully for the essential requirements of life and exerts a physical
influence on its habitat so limiting the performance of other species
which inhabit the same living area.
22
Predation: The act or practice of capturing another creature (prey)
as a means for securing food.
2.6
SELF ASSESSMENT QUESTIONS
1.
What is meant by species diversity? How would you
determine Shannon’s Index of species diversity? Discuss
various hypotheses related to species diversity.
2.
Discuss
briefly
the
inter-relations
between
different
organisms in a community.
3.
Discuss various types of succession and the process of
succession.
2.7
SUGGESTED READINGS
1.
Smith, R.L. (2001): Ecology and field biology. Harber Collins
College Publishers, USA.
2.
Colinvaux, P. (1996): Ecology 2. John Willey and Sons,
Canada.
3.
Beeby, A. (1993): Applying ecology. Chapman and Hall,
Madras.
4.
Chapman J.L. and Reiss, M.J. (2000): Ecology Principles and
Applications. Cambridge University Press.
5.
Karmondy, E.J. (2001): Concept of Ecology. Prentice Hall of
India Pvt. Ltd., New Delhi.
23
UNIT-II
PGDEM-01
ECO-SYSTEM, ITS STRUCTURAL AND FUNCTIONAL
ASPECTS, ENERGY FLOW, BIOGEOCHEMICAL
CYCLES AND REGULATION OF ECOSYSTEMS
Dr. Asha Gupta
STRUCTURE
3.0
Objectives
3.1
Introduction
3.2
Kinds of ecosystems
3.3
Aspects of an ecosystem
3.3.1 Structural Aspects of an Ecosystem
3.3.1.1 Abiotic Components
3.3.1.2 Biotic (living) Components
3.3.1.3 Trophic Structure
3.3.1.4 Ecological Pyramids
3.3.2 Functional aspects of an ecosystem
3.3.2.1 Productivity of Ecosystem
3.3.2.2 Food Chains in Ecosystems
3.3.2.3 Food Web
3.3.3 Energy Flow in Ecosystem
3.3.3.1 Fundamentals of Thermodynamics
3.3.3.2 Single Channel Energy Models
3.3.3.3 V-Shaped energy flow models
3.3.3.4 Classification of Ecosystem on the basis of
source and level of energy flow
3.3.4 Biogeochemical Cycles
3.3.4.1 Gaseous
3.3.4.2 Sedimentary
1
3.3.4.3 Carbon cycle
3.3.4.4 Nitrogen cycle
3.3.4.5 Phosphorus cycle (P cycle)
3.3.4.6 Water (hydrological) cycle
3.3.5 Regulation of ecosystems
3.3.5.1 Homeostasis
3.3.5.2 Feedback
3.0
3.4
Summary
3.5
Keywords
3.6
Self assessment questions
3.7
Suggested readings
OBJECTIVES
•
To understand the concept of ecosystem
•
To have an idea of structural and functional aspects of an
ecosystem
•
3.1
To understand regulation of ecosystem
INTRODUCTION
The term eco-system was proposed first of all by A.G. Tansley in
1935 and he defined ecosystem as “the system resulting from the
integration of all the living and non-living factors of the environment”. So
ecosystem includes not only the organism complex but also the whole
complex of physical factors forming the environment.
System resulting from the interaction of communities in a given
area with their physical environment in such a way that a flow of energy
leads to clearly defined trophic structure, biotic diversity and material
cycle within it is known as an ecosystem. An ecosystem may thus be as
small as a pond, a cropland or as large as an ocean, desert or forest.
2
Ecosystem represents the highest level of ecological interaction
which is energy based and this functional unit is capable of energy
transformation, accumulation and circulation. Its main function in
ecological
sense
is
to
emphasize
obligatory
relationships,
inter-
dependence and casual relations.
3.2
KINDS OF ECOSYSTEMS
ECOSYSTEMS
3.3
ASPECTS OF AN ECOSYSTEM
Two major aspects of an ecosystem are structure and function. The
structure consists of various components, their interactions which make
the ecosystem are ultimately, responsible for the functioning of the
ecosystem.
3
3.3.1 Structural Aspects of an Ecosystem
Abiotic components
Biotic components
3.3.1.1 Abiotic Components
1.
Amount of inorganic substances such as P, S, C, N & H etc
involved in material cycles. The amount of these inorganic substances
present at any given time in an ecosystem is designated as the standing
state.
2.
Amount
and
distribution
of
bio-chemicals
such
as
chlorophylls etc and of organic materials such as proteins, carbohydrates
and lipids etc present either in the biomass or in the environment i.e.
biochemical structure that link the biotic and abiotic components of the
ecosystem.
3.
Environmental factors are of three types
a)
Climatic
factors
such
as
rainfall,
humidity,
temperature, light.
b)
Topographic factors such as altitude, slope, direction
of mountain ranges etc.
c)
Edaphic factors such as soil composition, soil texture,
soil biota etc.
These
factors
bring
marked
distributional,
structural
and
functional changes in organisms. An organism requires harmonious
relationship with its immediate environment for its proper growth and
reproduction etc. The diversity between the vegetation of a desert and a
rain forest or diversity of consumers between them indicates the role of
environmental factors on the distribution and survival of organisms in
different ecosystems. These environmental factors exhibit diurnal,
4
seasonal, annual and cyclic variation to which the organisms are
subjected.
(A)
CLIMATIC FACTORS
Climate of any region is chiefly determined by such meteorological
influences such as relative humidity of air, temperature, wind pressures
and
evaporation
rates.
These
factors
include
light,
temperature,
humidity, precipitation and wind.
Light factor
Light is well known for its effects on such basic physiological
processes of plants such as photosynthetic, transpiration and seed
germination, flowering etc. Sun light is the ultimate source of energy for
biological world. Light intensity reaching the earth’s surface shows much
spatial variations being influenced by some factors such as atmosphere
chiefly:
i)
Atmospheric gases, nitrogen, O2 that absorb and disperse
small fractions of shorter wavelengths.
ii)
Suspended particles e.g. solid particles dispersed in air such
as dust and smoke or in water such as clay, silt and
plantation etc.
iii)
Topographic factors such as direction and slope of land
surface which bring marked variations in intensity and daily
duration of insulation. Radiant energy reaching the earth is
transformed to other forms of energy such as thermal,
mechanical, electrical etc which brings about changes in
several physical process in the atmosphere as well as on the
earth. Light intensities are closely related with atmospheric
temperature
and
relative
humidity.
organisms directly as well as indirectly.
5
Thus
light
affects
Large amount of solar radiations is absorbed in the atmosphere.
The amount of solar radiations is greater at higher altitudes than at sea
level. This is due to thinner layers of air at higher altitudes.
The layers of vegetation bring about variations in light intensities
reaching at various heights of mountains. It is higher in upper region in
comparison to lower region. Besides this, the layers of water also have
pronounced effects on light intensities. So, different zones of vegetation
are found in aquatic bodies. Besides layers of water, edaphic factors may
also play an important role in plant distribution.
Effects on plants
Light directly or indirectly affect the plants life in the following
ways:
1.
Chlorophyll productions
2.
Heating actions: Light results in increase in temperature of
the leaf.
3.
Effect on transpiration rate-Transpiration rate is increased in
presence of light.
4.
Stomatal movement- opening and closing of stomata is
related to light
5.
Distribution of plants- light conditions at poles are different
from other parts of earth. So amount of light reaching at
earth’s surface varies with latitude i.e. distance from equator.
This causes differences in vegetation at various latitudes.
6.
Overall vegetative development of plant parts- Plants are
classified on the basis of their light requirements and effects
of light on overall vegetative development.
a)
Heliophytes- growing best in full sunlight
b)
Sciophytes- growing best at low sunlight
6
c)
Facultative heliophytes- although grow best at lower
light intensities but can grow well in full sunlights.
d)
Facultative sciophytes- best at higher but can also
grow in lower light intensities.
7.
Phtoperiodism- Photoperiod is the total length of light period
to which plants are exposed.
i)
Short day- require less photoperiod (less than 12-14
hrs) e.g. Cannabis sativa, Salvia splendens.
ii)
Long day- require more photo period e.g. Sorghum
vulgare.
iii)
Day neutral- Photoperiod does not have any effect on
these plants e.g. Cucumis sativus.
8.
Succession: Some reports in literature indicates role of light
in plant succession. According to which pioneers (starting
organisms) require comparatively more light than climax
(final) communities.
Effects on animals
Light affects pigmentation, reproduction, development, growth,
locomotion, migration etc of animals as follows:
1.
Metabolism: More the light faster is the metabolism and viceversa e.g. animals living in caves show lethargic habits.
2.
Reproduction: In birds, light initiates breeding activities.
Some animals are short day animals e.g. sheep, deer, goats
while others are long day e.g. birds and ground squirrel while
guinea pigs are indifferent to light or day neutral.
3.
Development: Some larvae develop more in size in darkness
e.g. Mytilus while some under sufficient light condition e.g.
Salmon.
7
4.
Eyes: Animals living in caves and deep sea have rudimentary
eyes or are absent while in others eyes are present.
5.
Vision: Things visible only in natural or artificial light e.g.
man.
6.
Pigmentation: Due to high light intensity, complexion
becomes dark due to melanin.
7.
Locomotion: Speed in lower animals is regulated by light and
the process is known as photokinesis e.g. Mussel Crab larvae
move faster on exposure to light.
Phototaxis- Movement of animals in response to light.
Towards light e.g. Euglena and Anky from light e.g. earth
worm.
8.
Photoperiodism: Some animals respond to length of day e.g.
eels, salmons etc.
Daily rhythms of animals to light condition are known as
circadian rhythm. Circannual rhythms are the rhythms
occurring annually.
Temperature
Temperature also influences plants and animals in various ways as
discussed below:
1)
Metabolism: All metabolic reactions are influenced by
temperature. For every 10°C rise in temperature, rate of
chemical activity increases. Every physiological function has
temperature limits- optimum, minimum and maximum.
Temperature
affects
the
rate
of
transpiration,
photosynthesis, respiration and other metabolic processes in
plants and animals.
2)
Reproduction: Temperature also affects reproduction in both
plants and animals. Thermoperiodism is the response of
plant to temperature. Effect varies from species to species.
8
3)
Effect on growth & development: Both extremely low and
high temperature have adverse effects on growth of plants.
Low temperature causes dessication, chilling, injury and
freezing injury. Dessication is dehydration of tissues and
their injury due to rapid transpiration and slow absorption.
Low temperature effects
Chilling: Killing or injury of plants of hot climate on exposure to low
temperature.
Freezing injury: In some plants of temperate climates if exposed to
low temperature for sometime, water is frozen into ice crystals in
intercellular space causing cell injury. Some can tolerate extremely low
temperature so are cold resistant.
High temperature effects
Stunting and final death of plants occurs due to adverse effects on
a number of physiological processes such as respiration, transpiration,
protein metabolism etc.
In animals also high temperature affects growth and development
e.g. in oyster, length of body increases from 1.5 to 10.3 mm with increase
in temperature from 10-20°C.
1)
High temperature affects crossing over in chromosomes and
somatic expression of gene characters.
2)
Sex ratio is found to get changed due to high temperature
e.g.
rotifers
and
daphnids.
Under
normal
conditions-
daphnids give parthenogenetic eggs which develop into
females, with increase in temperature they give generative
eggs which after fertilization develop into males or females.
3)
Coloration: Cold climate favour fair complexion while high
temperature results in pigmentations.
9
4)
Morphology: Temperature affects absolute size of an animal
and relative proportion of various body parts e.g. birds and
mammals attain greater body size in cold regions than in
warm region.
Variations in temperature and its effect on distribution of plantsTemperature varies both with altitude (height above sea level) and
latitude (distance from equator). Accordingly, plant distribution also
varies. Plants are classified into various categories depending upon the
variation in temperature and its effect on plant distribution:
i)
Megatherms: Require high temperature throughout the year
and dominant vegetation is tropical rain forest.
ii)
Mesotherms: Require high temperature alternating with low
temperature for their growth. Dominant vegetation is tropical
deciduous forest.
iii)
Microtherms: Grow at low temperature and vegetation is of
mixed coniferous forest type.
iv)
Hekistotherms: Require very low temperature for their
growth and alpine like is the dominant vegetation.
Aquatic bodies-Have different layers
i)
Epilimnion: Have vertical gradient of gradually decreasing
temperature from surface.
ii)
Thermocline or Metalimnion: It is short zone of rapidly falling
temperature
iii)
Hypolimnion: It is bottom cold zone where no temperature
gradient is evident.
10
Effect of temperature on animal distribution
Homoeothermic
Poikilothermic or Ectothermic or cold
Or
blooded
Endothermic
Or
Body temperature fluctuates with changes
Warm blooded
in environment temperature
e.g. Amphibian, reptiles, fishes
Able to maintain their body
temperature at constt level
Irrespective of environmental temperature
e.g. birds and mammals etc.
Precipitation factor: Precipitation is the chief source of soil water.
It includes all moisture that comes to earth in the form of rain, snow, hail
and dew.
Rain in India is caused by monsoons. Amount as well as seasonal
distribution of rainfall affects the vegetation as well as animal population
of a particular region
High mountains
(the regions correlated with rainfall distribution).
Lower with scanty
Upper dry above clouds covered
rainfall
with snow and ice.
Middle with lot
of rain (cloudy region)
Annual rainfall determines the type of vegetation in any region e.g.
evergreen forests in tropical areas are found due to heavy rainfall
throughout year.
11
Sclerophyllous forests: Rainfall is heavy during winter and low in
summer e.g. grass lands.
Deserts: Have low rainfall throughout the year.
Humidity of air: Atmospheric moisture in form of invisible water
vapours is humidity or it is amount of moisture in air as % of amount
which the air can hold at saturation at the existing temperature.
Humidity is greatly influenced by intensity of temperature, altitude,
wind, exposure and water status of soil etc.
Daily variations in its values depend upon the type of habitat
conditions. In plains and deserts, it may show variation during day
whereas in oceans only little variation is seen, remains same throughout
the year. Humidity influences transpiration, absorption of water in plants
and animals.
Wind Factor: Air in motion is called wind. Involved in transpiration,
cause several types of mechanical damages, dissemination of pollen,
seeds and fruits. Modifies water relations and light conditions of a
particular area.
Velocity of wind is affected by geographical conditions, topography,
vegetational masses and position with respect to sea shores. Effects of
wind more pronounced in plants along with sea coast and at high
altitudes on mountains. Air moves from high to low pressure.
Effects of wind
1)
Breakage and uprooting
2)
Deformation
3)
Lodging- violent winds cause flattening of grasses against
ground.
12
4)
Abrasion: Soil particles or ice carried by wind may act as
strong abrasive forces by which buds and other parts of
plants may be eroded away.
5)
Erosion and deposition
6)
Salt Spray: along sea coast
7)
Desiccation: Increase in rate of evaporation and transpiration
8)
Dwarfing: Common in trees on sea coasts.
9)
Compression wood: Development of reddish type of xylem .
b)
Topographic factors
Climate of any area is chiefly determined by interactions of solar
radiations, atmospheric temperature, relative humidity and annual
rainfall. Each of these climatic factors, besides latitude and altitude is
greatly influenced by topography of area.
Topographic factors are concerned with physical geography of earth
in the area. Earth shows irregularities in different regions which produce
climatic variations which in turn give rise to characteristic local or even
micro climates or may even modify soil conditions. So effect of
topographic factor is indirectly through climatic factors:
1)
Height of mountain chains: Mountains, hills, valleys are
formed due to irregularities in earth’s surface. Effect of height (altitude)
can be seen on mountains with an increase in altitude. One can observe
changes in temperature, pressure, wind velocity, humidity, intensity of
solar radiations. Due to these changes, vegetation at different altitudes
differ.
2)
Direction of mountains and valleys: If direction of mountains
and valleys is same as that of monsoons then no rains and vice-versa. So
if more rain, more growth of plants and animals and vice-versa. Relative
humidity depends upon direction of air i.e. whether it comes from north
13
or south, as well as on habitat i.e. from sea, desert or forest. The heating
of earth’s surface is directly proportional to angle at which solar rays
arrive on ground. So mountains affect climate through rainfall and other
factors.
EFFECT OF DIRECTION OF MOUNTAIN CHAINS ON CLIMATE
THROUGH RAINFALL
3)
Steepness of Slope: Steepness of a slope affects the amount
of solar radiations received during day and soil characteristics primarily
through its effect on rate of water flow. In the northern hemisphere at
higher altitudes, steepness of slope increases the exposure of surface to
sun on the southern slopes whereas the northern slope remains cooler.
This is due to the fact that the steep southern slopes receive the rays of
mid-day sun almost at right angles whereas northern slopes receive only
oblique rays during morning and evening hours and sometime none at all
except for a short period during summer. This difference changes the
vegetation on two slopes.
Steepness of slope also influences speed of water flow. It affects soil
characteristics. Most water on slopping side flows down rapidly and little
is allowed to reach the soil. Thus two sides of mountain, in spite of
similar annual rainfall may bear different vegetation.
14
Soil erosion is also related with steepness of slope. Even with heavy
rainfall on a steep slope, active soil erosion and denudation due to run off
result into disappearance of plants from area.
EFFECT OF STEEPNESS OF SLOPE ON CLIMATE THROUGH LIGHT,
HEATING OF EARTH’S SURFACE AND RAINFALL
4)
Exposure of slope: The sun and wind affects very much the
kind of plants growing there. Vegetation is totally different in the exposed
and non-exposed regions
c)
Edaphic Factors
These include structure and composition of soil and nature of soil etc.
Importance of Soil: Soil is one of the most important ecological
factors. Plants, aquatic and terrestrial depend upon soil for nutrients,
water supply and anchorage. Depending upon the occurrence of plants in
soil, there are 5 ecological groups of plant:1.
Oxylophytes
:
Found on acid soils
2.
Halophytes
:
Found on saline soils
3.
Psammophytes
:
Found on sands.
4.
Lithophytes
:
Found on rocky surfaces
5.
Chasmophytes
:
Found on rock crevices.
15
Definition and composition of soil: Any part of earth’s crust in which
plants root is soil. Soil is not merely a group of mineral particles but
besides this it has biological systems of living organisms as well as some
other components. So it is known as soil complex.
Components of soil are:1)
Mineral matter: A matrix of mineral particles derived by
varying degrees of breakdown of rock.
2)
Soil organic matter or humus: An organic component derived
from long and short term addition of minerals, from
organisms growing above and below ground i.e. plants,
animals and micro organisms.
3)
Soil water/soil solution: All water contained in soil, together
with its dissolved solids, liquids and gases. Soil water in
reality is a dilute solution of many organic and inorganic
compounds which is the source of plant mineral nutrients.
4)
Soil atmosphere: It occupies the pore space between soil
particles. It differs from above ground atmosphere as it has
normally lower O2 and higher CO2 contents.
5)
Biological System: To the above, there may also be added,
the biological system as each soil has a distinctive flora as
well as fauna of bacteria, fungi, algae, protozoa, rotifers,
nematodes, oligochaetes, mollusks and arthropods etc.
The living organisms present in soil bring about nitrogen fixation,
secrete growth hormones, may act as antibiotics, help in soil formation
and cause decomposition of organic matter.
Soil pH: It affects distribution of organisms.
Calcicoles are the plants growing in soil having pH 6.5 while
Calcifuges grow in pH 3.8 to 4. Soil pH affects microbial activity and
16
below pH 5.0, bacterial and fungal activity is reduced. Plants grouping at
high salt concentration are called halophytes.
Soil texture represents the relative proportion of mineral particles
of different sizes present in soil. Soil texture indirectly influences soilwater
relationships,
aeration
and
root
penetration
through
its
relationship with inter particle pore space. It directly affects nutritional
status of soil. Sandy soils are nutrient deficient due to high porosity in
comparison to loamy soils which have less porosity.
So soil is a complex of several components and may be defined as
“the weathered superficial layer of the earth’s crust in which living
organisms grow and also release the products of their activities through
death and decay”.
3.3.1.2 Biotic (living) Components
This is indeed the trophic structure of any eco-system, where living
organisms
are
distinguished
on
the
basis
of
their
nutritional
requirements. From the nutritional point of view- ecosystem has two
components:
a)
Autotrophic components: These are the producers which
convert simple inorganic substances into complex organic substances
with the help of energy. They are of two types:
1)
Photosynthetic: They manufacture food with the help of
chlorophyll in presence of sunlight so energy utilized is
radiant energy. These constitute the major proportion of
autotrophic components. It includes green plants and
photosynthetic bacteria.
2)
Chemosynthetic: They manufacture food with the help of
chemical energy evolved during chemical reactions. They
17
contribute to lesser extent to the production of food in an
ecosystem.
b)
Heterotrophic components: These are known as the
consumers in which utilization, re-arrangement and decomposition of
complex materials predominate. They consume the food produced by
producers:i)
Macro consumers: Macro consumers occur in an order in the
food chain such as:1)
Primary consumers (Herbivores): Eat producers such
as green plants.
2)
Secondary
consumer
(Smaller
carnivores):
eats
herbivores (animals).
3)
Tertiary consumers (Larger carnivores): Eats smaller
carnivores.
ii)
Micro-consumers:
Known
as
the
decomposers
which
breakdown complex dead or living organic compounds,
absorb some of the breakdown products and release
inorganic nutrients in the environment, making them
available
again
to
autotrophs.
Bacteria,
fungi
and
actinomycetes are the decomposers.
3.3.1.3 Trophic Structure
It is the producer consumer arrangement where each food level is
known as trophic level.
Amount of living materials at different levels in different trophic
levels or in a component population is known as the standing crop. The
standing crop may be expressed in terms ofi)
Number of organisms per unit area.
18
ii)
Biomass i.e. organism mass in unit area which can be
measured as living weight or fresh weight, dry weight, ash
free dry weight, carbon weight or calories.
3.3.1.4 Ecological Pyramids
Graphic representation of trophic structures is known as ecological
pyramids. Trophic structure i.e. the interaction of food chain and the size
metabolism relationship between the linearly arranged various biotic
components of an ecosystem. In an ecological pyramid, producers
constitute the base of pyramid and the successive levels (herbivores,
smaller carnivores and larger carnivores), the tiers making the apex.
Ecological pyramids are of three types:
1.
Pyramids of numbers
2.
Pyramids of biomass
3.
Pyramids of energy.
Shape of ecological pyramid may change depending upon the
nature of food chain in the particular ecosystem.
1)
Pyramids of numbers: It shows the number of individuals at
each trophic level and it shows the relationship between producers,
herbivores and carnivores at successive trophic levels in terms of their
numbers e.g. in a grassland ecosystem, the producers are mainly
grasses, are always maximum in number. Herbivores like rabbits and
mice etc are lesser in number than grasses, the secondary consumers
such as snakes and lizards are lesser in number than the rabbits and
mice. Finally, the top (tertiary), consumers such as hawks or other birds
are the least in number. Thus the pyramid is upright.
19
GRASS LAND ECOSYSTEM
POND ECOSYSTEM
FOREST ECOSYSTEM
PARASITIC FOOD CHAIN
PYRAMIDS OF NUMBERS IN DIFFERENT TYPES OF ECOSYSTEMS
As evident from above figures, pyramid of number for grassland
and pond ecosystem is upright in shape while for forest ecosystem, it is
inverted as in an forest ecosystem, trees are major producers and less in
number while fruit eating birds i.e. herbivores are more in number. Then
snakes and lizards-smaller carnivores are lesser in number, than
herbivores and larger carnivores such as lion still less in number than
smaller carnivores, so the pyramid is upright in shape.
In a parasitic food chain, pyramid of number is always inverted as
a single plant-tree (producer) may support the growth of many herbivores
and each herbivore in turn may provide nutrition to several parasites
which support many hyper parasites.
20
Actually the pyramid of number does not give the true picture of
the food chain as they are not very functional. They generally vary with
different communities with different types of food chains in the same
environment.
2)
Pyramids of biomass: They show the total dry weight. They
represent the quantitative relationships of the standing crops. In
grassland and forest ecosystem, there is generally gradual decrease in
biomass of organisms at successive levels from the producers to the top
carnivores. Thus pyramids are upright. While in a pond ecosystem,
biomass shows a gradual increase towards the apex of the pyramid, so
pyramid becomes inverted.
POND ECOSYSTEM
PYRAMIDS OF BIOMASS IN DIFFERENT KINDS OF ECOSYSTEMS
3)
Pyramids of energy: Of all the three types of ecological
pyramids, pyramids of energy present the best picture of overall nature of
21
the ecosystem. In this type of pyramid, number and weight of organisms
at any level depends not on the amount of fixed energy present at any
one time in the level just below but rather on the rate at which food is
being produced.
tertiary
consumers
(carnivores)
Secondary
consumers
(carnivores)
Primary consumers
(hervibores)
Producers
PYRAMID OF ENERGY
Pyramids of numbers and pyramids of biomass represent the
standing situations while pyramid of biomass shows the rate of passage
of food through the food chain. Pyramid of energy is upright in shape as
at each trophic level, there is gradual decrease in energy.
3.3.2 Functional aspects of an ecosystem
1.
Rate of biological energy flow i.e. the production and
respiration rates of community.
2.
Rate of materials or nutrient cycles.
3.
Biological or ecological regulation.
22
Living and non-living components are interwoven. Radiant energy
is converted into chemical energy. Energy flows in a non-cyclic manner
whereas minerals flow in a cyclic manner in any ecosystem.
3.3.2.1 Productivity of Ecosystem
Rate of productivity i.e. amount of organic matter accumulated in a
unit time: two types:
1.
Primary Productivity: Rate at which radiant energy is
stored by photosynthetic and chemo-synthetic activity of producers e.g.
plant, photosynthetic bacteria and chemosynthetic micro organisms. It is
of two types:
(a)
Gross Primary Productivity: Total rate of photosynthesis
including organic matter used up in respiration during
measurement period. Also known as total assimilation or
total (gross) photosynthesis. It depends upon the chlorophyll
content and is estimated in terms of chlorophyll content per
gram dry weight per area or CO2 fixed per gram chlorophyll
per hour.
(b)
Net Primary Productivity: Rate of storage of organic matter in
plant tissue in excess of respiratory utilization by plants
during measurement period or energy remaining after
respiration and stored as organic matter is net primary
productivity or rate of increase of biomass is known as
apparent photosynthesis or net assimilation. It refers to
balance between gross photosynthesis and respiration and
other plant losses as death. Ratio of NPP (Net)/GPP (Gross)
ranges between 40-80.
The most productive terrestrial ecosystem are tropical forests with
high rainfall and warm temperature. Net primary productivity (NPP)
23
ranges between 1000 and 3500 g /m2/year. In case of temperate forests,
rainfall and temperature are relatively low and NPP ranges between 6002500 g/ m2/year.
Some ecosystems have high productivity due to an additional
energy subsidy to the system. This subsidy may be warmer temperature,
greater rainfall, circulating or moving water that carries food and
additional nutrients into community or in case of agricultural crops.
Net productivity also declines from a young ecosystem such as
weedy field or an agricultural crop to a mature plant community such as
forest. As plant community approaches a stable or steady state condition,
more of gross productivity is used for maintenance of biomass and less
goes into newly added organic matter. Thus ratio of gross production to
biomass decline with time. Productivity varies considerably not only
among different types of ecosystem but also among smaller ecosystem
and within the system from year to year. Productivity is influenced by
factors such as nutrient availability, moisture, especially precipitation,
temperature, length of growing season, animal utilization and fire etc.
2)
Secondary Productivity: Net production is the energy
available to heterotrophic components. These are the rates of energy
storage at consumers level. Since consumers only utilize food materials
in their respiration simply converting food matter to different tissues by
an overall process, secondary productivity is not divided into gross and
net amounts. Secondary productivity actually remains mobile and does
not live in situ like primary productivity.
Energy left over from maintenance and respiration goes into the
production of new tissues, fatty tissues, growth and new individuals. This
net energy of process is secondary production or consumer productivity.
Secondary productivity is the greatest when the birth rate of population
and growth rate of individuals are the highest.
24
Herbivores are the energy source for carnivores and when they are
eaten, not all of energy in their bodies is utilized. Part of it goes
unconsumed and some through metabolic losses can be accounted for.
At each transfer, considerably less energy is available for next consumer
level.
The energy budget of a consumer population is summarised as:C
=
Where,
A+F+U
C = Energy ingested or consumed
A = Energy assimilated.
F + U= Energy lost through faeces and nitrogenous
wastes.
The term A can be refined further as
A=P+R
Where,
P= Secondary productivity
R= Energy lost through respiration.
U=
Representing
nitrogenous
wastes,
should
be
included as part of A.
A= P+R+U, because they are involved in homeostasis of
organisms
Thus C = P + R + F + U or
Secondary Productivity P = C – R – F – U
Secondary productivity can be examined from the view point of
three different ratios:
i)
Ratio of assimilation to consumption; A/C- It is measure of
efficiency of consumer at extracting energy from food it
consumes. It relates to food quality and effectiveness of digestion.
ii)
Ratio of productivity to assimilation (P/A)- It is a measure of
efficiency of consumer in incorporating assimilated energy
into new tissues or secondary productivity.
25
iii)
Productivity/Consumption-
The
first
two
underlie
the
magnitude of III. III indicates how much energy consumed by
animal is available to next group of consumers.
3)
Net Productivity: Rate of storage of organic matter not used
by heterotrophs i.e. equal to net primary productivity or minus
consumption by heterotrophs during unit period in a season or year. It is
the rate of increase of biomass or primary production which has been left
by consumers.
3.3.2.2 Food Chains in Ecosystems
The transfer of food energy from the producers, through a series of
organisms with repeated eating and being eaten is known as a food
chain. There are different trophic levels in an ecosystem e.g. in grassland
ecosystem- food chain starts with grasses and goes through grass
hoppers, frogs, snake, hawk in an orderly sequential arrangement based
on food habits whereas in a pond the order will start with phytoplankton
going through water fleas, smaller fish, bigger fish, birds, larger animals
and so on.
In Marshes the chain is:
Marsh vegetationÆgrass hoppersÆshrewsÆMarsh hawks or owls.
Types of Food Chains
a)
Grazing food chain: It starts from living green plants, goes to
grazing herbivores and then finally to carnivores. Ecosystem
having this type of food chain are directly depended upon an
influx of solar radiations so depends upon the autotrophic
energy capture and its movement to herbivores. e.g.
Photoplanktons Æ Zooplanktons Æ Fish sequence
or
Grasses Æ Rabbit Æ Wolf
26
b)
Detritus food chains: In this type of food chain dead organic
matter goes into micro organisms and then to organism
feeding on detritus and their predators e.g. in the brackish
zone of Southern Florida, leaves of red mangroves fall into
warm, shallow waters. Fallen leaf fragments are eaten and
re-eaten by a key group of small animals. These animals
include crabs, copepods, insect larvae, grass shrimps,
mysids, nematodes, amphipods, bivalve molluses etc. All
these are detritus consumers. These animals are in turn
eaten by some minnows and small game fish i.e. small
carnivores, which in turn serve as main food for large fish.
Detritus
type
is
simply
a
sub-component
of
another
ecosystem. And these two chains are linked together
belonging to same ecosystem.
Supplementary food chains
Other feeding groups, such as parasites and scavengers form
supplementary food chains in community. Parasitic food chain is highly
complicated because of life cycle parasites. Some parasites are passed
from one host to another host by predators in food chain. External
parasites may transfer from one host to another. Other parasites are
transmitted from one host to another through blood stream or plant
fluids.
Food chains also exist among parasites themselves e.g. Fleas that
parasitize mammals are in birds, are in turn parasitized by a protozoa
Heptomonas.
3.3.2.3 Food Web
Food chains in natural conditions never operate as isolated
sequences but are interconnected with each other forming some sort of
27
inter-locking patterns known as food web e.g. in grazing food chain of a
grass land in the absence of rabbit, grass may be eaten by mouse. Mouse
in turn by Hawk or by snake first, which is then eaten by hawk. Thus, in
nature there are several alternatives.
DIAGRAMMATIC SKETCH SHOWING FOOD WEB IN A GRASSLAND
ECOSYSTEM
DIAGRAMMATIC SKETCH SHOWING FOOD WEB IN A POND
ECOSYSTEM IMPORTANCE OF FOOD WEB
Food webs are very important for the maintenance of ecosystem
stability e.g. decrease in rabbit population causes an increase in
population of alternative herbivore-mouse may decrease the population of
consumers (carnivore) that prefers to eat rabbit. Thus alternatives serve
for maintenance of stability of ecosystem.
28
If primary consumers are absent then producers will perish due to
overcrowding and competition. Similarly survival of primary consumer is
linked with the survival of secondary consumers and so on. Thus each
species of an ecosystem is indeed kept under check so that the system
remains balanced.
Food web theory has been given for food web- the theory considers
the size, organization and structure of food webs as influenced by
environment, number of invasions, loss of species and the relationship of
the trophic level to another.
The species in a food web with same diets and predators are known
as trophic species and classified as basal, intermediate and top species.
Basal species include both primary producers and detritus which are at
the bottom of food web and feed on no other species. The top species
occupying the apex of food web are ones on which no other species feed.
Intermediate species are neither basal nor top, they may feed on more
than one trophic level. Predators are the species that feed on other
species in the web and they are the species that are feed by some other
species.
Species in a food web are linked together by several food chains of
different energy efficiencies. Complexity of any food web depends upon
the diversity of organisms in system. Two main points are:1)
Diversity and length of food chain.
Diversity in organism based upon their food habits would
determine the length of food chain.
2)
Alternatives at different points of consumers in food chain
If more alternatives are present then more complex will be
interlocking pattern e.g. in deep oceans, seas etc., a variety
of organism are present so food webs are more complex.
29
3.3.3 Energy Flow in Ecosystem
Ecosystems
are
open,
non-equilibrium
and
thermodynamic
systems that exchange energy and matter with the environmental
constituents to decrease internal energy but increase external entropy.
3.3.3.1 Fundamentals of Thermodynamics
1)
Energy is the ability to do work. It is measured in term of
calories and joules. 1 cal = 4.184 J or IJ = 0.397 cal.
2)
First law of thermodynamics: Energy can neither be created
nor destroyed but it can converted from one form to another
e.g. plants convert solar energy to chemical energy during
photosynthesis.
3)
Entropy: Degree of randomness is a measure of unavailable
energy resulting from transformation. It is a general index of
the disorderliness associated with energy degradation.
4)
Second law of thermodynamics: No process involving an
energy transfer will occur spontaneously unless there is
degradation of energy from a concentrated dispersed form.
These two laws of thermodynamics are applicable to everything
including the organisms, ecosystem and our entire biosphere as they can
create and maintain a high state of internal order or low entropy which is
achieved by continuous dissipation of energy of high quality. Only due to
this ecosystem are open and in a state of non-equilibrium and there is
unidirectional flow of energy.
3.3.3.2 Single Channel Energy Models
About 30% of the sunlight-reaching the earth’s atmosphere is
reflected back into space, about 50% absorbed as heat by ground,
vegetation or water and about 20% absorbed by atmosphere. In fact, only
30
about 0.02% of the sunlight reaching the atmosphere is used in
photosynthesis.
ENERGY FLOW DIAGRAM FOR A LAKE (FRESH WATER ECOSYSTEM)
As evident from diagram, out of total 118,872 g cal/cm2/yr, 111 g
cal/cm2/yr taken by autotrophs with an efficiency of energy capture of
0.10%, 21 % of this energy (23 g cal) is consumed in metabolic reactions
of
autotrophs
for
their
growth,
development,
maintenance
and
reproduction. 15 gm cal is taken (consumed) by herbivores- 17% of net
production. Decomposition (39 cal) accounts for about 3.4% of net
production. Remaining 79.5% of net productivity i.e. 70 g cal not utilized
at all but becomes part of accumulating sediments. Much more energy is
available for herbivores than is consumed.
If the total energy incorporated at the herbivores level i.e. 15 g cal,
30% or 4.5 g cal is used in metabolic reactions. Thus there is more
energy, loss at herbivore level than at autotroph level.
Only 3.0 g cal or 28.6% of net productivity passes to carnivores.
This is more efficient utilization of resources than occur at autotrophs to
herbivore transfer level.
31
At the carnivore level, about 60% of carnivore energy intake is
consumed in metabolic activity and remaining becomes part of sediment.
From this energy flow diagram two things are clear:
1)
Energy flow is unidirectional.
2)
There is progressive decrease in energy level at each trophic
level.
ENERGY FLOW DIAGRAM DEPICTING THREE TROPHIC LEVELS IN A
LINEAR FOOD CHAIN
I = Total energy input
LA = Light absorbed by plant cover
PG = Gross primary production
A = Total assimilation
PN = Net primary production
P = Secondary production
NU = Energy not used.
NA = Energy not assimilated by consumer (egested).
R = Respiration
Out of 3000 K cal of total light falling upon green plants- 1500 K
cal is absorbed of which only 1 % i.e. 15 K cal is converted at 1st trophic
level.
32
Secondary productivity tends to be about 10% at successive
consumer trophic level.
3000 K cal — 1500 K cal Æ 15 K cal Æ 1.5 Æ 0.3
1%
10%
20%
From these two energy flow diagrams, it is clear that there is a
successive reduction in energy flow at successive trophic levels. Thus
shorter the food chain, greater would be the available food energy i.e.
smaller will be the energy loss and vice-versa. There exist no co-relation
between biomass and energy.
Energy represents the rate of function while biomass represents
standing crop e.g. 1 gm of an alga (low biomass) may be equal to many
gms of forest leaves ( more biomass) but if we see the energy fixation rate
then it is higher in case of alga with low biomass in comparison to forest
leaves with more biomass.
3.3.3.3 Y-Shaped energy flow models
These are two channel energy flow models in which each arm of Y
represents a different food chain. One arm represents the grazing food
chain while another represents detritus food chain. These two food chains
are not isolated from each other. This model is more realistic and practical
working model than the single channel energy flow models because it
confirms to the basic stratified structure of ecosystems, separates grazing
and detritus food chains and besides this the micro-consumers and
macro-consumers differ greatly in size metabolism relations.
33
Y-SHAPED OR 2 CHANNEL ENERGY FLOW MODELS REGARDING A
GRAZING FOOD CHAIN FROM A DETRITUS FOOD CHAIN
3.3.3.4 Classification of Ecosystem on the basis of source and
level of energy flow
Finally, the ecosystem can be classified according to source and
level of energy as follows:
1)
Unsubsidized natural solar powered ecosystems e.g. open
oceans, upland forests. These systems constitute the basic life support
module on earth Annual energy flow-100- 10,000 K cal/m2.
2)
Naturally subsidized solar powered ecosystem e.g. tidal
estuary, some rain forests. These are naturally productive system of
nature that not only have high life support capacity but also produce
excess organic matter that may be exported either to other system or
stored. Annual energy flow 10,000 to 20,000 K cal/m2.
34
3)
Human subsidized solar powered ecosystem e.g. agriculture,
aquaculture. These are food and fibre producing system supported by
auxiliary fuel or other energy supplied by human. Annual energy flow10,000-40,000 k cal/m2.
4)
Fuel powered urban-industrial system e.g. cities, industrial
estates etc. These are our wealth generating system in which fuel
replaces Sun as chief energy source. These are dependant upon others
for life support, food and fuel. Annual energy flow- 1,00,000- 30,00,000 K
cal/m2.
3.3.4 Biogeochemical Cycles
Exchange of materials (minerals) between biotic and abiotic
components is known as biogeochemical cycle. All organisms require two
types of nutrients: Macro-nutrients and Micro-nutrients.
Macro-nutrients: Required in large amounts e.g. C, N, O, H, S, P,
Ca, Mg etc.
Micro-nutrients: Required in small amounts e.g. Fe, Mn, Cu, Zn,
B, Co, Cl, Na, etc.
These
nutrients
go
on
circulating
between
organisms
and
environment in circular pattern called biogeochemical cycles. These are
of two types- (i) gaseous; and (ii) sedimentary.
3.3.4.1 Gaseous
Reservoir lies in atmosphere e.g. C, N, O cycle etc.
3.3.4.2 Sedimentary
Reservoir lies in earth’s crust e.g. P, S, Ca, etc.
35
3.3.4.3 Carbon cycle
By volume CO2 concentration in atmosphere is 0.03%.
Taken by
Food
Green plants
CO2
Respiration
Animals
Atmosphere
Photosynthesis
CARBON (C) CYCLE
However, there are some ramifications. Oceans regulate CO2
content in atmosphere so play a very important role. Sea water contains
50 times more CO2 than air in form of carbonates and bicarbonates.
CO2 + H2O Æ H2CO3 Æ H3O+ + HCO 3−
HCO 3− + H2O Æ H3O+ + CO 3−2
CO2 dissolves in sea water to form carbonic acid which converts
carbonates
into
bicarbonates
which
are
dissociated
during
photosynthesis, to precipitate carbonates in form of sediments on
bottom. In warm climates, greater salinity and alkalinity coupled with
36
high temperature favours the formation of coral reefs and thicker shells
of molluses.
3.3.4.4 Nitrogen cycle
Nitrogen is an essential element of all form of life. In atmosphere,
its concentration is 79%. Although nitrogen is very important for
organisms, it is never taken directly from atmosphere. Chief source of
nitrogen for plants are nitrates in soil. Nitrates are formed by nitrogen
fixing micro-organisms. Some of nitrogen is fixed by lightening also.
NITROGEN (Na) CYCLE
Nitrogen fixation is done by two ways: lightening and microbial.
Microbial nitrogen fixation is done symbiotically as well as asymbiotically,
e.g. Rhizobium fixes the nitrogen symbiotically while blue green algae,
e.g. Azotobacter and Clostridium are responsible for fixing the nitrogen
asymbiotically.
Ammonia Æ Nitrites Æ Nitrates
37
Produced
by
Nitrosomonas
&
Nitrococcus,
Nitrobacter
&
Nitrocystis, Bacillus & Actinomycetes by denitrification, nitrogen is
returned to the atmosphere and the organisms responsible for this is
Pseudomonas denitrificans etc.
3.3.4.5 Phosphorus cycle (P cycle)
For their nutrition plants require inorganic phosphates typically as
orthophosphate ions. Thus phosphates are taken by plants from soil.
Phosphate salts in soil Æ Plants Æ Animals Æ Decomposers.
Organic phosphate is made available for recycling through
mineralization and decomposition. Chief sources of phosphates in soils
are rocks. However, major portion of phosphates becomes lost to this
central cycle by physical processes such as sedimentation (seas, oceans
etc.) which take it out of reach of unwilling and major water circulation,
biological processes such as formation of teeth and bone, both very
resistant to weathering and excretion that accounts for considerable
losses from the major portion of cycle.
PHOSPHORUS CYCLE
38
3.3.4.6 Water (hydrological) cycle
Cyclic exchange of water between the earth’s surface and the
atmosphere via precipitation and evapo-transpiration constitutes the
water cycle. Water covers about 73% of the earth’s surface occurring in
lakes, rivers, seas, oceans, etc.
WATER CYCLE
From water bodies such as lakes, ponds, rivers, sea, ocean etc.,
water keeps on evaporating in the atmosphere. These water vapours after
cooling and condensation form clouds resulting into snowfall and rains
etc. Some amounts of water falling during rainfall percolates into the
deeper layers of soil (water table) and the rest moves as run-off water into
ponds, lakes, rivers etc. that finally pass on this water to oceans. The
water from soil is absorbed by plants which in turn, return, it to the
atmosphere through evapo-transpiration. Animals also take water which
is then returned to atmosphere through evaporation. In this way, there is
a continuous cycling of water, though its form changes between earth’s
surface and the atmosphere.
39
3.3.5 Regulation of ecosystems
3.3.5.1 Homeostasis
Environmental
parameters
exhibit
variations
which
may
be
diurnal, nocturnal, seasonal, annual, cyclic, regular or irregular. Living
organisms are subjected to these changing conditions. Hence living
systems always try to maintain a constant internal environment within
narrow limits irrespective of external change. This is called homeostasis.
So capacity or tendency of living system to maintain stable internal
environment is known as homeostasis.
Ecosystems also exhibit stability maintained by physiological,
genetic,
behavioural
and
ecological
adaptations
e.g.
the
rate
of
photosynthesis of a whole crop field may be less variable in comparison
to individual plants because when one individual speeds up its activities,
the other slows down leaving the average value more or less constant.
Ecosystems are also capable of self-maintenance and self-regulation.
System exhibits 3 states; growth, balance and ageing. Change in
state may occur with respect to time or due to environmental parameters
variation.
3.3.5.2 Feedback
The form in which environmental variation is controlled in
ecological system is through feedback which involves a process in which
information on the effect of a process is fed back to the component and
used to change the component behaviour. If increased output results in
increased input the feedback is positive e.g. increased number of females
in a population give birth to more young individuals and population
grows.
40
Comp. A
Comp. B
CONCEPTUAL FEEDBACK MODEL
Output
α
Output
α
Input —
1
Input
–
+ve the feed back
-ve feed back
Output = Input – Balance
Feedback may not be instantaneous since information takes time
to travel over network and so there is a time lag which may result in
oscillations in system behaviour. The oscillatory system remains stable
as long as oscillations do not exceed the limits defined for it. Thus
ecosystems operate through feedback interaction and homeostatic
mechanisms. If these interactions and mechanisms become weak or
absent, ecosystem function is damaged or destroyed e.g. if extensive overgrazing, fire or an air or soil pollutant destroys a grassland and exhibits
the growth of new grass, a chain of reactions may occur in system.
Grasses absent
Insect Herbivores absent
Snakes and Lizards
Frogs and toad becomes absent
Patten (1974) developed a theory of ecosystem stability with
regards to matters and energy constraints.
Stability involves (a) resistance to change; and (b) restoration of
original state before disturbances.
3.4
SUMMARY
Ecosystem represents the highest level of ecological interaction
which is energy based and this functional unit is capable of energy
transformation, accumulation and circulation. There are 2 aspects of an
41
ecosystem. Structural aspect of an ecosystem consists of abiotic and
biotic components while functional aspect of an ecosystem includes rate
of biological energy flow, biogeochemical cycles and biological or
ecological regulation.
3.5
KEYWORDS
Biotic factor: Refers to the influence exerted upon a habitat by the
plant and animal organisms which inhabit an area.
Chemotrophs: Organisms that obtain energy from oxidation or
reduction of inorganic or organic matter.
Consumer: A heterotrophic organism in a food chain that ingests
other organisms or organic matter.
Decomposer: Any organism of an ecosystem that feeds on plant
and animal protoplasm, breaking it down into its constituent parts so
bringing about decay.
Habitat: The place where an organism live.
Herbivore: An animal which get the most of food from plants.
Humidity: Amount of water vapour in the atmosphere.
Keystone: Species that play roles affecting many other organisms
in an ecosystem.
Latitude: The angular distance from the equator to the pole.
Obligate
anaerobes: Organisms for which the presence of
molecular oxygen is toxic.
42
3.6
SELF ASSESSMENT QUESTIONS
1.
Define ecosystem. Enumerate different types of ecosystems.
2.
What are the two major aspects of an ecosystem? Discuss in
detail the environmental factors affecting the ecosystem.
3.
How does the light and temperature affect the plants and
animals?
4.
Write a note on topographic factors.
5.
What is soil? Why it is important to us? Classify plants on
the basis of soil.
6.
Both Rajasthan and Himachal Pradesh have hills but still
Himachal Pradesh gets more rain-fall but Rajasthan does
not. Why it is so?
7.
What do you mean by the biotic components of an
ecosystem? Write down the biotic components in an aquatic
ecosystem.
8.
(a)
What is the difference between trophic level and
trophic structure?
(b)
Explain the types of ecological pyramids. Among these
which is the most important.
9.
Write a note on productivity of an ecosystem.
10.
(a)
How many types of food chains exist in nature?
(b)
Write a note on food web.
11.
Explain the different models of energy flow in an ecosystem.
43
12.
What is meant by biogeochemical cycles? Enumerate its
different types and discuss in detail Nitrogen Cycle.
3.7
SUGGESTED READINGS
1.
Odum,
E.P.
(1996):
Fundamentals
of
Ecology,
Natraj
Publishers, Dehradun.
2.
Miller, G.; Tyler (2002): Living in the Environment, Belmont
Brook/Cole, USA.
3.
Smith, R.L. (2001): Ecology and Field Biology. Harber Collins
College Publishers.
4.
Colinvaux, P. (1996): Ecology 2. John Willey and Sons,
Canada.
5.
Bebby, A. (1993): Applying Ecology, Chapman and Hall,
Madras.
6.
Karmondy, E.J. (2001): Concept of Ecology. Prentice Hall of
India Pvt. Ltd., New Delhi.
7.
Chapman, J.L. and Reiss, M.J. (2000): Ecology Principles
and Applications. Cambridge University Press, USA.
44
UNIT-III
PGDEM-01
HUMAN ACTIVITIES & ENVIRONMENTAL DEGRADATION
Prof. Anubha Kaushik
STRUCTURE
1.1
INTRODUCTION
1.2
ENVIRONMENTAL IMPACTS OF HUMAN ACTIVITIES
1.2.1 IMPACT OF AGRICULTURE
1.2.1.1 Traditional Agriculture
*
Deforestation
*
Soil Erosion
*
Nutrient depletion
1.2.1.2 Modern Agriculture
*
High yielding varieties
*
Fertilizers
*
Pesticides
*
Water logging
*
Salinisation
1.2.2 IMPACT OF OVERGRAZING
*
Land Degradation
*
Soil Erosion
*
Loss of useful species
1.2.3 IMPACT OF MINING
*
Sub-surface mining
*
Surface mining
*
Impacts
1.2.4 IMPACT OF INDUSTRIALIZATION
*
Resource Depletion
*
Urbanisation
*
Environmental Pollution
1.3
SUMMARY
1.4
KEY WORDS
1.5
SELF ASSESSMENT QUESTIONS
1.6
SUGGESTED READINGS
1
1.0
Objectives
After going through this unit you would be able to understand :
•
What have been the impacts of traditional and modern agriculture
including green revolution on environment.
•
What are the influences of other activities like overgrazing by cattle,
mining and industrialization on our environment.
1.1
INTRODUCTION
There are millions of species of living organisms on this earth. Advent of
man (Homo sapiens) took place quite late during evolution about 40.000
years ago on this 4.6 billion years old earth. However, this late comer,
after appearing on the scene has captured the key role on this planet. He
has become the master of the show. He has reached almost every part of
the earth and created a livable place of himself by inventing means to
control the harsh environmental conditions. He has succeeded in
harnessing the rivers, digging through the mountains, blasting the rocks
and harvesting the oceans.
Initially when the human population was small and the level of scientific
and technological development was low, the extent to which human
beings could interfere with the ecosystem was limited and the ecosystem
was able to sustain the effect of human intervention. But with the growth
of human population, the demands as well as interference with the
environment have increased many fold. Scientific and technological
advancement and human ingenuity have all been extensively used for
exploitation of the natural resources. The ecological balance has been
badly disturbed and the ecosystem is not able to cope up with the drastic
changes caused by the activities of human being. Anthropogenic
activities (caused by human actions) have started polluting and
degrading the environment.
2
In this unit, we well see how agriculture, overgrazing, mining and
industrialization have affected our environment. We shall see how our
land has been degraded and fertile soils have been rendered infertile. We
shall study the effects of large scale clearing of forests and the problems
associated with over- nourishment of lakes causing their degradation.
1.2
ENVIRONMENTAL IMPACTS OF HUMAN ACTIVITIES
1.2.1 IMPACTION AGRICULTURE
Early man was a hunter-gatherer. In fact, during 3/4th of our 40,000
years existence on the earth human beings acted as hunter and till then
human beings were quite like other animals on this planet.
Some 10,000 to 12,000 year ago, a cultural shift called ‘Agricultural
revolution’ took place in several regions of the world. This food producing
revolution changed the life- style of human from a wandering nomadic
life style to a settled one. People now learnt how to domesticate wild
animals and cultivate wild plants which were useful. To prepare for
planting, they cleared small patches of forests by cutting down trees,
clearing vegetation and burning the under-shrubs. This was slash and
burn cultivation. At that time people used to grow only as much food as
was required for their family. This was called subsistence farming. After
some years, the soil would get depleted of nutrients and then the growers
would leave that place and shift to some other area for cultivation- a
practice known as shifting cultivation.
With the exponential rise in population since 18.000 the demand for food
also increased. Therefore, the agriculture had also to be revolutionized.
Till the middle of this century, extension of agriculture was done on a
massive scale to feed the teeming billions. For this there was large scale
exploitation of natural resources like forests, grasslands, sea–coast and
water resources. The area under cultivation was increased from 700 m
ha in 1970 to 2000 m ha in the year 2000. There has been many times
3
more production of grain due to improved technology used in agriculture
which brought about the much acclaimed green revolution.
Agriculture can be broadly grouped into two types; based on over all
practices employed a) Traditional Agriculture; b) Modern Agriculture.
While traditional agriculture used to be only on a small plot, being
subsistence farming, it used to make use of simple equipments, naturally
available water, organic fertilizers and mix of crops. Modern agriculture,
on the other hand, uses hybrid seeds of single crop variety on a large
field, uses high-tech equipments, lots of energy subsidy in the form of
pesticides, fertilizers and irrigation water. Let us now examine the effects
of these two types of agriculture on our environment.
1.2.1.1 Traditional Agriculture
Traditional agriculture is still practiced in many parts of rural India. It is
usually characterized by low production, poor unrecognized cropping
pattern and more near to natural conditions. Traditional agriculture is
practiced by about 2.7 billion people i.e. almost half of the total people on
the earth.
Some of the important effects of traditional agriculture are as follows:
Deforestation
As discussed earlier, human beings have been clearing the forests for
creating land area for cultivation. With the cutting of trees there is
destruction of natural habitat for many species. Also, the land which
used to be covered under the forest cover is exposed and is liable to be
lost by erosion.
Shifting agriculture which has been practiced since ages, and is still
practiced in the North East parts of our country (known as Jhum
cultivation depletes the soil of important nutrients.
4
Soil Erosion
Clearing of forest cover exposes the soil to the action of rains, strong
winds and sun. These agents cause erosion of the land resulting in loss
of the top fertile layer. In traditional agriculture no measures are taken
for soil moisture management. Hence the loss of top soil is enormous.
This is more serious in hilly regions. We shall learn more about soil
erosion in the next chapter.
Nutrient depletion
In traditional agricultural involving shifting cultivation practice when all
the old vegetation in the area is burnt, the organic matter in the soil gets
destroyed.
As we grow the crop, the soil nutrients are taken up by the crop very
rapidly and the soil becomes deficient in those essential macronutrients.
Then the growers leave the area and shift to some other area for
cultivation. As the land is left to the mercy of nature to regenerate, there
occurs loss of many nutrients in the soil through erosion and leaching.
Thus shifting agriculture causes as a lot of damage to the ecosystem.
Still traditional agriculture is not so harmful as modern agriculture from
the point of view of it side-effects. Here, a mix of crops is allowed to
initiate natural secondary ecological succession. That’ s why when the
area is abandoned after cultivation, a secondary forest, though with
much less diversity, develops. Also, the planting of a mix of crops
provides habitats for natural predators of pest species. So there is better
ecological balance of pest and predator as compared to modern
agriculture.
5
1.2.1.2 Modern Agriculture
Modern agriculture makes use of too many technological inputs like
chemical fertilizers, pesticides, agricultural equipments, high yielding
varieties etc. We use various fungicides to protect the crops from fungal
diseases, weedicides for destroying weeds which compete for nutrients,
nematocides that kill nematodes which can attack the crops and
rodenticides to kill rodents which destroy our grain by eating them. This
intensive modern farming also known as industrial farming/ agriculture
has given rise to a number of environment problems. Let us discuss
these problematic off shoots of modern agriculture.
Impact related to use of High yielding varieties
Green revolution is directly related to high yielding hybrid varieties. Since
1950s most of the increase in global food has come from increase in gain
yield per hectare and this has been achieved using hybrid varieties.
During the first green revolution from 1950-1970, there was increase in
crop yield in most of the developed nations who used lots of energy
subsidies. During the second green revolution, fast
growing dwarf
varieties of rice and wheat, specially bred for tropical and sub –tropical
climates were introduced in many less developed countries. These high
yielding varieties (HYV) gave up to 80% more yield as compared to
traditional varieties, but was heavily dependent upon fertilizers, irrigation
water, pesticides etc. they were largely dependent upon human beings for
protection. Left to themselves they were not able to compete with their
wild counterparts.
The HYV’s encouraged monoculture, which means the same variety with
the same genotype is grown over vast areas. Such monoculture practices
are associated with a serious environmental problem. If any pathogen
attacks the crop, then there is total devastation of the whole crop
because it is all over the same genotype; so the disease becomes
6
epidemic. Had there been several varieties there would certainly be some
chances that some of the genotypes would resist the onslaught of the
pathogen.
Monoculture also results in loss of biodiversity, because the farmer
deliberately keeps off all the wide relatives of the crop and appearance of
any new combinations of genotypes through cross-pollination is totally
eliminated.
Fertilizer related impacts
Most of the chemical fertilizers use in modern agriculture have nitrogen,
phosphorus and potassium (N, P & K), which are the essential
macronutrients. Since these chemical nutrients are required by all crops
for their growth, the farmer uses these fertilizers in huge quantities for
boosting crop growth. Excessive fertilizers cause the crops plants to grow
fast and at the same time draw micronutrients from the soil at a much
faster rate than could be replenished naturally by the soil. Thus
excessive use of fertilizers causes micronutrient imbalance in the soil.
Thus excessive use of fertilizers causes micronutrient imbalance in the
soil. A very common example of micronutrient imbalance is the deficiency
of zinc in the soils of Punjab and Haryana caused due to excessive use of
fertilizers. This is now affecting soil productivity in these two states.
Another adverse effect of excessive application of fertilizer is that about
75% of the fertilizer is use by the crops while the remaining is lost from
the agroecosystem by leaching deep into the soil. On reaching the ground
water aquifers these fertilizers contaminate it with nitrates or phosphate.
The nitrogen containing fertilizers on entering the ground water become
serious health hazard. The nitrates get concentrated in the water and
when their concentration exceeds 100 mg/l, they cause a serious ailment
called ‘methaemoglobinemia’; a disease that affects the infants commonly
known as ‘blue baby syndrome’. Some times the disease becomes lethal.
7
Denmark, England, France Germany and Netherlands are know to have
this problem. In India also nitrate pollution is a serious problem in many
areas. Excess of nitrogen and phosphorus fertilizers when washed off
with rain water flow along the slopes of soil and along with run-off water
ultimately reach the water bodies causing over-nourishment of the lakes,
a process known as Eutrophication.
We will learn more about
eutrophication in the next chapter.
Pesticide related impacts
Modern agriculture uses several types of pesticides (biocides) to kill
insects, weeds, fungi, rodents etc in order to protect the crop. Some
70.000 different types of these chemicals are currently in use.
Indiscriminate use of these pesticides can lead to several environmental
problems.
The major pest –control revolution in agriculture began since 1939, when
Paul
Mueller,
an
entomologist,
discovered
DDT
(dichlorodiphenyl
trichloroethane) as an insecticide. Mueller also got Nobel Prize for his
discovery in 1948. DDT was the so called second generation pesticide.
First generation pesticides used earlier were chemicals like sulfur.
Arsenic, lead or mercury. However, after 1940 a large number of
synthetic chemicals came into use. These pesticides have been found to
be quite effective in controlling some pests. However, they also have a
number of environmental problems as discussed below.
•
Creating resistance in pests and producing new pests:
Spraying with a pesticide kills most of the pests. But, a few individuals of
the target species survive which reproduce and produce a more resistant
generation. Each time the resistant survivors are sprayed, the next
generation contains a higher percentage of resistant organisms which is
the natural selection process. Most insects develop genetic resistance to a
8
widely used chemical poison within 5-10 years and even sooner in
tropical areas.
Since 1950 more than 500 major insect pests have developed genetic
resistance to one or more insecticides. At least 20 species are known
which are now immune to all types of pesticides. They are called as
Superpests.
Sometimes new pests also appear after pesticide spray due to imbalance
in population. In Malaysia a large number of pests appeared on Cocoa
plantation after natural pesticide spray and ultimately the pesticide spray
had to be abandoned.
•
Death of non-target organisms
Many insecticides are broad spectrum that may have been maintaining
the pest species at a reasonable level. Several useful species playing
important role in the agroecosystem also get killed due to the pesticide
spray.
•
Biological magnification
Many of these pesticides are non- biodegradable and they keep on getting
accumulated in the food chain. As they move from the lower trophic level
to higher trophic level, they get more and more concentrated-a
phenomenon known as biological magnification. Man is situated at the
higher trophic level and hence biological magnification is more in man.
The new born infants are found to receive concentrated dosage of
pesticides through mother’s milk. These pesticides are known to cause
several serious ailments in human beings ranging from indigestion and
nervous disorders to cancer, and in many cases even cause instant death
e.g. due to malathion.
9
Water-logging
Due to excessive irrigation by the farmers, without proper drainage, soil
becomes drenched with water, a situation known as water logging. These
soils cannot support good plant growth because they lack air in the pore
spaces which is very important for the roots. Such soils also lack
mechanical strength. Many irrigation canal projects have rendered large
land areas water logged which have become unfit for crop production.
Salinisation
Excessive irrigation in arid regions without proper drainage also causes
secondary salinisation due to intense heat. Water from the soil
evaporates very fast and leaves behind salts in the soil profile resulting in
salinisation of the soils. In India more than 7 million ha of land are saltaffected which render the soil unfit for crop production.
1.2.2 IMPACT OF OVERGRAZING
Livestock wealth plays a crucial role in the rural life of our country.
Domesticated animals are an important source of milk and meat. They
also provide fuel, organic manure, hides and hoofs, phosphorus fertilizers
etc. Wool and shoe industry are also dependent on livestock. India leads
the world in livestock population. This huge population of livestock needs
to be fed, whereas the grazing lands or pasture areas in our country are
not adequate to support our huge livestock population. Very often we find
that the livestock grazing on a particular piece of grassland or pasture
exceed the carrying capacity. Carrying capacity of any system is the
maximum population that can be supported by it on a sustainable basis.
However, most often, the grazing pressure is so high that its carrying
capacity is crossed and the sustainability of the grazing land fails. Let us
see what are the impacts of over-grazing.
10
Land Degradation
Overgrazing removes the vegetal cover over the soil and the exposed soil
gets compacted. As a result of this compaction the operative soil depth
declines. So the roots cannot go much deep into the soil and adequate
soil moisture is not available. Organic recycling also declines in the
ecosystem because not enough detritus or litter remains on the soil to be
decomposed. The humus content of the soil decreases and over-grazing
leads to organically poor, dry, compacted soil. Due to trampling by cattle
the soil loses infiltration capacity which reduces percolation of water into
the soil and as a result of this more water gets lost from the ecosystem
along with surface run-off. Thus overgrazing leads to multiple actions
resulting in loss of soil structure, hydraulic conductivity and loss of soil
fertility.
Soil erosion
Due to overgrazing by cattle, the cover of vegetation almost gets removed
from the land. The soil becomes exposed and gets eroded by the action of
strong wind, rainfall etc. The grass roots are very good binders of soil.
When the grasses are removed, the soil becomes loose and is susceptible
to the action of wind and water.
Loss of useful species
Overgrazing adversely affects the composition of plant population andtheir regeneration capacity. In the original grassland there are good
quality grasses and forbs with high nutritive value. When the livestock
graze upon them heavily, even the root stocks which carry the reserve
food for regeneration get destroyed. Hence these species cannot get
regenerated in the ecosystem. Then some other species appear in their
places; These secondary species are more hardy and are less nutrititive
in nature. Some livestock keep on overgrazing on these species also.
11
Ultimately the nutritious, juicy fodder giving species like Cenchrus,
Dichanthium, Panicum and Heteropogon etc. are replaced by unpalatable
and sometimes thorny plants like Parthenium, Lantana, Eupatorium,
Xanthium etc. These species do not have a good capacity of binding
together the soil particles and therefore the soil becomes more prone to
soil erosion.
As a result of overgrazing vast areas of Arunachal Pradesh and
Meghalaya are getting invaded by thorny bushes, weeds etc. of low fodder
value. Thus overgrazing makes the grazing land lose its regenerating
capacity and once good quality pasture land gets converted into an
ecosystem with poor quality thorny vegetation.
1.2.3 IMPACT OF MINING
Mining is done to extract minerals and fossil fuels like coal from the deep
deposits in soil by using subsurface mining and from shallow deposits by
surface mining.
•
Sub-surface mining
Subsurface mining disturbs less than 10% as much land as surface
mining and usually produces less waste material. However, sub-surface
mining leaves much of the resource in ground and is more dangerous
and expensive than surface mining. Very often trapping, and killing of
miners occurs due to collapse of mine walls, explosions of dust, fire or
flooding.
•
Surface mining
Surface mining makes use of mechanized equipments which strip off the
overburden of soil and rock and usually discard it as waste material,
which are referred to as spoil. Extraction is 90% in this for minerals and
60% for coal. For surface mining, depending upon the topography we can
12
have three types of mining: first, open-pit mining in which, machines dig
holes and remove ores (copper, iron, gravel, limestone, sandstone,
marble, slate, granite). The second method is dredging, in which we use
chained buckets and draglines which scraps up under water mineral
deposits. The third method is known as strip mining in which bulldozers,
power shovels or stripping wheels remove the ore (coal & phosphate
rocks).
1.4.3 Impacts
Various environmental impacts of mining are discussed below:
(i)
Defacing of landscape:
During mining, the top soil and vegetation are removed to get access to
the deposit. So there are noticeable scars and disruptions on the land
surface and the ugly spoil heaps and tailing become an eye-sore spoiling
the aesthetic value of the area.
(ii)
Underground fires:
In the coal mines fires are known to occur quite often and sometimes this
cannot be put off, which causes a lot of damage and sometimes can cost
even human lives.
(iii)
Subsidence of land :
Land above underground mines often collapses and subsides and this
may result in tilting of houses, buckling of roads, bending of rail-tracks
and leaking of gas by breaking of gas pipe-lines.
(iv)
Ground water contamination:
Mining disturbs the natural hydrological processes and contaminates the
underground water. Very often sulphur present as an impurity in many
ores is converted into sulphuric acid by the action of bacteria like
13
Thiobacillus and the groundwater in the vicinity of the mined area can
become acidic. Some heavy metals get leached, into the groundwater and
contaminate it to an 'extent that it can pose health hazards.
(v)
Surface Water Pollution:
Near many mines (coal or S-containing ores), the aerobic bacteria
produce sulphuric acid by acting with the iron sulphide present in the
ore. This produces acidic waters. The acid mine drainage then seeps
through the mine and these are carried along with rain water to nearby
streams.
The
acidic
water
destroys
the
aquatic
life.
Sometimes
radioactive substances like uranium also enter into the water bodies
along with minewastes and kill the animals. Lead, arsenic and cadmium
are often found to contaminate the water bodies near the mines.
(vi)
Air Pollution:
Most ore minerals do not consist of pure metal. So smelting has to be
done to separate the metal from the other elements. Smelting emits
enormous quantities of air pollutants which damage vegetation and soils
in the surrounding area. The pollutants include sulphur dioxide, soot,
arsenic particles, cadmium, lead etc. Decades of uncontrolled SO2
emissions
from
the
copper-smelling
operations
near
Coppertill,
Tennessee killed all vegetation around the smelter.
Burning of mined coal raises the levels of carbon dioxide and carbon
monoxide levels along with oxides of sulphur and nitrogen. This also
contributes to rise in greenhouse effect resulting in global warming.
Suspend particulate matter (SPM) is also raised enormously in the mined
areas which causes several health problems.
vii)
Occupational Health Hazards:
Many of the mines are found to suffer from various diseases because they
are constantly exposed to the suspended particulate matter and toxic
14
substances. Black lung disease of coal miners, silicosis and asbestosis
etc. are some of the common diseases found in the miners.
1.2.4 IMPACT OF INDUSTRIALIZATION
During the mid 1700s industrial revolution started in Europe which
spread in USA in 1800 and brought a major change in human
civilization. There was a radical change in the life style and standards of
living. Industrialization multiplied energy consumption many times and
enabled the human beings to shape the earth at their will. When
industrial revolution occurred in England, there occurred a shift from
renewable forest resources (like wood fuel) to nonrenewable fossil fuels
(like coal). The furnaces and foundries were fed with coal instead of wood.
With the availability of abundant coal invention of steam engine came in
which performed several functions. Gradually small scale industry got
changed to large scale industries producing huge quantities of goods. Let
us now examine various impacts of industrialization:
•
Resource Depletion
With rapid industrial growth there was more and more exploitation of
mineral resources and other natural resources. The non-renewable
resources being limited in reserves are getting fast depleted due to overconsumption. Renewable resources (e.g. wood for paper and pulp
industry) are also consumed at such a fast rate that the consumption
rate has started exceeding regeneration rate. Due to such activities, even
renewable resources are getting converted into non-renewable ones. With
increasing industrial growth, the energy use has also increased many
times. Every technological development was is dependent on one or the
other form of energy. The reserves of coal and petroleum are being rapidly
getting exhausted and in near future we are likely to run out of these
energy resources.
15
•
Urbanisation
Industrialisation has led to urbanization. People in large number have
migrated from rural areas to cities in attraction of employment in the new
factories. About 67% of the world's population lives in cities. India has
the fourth largest urban population in the world, next to USA, USSR and
China.
To meet the demands of increasing urban population, residential areas
are constructed encroaching upon the agricultural lands. These lands
with all their biological resources get irreversibly lost. Water requirement
of the urban population has also been increasing many times. Due to
extensive built-up areas, the local ground water recharge declines. Just
imagine how much water is in demand in residential areas where every
building is multi-storeyed with hundreds of people living in. Since ground
water becomes inadequate, water from rivers and canals is to be drawn
from long distances at the cost of cultivation and rural demands.
Industries too draw lots of water for cooling, washing and processing.
Slums are the worst type of environmental degradation that has resulted
from industrialization. About 20% of India’s urban population lives in
slums. Delhi records the highest of about 50% slum dwellers. The slumdweller have to face stringent environmental conditions. They do not have
adequate, clean living space and sewerage facilities. They do not get clear
and safe drinking water and as a result of this suffer from various
contagious diseases.
Environmental Pollution
The most serious impact of industrialization is environmental pollution,
be it air, water, soil or noise pollution. Pollution of fresh water through
industrialization and urbanization is just colossal. About 90% of the
drinking water in the country comes from rivers polluted by these human
16
activities. Seventy percent of the effluents come from large polluting
industries while about 30% comes from small-scale and cottage industry.
Indian cities are not having proper sewage facilities and thus sewage
either leaks into soil and pollutes the ground water or it flows into
streams and rivers. Most of our rivers have been badly polluted. Just look
at Yamuna, which has just become an open sewer after reaching Delhi.
Most of our industries release enormous quantities of toxic emissions like
oxides of sulphur; nitrogen, hydrogen sulphide, oxides of carbon,
hydrocarbons, fly ash and suspended particulate matter. Earlier our
automobiles were responsible for large quantities of lead emissions along
with vehicular exhausts. Now unleaded petrol has replaced the earlier
fuel, but now benzene is found to be coming out of the vehicular exhaust
and causing pollution. Most of these toxic emissions· result in serious
respiratory and skin ailments. The green-house gases result in global
warming while acid rains are caused by the oxides of sulphur and
nitrogen. Several ozone depleting substances like CFC's released in the
atmosphere are depleting the Protective ozone layer of our atmosphere
which shield us from the harmful ultraviolet radiations. Noise is another
form of pollution which results in a number of stresses ranging from
psychological to physical. We shall learn in detail about all these aspects
of pollution in another paper.
In sum, we can say that Agricultural Revolution took place over 10,000
years ago and the Industrial Revolution over more than 200 years ago.
Together they have brought in enormous changes in our environment
causing resource depletion, pollution and environmental degradation.
Our life-styles have been changed. Our social, cultural and economic
systems have been transformed. Now there is a need for Environmental
Revolution so that we can protect the planet earth from disaster, only
then can human kind survive.
17
1.3
SUMMARY
Agricultural revolution took place about 10,000 to 12,000 years ago.
Traditional agriculture usually involved small areas, simple tools and
mixed cropping. One common practice was shifting cultivation of the
slash and burn cultivation. Nutrients in the soil were soon depleted by
the crops grown in the deforested areas, which were then abandoned.
Modern agriculture on the other hand, involves intensive farming using
high yielding varieties, lots of fertilizers, pesticides, irrigation water etc.,
which help in boosting crop yield. However, simultaneously they have
several environmental effects like loss of soil fertility, eutrophication of
water bodies, outbreak of diseases, biological magnification of pesticides,
water logging and salinisation problems. Overgrazing by cattle leads to
soil erosion, land degradation and loss of superior species. Mining
activities lead to land subsidence, pollution and occupational hazards.
Industrialization has been mainly responsible for rapid depletion of
resources and large scale air, water and soil pollution.
1.4
KEY WORDS
*
Anthropogenic
-
human generated
*
Shifting cultivation
-
A type of agricultural practice in which
forest area is cleared and used for
raising crops. As the nutrients get
depleted, the farmers shift to another
area.
*
Soil erosion
-
Loss of top fertile layer of soil.
*
Pests
-
Organism causing damage to crops.
*
Biological magnification-
increasing
accumulation
of
non-
biodegradable substances in the food
chain.
*
Water logging
-
drenching of soil with water so that all
18
capillary pores in the soil are filled with
water.
*
Salinisation
-
Formation of salt-affected conditions in
soil.
*
Carrying capacity
-
Maximum
population
that
can
be
support by it on sustainable basis.
1.5
SELF ASSESSMENT QUESTIONS (SAQ)
I.
Fill in the blanks :
(i)
Farming on a small land to grow just enough food for a small
group of individuals is called __________________.
(ii)
Which of the two favours greater disease outbreak –
Monoclature or traditional farming?
(iii)
Over-nourishment
of
water
bodies
with
nitrogen
and
phosphorus due to surface run-off is called _________ .
(iv)
The pests that can withstand all types of pesticides are called
_____________.
(v)
Excessive irrigation often leads to _________ and ________ .
(vi)
Good quality herbaceous plants are replaced by poor quality
thorny shrubs as a result of ___________.
(vii)
Collapsing and downward movement of land resulting in
tilting of houses and buckling of roads is called ___________ .
(viii) Ozone layer protects the living organisms on the earth from
the harmful effects of _________________ .
(ix)
Even renewable resources can become non-renewable when
consumption is greater than ________________ capacity.
19
(x)
___________ are the worst type of urban settlements showing
highly degraded environment.
II.
Answer the following :
(i)
Compare the effects of traditional Vs. modern agriculture.
(ii)
Discuss the impacts of over-grazing by cattle.
(iii)
How do sub-surface mining and surface mining affect the
environment?
(iv)
Discuss, “Industrialisation leads to urbanization”.
2.6
SUGGESTED READINGS
1.
Living in the Environment – T.J. Miller
2.
Perspectives in Environmental Studies, 2004 – A. Kaushik & C.P.
Kaushik, New Age Publication, New Delhi.
20
UNIT-III
PGDEM-01
ANTHROPOGENIC IMPACTS – LAND DEGRADATION,
DEFORESTATION AND EUTROPHICATION
Dr. Anubha Kaushik
STRUCTURE
2.0
OBJECTIVES
2.1
INTRODUCTION
2.2
ANTHROPOGENIC IMPACTS
2.2.1 Land Degradation
*
Soil erosion
*
Soil Conservation Practices
*
Water logging
*
Soil Salinity
2.2.2 Deforestation
*
Causes and consequences
*
The Indian Scenario
2.2.3 Eutrophication of Water Bodies
2.3
SUMMARY
2.4
KEY WORDS
2.5
SELF ASSESSMENT QUESTIONS
2.6
SUGGESTED READINGS
2.0
OBJECTIVES
•
After going through this unit you will be able to understand various
land degradation processes.
•
Deforestation, its causes and effects particularly with reference to
India.
•
Eutrophication of water bodies due to anthropogenic activities.
1
2.1
INTRODUCTION
Human activities leading to various serious impacts on our environment,
known as anthropogenic activities, have disturbed the ecological balance
of our environment. In the last chapter we have learnt about various
impacts associated with our agricultural, ranching (grazing) and
industrial activities. Now we shall study about three major environmental
problems caused by human activities, namely degradation of land,
causes and consequences of deforestation and degradation of water
bodies due to eutrophication.
2.2
ANTHROPOGENIC IMPACTS
2.2.1 LAND DEGRADATION
While human population is increasing rapidly and there is more and
more demand for arable land area for producing food, fibre and fuel
wood, we find that the valuable land is, being degraded due to overexploitation for various purposes. Soil degradation is a real cause of
alarm because soil reformation is an extremely slow process. It takes
about 200 to 1000 years to renew one inch of soil in tropical and
temperate areas and the average annual erosion rate is 20-100 times
more than the renewal rate.
Soil erosion, water-logging, salinization and contamination of the soil
with industrial wastes like fly-ash, press-mud or heavy metals all cause
degradation of land. Let us consider the important land-degrading
activities :
•
SOIL EROSION
The literal meaning of ‘soil erosion’ is wearing away of soil. Soil erosion is
defined as the movement of soil components, especially surface-litter and
topsoil from one place to another. Soil erosion results in the loss of top
fertile layer.
2
Soil, especially the top soil, is classified as a renewable resource because
it is continuously regenerated by natural process at a slow rate; i.e. 2001000 years are needed for the formation of one inch or 2.5 cm soil,
depending upon the climate and the soil type. But, when rate of erosion
is faster than rate of renewal, then the soil becomes a non-renewable
resource.
If we see the world situation we find that one third of the world’s
cropland is getting eroded. Two thirds of the seriously degraded lands lie
in Asia and Africa.
Soil erosion is mainly of two types :
(i)
Normal erosion or Geologic erosion : caused by the gradual
removal of top soil by natural, physical, biological and hydrological
equilibria which maintain a natural balance between erosion and
renewal.
(ii)
Accelerated erosion : Here the rate of erosion is much faster than
the rate of formation of soil. Overgrazing, deforestation and mining
are some important activities causing accelerated erosion.
There are two main agents which cause soil erosion, namely climatic and
biotic.
a)
Climatic agents : These are water and wind. Water affects soil
erosion in the form of torrential rains, rapid flow or water along
slopes, run-off, wave action and melting and movement of snow.
Water-induced soil erosion is of the following types :
•
Sheet erosion – when there is uniform removal of a thin
layer of soil from a large surface area, it is called sheeterosion. This is usually due to run-off water.
•
Rill erosion – When heavy rainfall and rapidly running water
produces finger-shaped grooves or rills over the area, it is
called rill erosion.
3
•
Gully erosion – It is a more prominent type of soil erosion.
When the rainfall is very heavy more deep cavities or gullies
are formed, which may be U or V shaped.
•
Slip erosion – This occurs due to heavy rainfall in slopy
areas like hills and mountains.
•
Stream bank erosion – During the rainy season, when fast
running water streams take turn in some other directions,
they cut the soil and make caves in the banks.
•
Wind erosion – Wind causes the following three types of soil
movements.
•
Saltation – This occurs under the influence of direct
pressure of stormy wind and the soil particles of 1-1.5
mm diameter move up in vertical direction.
•
Suspension – Here soil particles (less than 1 mm
diameter) which are suspended in the air are kicked up
and taken away to distant places.
•
Surface creep – Here larger, particles (5-10 mm
diameter) creep over the soil surface along with wind.
b)
Biotic agents : Excessive grazing, mining and deforestation are the
biotic agents responsible for soil erosion. Due to all these processes
the top soil is disturbed or is rendered devoid of any vegetation
cover. So the land is directly exposed to the action of various
physical forces facilitating erosion. Overgrazing accounts for 35% of
the world’s soil erosion while deforestation is responsible for 30% of
the earth’s seriously eroded lands. Unsustainable methods of
farming cause 28% of soil erosion.
4
The practices which leave the top soil vulnerable to erosion and drying
include :
Deforestation without reforestation, overgrazing by cattle, surface mining
without land reclamation, irrigation techniques that lead to salt build-up,
waterlogged
soil,
farming
on
land
with
unsuitable
terrain,
soil
compaction by agricultural machinery, action of cattle trampling etc.
•
SOIL CONSERVATION PRACTICES
In order to prevent soil erosion and conserve the soil, the following
conservation practices are used :
•
Conservational tillage farming :- In traditional method, the land
is ploughed and the soil is broken up and smoothed to make a
planting surface. However, this disturbs the soil and makes it
susceptible to erosion when fallow (i.e. without crop cover).
Therefore, now conservational tillage farming is employed which is
no-till farming i.e. minimum disturbance is caused to the top soil.
Here special tillers break up and loosen the sub-surface soil
without turning over the topsoil. The tilling machines make slits in
the unploughed soil and inject seed; fertilizers, herbicides as well
as leave a little water in the slit, so that the seed germinates and
the crop grows successfully without competition within weeks.
Conservation tillage is now used in about 1/3rd of croplands in
United States. However, this type of farming has not yet become
popular in other parts of the world.
•
Contour farming : On gentle slopes, crops are grown in rows
across, rather than up and down, a practice known as contour
farming. Each row planted horizontally along the slope of the land
acts as a small dam to hold the soil and slow down loss of soil
through run-off water.
5
•
Terracing : It is used on still steeper slopes. The slope is converted
into a series of broad terraces which run across the contour.
Terracing retains water for crops at all levels and cuts down soil
erosion by controlling run-off. In high rainfall areas, behind the
terrace ditches are also provided to permit adequate drainage.
•
Strip cropping: Here strips of crops are alternated with strips of
soil saving cover-crops like grasses or grass-legume mixture.
Whatever run-off comes from the cropped soil is retained by the
strip of cover-crop and this reduces soil erosion. Nitrogen fixing
legumes also help in restoring soil fertility.
•
Alley cropping : It is a form of inter-cropping in which crops are
planted between rows of trees or shrubs. This is also called
Agroforestry. Even when the crop is harvested, the soil is not fallow
because trees and shrubs still remain in the soil holding the soil
particles and prevent soil erosion.
•
Wind breaks or shelterbelts : They help in reducing erosion
caused by strong winds. The trees are planted in long rows along
the cultivated, land boundary so what wind is blocked. The wind
speed is substantially reduced which helps in preventing wind
erosion of soil.
Thus, soil erosion is one of the world’s most critical problems and, if not
slowed, will seriously reduce agricultural and forestry production, and
degrade the .quality of aquatic ecosystems. Soil erosion, is in fact, a
gradual process and very often the cumulative effects becomes visible
only when the damage has already· become irreversible. The best way to
control soil erosion is to maintain adequate vegetational cover over the
soil.
6
•
WATER LOGGING
In order to provide congenial moisture to the growing crops, farmers
usually apply heavy irrigation to their farmland. Also, in order to leach
down the salts deeper into the soil, the farmer provides more irrigation
water. However, due to inadequate drainage, water tends to accumulate
underground and gradually it forms a continuous column with the water
table. We call these soils as waterlogged soils. The pore-spaces between
the soil particles get fully drenched with water and soil air gets depleted.
This causes an imbalance in soil-water-air ratio in the earth and badly
affects the plants and microorganisms. The water-table rises in these
soils and the roots of plants do not get adequate air for respiration. Also,
the mechanical strength of the soil declines and the wet soil with little
strength is unable to physically support the roots and the weight of the
plants. Plants get submerged and the crop yield sharply falls.
Water logging is a serious problem in the heavily, irrigated areas of the
world and at least 1/10th of all the irrigated lands suffer from water
logging and the problem is getting more severe every day. The farmer only
thinks that he is providing and ensuring enough water to his crop, but
hardly does he realize that excessive irrigation is spoiling his crop by
creating water logged conditions. In our country also, we find water
logging emerging as a serious problem affecting crop growth and yield,
wherever we have enough irrigation water in the form of canal water and
tube well water.
In Punjab and Haryana, extensive areas have become water logged and
unfit for crop production. The topographical conditions also play a
significant role in creating water logging. For instance, the state of
Haryana is saucer-shaped and whatever run-off or leaching occurs, it
gets accumulated at the base of the saucer-shaped land and there is no
natural
drainage
outlet.
Hence,
underground causing water logging.
7
water
keeps
on
accumulating
It is very important to prevent water logging by avoiding excessive
irrigation. Sub-surface drainage also helps in artificially removing the
excess water. However, this is a very expensive practice. Recently, biodrainage concept has come where we make use of trees that can absorb
excess of water from the soil. Eucalyptus plantations can prove quite
useful in this direction because this tree absorbs and transpires huge
quantities of water.
Some of the kharif crops (summer crops) like maize, millet, cotton etc.
cannot stand water logging. Rabi crops (winter crops) also often suffer
badly due to water logging. India has the largest irrigated area in the
world measuring 56 M ha, followed by China. It has been estimated that
30-35% reduction in total crop yields has occurred due to the water
logging of our land area.
While water logging leads to crop failure, it also causes deterioration in
general health of people. There is chronic occurrence of fevers, malaria
and other diseases in such areas. Water logged conditions are known to
be the breeding grounds for vectors of many diseases. e.g. Anopheles
mosquito which is the vector of malarial parasite-causing malaria, snail
which is the vector of Schistosomiasis etc. are found to breed under
water-logged conditions.
•
SOIL SALINITY
Soil salinity and alkalinity are two serious problems posing a threat to
crop production the world over. A soil is said to be saline when it has a
high concentration of soluble salts like sodium chloride, sodium
sulphate, calcium chloride, magnesium chloride etc. in the soil profile.
These soils show a high electrical conductivity exceeding 4 mmhos/car 4
deci Siemens per meter or dS/m). The pH of these soils is usually near
neutral. Alkali soils, on the other hand, are formed due to the
accumulation of basic salts like carbonates and bicarbonates of sodium
8
and calcium. These soils are characterized by high pH often exceeding
8.0. When the soils have exchangeable sodium percentage (ESP)
exceeding 15%, we call these soils as sodec. These soils have a highly
dispersed structure, poor hydraulic conductivity and poor permeability
making root penetration difficult.
At present, one third of the total cultivable land area of the world is
estimated to be salt-affected. In India alone, about 7 million hectares of
land are affected by salinity and alkalinity. A major cause of salinisation
of soil is excessive irrigation. About 18% of the world’s cropland receives
irrigation with canal water or groundwater. However, unlike rain-water,
these waters contain dissolves salts. In arid (dry) climates much of the
water evaporates from the soil due to high evaporative rate under the
prevailing hot dry conditions and this leaves behind the salts in the
upper soil profile.
Thus the top soil, where the roots of the crops penetrate is having a high
concentration of soluble salts of sodium chloride, sodium sulphate etc.
The most common salt found in saline soils is sodium chloride.
Accumulation of soluble salts in the top soil is known as salinization.
Salinity in the soil causes stunted plant growth, lowers yield and
eventually kills plants and thus there is large scale land-degradation and
crop failure.
When irrigation water is repeatedly withdrawn from a stream and
subsequently returned to a stream, the salinity of the water steadily
increases. Using this more salty water for irrigation, salt-build-up takes
place in the soil. For example, when the Colorado River flows from its
headwaters in Rockey Mountains to Mexico, it causes an increase in salt
concentration in the soil along which it flows. Over the years its salinity
has risen about 20-fold and the water has become so much saline that it
cannot be used for irrigation any more.
9
The most common method for getting rid of this excessive salt build-up
from the top soil is to flush out the salts from the soil by applying much
more irrigation water than is actually needed by the crops for their
growth. However, this practice increases pumping cost and thus crop
production becomes expensive.
Another method is laying an underground network of perforated drainage
pipes and flushing the soil with large quantity of low-salt water. This
costly scheme only helps in slowing down the process of salt build-up but
does not stop the process. While flushing of salts is carried out by using
excess irrigation water, it is found that down stream irrigation water
becomes saltier. Now instead of allowing the drainage water or run-off
water to enter a canal or stream, large evaporation ponds are built where
the saline water is drained.
These days restoration of saline soils is being tried by planting salttolerant trees shrubs or grasses which can take up salts from the soil,
thus lowering down salinity in the soil. Blue green algae are also used as
biofertilizers to reclaim salt affected soils.
2.2.2 DEFORESTATION
Cutting trees from large areas without adequate planting is called
deforestation. Every year about 1.7 × 105 km2 of the forests are cut down
and an equal area of forest lands are degraded. In the developing nations
old forests have been cleared at many places and substituted by singlespecies tree farms, thereby greatly reducing the biodiversity and wild-life
habitat.
Tropical forests cover about 6% of the global land area. It is really an
alarming situation that we have already damaged more than half (56%) of
the world’s tropical forests. Tropical forests are the home of 50% of the
wild-life on this earth. Some scientist estimate it to be upto 90 percent.
10
The tropical rainforests not only provide the natural home to millions of
species of wild life, but also provide us with hardwood, food, beverages,
drugs, medicines, gum, resin, chocolate and so many other things.
Besides, they regulate the hydrological cycle, prevent soil erosion, purify
the air by producing oxygen and absorbing carbon dioxide and many
toxic gases. Yet, we are ruthlessly cutting down the forests.
•
Causes and consequences
The main reason for deforestation include clearing down of forests for
providing agricultural land for the ever-increasing demands of food by the
growing population, cutting of trees for fuel-weed, over-grazing by animal
in the forests areas, urbanization etc. Thus population growth is
indirectly the most important factor responsible for clearing of forests.
The total forest area of the world estimated in 1900 was about 7,000
mha. By 1975, it was reduced to 2890 mha and by 2000 it fell down to
just 2,300 m ha. While deforestation in temperate countries is much
lower (Just 0.6 percent) as compared to tropical countries where the
deforestation rate is of the order of 40-50 percent. If the present state of
deforestation continues, it is quite possible that in the next 60 years we
would lose more than 90 percent of the tropical forests. Thus the major
causes of deforestation are as follows :
*
Shifting Cultivation :
In this type of practice a patch of land is cleared, vegetation is burned
and the ash is mixed with soil and now the land is used for cultivating
crops for 2-3 years. As the soil fertility declines, this area is abandoned
and the growers move to some other area of the forest. This practice is
more prevalent in the North-East like Assam, Meghalaya, Manipur,
Tripura, Arunachal Pradesh, Mizoram and in Andaman & Nicobar
islands. To some extent it is practiced in Andhra Pradesh, Bihar and
11
Madhya Pradesh also. On a whole, more than 5 lakh ha of forest is
cleared annually for shifting cultivation.
*
Fuel requirements :
The increasing demands for fuel wood for the growing population is also
putting a lot of pressure on forests. It is an interesting fact that 50
percent of all the wood cut in the world is actually put to use as fuelwood. The fuel-wood consumption rate in India has shooted upto 300500 million tones in 2000-2001 as compared to 65 million during
independence. For this purpose only, 15-30 million ha of forest cover is
being stripped off.
*
Raw materials for industrial area :
The multi-purpose use of forests has been another major cause for its
exploitation. The wood is used for making boxes, crates, packing boxes,
match-boxes, furniture, railway sleepers paper, plywood etc. Paper
industry is one of the largest consumers of wood-pulp, particularly
bamboo. This has exerted tremendous pressure on the bamboo
plantations of the country. Besides this apple industry in Himachal
Pradesh and Jammu & Kashmir require a large number of packing boxes
for the transportation of the apples. The most common tree used for
providing wood for these packing cases in Abies. Tea industry of Assam
and Nilgiris also have a high demand for packing cases. There plywood is
more extensively made use of.
*
Development Projects :
Various hydroelectric projects, large dams, reservoirs, construction of
roads, railway line laying, construction of buildings etc. are all
responsible for clearing of forested areas. Some of these projects require
massive deforestation. Earlier there were seldom any reports of landslides
in the areas between Rishikesh and Byasi in Badrinath Highway. But,
12
after the highway was constructed 15 landslides occurred in a single
year. During construction of roads, huge portions of fragile mountainous
areas are cut or destroyed by dynamite and thrown into adjacent valleys
and streams. These land masses not only weaken the already fragile
mountain slopes but also increase the turbidity of various nearby
streams.
*
The Indian Scenario :
Our National Forest Policy (1952) has recommended that 33 percent of
the total geographical area of our country should be covered by forests.
In 1984-85, it was estimated that 22.8 percent of our land is under forest
cover. However, later on the satellite data indicated that only 11 percent
of our land is under good forests cover. With massive afforestation
programmes launched all over the country the forested area has now
increased.
The Himalayan forests were largely cleared in the past few decades
resulting in devastating effects. Denudation of Himalayas has disturbed
the water-sheds and has threatened the rivers flowing down hills into our
Indo-Gangetic plains. Shivalik forests having pre-dominantly ‘Sal’ trees
(Shorea robusta) have been over exploited for preparing railway-sleepers.
The foot-hills of Shivaliks which were once covered with dense forests are
now facing acute shortage of water and almost semi-desert like
conditions have appeared in many areas. Trees with their roots in the soil
have the property to absorb the water and retain it like a sponge during
heavy rains which is slowly released later on. The tree leaves release
moisture in the air through transpiration. Thus tree cover helps in
maintaining moisture in soil as well as in the atmosphere. And, in the
absence of trees the climate becomes drier and drier. The soil too
becomes loose, more prone to erosion by the action of wind or water and
this ultimately leads to desertification.
13
Big River Valley projects are also responsible for the destruction of
forests. When the Silent Valley Project was to start in Kerala, a hue and
cry was raised by the environmentalists that we would lose our rain
forests that present only in that area. Ultimately, however, the projects
was stopped. Had we gone for the hydroelectric project in Silent Valley we
would have lost all the biodiversity that has evolved and has been
preserved in the rain forests over millions of years. And, the interesting
fact is that we have not yet known or explored the major proportion of
those species. Who knows what marvellous gifts of nature are there in
store for us in these rain forests! Other important River Valley Projects
are Tehri Dam across Bhagirathi ad Sardar Sarovar Dam on Narmada
which have become controversial for several environmental impacts
including deforestation. When the forests are devastated, ecological
balance maintained by nature breaks away. Floods, droughts and
landslides become common. The trees not only increase rainfall of an
area, but also conserve the water which falls on the ground as rain.
Plants also reduce evaporation thus allowing water to remain in soil for a
long time.
Large-scale deforestation particularly during post-independence period
badly affected the whether, particularly rainfall. Along with cutting of
trees, there is also over grazing by cattle which reduces the regenerative
capacity of forests to an extent that forest cover cannot be restored easily.
Deforestation and over grazing have been causing tremendous soil
erosion. On an average India is losing about 6,000 million tons of top soil
annually due to soil erosion. Devastating landslides have been found to
occur in the hilly areas due to felling of trees in Risikesh, Alaknanda etc.
2.2.3 EUTROPHICATION OF WATER BODIES
Eutrophication means over-nourishment (eu = well, trophic = food). When
the nutrient levels become too high in the fresh waters, it is said to be
14
eutrophicated. Eutrophication of lakes is a natural process, in which the
nutrients are being carried by the run-off water from land to lakes
through a gradual process which takes hundreds of years. This is called
natural eutrophication. However, of late, instead of a slow and gradual
process of nutrient inputs there is a rapid flow of nutrients into fresh
water lakes, which is called cultural eutrophication. This type of
eutrophication is a dramatically fast process caused by human activities.
The sewage of urban population and the excess of nitrogenous and
phosphorus fertilizers used by our farmers in their agricultural fields
ultimately enter into the water bodies along with surface run-off. Since
the sewage is also rich in organic matter containing nitrogen (N) and
phosphorus (P), hence these two nutrients become very high in
concentration in the water body. The sudden high supply of N and P,
both of which are the major essential macronutrients boost up the
growth of some algae, which are microscopic green plants as well as some
large aquatic weeds like duckweed and water hyacinth. Some of the algae
also secrete certain toxic substances which are harmful to many of the
aquatic fauna. The earlier aquatic flora is now replaced by fast-growing
species. These new plants are often not the normal food of many of the
aquatic ecosystem. The new algal species become the dominant flora and
over-night the whole water body gets lovered with an algal-bloom,
converting the water into green or dark coloured soup like thing. The
turbidity of water increases. The water body is now having large
populations of blue-green algae, dinoflagellates etc. zooplanktons and
some fishes. Very soon the plants complete their life cycle and die. Then
they start decomposing or degrading. As we know, decomposition is a
process in which the complex organic matter is broken down into simpler
inorganic forms by making use of oxygen. Very fast depletion of oxygen
occurs due to this fast consumption of oxygen as a result of
decomposition of dead organic matter in the water body. Due to
unavailability of adequate oxygen in the water body, the aquatic fauna
15
too starts dying. This further depletes oxygen from the water-body and
under the prevailing anaerobic conditions (oxygen-deficient or oxygenless conditions), the normal flora and fauna disappear and the water
body now becomes a place suitable for the growth of anaerobic bacteria.
Most of these anaerobic bacteria are pathogenic in nature. Thus we see,
how a good quality water body gets totally degraded in water quality due
to eutrophication problem. A simple Schematic diagram would help
illustrate the process.
Sewage and/or fertilizer (N,P)
runoff
Nutrient enrichment in river
lakes
Algae (Algal blooms) and
some macro-plants
predominate and grow too fast.
Rapid consumption of oxygen
due to decomposition of dead
plants
Creation of anaerobic
conditions
Anaerobic bacteria flourish and
spread diseases
Eutrophication of the famous Dal-lake in Kashmir has spoilt to a large
extent its water quality and has badly affected navigation and tourism.
16
Many of our rivers like Yamuna are eutrophicated at certain places, for
example from Okhla, Delhi onwards to Agra, the river is highly
eutrophicated.
Remedy to eutrophication could be possible through anyone of the two
approaches :
a)
Input Approach : Here there should be a check on the excessive
inflow of nutrients into the water body. Discharge of organic wastes
into water bodies should be banned. The farmers must not use
excessive fertilizers because the excess nutrients do not become
available to the crops, rather enter into the water bodies through
surface run-off.
b)
Output Approach : In this type of approach the eutrophicated
waters are pumped out and fresh water with low nutrient levels are
pumped into the water body.
Since prevention is better than cure, hence it is more desirable to
have the input approach.
2.3
SUMMARY
Three major environmental impacts related to anthropogenic activities
are land degradation, deforestation and eutrophication. Soil erosion is
one of the most important land degrading process which is caused by
climatic agents like water and wind and also due to biotic factors,
particularly mining, deforestation and overgrazing. Soil erosion can be
prevented or minimized by adopting several soil conservation practices
that involve growing plants in specific patterns. Irrigation without proper
drainage is responsible for creation of water logged soils, which show
build-up of high concentration of soluble salts resulting in secondary
salinisation.
Saline
soils
have
serious
adverse
impacts
on
crop
production. Management of salt-affected lands is a major concern these
17
days. Human activities like mining, logging, hydro-electric projects and
clearing areas for crop production have been responsible for rapid
deforestation all over the world. Deforestation has serious environmental
impacts like loss of habitats for wildlife, soil erosion, desertification and
landslides. Another major impact of human activities is eutrophication of
water bodies caused by excessive addition of nitrogen and phosphate in
water bodies with surface run off and industrial discharge which result in
algal blooms and degrade the water body.
2.4
KEY WORDS
*
Soil erosion
-
Loss of top fertile layer of soil.
*
Water logging
-
drenching of soil with water so that all
capillary pores in soil are filled with water.
*
Saline soil
-
Soil having high concentration of soluble
salts with electrical conductivity exceeding
4 mmhos/cm.
*
Alkali soil
-
Soils with pH exceeding 8, formed due to
carbonates and bicarbonates of sodium or
calcium.
*
Deforestation
-
Cutting trees from large areas without
adequate planting.
*
Eutrophication
-
Overnourishment
of
water
bodies
with
nitrogen and phosphorus.
*
Algal blooms
-
Rapid growth of some species of algae in
eutrophicated lakes covering the whole
water body.
18
2.5
SELF ASSESSMENT QUESTIONS (SAQ)
I.
Fill in the blanks :
(i)
Removal or loss of fertile top soil is called …………..
(ii)
Heavy rainfall resulting in deep cuts in the soil is known to
cause ……….. erosion.
(iii)
Shelter-belts provide protection against ……….. erosion.
(iv)
The prominent appearance of ……….. in the water bodies
indicate eutrophication.
II.
Answer the following :
(i)
Discuss various types of soil erosion.
(ii)
How can soil be conserved?
(iii)
What are the main factors responsible for causing water
logging?
(iv)
How can health be affected by water-logging?
(v)
What are the differences between saline and alkaline soils?
(vi)
How
does
salinization
occur
through
faulty
irrigation
practices?
(vii)
What are the major causes and effects of deforestation?
(viii) Discuss the degradation of an aquatic ecosystem due to
eutrophication.
2.6
SUGGESTED READINGS
1.
Perspectives in Environmental Studies A. Kaushik & C.P. Kaushik,
2005, New Age Publication.
2.
Cunningham
W.
&
Cunningham,
M.A.
2001,
Principles
of
Environmental Science : Enquiry and Application, Tata MGH, New
Delhi.
19
UNIT-IV
PGDEM-01
HUMAN EVOLUTION AND POPULATION TRENDS
Prof. Anubha Kaushik
STRUCTURE
1.0
OBJECTIVES
1.1
INTRODUCTION
1.1.1 HUMAN EVOLUTION
a)
Evolutionary trends
b)
Origin of the Hominids
1.1.2 HUMAN POPULATION GROWTH
a)
Historical trends of population growth
b)
Characteristics of human population
c)
Demography
1.2
SUMMARY
1.3
KEY WORDS
1.4
SELF ASSESSMENT QUESTIONS
1.5
SUGGESTED READINGS
1.0
OBJECTIVES
After going through this unit you would understand the following :
*
How evolution of humans has taken place on the earth.
*
How human population growth has taken place over the ages.
*
What are the characteristics of human population and growth.
1.1
INTRODUCTION
Human beings or Homo Sapiens evolved on this earth about 40,000 years
ago. The story of human species however, began around 65 million years
ago with the origin of primates in tropical forests. Homo sapiens is the
1
scientific name given to human beings which belong to Class-Mammalia
and order-primates of the animal kingdom.
There are two sub-orders of order primates, viz. Prosimii (before apes and
Anthropoidea
including
monkeys,
apes
and
human
beings).
The
Prosimmi includes lemurs, tree-shrews, tarsier etc.
Apes resemble human beings more than do monkeys. So apes and recent
human ancestors are classified together as hominoids. However, it was
millions of years ago that the divergence from the common ancestors is
supposed to have taken place and the evolutionary line that led to the
origin of humans is called as hominids.
1.1.1 HUMAN EVOLUTION
From primates to humans, some distinct evolutionary trends can be
observed which helped them to become adapted to live on land than on
trees. The primitive primates lived primarily on trees showing arboreal
habits. A long dry spell lasting for a period of about 56 million years
during 60 million to 4 million years ago is believed to have forced the
arboreal primates to move down to land. Due to non-availability of water
the growth of tree cover was reduced and a decline in fruit production
followed. During human evolution the following trends were observed:
(i)
Precision grip and power grip
The primitive primates were adapted for an arboreal life with well
developed muscles, grasping power of limbs to hold on the branches and
a sharp vision. All primates have 5 digits on each limb and at least one
digit is placed in such a way that it is opposable to the rest of the digits
(Fig. 1). This helps them to have a grasp of the tree-trunk or branches
during climbing. When we see the pattern of placement of the 5 digits in
primates, we observe a clear gradation of opposability. In tree-shrew and
Tarsier the 5th digit is not so opposable to the rest, but in monkey, and
2
particularly in human beings, the 5th digit is fully opposable and distinct
from the rest, which we call as the “thumb” (Fig. 1). The fully opposable
thumb in humans help in better grasping capability and it is this faculty
of man which has enabled him to handle and use various tools that have
made life easier for him. In fact, refinement in hand movements was the
foundation for unique cultural and technological development of human
beings.
Monkey
Fig. 1 : Evolution in primate hands from relatively immobile digits
in tree shrew to human hands with fully opposable thumb.
(ii)
Steroscopic Vision
It is another evolution that took place in primates. Primates have both
the eyes placed on the face forward in contract to the other mammals in
which the two eyes are on two sides of the face. If we look at the same
object with two eyes simultaneously, we get a super-imposed image
which gives us the ability to perceive depth. In human beings this
stereoscopic vision became still more sharp as there was further
flattening of the snout and shortening of the jaws, placing both the eyes
in a better stereoscopic position. This improved vision in human beings
helped him to carry out several such activities which other primates
could
not
due
to
lack
of
perception
of
depth.
Through
other
modifications, eyes were able to respond to variations in colour and light
intensity. This further helped him by enhancing day time vision because
the eyes were able to respond to variations from dim to bright light
intensity.
3
(iii)
Upright posture and bipedal locomotion
Upright posture and bipedal locomotion is yet another evolutionary
feature that made human beings better adapted for terrestrial habit. The
habitual 2-legged locomotion in humans is called bipedalism. Due to the
change in posture his two forelimbs were freed for performing so many
extra activities in contrast to other primates. Monkeys are adapted to life
on trees. There skeleton is such which permits rapid climbing, leaping
and running along branches. Their arm bones and leg bones are of
almost the same length. So the monkeys can run easily with palms down.
Unlike monkeys, apes can hand into overhead branches and use their
long arms to carry some of their body weight. The arms often support the
body weight when an ape is one the ground. Because of the way their
shoulder blades are positioned, apes can swing their arms freely above
the head, when the body is erect or semi-erect. Compared to monkeys
and gorilla, humans have a shorter S-shaped and somewhat flexible
backbone. The position and shape of their backbone, shoulder blades
and pelvic girdle are the basis of their bipedalism. These skeletal traits
evolved when divergence took place leading to the evolution of hominids.
(iv)
Teeth for all occasions
Monkeys have long canines and rectangular jaws. Man has smaller teeth,
which are of about the same length ad he has a bow-shaped jaw. Teeth
and jaws became modified in such a way that from eating insects they
shifted to fruits and leaves and ultimately to all kinds of food available.
(v)
Well developed Brain
A well developed brain in Homo sapiens provided him with much more
intelligence than all other ancestral primates. This established his
superiority over all other animals on the planet. He was also provided
with some degree of physical ability for a large number of activities. He
4
could run, walk, climb and swim. Besides this, due to better developed
grasping power, a sharp vision, a well developed brain and two free limbs
he was able to perform various activities by using his intelligence that
other animals could not.
(vi)
Better speech
Early hominids had a skull with flattened base. Their larynx (the tube
leading to lungs)j was not much below the skull. Hence throat volume
was small (see Fig. 2). In modern man, the base of skull angles down
sharply. This moves the larynx down. So the volume of throat increases
and better sounds can be produced. That is how better speech faculty
developed in humans.
Throat
Throat
Larynx
Larynx
Modern Human
Early Human
Fig. 2 : Variations in structure of basal skull : early human shows larynx
not far below the skull. In modern humans larynx is pushed down and
volume of throat becomes more for production of better sound (speech).
Also, his social and interactive nature helped him in furtherance of his
development. Whatever new was discovered by one man was gradually
available to the rest of his species, thus moving collectively for better
adaptations to the harshness of the environment. Due to his improved
adaptive innovations human species became the most successful species
on the earth. In sum, the key evolutionary trends leading to the
emergence of modern man are :
1.
Skeletal changes leading to upright walking, which freed the hands
for new functions.
2.
Changes in bones and muscles leading to refined hand movements.
5
3.
Less reliance on a sense of smell and more on day time vision.
4.
Changes leading to fewer, less specialized teeth.
5.
Elaboration of brain and changes in the skull that led to the faculty
of speech in humans.
Fossil record of human evolution is not adequately complete and there
are several gaps which limit our understanding on the subject.
Nonetheless, it is understood that the primates emerged during the
coenozoic era and gave rise to modern primates
Origin of the Hominids
The origin of first hominids (i.e. the evolutionary line leading to humans)
took place somewhere in the Miocene-pliocene boundary i.e. about 5
million years ago. They showed several diversions and branching and
their evolution. However, they shared three characteristics in common,
including bipealism, omnivorous behaviour and brain development. They
were flexible in nature and could adapt to changing environmental
conditions. The African rain forests started giving way to mixed
woodlands and grasslands due to the dry spell during that period.
Australopithecus africancis, which showed bipedal locomotion existed
about 500,000 to 1000,000 years ago. Homo erectus, the human type of
primate evolved from this species in a period of about 2 million years.
The modern man Homo sapiens emerged from Homo erectus sometime
100,000 to 40,000 years ago. Homo erectus has got extinct whereas Homo
sapiens is successfully surviving because of the several advantageous
evolutionary changes discussed above. A well-developed brain in Homo
sapines further gave him a competitive advantage. The size of the brain
gradually increased from 500 cc in Chimpanzee to 770 cc in Homo
erectus and it almost doubled to 1400 cc in Homo sapiens.
The changes include :
6
(a)
Gradual increase in the size of the brain.
(b)
The joint between neck and head moved as posture became more
upright.
(c)
Reduction in the size of teeth and jaws occurred as the purely
herbivorous habit changed to omnivorous.
From 40,000 years ago to the present times, human evolution has taken
place in a way that is almost entirely cultural evolution and not biological
evolution. Biologically speaking, we can say that Homo sapiens have
spread to all parts of the world. Homo erectus had originated in Africa
and from there they migrated to South-East Asia, Europe and China.
Homo sapiens (the modern man) evolved in Europe, the North East and
China, but later reached every part of the earth with varied climatic
conditions. This cultural evolution due to well-developed brain enabled
him to adapt to varied conditions. From the stone-age technology, he has
evolved to the high-tech age of 21st century. Although he is on the zenith
of technological civilization today, yet there are certain ethnic groups
which still act as hunter-gatherers. This shows the range of human
adaptations evolved on the earth.
1.2.2 HUMAN POPULATION GROWTH
a)
Historical trends of population growth
The success of human species on this earth is mainly attributed to its
great flexibility and adaptability which allowed him to survive even under
a wide range of stress conditions. His intelligence led him to devise new
tools that subsequently converted a huner-gatherer man into an
agriculturist man. With the development of agricultural societies the
human populations became more stable and secure. The wandering man
became settled and the concept of family emerged. While the nomadic
man did not use to have more babies since it was difficult for him to
move along with them-for hunting. However, in the agricultural society,
7
which led to more stationary life style, it was in the interest of human
beings to have more children, because it was not so difficult to bring
them up and these children on growing up helped in the farm. Food was
no problem because lots of crops were grown by them. Because of this
change in life style, there was an increase in population between 10,000
B.C. and 10 A.D. coinciding with the agricultural growth.
Human population remained quite stable during the beginning of human
civilization i.e. the stone age. The environmental conditions at that time
were hostile and humans had not yet developed adequate artificial means
for adaptations to those stresses. Food would often become scarce due to
drought and our ancestors were very susceptible to outbreak of diseases.
High death rates thus kept the human population quite low. The 14th
century A.D. is believed to have experienced large scale mortality of
human being due to the outbreak of bubonic plague. More than fifty per
cent of the human population is estimated to have died in Asia and
Europe due to this plague.
Fig.-3 : Growth of human population in the last half-million years
If we look at the population growth trends during the past years, we find
that human population remained less than I million for thousands of
years. It reached 1 million only after 1850, the period coinciding with
industrial revolution. Scientific and technological advancement improved
the life expectancy of humans. Sanitation conditions improved, people
started living in definite settlements leading a more stable life with better
facilities for health care. A sharp fall in mortality rates led to an
8
exponential growth of human population. It is interesting to note that it
took about 39,000 years to reach I billion population whereas it took only
80 years to reach the 2 billion mark. The next doubling occurred in mere
45 years. Today we have crossed the 6 billion mark and are heading for a
next doubling which is likely to occur in a short span of a few decades.
Unless death rates rise sharply, the world human population may reach
11 billion by 2045 and 14 billion by 2100 as per World Bank estimates.
Fig. 4 Growth of human population during the last 400 years
b)
Characteristics of Human Population
Human population of the world today has crossed the 6 billion mark. The
population growth has bee found to be more in recent decades as
compared to the beginning of civilization. Also, the population growth in
different parts of the world is very different. Average growth rate varies
depending upon various socio-economic factors, medial facilities and
cultural beliefs. Let us see how the human population has been growing.
*
Exponential growth
When a quantity increases by a constant amount per unit of time e.g.
1,2,3,4 or say 2,4,6,8 or 1,3,5,7 it is known as linear growth. But, if a
quantity increased by a fixed percentage (say 100%) per unit of time,
then it is called exponential growth. Here the quantity will increase as
9
2,4,8,16,32 or it could be 10,100,1000,10000 etc. This exponential
growth results in very fast increase in the quantity over a period of time.
The human population does not grow at a uniform or linear rate. It is an
example of exponential growth. However, the rate of increase also keeps
on changing. In other words, we can say, the time taken by human
population to become double keeps on changing.
*
Doubling Time
The time needed for a population to double its size at a constant annual
growth rate is called its doubling time. A very quick way of finding out the
doubling time is the rule of 70 i.e.
Doubling time
=
70 / Annual growth rate
e.g. in 1993, the world’s population was growing by 1.7%. At that very
rate, we could calculate how much time would it take to grow the
population double the size of that of 1993 as 70 ÷ 1.7 = 41 years.
Exponential growth is deceptively fast. It took the world 300 years to
grow from 0.5 billion to 4.0 billion. But, the number of doublings
increase the growth factor enormously. A population size of 1 will be 2
after first doubling, 4 after second doubling, 8,16 after 3rd, 4th and 5th
doubling and it will become 1024 after 10th doubling and 1.05 x 106 after
20th doubling.
*
Growth Curve of Human Population
The exponential growth of human population can be modeled to fit into a
growth curve. In the first several million years, our population grew at an
average rate of 0.002% per year. So the shape of the growth curve was
flat or horizontal part of the curve, which suddenly tok off to give rise to a
J-shaped curve around 1970s. In 1970 the record highest of 2.06%
annual growth rate (global average) was observed which later on declined
to 1.7% in 1985.
10
However, the growth curve for human population cannot continue to
follow the J-Shape indefinitely. It has to stabilize after attaining a
particular size, depending upon the Carrying capacity of our earth.
Such a curve is known as the logistic curve. Mathematically, the logistic
curve is derived from the equation :
dN
––––
dt
=
1–N
–––––
R
rN
Where N = population size, r = growth rate, k = carrying capacity , t = time.
If the population growth is logistic, then n = k/2 i.e. we get maximum
sustainable yield when the population is half the carrying capacity.
Carrying capacity is the number or population that can be sustained
indefinitely by any system (e.g. earth) depending upon its resources
consumption demands of the population, waste generation and pollution
and regenerating capacity of the resources.
1.2.3 Demography
The study of human population dynamics is called demography. There
are two basic components affecting demography, viz. birth rate (natality)
and death rate (mortality). Usually, crude values of birth rates and death
rates are taken for demographic studies.
Crude birth rate
(natality)
Number of live birth
––––––––––––––––––––––
Mid year population
Crude death (mortality) rate
× 1000
Number of deaths
––––––––––––––––––––––
Mid year population
× 1000
Mid year is taken as July 7 of that year.
Rate of Natural Increase = Crude birth rate – Crude death rate.
11
Total fertility rate is the average number of children that would be born
to a woman if the age-specific birth rates remain constant. This varies
from country to country. For example, the total fertility rate in developed
countries was 1.9 while it was 4.7 in less developed countries in 1990.
Another important parameter is infant mortality that affects future
growth of a population.
Infant mortality (%)
Number of infant deaths
––––––––––––––––––––––––
Number of live births
× 100
Due to the death of many children before reaching adulthood, very often
there is change in the expected growth pattern.
The concept of replacement level is also important in population
dynamics. It is usually believed that two parents bearing two children will
be replaced by their offspring. But, due to infant mortality this
replacement level is usually changed. For developing countries, where
infant mortality is high and life expectancy is low, the replacement level
is approx. 2.7, while in developed nations the replacement levels is 2.1.
*
Age Structure
Age structure of human population in different countries is found to be
very different. Age pyramids are constructed based on the percentage of
people belonging to different age classes like :
a)
* Pre-reproductive
(0-14 years)
* Reproductive
(15-44 years)
* Post-reproductive
(45 years and above)
Pyramid shaped
This type has a broad base indicating a high percentage of young people
in the total population and indicates a growing population. This type of
12
age structure is found in India. The population is going to expand in the
years to come, because the large number of individuals in the young
stage would reach the second reproductive class in near future, thus
causing growth of population. The top narrow part consisting of old
people indicates relatively less loss of population due to death (Fig. 6a).
b)
A Bell shaped polygon
This type indicates a moderate proportion of young to old individuals in
the total population. This type is characteristic of many countries like
France, where the birth rates have declined so that almost equal number
of people are found in the age group between 0 to 35 years of age. In the
next 10 years the number of people in the reproductive age group is not
going to change much. Thus the population is stable (Fig. 6b).
c)
Urn shaped
Here the number of individuals in younger class is much less than young
adults. In the next 10 years there would be fewer people in the
reproductive age-group than before. This type of trend indicates declining
population, as found in countries like West Germany (Fig. 6c).
A) India
C) Germany
B) France
Fig. 5 : Age structure of three nations (a) India showing an expanding
population (b) France showing a stable population and (c) Germany showing
a declining population.
13
•
Zero Population Growth : When the birth rate equals the death
rate, it leads to zero population growth. Such a situation-occurred
in many European countries like Hungary, Denmark, Sweden and
also Australia in the Post-industrial stage.
•
Demographic Transition : A decline in death rate followed by a
decline in birth rate leading to low population growth is called
demographic
transition,
urbanization
and
which
technical
is
usually
development
associated
in
the
with
society.
Demographic transition takes place in four phases:
a)
Pre-industrial Phase : Here birth rate as well as mortality
rates are high. Thus, net population growth is low.
b)
Transitional
Phase
:
This
occurs
just
after
the
industrialization begins in a society. Here the death rates fall
due to adequate food production and better hygiene and
medical facilities. However, birth rates remain high and
population shows 2.5 to 3% growth rate.
c)
Industrial Phase : Here birth rates fall and eventually tends
to equal death rate. The growth rate is lowered down
substantially.
d)
Post-industrial Phase : Here the zero population growth is
achieved and decline in population size occurs.
As a result of demographic transition the developed nations are now
growing at about 0.5% rate with a doubling rate of 118 years. However
more than 90% of the global population is concentrated in developing
nations which have a growth rate nearing 2%.
1.3
SUMMARY
Human beings (Homo sapiens) evolved on earth about 40,000 years ago.
Evolution of human beings has taken place from primates including apes
and monkeys. There was a change in habit from trees to land and
14
accordingly changes took place in more accurate and power grip of
hands, development of stereoscopic vision giving perception of depth,
upright posture and walking on two limbs. A variety of teeth, well
developed brain and better speech marked further evolutionary trends.
Apes resemble human beings more than monkeys and are together with
humans placed in a common section called hominids. Modern man
emerged from homo erectus.
Human population is found to have been more stable in the beginning,
being less than 1 million for thousands of years. With industrial
revolution in 1850 it reached 1 million. After that there was a rapid
increase and now we have crossed 1 billion mark. Human population
grows exponentially. Doubling time used to be quite high earlier but now
with fast growth rate the doubling time has been reduced markedly. The
study of human population dynamics or demography has essential
components of natality, mortality, replacement level and age structure. A
pyramid shaped age structure indicates expanding population, while bell
shaped shows a stable population and urn-shaped structure indicates
declining population. There are several models to predict human
population growth in the coming years. Population explosion in the last
few decades has led to rapid resource depletion and several problems. In
order to achieve stabilized population, zero population growth rate will
have to be achieved.
1.4
KEY WORDS
Hominoids
-
Apes
and
human
ancestors
classified
together
Hominids
-
ancestors of human beings
Exponential growth
-
when
a
quantity
increased
by
a
percentage per unit time.
Demography
-
Zero population growth -
Study of human population dynamics.
when death rate equals birth=rate.
15
fixed
1.5
SELF-ASSESSMENT QUESTIONS (SAQ)
1.
Fill in the blanks :
(i)
The first primates originated about _____________ million
years ago in ___________.
(ii)
The ability to tie a knot in the neck-tie is a consequence of
the adaptation in the form of _____________ vision.
(iii)
The first hominid species to live a human type of existence
on this earth was __________.
(iv)
Monkeys, apes and humans belong to the order ______ of
primates.
(v)
Increase in human population was first observed when there
was a change from hunter-gatherer man to ______ man.
(vi)
The population of humans reached 1 billion in the year _____
(vii)
The world population now is over __________ billion.
(viii) Exponential growth in population coincided with the ______
revolution.
II.
Write True or False :
(i)
Only apes and humans are hominoids.
(ii)
Homo sapiens have been successful on earth because of their
highly rigid and elastic (nature.
(iii)
The fully opposable thumb in the limbs is an adaptation for
climbing the trees.
(iv)
Flattening of snout, shortening of jaws and placing of the two
eyes on front led to stereoscopic vision in hominids.
III.
Choose the correct answer :
(i)
Human Population explosion has resulted mainly due to :
-
Increased birth rate
-
Agricultural revolution
16
-
Industrial revolution
-
Decreased mortality rate due to improved medical
facilities and hygiene.
(ii)
IV.
Doubling time of human populations, over the year is :
-
Decreasing
-
Increasing
-
Stable
Answer the following :
(i)
Discuss the role of agricultural and industrial revolution on
population trends.
(ii)
What are the key evolutionary trends that led to the origin of
Homo sapiens?
(iii)
What environmental conditions were responsible for the
adaptive radiation from primates to hominids?
(iv)
What is the difference between hominoids and hominids?
(v)
Discuss any three important faculties which you think the
Homo sapiens developed by making use of their brain.
1.6
SUGGESTED READINGS :
(i)
Principles of Environmental Science : Enquiry and Applications. By
Cunningham, W. & Cunningham, M.A. 2003. Tata McGraw Hill
Publication, New Delhi, 2nd ed.
(ii)
Living in the Environment – Tyler & Miller. 2002 Brooks/Cole.,
Thomas Learning, Inc. USA, 12th ed.
17
UNIT-IV
PGDEM-01
HUMAN POPULATION GROWTH AND ITS IMPACT
Prof. Anubha Kaushik
STRUCTURE
2.0
OBJECTIVES
2.1
INTRODUCTION
2.2.1 FACTORS AFFECTING HUMAN POPULATION SIZE
2.2.2 MODELS FOR HUMAN POPULATION GROWTH
2.2.1 Population Crash Model
2.2.2 Gradual transition to zero population growth
2.2.3 Modified Irish Model
2.2.3 POPULATION EXPLOSION
2.4.1 People over-population
2.4.2 Consumption over-population
2.2.4 POPULATION STABILIZATION
2.2.5 THE INDIAN CONTEXT
2.6.1 The Kerala Model
2.2.6 ENVIRONMENTAL PROBLEMS RELATED TO POPULATION
GROWTH
2.7.1 Resource depletion
2.7.2 Land degradation
2.7.3 Degradation of water resources
2.7.4 Air pollution
2.7.5 Quality of life
2.3
SUMMARY
2.4
KEY WORDS
2.5
SELF ASSESSMENT QUESTIONS
2.6
SUGGESTED READINGS
1
2.0
OBJECTIVES
After going through this unit you would understand the following :
*
Various factors influencing population growth.
*
Population explosion and over-population concepts
*
Population stabilization
*
Population related problems of environment
2.1
INTRODUCTION
Net population growth of the world over a period of time is the difference
between the total number of births and total number of deaths during
that time or it is the difference between crude natality rate and crude
mortality rate.
Population growth usually follows either a J-curve or an S-curve.
J-curve
S-curve
Carrying Capacity
(K)
Fig. 1 Exponential Growth or J-shaped curve (characteristics of unchecked
population) and Sigmoid Population Growth or S-shaped curve
(characteristics of many species when introduced into a favourable new
environment, K=carrying capacity).
With
unlimited
resources
and
ideal
environmental
conditions,
a
population can reproduce at its maximum rate called biotic potential.
Large species like Homo Sapiens (man) have low biotic potential. Various
limiting factors tend to regulate the maximum allowable size of a
population known as carrying capacity. These limiting factors or
environmental resistances do not allow the exponential growth to
continue indefinitely, rather cause its leveling off near the point of
2
carrying capacity. Thus, environmental resistance converts a J-curve to
an S-curve.
Population regulation takes place in a cybernetic manner through
feedback mechanisms. Most animal species experience ups and downs in
their population size. However, human species generally has shown a
long upward trend.
2.2
1.
FACTORS AFFECTING HUMAN POPULATION SIZE
Population size of the early humans who lived in isolated groups
consisting of a few people was density dependent, that is limited by
the availability of food resources. In the pre-agricultural humans the
population size was reduced due to some other factors like short life
span of the individuals (about 30 years) resulting in a short
reproductive period, a long nursing period causing delay of renewed
fertility and a very high intensity of infant mortality.
2.
For the agricultural, man, the availability of food resources was
regular and thus mortality due to starvation got reduced. Permanent
settlements protected man from other hazards and thus increased
population size.
3.
The effect of urbanization on the rate of population growth was
rather negative since the massing of people together at one place
made it possible for diseases to be epidemic than it could have been
in scattered nomadic isolates. Thus density-dependent factor of
starvation and spread of disease both are important in population
regulation.
4.
In modern times, revolutionary advancements in the field of
medicine has increased the life-span and decreased infant
mortality. As a result, population density has increased, which on
the other hand had a regulating effect due to density-dependent
3
starvation. The survival of a high percentage of females through
their potential child bearing years has doubled the number of
offspring that can be expected, as compared to early man.
Thus, the doubling time of world population has shown dramatic changes
as shown below :
Year (A.D.)
1000
1650
1850
1930
1975
2010
Population
(in billions)
0.25
0.50
1.1
2.0
4.0
8.0
No. of years taken for
doubling
650
200
80
45
35 (?)
It is quite evident how the doubling time of the population is drastically
getting reduced. Perhaps it will show a doubling to 8 billion in 30 years.
Thus, ever since our nomadic ancestors abandoned their hunting and
food gathering mode of existence, the first step was taken, towards the
rapid boost in population growth which is know as population explosion.
Man has now reached a most extraordinary and abnormal period of
development, where millions are permanently undernourished and are
likely to die due to starvation. Before world-war-II, 49% of the total world
population was under-nourished, while now more than 66% are starving.
This is a clear case of density-dependent population starvation.
2.3.2 MODELS FOR HUMAN POPULATION GROWTH
There are three models to interpret the fate of this increasing human
population; which are expressed as follows :
Population Crash Model
Population would continue to double at the current rate in this century
until we over-shoot the carrying capacity of the earth. Then there will be
4
a catastrophic population crash due to extreme famine, excessive
pollution, communicable diseases, social chaos and wars in order to cut
down the scarce resources of the earth. According to resource experts
the, earth cannot sustain more than 48 billions, which is likely to be the
world population in 2100 A.D., if the population explosion continues at
the present rate of growth.
Gradual transition to zero population growth
The basic assumption here is that we have not already crossed the
population level which the earth can continuously sustain. If man slows
down his rate of population growth, then it will be within a limit which
can be sustained constantly by the earth’s resources. In order to achieve
the steady state (i.e. the leveling off of the growth curve) of population,
zero growth rate will have to be achieved when births would match but
not exceed deaths.
This model predicts that the steady state level of population can be
reached 70 years after the fertility rates reach the replacement level,
expected in 2055 A.D.
Modified Irish Model
This model predicts that the population will be reduced by droughts
blights and famines. This model is based on the population growth rates
of
Ireland
of
1840’s,
when
the
potato
famine
caused
due
to
Phytophthora infestans caused 24% decrease in Ireland population.
When there is a sharp fall, it takes quite some time to regain the original
level and for centuries the population would keep on fluctuating. If we
apply a great deal of intelligence and foresight, then a steady state level
can he achieved where without famines or chaos, a balance is maintained
with the earth's ecology and the population is sustained within some
fluctuating limits.
5
2.2.3 POPULATION EXPLOSION
Human population has grown faster in the 20th century than ever before.
World population had doubled during 1950 to 1990 (i.e. in 40 years)
crossing five billion. There is an addition of 92 million every year. We may
say, we are adding the population equivalent to that of Mexico every year.
In 2000, the world population was 6.3 billion. It is predicted that the
population
will
become
4
times
in
the
next
100
years.
This
unprecedented growth at an alarming rate has resulted in explosive in
growth of human populations, which we usually refer to as ‘PopulationExplosion’.
The statistics of Indian population shows that in post independence era
(1947-81), in just 35 years, we added a second India, or in other words,
we doubled our population. It has already reached 1 billion mark on 11th
May, 2000 and now India is the second most populous country in the
world, the 1st being China. This population explosion is more pronounced
in developing nations.
Every year India is adding to its population, a number equal to that of
Australia’s population. You can well imagine how many problems we are
going to face in view of this explosive population growth. There are two
types of over-population based on resources and area:
People Over-population
When there are more people than the available supplies of food, water
and other important resources in the area. Excessive population result in
degradation of the resources and there is absolute poverty, premature
deaths and under-nourishment of population. This is found in the LDCs
(less developed countries).
Consumption Over-population
It occurs in the MDCs (More developed countries). Here symptoms of
6
Over population occur in the society due to over-exploitation of
resources. This occurs where a little proportion of the world's people are
found to use a large share of the world's resources and cause a lot of
environmental degradation by resource depletion and pollution.
This concept of two types of over-population can be, explained with the
help of the Fig. 2.
Environmental Impact
×
=
A)
×
=
B)
Resource use
per person
Resource per
person
No. of People
×
People
overpopulation
×
Consumption
Overpopulation
Fig. 2 : Two types of over-population on the basis of resource use.
Some important statistical figures which can further help in illustrating
this point are:
*
USA is having the world's highest consumption over population. It
has 4.7% of world's population. It uses 25% of world’s energy and
mineral resource, causes 25% of total pollution, produces 22% of
ozone destroying CFC's and emits 18% of total greenhouse gases.
*
Consumption by 258 million Americans is equal to 12.9 billion
Indians.
*
A baby born in U.S.A. damages the planet earth 20-100 times more
in his life-time as compared to a baby born in a less developed
country.
Thus people over-population is a matter of direct concern and is a visible
population problem. But, consumption over-population is no less serious
a problem from the point of view of environment
7
2.2.4 POPULATION STABILIZATION
With the exponential growth of population it seems that the population is
going to overshoot the carrying capacity of the earth. However, nature
has its own checks and balances. It is thus believed that the growth
curve will suffer a dieback and reduce the population to a sustainable
size. How long can we continue our exponential growth in population and
resource use without suffering overshoot or dieback? It is not known; but
we are getting the warning signals that if not controlled, the increasing
population is going to deplete all the resources beyond their regeneration
capacity. A catastrophic dooms-day model warns us that the earth
cannot sustain more than two more doublings. It is therefore, very
important to adopt the sustainable growth concept. We have to reduce
our global environmental impact by just half in the next 50 years.
For stabilization of population the important factors are birth rate and
death rates. The birth rates are mainly affected by:
*
Average level of education and affluence which are found to be
more in developed countries.
*
Family planning services and their easy accessibility.
*
Cost of raising and educating children
*
Social structure and religious beliefs influence the family size.
*
Average marriage age.
Death rates are affected by factors like :
*
Nutrition availability
*
Hygeine and medical facilities
*
Educational facilities in rural areas.
Change in population size is also influenced markedly by migration from
one country to another.
8
2.2.5 THE INDIAN CONTEXT
India accounts for 16 percent of the world population but only 2.4
percent of the land area. Hence per capita land availability is just 0.48
hectare as against 4.14 hectare in USA and 0.98 hectare in China.
As population multiplies, there is more and more pressure on the
resources. The future population growth is to be related to the resource
base in order to have sustainable development. Population stabilization
(i.e. keeping the population much below carrying capacity) is thus very
important for a country like ours. Population explosion is like a time
bomb that must be diffused in time.
2.6.1 The Kerala Model
Kerala has earned the distinction of having lower birth rates among all
states of India. The main parameters deciding the effectivity of Kerala
Model include higher age of marriage for women at 21 years (as against
an Indian average of 17 years); female literacy rate of 53 percent (Indian
average being 13 percent); greater emphasis on primary education (60%
of budget expenditure on primary education as against 40-50% in many
other states); better public distribution system of food among 97 percent
of population, better health facilities in rural areas and greater success of
family planning programmes. Thus, Kerala shows the way to population
stabilization in India.
India started the family planning programme in 1952 when its population
was nearly 400 million. In 1993, after 41 years of population control
efforts; India was the world's second most populous country with more
than I billion population. The population of India was projected to be 1.4
billion by 2025, but there has to be a reduction in the current growth
rate.
In 1978, The Government of India took a new approach raising the legal
minimum age for marriage from 18 to 21 for men and from 15 to 18 for
women, The 1981 census, however, showed no drop in the population
9
growth rate between 1971 and 1981, Since then the government has
increased family planning efforts and funding. The government is also
considering job and housing incentives for couples who agree to meet
family planning goals. There is also a plan to push up the legal marriage
age a little more.
It is suggested by the World Watch Institute that world population should
be reduced by lowering the average growth rate from 2.6 to 1.8 percent if
population stabilization is to be achieved on sustainable basis.
2.2.6 ENVIRONMENTAL
GROWTH
PROBLEMS
RELATED
TO
POPULATION
The environmental impact of population in a given area depends upon
three factors: (i) number of people; (Ii) the resource used by each person
(iii) the environmental degradation and pollution resulting from each unit
of the resource used.
Environmental
Impact of
Population
=
Number
of People
×
Per capita
Resource
use
×
Environmental
degradation &
Pollution per
Unit of
Resource used
This three factor model is rather simple, yet it explains the environmental
problems related to population.· The population size determines the
extent of resource exploitation but keeping in view the two types of overpopulation, which we have already discussed, it is quite clear that the
resource consumption would vary with the country. U.S.A is known for
its maximum consumerism. The throw away attitude and luxurious lifestyle of the west exerts much more pressure on our resources as
compared to less developed countries. More the resource exploitation per
capita more is the degradation of environment. With every unit of energy
or mineral resources consumed by population there is pollution caused
in the environment. Thus, population, resource use and environmental
pollution are all correlated.
In LDCs where people over-population occurs, the major environmental
10
problems arise mainly due to poverty, unhygeinic conditions, diseases
and waste disposal problems. In MDCs where consumption overpopulation, occurs, there is more liberal consumption of energy
resources, mineral and biological resources and this in turn, results in
more emission of toxic gases and other pollutants in the environment.
Percent of global values
U.S.A.
India
Population
4.7%
16%
Production of goods
21%
1%
Energy use
25%
3%
Pollutants generated
25%
3%
CFCs production
22%
0.7%
Thus, although the population of India is 3.4 times more than that of
U.S.A., its energy use and polluting role is significantly less than U.S.A.
In less developed countries, poverty related with over-population is the
main contributor to environmental degradation. These are some of the
highlights of poverty in LDCs :
•
One out of 5 persons is hungry and malnourished, lacks clean
drinking water, decent housing and adequate health care.
•
One out of 3 persons does not have enough fuel to cook food and
keep warm.
•
One out of 4 adults cannot read or write
•
One out of 2 lack proper sanitary facilities.
These desperately poor people live in urban slums or in rural areas under
filthy conditions, eating unhygienic food, drinking contaminated water
and breathe in air full of stench of refuse and open sewers. The people
suffer from malnutrition and several contagious diseases. About 40
million of the desperately poor die from these diseases every year.
11
Because the world population is growing, the demand for resources like
water, food, minerals and fuels is also increasing. But these resources
cannot be generated beyond a limit. The water, land and mineral
resources are finite. Even energy resources in the form of fossil fuels are
limited. If we keep on consuming these resources at the present rate, very
soon the reserves will be exhausted.
The future population growth will occur more in the less developed
countries and yet many of them do not have enough resources to meet
even the demands of their current population. What will they do when
there will be millions of more mouths to feed? In India we have to double
every facility and resource within the doubling time which is an uphill
task.
The 3 P's - Population, Poverty and Pollution are inter-related and
have to be regulated and stabilized so that life on the earth becomes
sustainable. The population related problems can be listed as follows:
(a)
Resource Depletion
Over-population exerts more pressure on our resources. The limited nonrenewable resources like fossil fuels and minerals are getting depleted
very fast and if we exhaust these resources, what our future generations
would do?
(b)
Land Degradation
Over population has been putting more demands on land, because more
and more food is required to feed the population and this is achieved by
bringing more land under cultivation. In order to create more land for
agriculture,
forests
are
cleared
and
this
ultimately
leads
to
desertification. Over-grazing by cattle, which give us milk, wool etc also
cause land degradation. Mining, quarrying and related activities are also
responsible for fast degradation of land. Thus, in order to meet the
demands of increasing population there is degradation of land in several
ways.
12
(c)
Degradation of Water Resources
Although 3/4th of the earth's surface is covered with water, yet water is
considered to be a limiting resource. This is because out of all the water
existing on the earth 97% is found in oceans and it too salty for drinking,
irrigation or industry. The remaining 3% is freshwater, but out of that
2.997% is locked up in ice caps and glaciers, only 0.003% of total volume
of earth's water is exploitable as fresh water.
We can give an analogy for this freshwater supply of the world. If the
world's water supply is taken as 100 litres, our usable freshwater is only
one half of a teaspoon.
Nature has its own hydrologic cycle through which natural recycling and
purification process continues. But, this can occur effectively as long as
we do not overload it with wastes or withdraw water from underground
supplies faster than it is replenished. Unfortunately, due to overpopulation, we are doing both. Also, we are dumping lots of industrial
wastes into our rivers and streams. Thus, the small amount of fresh
water available on this earth is getting polluted and becoming unfit for
consumption.
(d)
Air Pollution
The atmosphere has no territorial boundaries. The toxic gases emitted by
our industries at one place are dispersed into the atmosphere. To meet
the demands of the increasing population there is need to produce more
and more material goods and hence industrialization is on an increase. A
large number of pollutants are released-into the environment by these
industries. Oxides of sulphur and nitrogen, volatile organic compounds,
suspended particulate matter and heavy metals released by the
industries are increasing in concentration in the air and posing serious
threat to human health and other forms of life.
13
Increased concentration of greenhouse gases like carbon dioxide,
methane, CFC's etc are raising global concern as they are resulting in
global warming. Similarly, ozone depletion and acid rains have become
serious threats for living organisms. In nut shell, we can say that more
population has led to more industrialization and has ultimately led to
greater pollution of the environment.
(e)
Quality of Life
Over-population has increased the misery of the poor people in LDCs.
With limited resources and increasing population pressure, there is more
and more exploitation of the available resources which get polluted and
degraded due to over-burden.
In
MDCs
where
consumption
over-population
occurs,
there
is
improvement in standards of living. Because, more and more goods are
produced and life has become more luxurious and prosperous. However,
use of excessive energy and other resources is increasing the pollution
load on the environment, which ultimately is going to have its impact on
the health of the people.
Thus poverty as well as over-consumerism are the off-shots of overpopulation in LDCs and MDCs which affect the overall quality of life.
In view of the foregoing discussion it is quite clear that the world
population must get stabilized and we have no other option but to adopt
the principles of sustainable development. Only then the life-support
system of this planet earth can sustain the mankind.
2.3
SUMMARY
Population growth depends upon the total birth rate and death rate of the
people. Growth can take a J-shaped or S-shaped form. Human
population is governed by several factors like availability of food
14
resources and disease outbreak. Settled life and development of medical
facilities have remarkably increased the birth rates and reduced mortality
rate thereby increasing human population in the last 150 years or so,
and the doubling time reduced from 200 years in 1650 to 80 years in
1850. In 1975, it was 35 years and now it has been further reduced to 30
years. There are different models to project human population growth.
The population crash model predicts exponential growth resulting in
about 48 billion population followed by a crash. The Irish model predicts
that due to famines, droughts etc. the population will be reduced from
time to time. The zero population growth model predicts than rate of
growth would be gradually reduced to zero population growth so that
population would reach a stabilized level in the next 50 years.
Population in the 20th century has already assumed an explosive
situation showing addition of 92 million each year. Over population has
led to many environmental problems, in developing countries. In
developed countries another type of over population called consumption
over population is observed due to too much resource used. There is a
need to stabilize the population adopting measures of family planning,
better medical and educational facility.
2.4
KEY WORDS :
Cybernetics
:
Science of regulation or control.
Doubling time
:
time
required
for
doubling
a
population.
Population explosion
:
Growth of human population at an
alarming rate resulting in a huge
population in 20th century.
15
People over-population
:
when population is more than the
available supplies of food and other
resources.
Consumption over-population
:
when over-population of resources
occur due to too much consumption
by people.
Population stabilization
:
to make the population stable before
it crosses the carrying capacity.
Carrying capacity
:
Capacity of a system (e.g. earth) to
sustain a given population for long.
2.5
SELF-ASSESSMENT QUESTIONS :
I.
Fill in the blanks :
(i)
Doubling
time
has
shown
_________
over
the
years
(Increase/decrease /no change).
(ii)
The carrying capacity of the earth is estimated to be _________ .
(iii)
When birth rate matches death rate in a society it is called ______
(iv)
Consumption over population occurs due to ______ of resources
(scarcity/ over-exploitation/ restricted use).
(v)
_________ state in India has lowest birth rates.
II.
Answer the following :
(i)
What are the factors responsible for increase in human population?
(ii)
Discuss various models predicting human population growth.
(iii)
Distinguish between people over-population and consumption overpopulation.
(iv)
Write a short note on population stabilization with emphasis on
Kerala model.
(v)
What are the population related environmental problem?
16
2.6
SUGGESTED READINGS :
1.
Sharma, H.S. & Khan, T.I. 2003. Environmental conservation,
depleting
resources
and
sustainable
development.
Aavishkar
Publishers, Jaipur.
2.
Kaushik,
A.
and
Kaushik,
C.P.
2004.
Perspectives
in
Environmental Studies, New Age Publishers.
3.
Chary,
S.N.
and
Vyasulu,
V.
(eds.)
2001.
Environmental
Management. An Indian Perspective. MacMillan India Ltd.
17