Lessons from ecology
To understand environmental problems and their solutions, one must have a good
understanding of ecology. Ecology is a science that studies how organisms interact
with one another and how they interact with their environment.
Ecology can be studied at different level of organization. Population is defined as
number of organisms from one species that live within an area (e.g. human population
in Fiji, myna population in Australia). Community is defined as all living organisms from
different species occupying a given locality. For example all the plant, animal and
microorganisms that live in a certain forest. Ecosystem is composed of all the
organisms and their chemical and physical environment. For example, in a river, all the
fish, water birds, plants, microorganisms, water and river sediment (e.g. mud or sand)
form a river ecosystem. Therefore, all the ecosystems consist of two components, the
living (biotic) and the nonliving (abiotic).
The flow of energy through ecosystems
The sun is the source of virtually all energy in the biosphere. It heats the Earth and
causes winds that can be used to produce electricity. The energy of flowing water can
also be trapped by dams and generators to produce electricity. For living organisms,
sunlight is captured by plants and stored in the molecules they make.
Sun produces different forms of radiant energy and they form an electromagnetic
spectrum. The types of energy represented in the spectrum range from low-energy
radio waves to high-energy gamma waves. From the entire electromagnetic spectrum
produced by the sun, visible light and infrared radiation are generally most useful to
living organism. Visible light is detected by organisms and some colors of visible light
are absorbed by plants during photosynthesis. Gamma rays, X-rays, and ultraviolet
radiation from the sun are all harmful to living creatures.
Solar energy flow
Not all of the sun’s radiant energy makes it to Earth’s surface. In fact, 32% is reflected
back into space by dust and clouds in the atmosphere and from Earth’s surface – either
from land, water, soil or vegetation. This reflectivity is known as albedo.
67% of the incoming solar energy is absorbed by Earth’s atmosphere, land, water and
vegetation. It is converted to heat. Although this heat will eventually dissipate into
space, it performs some vital functions before it escapes. It warms Earth’s surface (both
land and water), it evaporates water. Heating of Earth’s surface also produces winds,
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waves and weather. Only tiny amount fraction of the sunlight striking Earth is used by
photosynthetic organisms – somewhere around 1-2%.
Solar energy flow and heat loss can be altered by human activities. For example,
overgrazing of rangeland and pastures can result in a loss of vegetative cover and
therefore decrease albedo, causing Earth’s surface to heat up (less light is reflected
back into space). Tropical deforestation has similar effects. Pollutants in the atmosphere
can also alter these energy flows.
Over 100 years ago, scientists found that certain naturally occurring chemicals in the
atmosphere, such as carbon dioxide molecules and water vapor, retard the release of
heat into space. Like a giant blanket surrounding Earth, these chemicals cause heat to
be retained in the Earth’s atmosphere, creating climate suitable for life.
However, in the past 100 years, humans have burned massive quantities of fossil fuels.
This has lead to substantial increase in the concentration of carbon dioxide in our
atmosphere. This has lead to global warming of the atmosphere and oceans. This
could have dramatic effects on ecosystems, economies and human civilization.
Photosynthesis and respiration
All the energy that powers the living world can be traced back to its original source, the
sun. Sunlight is captured by plants during photosynthesis. During photosynthesis,
plants use solar energy to convert carbon dioxide and water into sugar, mostly glucose.
This process can only occur in the presence of chlorophyll, a green pigment found in
algae, some bacteria and plants. Chlorophyll acts like a solar collector in a cell,
gathering up sunlight energy. The general equation for photosynthesis is:
Solar energy + 6CO2+6H2O
C6H12O6 + 6O2
carbon dioxide + water
sugar + oxygen
Glucose is used as a source of energy by the plant itself and by animals who feed on
plants. Some of the oxygen produced by photosynthesis is used directly by the plant,
but the rest passes from the leaf through leaf microscopic pores into the atmosphere.
The breakdown of sugars by plants and animals to produce energy is called cellular
respiration. The general equation of it is:
C6H12O6 + 6O2
6CO2+6H2O + energy
This formula is the exact opposite of the equation for photosynthesis. So, in the living
world, plants produce glucose and oxygen which are inputs of cellular respiration. The
products of cellular respiration, in turn, become raw materials of photosynthesis,
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creating a recycling of materials. An understanding of photosynthesis makes it clear
why plants are so important to us and why it is important to protect Earth’s terrestrial
and aquatic plant life.
Food webs
The organic matter produced by terrestrial and aquatic plants is known as primary
production. This organic matter serves as source of organic building blocks and energy
for other organisms. A food chain is a feeding sequence in ecosystems. It illustrates
who eats who, and thus, the path in which nutrients and energy move in the
ecosystems. An example of food chain would be grass – grasshopper – frog – snake –
hawk (bird of prey). Plant eaters in such food chains (grasshopper) are known as
herbivores and meat eaters are known as carnivores.
Food chains are actually simplifications of the actual situation in the ecosystem. They
are usually part of larger food webs, or more complex feeding relationships. Also as a
rule the greater the number of channels through which energy can flow, the greater the
stability of the food web and ecosystem. This is so because with more complex food
webs, removal of one component does not disrupt the whole ecosystem (other
organisms can assume its place).
Another type of food chain involves detritus feeders. These organisms (certain
bacteria, fungi and termites) obtain their energy and nutrients from waste materials and
dead bodies of plants and animals – the detritus. As a result of activities of these
organisms, large, complex molecules in plant and animal remains and waste products
are broken down into smaller molecules such as nitrates. These compounds are
released into soil and water and are absorbed by plants. Detritus feeders therefore are
essential for nutrient recycling.
Human activities can remove components from a food web, and simplify it. This
happens for example when we create a single-species crop field (e.g. sugarcane filed).
In such simplified ecosystem, all the species that would normally be present in natural
grassland (different species of grasses, insects, birds etc) are missing or replaced with
only few species that can survive on sugarcane. These simplified ecosystems are very
vulnerable.
The Irish potato famine illustrates this phenomenon. The potato was introduced to
Ireland in the late 16th century. For many years, it was a staple of the Irish diet and
yielded more calories per km2 than any other crop. However, in 1845 a fungal disease
called the potato blight began attacking Ireland’s potato crop. The fungus spread quickly
from field to field, causing massive destruction to potato plants. This was possible
because most potatoes grown were of a single variety “Lumper” (i.e. there was little
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genetic variation in the potato plants grown in Ireland). Because the Irish were highly
dependent on only potato, and because few alternative foods were available, more than
1 million people died of starvation or disease (people get sick more easily when they are
starving). The Irish were so dependent on potato because they had to export other
foods to England that had colonized Ireland. Another 1 million Irish left the country. All
this was the result of ecosystem simplification brought on by the planting of huge crops
of a single food that allowed the disease organisms to proliferate.
Trophic levels and pyramids of energy and biomass
In the food chains and food webs, each feeding level is known as trophic level. So, for
example in a lake ecosystem, all the plants and algae form the first trophic level, all the
herbivores form the second trophic level and all the carnivores form the third trophic
level.
However, not all the energy and biomass (organic matter) produced in a given trophic
level is transferred to the next. Usually only 10% is transferred to next trophic level.
Why? Because not all material is eaten (e.g. roots of plants are usually not eaten by
animals) and also because the organism who is eaten themselves respire (e.g. they use
some of the energy to keep themselves alive). Cellular respiration explained above is
not very efficient process and usually around 40% of the energy in sugar molecule can
actually be used by the organism. The rest is lost as heat.
The energy and biomass in various trophic levels can be graphically represented as
energy pyramid and biomass pyramid. Hypothetical biomass pyramid with humanpig-corn illustrates that 1000 kg of corn are needed to produce 100kg of pig and this can
be converted to only 10kg of human flesh (this example assumes that human eats only
pig). The 10% conversion factor between trophic levels also means that if you have 20
000 calories from corn and if humans directly eat corn, then you can have 2000 calories
available for human. If you add one trophic level, cow for example, then from the same
20 000 calories from corn, the cow gets 2000 calories and human who eats cow will
only get 200 calories. It follows that eating more plant food feeds more people when
compared to meat diets. It also shows that humans (and some other omnivores –
organisms that eat both plants and animals) can occupy different trophic levels
depending on what they eat (meat or plant).
Nutrient cycles
Scientists have discovered that matter cycles over and over again in global nutrient
cycles. This can be illustrated by looking at Earth mass and the mass of all organisms
that have lived on Earth since the beginning of life 3.5 billion years ago. The mass of all
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the organisms ever lived on Earth is greater than the mass of Earth itself. The only
explanation for this is that matter is recycled.
Of 103 elements known to science, only about 35 contribute to the formation of living
tissue. Of these, carbon, hydrogen, oxygen, and nitrogen form about 96% of the human
body. Elements such as sodium, calcium, potassium, magnesium, sulfur, phosphorus,
zinc, iron, iodine and others, occur in smaller amounts.
The circular flow of an element from nonliving (abiotic) environment, such as rocks, air,
and water, into bodies of living organisms and then back into the nonliving environment
is known as nutrient cycle. Such cycles are also known as biogeochemical cycles.
The Nitrogen Cycle
Nitrogen is an essential component of many important compounds (chlorophyll, amino
acids that are part of proteins, DNA). Nitrogen in the bodies of plants and animals
comes from atmospheric nitrogen. Nitrogen gas is extremely abundant; it constitutes
about 78% of the atmosphere. However, nitrogen gas is chemically inactive and it is not
easy for living organisms to capture it from the atmosphere.
Nitrogen (N2) in the atmosphere can be fixed through atmospheric fixation. It is
naturally occurring phenomenon whereby the energy from lightning and sunlight causes
nitrogen to combine with oxygen to form nitrate (NO3-). The nitrate is then washed to the
Earth by rain and snow and is absorbed by the roots of growing plants.
A second type nitrogen fixation is biological fixation. Biological fixation is much more
important than atmospheric fixation; about 7 times more nitrogen is fixed through
biological fixation. It is performed by bacteria that live in soil and water; they convert the
atmospheric nitrogen to form ammonia. This can be used by plants. Nitrogen-fixing
bacteria also live inside the root systems of many plants. These include peas, beans,
and soybeans. By growing soybeans, a farmer may increase the nitrogen content (and
hence the fertility) of his or her soil.
Third type of nitrogen fixation is industrial fixation, a process in which nitrogen is
combined with hydrogen to form ammonia. Later the ammonia is converted into
ammonium salts that can be used as fertilizers (they are used by plants).
Animals excrete nitrogen back to soil and water in their wastes, mainly as urea. The
animal wastes and also the dead bodies of plants and animals are converted into
nitrates in the soil by bacteria, in a process called nitrification.
Nitrogen can also flow from one ecosystem to another, for example from terrestrial
ecosystem to aquatic ecosystem. Also, all nitrogen in plants, animals, soil and water
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eventually reenters the atmospheric reservoir from which it originally came. This is
again accomplished by bacteria in the soil and water that break down nitrates and
release gaseous nitrogen as a by-product. This process is known as denitrification.
This nitrogen cycle can be disrupted if we poison the soil bacteria (by using chemical
pesticides) or if we add too much nitrates into soil (as fertilizers). Also burning of any
fuel (e.g. coal, oil) causes the formation of nitrogen dioxide (nitrogen combines with
oxygen, the reaction is driven by the heat), which can later be converted into nitric acid.
Nitric acid when it rains down together with water is known as acid rain.
The carbon cycle
Carbon is a key element in all organisms from bacteria to humans, where it forms the
backbone of many biologically important molecules, including DNA, protein, and sugars.
Carbon atoms constitute 49% of the dry weight of the human body. Carbon dioxide in
the atmosphere acts as a thermostat for the planet - more carbon dioxide leads to
warming and lower carbon dioxide concentration in the atmosphere would lead to
cooling.
Carbon dioxide from the atmosphere is used by plants to manufacture sugars
(photosynthesis). If plant material is consumed by animals then some of the carboncontaining molecules are digested and converted into animal tissue.
In plants and animals, carbon dioxide is released back into the atmosphere during
cellular respiration. Carbon also returns to the atmosphere when an organism dies and
decomposes. Also animal wastes are broken down in the soil and water by bacteria,
and the carbon returns to the atmosphere. In both cases, carbon dioxide is released
back into the atmosphere by bacteria.
About 250-300 million years ago, during Carboniferous Period, giant tree ferns and
other plants were buried into sediment before they could become decomposed. They
were eventually converted into coal. Photosynthetic marine organisms that died and
settled at the bottom of the seas at that time were converted into oil and natural gas.
Thus, the carbon was removed from carbon cycle by these processes.
When humans discovered fossil fuels and started burning it, the carbon entrapped in
those fuels is returned to the atmosphere. Also, because plants take up carbon dioxide
from the atmosphere, human activities that remove vegetation (e.g. deforestation) lead
to increase in atmospheric carbon dioxide concentrations.
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The ocean also serves as carbon reservoir. Carbon dioxide dissolved in the water may
move through the food chain. Some of this carbon ends up locked in coral reefs or in
deep ocean sediments.
Also phosphorus cycles between earth’s crust, water, soil, and living organisms.
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