03
Dynamic nature of the
environment
By the end of this chapter you will be able to...
•
observe the interactions of different organisms based on their life
cycles.
•
control various stages of life cycles of animals for the benefit of man.
•
investigate the effect of abiotic and biotic factors on the existence of a
natural environment
•
manipulate the factors which affect crop cultivation for optimum use.
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3.1 Interactions based on life cycles
How beautiful is the environment around us! The green plants, beautiful birds as
well as the other animals around us, the blue sky, flowing rivers, streams and the fish
living in them all add beauty to the environment.
This beautiful environment is subjected to constant change. They may be shortterm or long-term changes. On a bright sunny day, within a very short time the sky may
get cloudy and rains will come, or a barren land over many years will become a lush
forest through a plant succession.
Certain changes which are not so apparent to
the naked eye also constantly occurr in nature. This
shows the dynamic nature of the environment.
The dynamic nature of the environment can be
well understood by studying the life cycles of certain
animals.
Fig 3.1
A butterfly (Fig 3.1) is a pleasing sight to any
person, but the caterpillar has quite a different structure from that of a butterfly. You
already know that caterpillar is just another stage in the life cycle of the butterfly. There
are types of animals which show very different external appearances at various stages
of their life.
Butterfly, mosquito, housefly and frog are some examples of such animals. They
show a phenomenon called ‘metamorphosis’ during their life cycle.
Life cycle of an organism is the series of stages that an organism will undergo
through its life from birth to the stage of producing another generation.
The different stages in the life history of the same organism showing different
external features is collectively known as metamorphosis.
All insects living in the world show metamorphosis.
Metamorphosis of insects is of two types.
1. Complete metamorphosis - Life cycle has 4 stages namely egg, larva, pupa
and adult.
2. Incomplete metamorphosis - Life cycle has only 3 stages namely egg, nymph
and adult.
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Eggs
Eggs
Adult
Incomplete
metamorphosis
Complete
metamorphosis Larva
Adult
Nymph
(half-grown
adult)
Pupa
e.g. : Cockroaches, bugs
e.g. : Butterflies, beetles, flies
Let us consider the life cycles and the various interactions of some animals
around us.
Eggs
Life cycle of butterfly
The life cycle of a butterfly shows
a complete metamorphosis. In each
of the stages in the life cycle the
external appearances, habitat, the life
styles and the food they take are
different.
Adult
butterfly
Larva
The female butterfly lays eggs
on the underside of leaves. The
Pupa
larva that comes out of the eggs has
a soft body. They often have the
same colouration as the background
Fig. 3.2 - Stages in the life cycle of Butterfly
as they are easily preyed upon by
enemies. At times they may possess poisonous hairs for protection. The butterfly larvae are known as caterpillars. Main food of caterpillars is young leaves of plant.
Hence they have mouth parts which can bite off the
leaves.
The stage following the larval stage is the pupal
stage or inactive stage. This stage is spent inside a
cocoon. The adult butterfly which comes out of the
cocoon feeds on nectar of flowers. In order to suck
nectar from the nectaries of the flower, it has a specially
adapted tubular proboscis.
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Proboscis
Fig 3.3 - Proboscis of butterfly
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They have wings to fly around. Further, they have colouration to suit the background
to protect themselves from enemies.
The change from the leaf eating caterpillar into the adult butterfly which flies around
sucking nectar from flowers is truly dramatic.
Life cycle of mosquito
Mosquito too is an insect that shows complete metamorphosis in its lifecycle. The
speciality about this insect is that each stage in the life cycle is found in different habitats
and it feeds on different kinds of food.
Mosquito
Mosquito lays eggs in
stagnant water. The type of
water selected for laying eggs
differs according to the species
Eggs
of mosquito. The larvae which
come out of the eggs spend
Pupa
their time in water, breathing
atmospheric oxygen through
Larva
special organs called siphon
tubes. Their food is mainly
Fig 3.4 - Stages in the life cycle of mosquito
micro-organisms in the water
and organic matter. The larva
moults many times inside the water and finally becomes a pupa. The pupal stage is
spent hanging on the surface of the water.
Adult mosquito coming out of the pupa is a winged insect that can fly. The food of
male mosquito is plant juice while the female mosquito feeds on blood of animals.
The female mosquito has a short, sharp stylet which
can pierce the skin and suck blood.
Among the different species of mosquitoes that are
found in Sri Lanka, some are responsible for spreading
various diseases.
Fig 3.5 - Stylet of mosquito
Do you know?
The Mammalian blood helps to maintain the requirements needed for
the female mosquito to lay eggs
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Life cycle of house fly
House fly is also an insect which shows a complete metamorphosis. The female
house fly lays eggs in decaying organic matter. The eggs hatch and light yellow coloured
larvae come out.
These larvae are commonly known as maggots. The larva eventually becomes a
pupa. The larva feed on the organic
Adult house fly
matter. Both the larval and pupal stages
are spent in soil. The adult that comes
out of pupa is the winged house fly.
House flies are attracted to excreta,
decaying food and dead bodies of
Eggs
Pupa
animals. They can sense such environments around them with special sense
organs. When flies settle on these
substances, they get rubbed on the feet,
Larva
and mouth parts of the flies. When the
flies rest on our food substances,
Fig 3.6 - Life cycle of house fly
these substances are transfered on to
the food. This causes diseases such as
diarrhea, chlolera, infectious hepatitis in man and other animals.
The special sense organs on the feet of house fly help them in sensing taste. They
suck the juices from food using the short stylets.
Do you know?
The mosquito which is constantly found arround unclean habitats, is very concerned about it’s own cleanliness. It cleans its body at least four times per hour to
get rid of the waste matter on its body.
Life cycle of frog
Frog is an amphibian with adaptations to live in aquatic as well as terrestrial habitats.
The life cycle of a frog consisting of several stages too highlights the dynamic nature of
the environment.
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Frogs lay their eggs in water,
which are covered with a gelatinous
covering and laid in clusters. The
‘tadpole’ leaving the egg is a
creature which can swim, and
breathe by gills.
Adult frog
Eggs
The stream- lined body helps
to swim in water. Food consists of
aquatic plants. After a series of
structural and physical changes the
tadpole becomes an adult frog.
Adult frog is an insectivore and has
a special type of tongue to catch
insects. Also it has legs for movement, a moist skin and lungs for
breathing air.
Early
tadpole
Mature
tadpole
Fig 3.7 - Life cycle of frog
Fig 3.8
Tongue adapted for catching insects
Fig 3.9
Feet adapted for swimming in water
and jumping on land
Assignment - 1
Get involved in a field survery to observe the various changes of life cycles of
several animals. Observe them in their own environments and note your observations.
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3.2 Controlling the different stages in the life cycles of
organisms
11976 Dengue patients were reported from various places in the
island in the year 2006
Shown above is a heading from a daily newspaper. Dengue is a disease spread by
mosquitoes. You already know that not only mosquitoes, but house flies also contribute
to spreading various diseases. Certain other animals destroy cultivations and reduce
the harvest of various crops Let us consider how the knowledge of life cycles can be
used to control these insects and to prevent these diseases.
Control of diseases
a)
Diseases caused by mosquitoes
By now about 18 species of mosquito have been identified in Sri Lanka. Amongst
these, certain types spread diseases to man.
Table 3.1 Types of mosquitoes spreading diseases to man
Disease
Malaria
Filaria
Dengue
Chikun gunya
Japanese encephalitis
Type of mosquito spreading the disease
Anopheles
Culex
Aedes
Aedes
Culex
Among the above diseases, some can be fatal, hence the state has to bear a huge
expenditure for their control and treatment of patients.
Organisms which help to spread diseases are called vectors, while
micro-organisms which cause diseases are called pathogens.
Controlling the vector is an important step in prevention of diseases. In this context,
a knowledge of their life cycles as well as their behavioral pattern and their activities is
useful.
Malaria was at one time at an epidemic level in the dry zone of Sri Lanka. The
disease causing pathogen is a micro-organism called Plasmodium.
A vector mosquito which sucks blood from a malaria patient, can bite a healthy
person and introduce the pathogen into his blood, spreading malaria to him. Filaria,
Dengue are also spread by the above methods. At present, Dengue haemorrhagic
fever and Chickun gunya are serious health problems in Sri Lanka.
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By the end of each rainy season, there is a rapid increase of these diseases. The
vectors of all of the above diseases are different mosquito types. Two main strategies
are adopted for protection from these diseases.
1. Controling the various stages in the life cycle of these mosquitoes.
2. Protecting oneself from mosquito bites.
Stagnant water is essential for the mosquito to lay eggs. Culex species of mosquito
lays eggs in polluted water which is not very deep.
eg. coconut husk pits, drains etc.
Anopheles lay eggs in fairly deep, pure water. But for the Dengue vector, Aedes
even small amounts of water is sufficient for laying eggs. They lay their eggs in coconut
shells, yoghurt cups, gutter pipes, flower pots and even in small quantity of water
collected at leaf bases of certain plants. Therefore such places should not be made
available for mosquitoes. Another speciality is that the eggs of the Dengue mosquito
can remain alive without water for a long time. Therefore vessels containing water
should be well rubbed and washed with soap and water. Since all three stages of the
life cycle of a mosquito are spent in water, various methods are adopted for destroying
them.
To investigate the methods used for controlling the breeding of mosquitoes,
conduct the following activity.
Activity - 3. 1
Find out the methods used in your area to control mosquito breeding and
those of other areas. Practise some of those methods and investigate how these
practices help to control mosquitoes.
You can get some idea of the ways to be adopted for controlling the various
stages of the life cycle of a mosquito.
Several methods can be adopted to destroy them;
•
Breeding various types of carnivorous fish which feed on mosquito
larvae. (in fish tanks, ponds and water bodies where mosquitoes breed.)
e.g. ‘Thithayas’, ‘Bulath hapaya’
Adding various types of oils onto the surfaces of drains where water collects so
that respiration of larvae and pupa is hindered.
Adding substances like salt, soap etc. to the places where water collects in the
house like flower pots, ant-traps etc.
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Spraying chemicals such as DDT and Malathion. (This method is not
much in use due to environmental pollution they can bring about).
The adult stage of the mosquito is the flying form. Hence it is difficult to control.
Yet can be controlled by cleaning up dark places around the house, where mosquitoes
are commonly found, Fumigation is another method we can use for that purpose which
helps to drive away mosquitoes from around your house.
Do you know ?
Traditional methods used for driving away mosquitoes were burning cashew
- nut shells and herbs such as ‘Madurutala’.
However, taking care not to be bitten by mosquitoes is the best way to avoid the
diseases.
Assignment - 2
Prepare a leaflet on the topic ‘ Let us prevent the mosquito menace‘. The
leaflet should include information such as the diseases caused by mosquitoes,
controlling their breeding, and methods of avoiding their bites.
b)
Diseases caused by house-flies
The house-fly grows and breeds in decaying organic matter. You already know
that house-flies cause infections such as diarrhoea, cholera. The eggs, larvae and pupa
of the house, fly can be destroyed by removing garbage heaps around houses, by
burying them or converting them to compost. Keeping the garbage bins closed, keeping
your food covered will help to prevent ourselves from diseases caused by flies.
Fig 3.10 - Keeping food covered
Fig 3.11 - Keeping garbage bins closed
Control of pests
Pests are the organisms which destroy useful crops or their products.
Do you know?
In Sri Lanka, as much as 15% - 20.% of the total paddy harvest is
wasted away due to pest attacks.
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Loss of harvest
Damages caused
by pests
Extra expense incurred for pest control
Market value of the crop is reduced
Profit for the farmer is reduced
Because of the above reasons, it is important to control pests. Pests that
destroy plant crops are of two types.
• Insects
• Non-insect pests
Among them destruction caused by insects is the most common. Let us consider
how a knowledge of the life cycles of the insect pests can help in controlling them.
1. Coconut black beetle
This is an insect pest showing complete metamorphosis. The adult beetle eats up
the tender part of the shoot and cause harm. Adult beetle lays eggs in decaying organic
matter. Larva is a white grub.
Methods of control
•
Garbage heaps should be turned around often. This exposes the grub to light
and heat making them dry up, this also exposes them to their predators such as
birds.
•
Pick up the adult beetles by hand fork and destroy them.
Fig 3.12 - Coconut beetle
Fig 3.13 - Coconut tree attacked by coconut beetle.
2. Fruit fly
A pest showing complete metamorphosis. Adult pierces fruits such as mango,
banana and guava and lays eggs inside the fruit. The maggots (larvae) remain inside
and feed on the fruit. As a result, the fruit rots and the quality of the fruit decreases.
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Methods of control
•
•
•
Cover the fruits so that the adult is prevented from laying eggs in the
fruit.
Place insecticides mixed with sugar and trap the insect.
Destroy any infected fruits fallen under the trees.
Fig 3.15 - Damaged fruits.
Fig 3.14 - Fruit fly
3. Paddy bug
This insect shows an incomplete metamorphosis. The adult as well as the
nymph pierce the paddy sheaf and suck juice causing empty paddy seeds.
Methods of control
• These insects are attracted to light. Therefore, light traps are used to
control them.
• Application of recommended amount of insecticide, if considered
necessary.
Fig 3.16 - Paddy bug
Fig 3.17 - Paddy affected by paddy bug.
Do you know?
Using insecticides for controlling pests is a major cause for environmental
pollution. Therefore they should not be used unless it is absolutely essential.
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Activity - 3. 2
Discuss with your teacher the scientific background of the following
traditional pest control methods used in controlling paddy pests in fields.
• Adding red-ant-colonies to paddy fields.
• Lighting ‘Kekuna-Oil’ lamps in the night.
• Sprinkling milk-rice to the field.
4.
Aulacaphora beetle
This is a pest showing complete metamorphosis. The adult eats up the leaves of
crop plants. The larval and the pupal stages are spent in soil and eat up the roots.
Methods of control
• Prepare the soil well by turning and ventilating.
• Mix kitchen ash with soil
Fig 3.18 - Aulacaphora beetle
Fig 3.19 - Damaged leaves of plants
Activity - 3.3
•
•
•
Pay a visit to your home-garden at school. Identify the plant pests that visit the
home garden with the help of your science teacher or agriculture teacher in the
school.
Make illustrations of the pests.
Design a poster including them, as well as the damage and methods of controlling
the particular pests.
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3.3 Effect of biotic factors on the environment
Go out to your garden and look
around the environment. You may
notice the birds flying in the air, the
butterflies sucking nectar from flowers
and the squirrels feeding on the
mangoes on the tree. You may feel
warm due to the heat of the sun, and
see the trees swaying in the wind. May
be your pet cat is preparing itself to
catch a bird or a squirrel perched on a
nearby tree.
Think further about the above
incidents. The environment is made
up of a collection of living (biotic) and
non-living (abiotic) components. You
will understand that there are constant
interactions between the living and
living, the living and the non-living and
the non-living and non-living
components in the environment.
Fig 3.20 - Various interractions in a natural
environment.
These continuous interactions help in maintaining the natural balance in the
environment and its existence. Let us consider how the biotic factors in the environment help the survival of the environment and its balance.
Activity 3.3
Observe the various interactions that take place in your environment and
note them down, under following topics:
Interactions between
* Biotic
- Biotic factors
* Biotic
- Abiotic factors
* Abiotic
- Abiotic factors
Diversity of organisms
There are many types of organisms in our environment. They range in size from
tiny micro-organisms such as virus, bacteria, plant plankton to enormous animals such
as the blue whale, elephant and trees such as the Redwoods and “Giant Nawanda”.
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Size
Method of
locomotion
Body
Structure
Diversity of
Organisms
Method of
feeding
Habitat
Type of
food
Fig 3.21 - Diversity among organisms
The habitats they live in also range from the surface of the soil to the deep interior
of the soil, on ice caps, warm deserts, bottom of the sea and in high mountains where
many organisms are found.
Diversity among organisms is the enormous diversity shown by plants,
animals and micro-organisms in their shape, size, colour, life style, type of
food and the habitats they live in.
Diversity of organisms helps their survival as well as the maintenance of balance in
the environment.
a) Competition
If you throw a piece of food to a pond or lake
with fish, you will note that a large number of fish
flock round the food fighting to get it. This is a very
simple example of competition.
Among the organisms in an ecosystem, there is
constant competition for food, water, habitat and
other basic needs. This competition can be among
organisms of the same species or of different species.
Fig 3.22 - Competition for food
among fish
Organisms living in the same environment show various adaptations to reduce
competition for their needs. In a tropical rain forest such as Sinharaja, the large trees
show stratification in order to reduce the competition for light. You have learnt about
this in Chapter 1.
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Certain plants have their branching limited to the upper parts of the stem. This
reduces competition for space. Competition for space is further reduced by creepers
living round large trees to reach light and the epiphytes which grow on the branches of
large trees.
Think!
Many seeds of Jak fall under the mother Jak tree. But they do not all grow
into new plants. Here you ever thought of the reason for this?
The various organisms which live in our surroundings are adapted to live in that
environment. Some organisms are adapted to live in a terrestrial habitat while some
are adapted to live in aquatic habitats. Some organisms are adapted to aerotic habitats
and some others to live under soil.
Activity 3.4
Fill the table below considering the different adaptation of organisms to live in different
environments. (copy down the table in your exercise book )
Organism
Habitat/ environment
in which it lives
1.Toad / Frog
2. Earthworm
3. Deer
4. Fish
5. Cactus
6. Orchids
7. Humming bird
Water/ Land
Under soil
Land
---------------------------------------------------------------------
Special adaptations
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Since different organisms have adaptations to live in different environments, it
reduces the competition for habitats.
The root system of the plant is responsible for absorbing water and minerals needed for the plant. There will be
a competition among plants in trying to acquire the necessities listed above.
This competiton is reduced to a great deal by the different types of the root system in different plants.Tap root
tap root
system of large trees goes deep into the soil while fibrous fibrous roots
roots spread out near the surface, in search of water. This Fig. 3.23 - Root systems of
plants
enables plants to absorb water from different levels of the
soil.
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s
Herbiv
ore
Tiger
Lion
Wolf
Eagle
Shark
Cow
Goat
Elephant
Rabbit
Deer
res
ivo
rn
Ca
There is a constant competition
among animals living in the environment
to obtain their food. Due to the diversity
among them, animals which depend on
different type of food in the same environment can be seen. Recall what you
learnt about herbivores, carnivores and
omnivores in Grade seven.
Man
Cat
Cock
Dog
Ominivors
Activity 3.5
Shapes of the beaks of birds differ according to the type of food.They take
name the type of food and the adaptations shown in the shape of the beak from the
pictures shown below.
Parrot
Eagle
Heron
Silver bill
Humming Bird
Duck
The competition for food is reduced among animals living in the same environment because of the differences in their choice of food.
Predation
Organisms in a particular habitat show various interactions. In this context the
interractions based on food have resulted in food chains and food webs.Those animals which occupy the third and fourth links are carnivorous animals which take other
animals as food. This is known as ‘predation’, the carnivore is called the ‘predator’
and the victim the ‘prey’.
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In a natural ecosystem, the numbers of
individuals are kept under control and a
balance is maintained by these predators.
In a particular environment, the
number of snails are controlled by the
greater coucal who feed on them. Also,
the number of deer in a jungle is kept under
control by the carnivorous leopards. When
the numbers of carnivores increase, the deer
3.24 - Lion hunts a prey
decrease in numbers resulting in a food
shortage which would again reduce the numbers of leopards.
Predators have special structural and behavioural adaptations for catching their
prey.
Tiger
Sharp teeth, claws
Python
Sharp teeth, expandable jaw
Eagle
Sharp claws, sharp beak
Fig 3.25 - Various adaptations of predators
Do you know?
A cheetah can run at a speed of 150 km per hour. A python can swallow
animals 5 times the diametre of its’ mouth.
Parasitism
Ticks, fleas and lice live on the bodies of household pets,
farm animals and even human beings. These live on the body
and derive not only nutrients but also protection. This too is
an interesting inter-relationship between different species of
animals. Such a relationship is called parasitism.
Fig 3.26 - Ticks living
on bodies of cattle
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Parasite is invariably smaller than the host. In this relationship, one partner that is
the parasite only is benefited. Very often, some degree of damage is caused to the
host. The damage can be a reduction in growth, a disease or even death. Parasites are
of two types.
• External parasites
• Internal parasites
External parasites live on the outside of the body of the host, gets its’ nourishment as well as protection from the host. Also, at certain times they act as
vectors for various diseases.
Table 3.2 - Some external parasites and their harm
Parasite
Host
Damage / Disease
Ticks
Lice
Fleas
Dogs, Cats, Rabbits
Man, Birds
Cats, Dogs
Sucks blood - Tick fever
Sucks blood
Sucks blood
Internal parasites live inside the host’s body, and cause various harm to the host.
Table 3.3 - Some internal parasites and their harm
Parasite
Host
Round worm
Plasmodium
Hook worm
Man (intestine)
Man (liver)
Man (intestine)
Damage / Disease
Sucks blood
Malaria
Sucks blood
Think!
Parasites cause a great deal of harm to the hosts, but hardly causes its death.
Have you ever thought why?
Not only animals, plants too show parasitism. You have
already learnt about Loranthus and Cascuta which are parasitic
on plants.
Loranthus has green leaves. They absorb only water and
minerals from the host plant and prepare their own food. These
are called ‘partial parasites’.
Fig 3.28 - Loranthus
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Cuscuta
Cuscuta has no chlorophyll, hence cannot
prepare their own food. They totally depend
on their host for water as well as food. They
are called ‘total parasites’.
Fig 3.29 - Cuscuta
Activity - 3.6
List out the adaptations shown in parasitic plants to maintain their life.
Further some micro organisms live as parasites inside plant and, cause many diseases
to the plant.These micro organisms are known as ‘Pathogens’.
Table 3.4 Various diseases of plants
Host plant
Parasite/Pathogen
Disease
Paddy
Fungi
Brown spot disease
Bannana
Virus
Leaf curling
Chillie/Tomato
Bacteria
Bacterial wilting
Activity - 3.7
Observe your home garden for the various diseases of
plants. Find out the symptoms and the causative factors of
those diseases
Fig 3.29 - Brown
spot disease
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3.4 Effect of abiotic factors on the environment
We are used to start our day’s activities with the rise of the sun. Animals as well
as plants get activated with the light and heat of the sun. Imagine on a certain day, if the
sun did not rise, what would be the situation? All activities of man, animals as well as
the processes of plants would be completely disrupted.
The above description shows how abiotic factors such as light and heat from the
sun are so closely tied up with the survival of organisms. In addition to heat and light,
other factors such as soil and water too play an important role in the existence of the
environment. Let us see how each of these factors affects the environment.
Temperature
The temperature of the environment is affected by the
heat from the sun. In Sri Lanka, places such as NuwaraEliya have a low temperature while places such as
Hambantota have a high temperature. Have you observed
the differences in the flora of these areas while travelling
through? In the same way, a variety of differences can be
observed among the plants growing in warm desert areas
and the cool temperate regions of the earth.
It is obvious that the environmental factor, like temperature
has limited certain ecosystems to certain specific areas.
Fig 3.30 - Plants adapted
to a cool climate
Example: Xerophytic ecosystem. Coniferous plants
Temperature affects the growth
of plants directly. Plants grow in
height and in girth due to increase
in the numbers of cells.
An optimum temperature is
needed for growth of cells.
Winter
Fig 3.31 - Plants
adapted
to a warm climate
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Summer
Fig 3.32 - Growth of plants
in winter and summer
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If we examine a cross-section of a tree growing in temperate
region, growth rings as in Fig 3.33 will be clearly seen. The
broader rings with light colour show the growth of the trunk in
the warmer seasons.
An optimum temperature is essential for all physiological
activities of plants, such as photosynthesis, respiration and
transpiration.
Fig 3.33 - Growth
rings of plants
In a plant, photosynthesis takes place best at a
range of 30 - 35 oC. Rate of photosynthesis drops
at very high temperatures or very low temperatures.
Rate of respiration too increases with increase of
temperature.
In plants, the rate of transpiration increases with
temperature. This causes a lot of water to be lost
Fig 3.34
from the leaf surfaces, resulting in wilting of the
Plant wilting on warm days
plant.
Temperature affects the behavioural patterns of animals
too. You would have noticed how buffaloes wallow in mud
on warm days. Also how little chicks cluster together on
cold days. We too respond to rise in temperatures by sweating. Certain animals and plants are limited to areas with
high or low temperatures.
In the zoological gardens of our country, there are cool
caves for animals such as the Polar Bear and Penguin while
Fig 3.35 - Chicks
in temperate countries warm areas are built for animals
Flocked together
such as Camels.
Polar bear
Penguin
Camel
Fig 3.36 - Animals adapted for various climate conditions
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Temperature has an effect on the various natural activities of the environment too.
Soil is made as a result of
weathering of rock. This activity takes
place best in high temperatures. Decay
of organic matter too takes place best in
warm temperatures. The reason for this
is that the activity of decomposers is high
when temperature is high.
Fig 3.37 - Seed germinating
Germination of seeds too needs an optimum temperature. In countries where
seasonal changes occur, seeds stay dormant till suitable temperatures are reached.
Earth gets rain by the activity of the water cycle. For the two main activities
in the water cycle, namely evaporation and condensation temperature is an
essential factor. Conduct the activity given below to observe these effects.
Activity - 3.8
Water drops
•
Set up the apparatus as shown in the diagram.
•
Observe the changes on the underside of the
watch glass when water gets heated up.
Watch glass
Water
Heating
It can be observed that there are drops of water on the underside of the watch
glass.This explains the effect of temperature on rainfall.
Assignment - 4
List out the changes in behaviour of animals when environmental temperature
• goes down
• goes up
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Light
The main natural source of light for us
Carbondioxide
is the sun. Green plants prepare the food
Oxygen
Water
they need by photosynthesis.Other
heterotrophic animals in the environment
Water
depend on this food. Light is an essential
factor for photosynthesis. Blossoming of
Fig 3.38 - Light is essential for
flowers too is affected by light. You know
photosynthesis
that flowers such as Nelumbium (Nelum)
and Sun-flower blossom in the morning
and close up in the night.
Further more the process of flowering in certain plants is affected by the
day-length. This is more clearly seen in
plants which grow in temperate countries,
but in Sri Lanka too certain plants such as Fig 3.39 - Flowers that bloom with sun rise.
winged bean (dambala), old varieties of
paddy show this character.
Do you know?
Paddy varieties such as Heeneti, Mavee, Dewaraddiri which are the original breeds of paddy take about one year to produce the harvest. The reason is
that their plants flower and produce fruits only once during the year.
Movement of plants too are affected
by light. You may have seen how plants
bend towards light as a response to light.
Certain plants in our surroundings have
leaves that close up when light intensity is
low. e.g. Sesbenia (katurumurunga),
Moringa (murunga).
Fig 3.40 Katurumurunga
Think!
Have you seen how the coconut trees along
the sea shore bend towards the ocean? Did
you wonder why?
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Among the plants around us, there are some which are fond of strong light and
some which prefer weak light.
Orchids, begonia, ferns grow well in low light intensity.
But plants such as ixora, maize, kurakkan and chillies, tomato need high light
intensity to flower.
Ixora
Tomato
Fig 3.41 - Plants that prefer bright light
Begonia
Ferns
Fig 3.42 - Plants that prefer weak light
Light has an effect on the production of colour pigments too in plants.
Do the following activity and observe the results.
Activity - 3.9
Select a lawn and place an inverted coconut shell at one point. Observe
after 3 - 4 days. What can you observe?
You will note that the grass has become yellow,
showing that sunlight is needed for the production of the
green colour of the leaves. Also, the red colour of fruits
and the various colours of flowers are enhanced by sunlight.
e.g.: Redness of Willard variety of mangoes.
Fig 3.43 - Willard mangoes
Water
All living beings on the face of the earth need water for survival. About 65% of the
weight of a plant is water. A watery medium is essential for absorbing minerals needed
for plant growth as well as to transport them through the plant. Land plants get their
water from the water in the soil (soil water). Epiphytes are adapted to get their water
from the water vapour in the atmosphere.
Water is an essential factor for the process of photosynthesis in plants.
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Most plants need water for their mechanical
support too. You may have observed how small
plants or herbs wilt if water is not provided for a
day or two. If water is provided, the plant
straightens up again.
Water provides the habitat for aquatic
organisms. They acquire oxygen, food and other
essentials from the water.
Fig 3.44 - Aquatic organisms
About 70% of the body weight of animals is composed of water. Animals
cannot survive for more than two or three days without water. All metabolic activities
inside the body such as transport of materials, elimination of waste products take
place in a watery medium. In conditions such as severe vomiting or diarrhoea excessive loss of water takes place and may even result in death.
Certain other activities that take place in the environment also need water.
Weathering of rock to produce soil is another such activity.
Soil
We come across various types of soil in different localities. Any type of
soil is basically made up of mineral parts such as sand, clay and silt, organic
matter, soil water, soil air and soil organisms in various proportions. Soil acts as
a medium of support for plants and also to provide the necessary minerals and
water. In discussing the importance of soil for plant growth, we can talk of three
types of soil. They are sandy soil, clayey soil and loam.
Table 3.5 - Characteristics of various soils
Sandy soil
• Has more of sand
Clayey soil
• Has more of clay
• Does not retain water • Does not retain
much air
• Lacks nutrients
• Fewer micro• Fewer micro-organisms
organisms
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Loam
• Has sand, clay and silt
in equal proportions.
• Retains water and air
well.
• Is rich in nutrients
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According to the above table you will understand that loam is the best kind
of soil for plant growth. A loamy soil retains sufficient water and air for plant
growth and the ease with which they can be obtained is also more.
Activity - 3.10
Take a sample of soil. Spread it out on a white sheet of paper and
identify the organisms found there. You may use a hand lens to identify the
small organisms.
Soil provides a habitat for many micro-organisms as
well as macro-organisms. Animals such as earth worms,
centipedes and termites live in burrows in the soil. Earthworm
makes burrows in the soil and also contributes to digestion of
organic matter. Therefore a soil with many earthworms can
be considered a rich soil.
Soil may have certain harmful effects too on plant
growth. Some micro-organisms living in soil may cause
diseases in plants.
Fig 3.45 - How roots
spread in soil
Some plant pests too have some of the
stages of their life cycle under the soil. They
live inside the soil and cause damage to the roots
of plants.
Fig 3.46 - Earth worm
Think!
Before preparing a plot for cultivation, farmers set fire to the ground.
Discuss the advantages and disadvantages of this practice.
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3.5 Controlling the factors affecting the
cultivation of crops
The role played by plants for the maintenance of the entire environment and for its
balance is enormous. You have learnt some aspects of it in Grade 6 and 7.
In pre-historic times man survived by eating plant materials such as vegetables,
fruits, leaves and yams. They were eaten raw but, as he passed from the hunter,
through the gatherer stage to become a farmer, man began to produce most of the
food he needed. At the beginning, farming was a simple activity, but later with more
diversification he managed to produce all of his food needs by himself. Thereby
agriculture became a more complex activity.
At present, with the increase in population and urbanisation, land has become a
limited resource. Along with it, the demand for food is also more. This resulted in
trying to get a higher yield from a smaller area of land. In order to overcome this
challenge, in the modern world many advanced agricultural practices are used.
Accordingly, let us examine how the agricultural methods advanced from
traditional ways to the modern ways in Sri Lanka.
Do you know?
In Sri Lanka about 29% of the whole land area is suitable for cultivation.
Traditional cultivation methods
There are many environmental factors that are needed for a plant to grow and
produce a crop. When the plant grows in a natural environment, these factors are
supplied to plants in more or less quantities. However, if these factors can be provided
at optimum level, plants can be made to show optimal growth and to give maximum
harvest.
In traditional agriculture, agricultural practices are conducted in harmony with natural
conditions. Here instead of controlling the environment, the practice was to select
agricultural methods in keeping with the prevailing environmental conditions.
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Water
Plant nutrients
Maximum harvest
and optimum growth
of plant
Light
Temperature
Humidity
CO2 and O2
Favourable soil
conditions
Fig 3.47 - Environmental conditions affecting cultivation.
Let us consider chena cultivation which is a traditional cultivation method practised
in the past and even today to some extent.
Chena cultivation
In chena cultivation it is the land that is changed instead of the crop. It is also
known as ‘Sala cropping’.
First a jungle land is cleared and the leaves etc. are allowed to dry. Then it is set on
fire. The remains of the fire such as the stems are used to put a fence round the plot. A
large tree is left in the middle of the ‘chena’ on top of which a hut is built. This is a
speciality in ‘chena’ cultivation. With the onset of rains the ground is tilled, soil is mixed
with wood ash and seeds are sprayed on the soil. Seeds are spread out without much
gaps. Since ‘chena’ cultivation is commonly done in the dry zone, the crops selected
are short-term crops and those which need little water.
Example: maize, gingelly, meneri, pumpkin, komadu ( sweet melon), cowpea,
kurakkan.
In this kind of cultivation water, fertiliser, pesticides are not used. The only problem
is to protect the cultivation from wild animals. This is done by a person keeping vigil
throughout the night, often with a large bon-fire accompanied by a particular type of
‘kavi’ or songs rendered loudly. This helps to chase away the wild animals.
Since the left overs from the cultivation are not removed, water and soil are
retained. The left over materials decay and add fertility to the soil.
Since mixed crops are used nutrient cycles take place effectively.
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Fig 3.48 - Traditional ‘chena’ cultivation’
When a chena has been continuously cultivated for about 5 years, the fertility of
the soil decreases gradually. Then the chena farmers abandon that plot and move into
another plot.
Do you know?
An abandoned ‘chena’ takes about 100 years to get back to a natural
forest.
Assignment - 5
1. Find out the regions where ‘Chena’ cultivation is done in Sri Lanka.
2. List out the favourable and unfavourable features of chena
cultivation.
Special cultivation methods
The modern cultivation methods are based on controlling the environment where
plants grow artificially by supplying the above factors (Fig 3.47) to optimum level in
order to get a maximum harvest of high quality.
Let us find out some information about some of the modern cultivation methods
currently being practised in Sri Lanka.
Green house cultivation
‘Green houses’ are areas where crops are cultivated by providing factors such as
suitable temperatures and correct composition of air artificially.
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In modern ‘green houses’ not only the factors such as temperature, composition
of air and humidity are controlled artificially, but also use special equipment to measure
them .
Most green houses are built in the shape of an arch. Special types of glass, plastic
and polythene are used to cover them. At present UV treated polythene is popularly
used.
Fig 3.49 - External view of a green - house
Fig 3.50 - Inside view of a green - house
Green house cultivation is a costly method. It is mostly used in temperate countries
in order to get the necessary warmth for crops in the cold seasons. In addition, this
method is used for research purposes and to cultivate crops with a high market value.
e.g.: Bell pepper varieties, Salad Leaves, Tomatoes and Strawberries.
Hydroponics
In crop cultivation, soil is an important factor for plant growth. It acts as a medium
of support to the plant as well as providing a media for water, nutrients and air. But in
order to overcome various disadvantages that may arise when using soil as the
medium, cultivation without soil (soil-less culture) has been introduced at present.
Here other media, are being used in place of soil.
When we use water in place of soil, that particular method is called as
hydroponics.The main principle of hydroponics is to supply the nutriemts to the plants
in liquid media. To provide a substratum to the roots, porous media such as coir dust,
saw dust or paddy husk is used.
These methods may vary from simple methods which we can practise in our home
to the complex methods which need advanced technology. Let’s identify these method
by observing the following illustrations.
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(b)
Hanging bag method
leafy vegetables, salad leaves, ornamental plants
(a)
Circulation method
chillies, tomatoes, green leafy vegetables
(c)
Styro foam box method
Salad leaves, ‘Kankun’, centella (Gotukola)
(d)
Floating method
Beet root, Carrot, radish
3. 51 - Various methods of hydro-ponic cultivation
Nutrient medium used in water culture
The success of water culture methods depends on the nutrient quality of the watery
medium used. The most commonly used medium for this is the ‘Albert solution’.
‘Albert solution’ has the properties given below:
• Contains all the nutrients needed for the plant
• Dissolves easily in water
• Non-poisonous to roots of plants
• Does not react chemically with the material of the containers.
Do you know?
Centuries ago, countries such as Babylonia, Egypt, China and India have
practised water culture methods.
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Activity - 3.11
Let us grow a ‘Gotukola’ pot using a throw away ice-cream container.
Materials needed.
•
An empty ice-cream container with a lid (4 Litres)
•
Albert solution’ prepared according to instructions
•
Steamed coconut fibre
•
Throw-away Yoghurt cup (with holes at the bottom) and sides
•
Few plants of ‘Gotukola’
Method:
Fill the ice cream container with the solution. Fill the yoghurt cup with coconut
fibre and place 2 plants of Gotukola in it. Make a hole in the cover of the ice
cream container, and place the plastic cup with the Gotukola plants on it, as shown
in the diagram.
.
Ice Cream container filled
with nutrient solution
Container with
gotukola plant
Well grown gotukola
plant
Place the prepared container in a place with good sunlight. Spray water on it
daily. When the liquid level goes down, fill it again with prepared Albert solution.
In a few days you will be able to get a well-grown shrub of Gotukola. You can
grow other green leaves such as Kan-kun, Salad leaves or certain flowers too in
this manner.
Mushroom cultivation
From the past, man has been in the practice of adding naturally growing mushrooms
to his diet. In Sri Lanka too natural mushrooms were popular.
Example: ‘Lena hathu’, ‘Kandan hathu’, ‘Uru hathu’, ‘Weli hathu’.
In recent times mushrooms are grown at commercial scale. The main reason for
this is the nutrient value and pleasant taste of mushrooms.
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Fig 3.52 - Different local varieties of edible mushrooms
Do you know?
Mushrooms are type of food with high nutritive value.
The nutrient value in 100 g. of mushrooms is as follows;
Water
- 91%
Iron
- 1 mg.
Energy
- 13 KJ
Thiamine
- 120 mg.
Protein
- 2.5 g.
Riboflavin
- 500 mg.
Fat
- 0.3 g.
Niacin
- 5.8 mg.
Calcium
- 20 mg.
Vitamin C
- 3.0 mg.
The ‘mushroom’ is the reproductive structure of ‘Fungi’ which is considered a
lower plant. Since fungi do not possess chlorophyll, they cannot prepare food like
other ordinary green plants. Therefore fungi are heterotrophic. Some of them grow on
decaying organic matter. They are known as saprophytes. Only a few varieties of such
saprophytic mushrooms are suitable for human consumption.
Mushroom cultivation needs a very small area of land. The medium for growing
can be some cheap materials such as straw, wood shavings. Culture medium contains
fungal spores.
In our country, two varieties of mushrooms are popularly grown.
1. Oyster mushrooms
2. Straw mushrooms
The more popular cultivation is the Oyster mushrooms
a. Golden coloured variety
b. American variety
Fig 3.53 - Different varieties of Oyster mushrooms
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Important factors in mushroom cultivation
•
•
•
•
•
Getting healthy culture media.
Sterilizing the growing medium (wood shavings/straw) well.
Keeping the medium in the dark till the fungi mycelia grow (Special enclosures
are prepared for this)
After the mycelium has grown well, they are placed in conditions of high
humidity. For this water is sprayed as a mist in the enclosures.
Harvesting at the correct time.
Do you know?
Most naturally growing mushrooms are not suitable
for human consumption ‘Poisonous mushrooms’ have
the following features.
• Dark Coloured
• Strong Smell
• Do not attract insects
Velum
Annulns
Assignment - 6
• Visit a place where mushrooms are grown in your area ( With your teacher)
• From the information gathered, prepare a booklet on ‘Mushroom Cultivation’
• More information about mushroom cultivation can be obtained from agriculture
instructor of your area or from Agriculturel Department publications.
Cultivation of ornamental plants
Cultivation of ornamental flowers is popular in Sri Lanka for earning foreign
exchange as well as a method of self employment.
Following kinds of flowers are popularly grown in Sri Lanka
• orchids
• anthuriums
• roses
• carnations
These varieties need vary environmental conditions and soil conditions for
maximum production. If we can supply these factors at an optimum level, it is possible
to get high yields.
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Orchids
Orchid is an epiphytic plant.
There are several naturally
growing orchids in Sri Lanka.
For export purposes Dendrobium
and Palanopsis are mainly used.
Possessing nice coloured petals
and ability to stay fresh for a
longer period are the special
characteristics of these.
Fig 3.54 - Types of Orchids
Orchid plants need a porous medium for root growth. As the potting medium a
mixture of charcoal, pieces of brick and dried moss are used. To obtain more colourful
flowers, humidity in the environment should be high. To maintain this high humidity
water should be sprinkled frequently in the form of a mist. Orchid plant always prefers
low light conditions. Therefore, Orchids should be grown in a shaded plant house.
Anthuriums
There is a good local and
foreign market for Anthuriums.
Various colours of Anthuriums
such as red, white, pink,
orange, pearl and bi - colour are
Fig 3. 55 - Various coloured Anthuriums
found. Demand for red colour
flowers is high. Anthuriums too tend to grow in shady places. When cultivating
Anthuriums, naturally or artificially shaded places are essential. A mixture of coconut
husk, pieces of brick, dried cowdung and fine sand are used as
the potting media.
Other flowers
Local demand for roses and carnations is high. To cultivate
these flowers high light intensity and low temperature is
preferable.
By cultivating these plants under optimum environmental and
soil conditions we can get high quality flowers from these plants.
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Fig 3.56 - Roses
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Exercises
1. Life cycle of a frog is indicated below.
Frog
B
a)
b)
c)
d)
A
Name ‘ A ’ and ‘ B’ stages of the above life cycle.
List out the interactions that take place between B and the environment in which
it lives.
Adult frog is an “Amphibian” as well as an “insectivore”.What adaptations does
it bear to suit that particular life style?
Life cycle of a frog is different from that of an insect. What are the two main types
of insect life cycles?
2. Read the following passage and answer the questions.
Malaria is a widely spread disease in the world. Annually about 250 million
people suffer from this disease and 2 million among them face fatal conditions. These
patients are mostly found in the areas where animal husbandry is done and has polluted
water. Vector for Malaria is the mosquito Anopheles while causative agent (pathogen)
is the micro organism called Plasmodium.In the past, DDT was used for vector control. But nowadays alternative methods are used to control the vector mosquito. However it is difficult to eradicate malaria from the world due to lack of education
and negligence of people. (WHO report)
a) Give definitions for the following ;
Š Vector
Š Pathogen
b) Name two diseases spread by mosquitoes other than that mentioned in the
above paragraph.
c) Which stages of the mosquito’s life cycles could be controlled by applying DDT
to stagnant water.
d) Suggest two more methods we can use to control that stage/stages and which
will not cause environmental pollution.
e) Write your comments about the sentence in bold in the given passage.
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3. There are various interactions between organisms in an eco-system .
These interactions help the existence of the enviroment. Predation and
parasitism are examples for such interactions.
a) Define the following words and give three examples for each.
Š Predation
Š Parasitism
b) Describe how the diversity of organisms helps to minimize competition for food
among them.
4. Give scientific explanations for the following ;
a) Milking cows imported from European countries give high milk yields only in
areas like Nuwara Eliya.
b) Yellowish coloured leaves can be seen in plants which grow in dark places.
c) Loam soil is more suitable for plant growth.
d) A layer of undecayed plant material can be observed on the soil of cool mountain
forests.
e) In countries where seasonal changes occur, plants show a rapid growth in Spring.
²).
5. Are these statements ‘True’ ( 9) or ‘False’ (²
(Copy down these statements in your excercise book)
a)
b)
c)
d)
e)
f)
g)
h)
i)
j)
In chena cultivation only harmful micro organisms are destroyed by burning the
soil. ( )
Green house cultivation is more suitable for temperate countries. ( )
All naturally grown mushroom varieties are edible. ( )
Any type of fertilizer is suitable for hydroponic culture. ( )
Orchids are suitable for export, because the flowers stay fresh for a longer
period ( )
In mushroom cultivation, high light intensity is required for the growth of the
fungus ( )
Photosynthesis is more effective at low temperatures. ( )
In Parasitism, both host and the parasite are benefited. ( )
Using insecticides should be minimized in pest control. ( )
Only the adult paddy bug is harmful for paddy sheaves.( )
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