An ecosystem can be visualised as a functional unit of
nature, where living organisms interact among themselves
and also with the surrounding physical environment.
Ecosystem varies greatly in size from a small pond to a
large forest or a sea. Many ecologists regard the entire
biosphere as a global ecosystem, as a composite of all
local ecosystems on Earth. Since this system is too much
big and complex to be studied at one time, it is convenient
to divide it into two basic categories, namely the
terrestrial and the aquatic. Forest, grassland and desert
are some examples of terrestrial ecosystems; pond, lake,
wetland, river and estuary are some examples of aquatic
ecosystems. Crop fields and an aquarium may also be
considered as man-made ecosystems.
We will first look at the structure of the ecosystem, in
order to appreciate the input (productivity), transfer of
energy (food chain/web, nutrient cycling) and the output
(degradation and energy loss). We will also look at the
relationships – cycles, chains, webs – that are created as
a result of these energy flows within the system and their
inter- relationship.
CHAPTER 14
ECOSYSTEM
14.1 Ecosystem–Structure
and Function
14.2. Productivity
14.3 Decomposition
14.4 Energy Flow
14.5 Ecological Pyramids
14.6 Ecological Succession
14.7 Nutrient Cycling
14.8 Ecosystem Services
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14.1 ECOSYSTEM – STRUCTURE AND FUNCTION
In chapter 13, you have looked at the various components of the
environment- abiotic and biotic. You studied how the individual biotic
and abiotic factors affected each other and their surrounding. Let us look
at these components in a more integrated manner and see how the flow of
energy takes place within these components of the ecosystem.
Interaction of biotic and abiotic components result in a physical
structure that is characteristic for each type of ecosystem. Identification
and enumeration of plant and animal species of an ecosystem gives its
species composition. Vertical distribution of different species occupying
different levels is called stratification. For example, trees occupy top
vertical strata or layer of a forest, shrubs the second and herbs and grasses
occupy the bottom layers.
The components of the ecosystem are seen to function as a unit when
you consider the following aspects:
(i) Productivity;
(ii) Decomposition;
(iii) Energy flow; and
(iv) Nutrient cycling.
To understand the ethos of an aquatic ecosystem let us take a small
pond as an example. This is fairly a self-sustainable unit and rather simple
example that explain even the complex interactions that exist in an aquatic
ecosystem. A pond is a shallow water body in which all the above
mentioned four basic components of an ecosystem are well exhibited.
The abiotic component is the water with all the dissolved inorganic and
organic substances and the rich soil deposit at the bottom of the pond.
The solar input, the cycle of temperature, day-length and other climatic
conditions regulate the rate of function of the entire pond. The autotrophic
components include the phytoplankton, some algae and the floating,
submerged and marginal plants found at the edges. The consumers are
represented by the zooplankton, the free swimming and bottom dwelling
forms. The decomposers are the fungi, bacteria and flagellates especially
abundant in the bottom of the pond. This system performs all the functions
of any ecosystem and of the biosphere as a whole, i.e., conversion of
inorganic into organic material with the help of the radiant energy of the
sun by the autotrophs; consumption of the autotrophs by heterotrophs;
decomposition and mineralisation of the dead matter to release them back
for reuse by the autotrophs, these event are repeated over and over again.
There is unidirectional movement of energy towards the higher trophic
levels and its dissipation and loss as heat to the environment.
14.2. PRODUCTIVITY
A constant input of solar energy is the basic requirement for any ecosystem
to function and sustain. Primary production is defined as the amount of
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biomass or organic matter produced per unit area over a time period by
plants during photosynthesis. It is expressed in terms of weight (gm
–
2
) or
energy (kcal m
–
2
). The rate of biomass production is called productivity.
It is expressed in terms of gm
–2
yr
–1
or (kcal m
–
2
) yr
–1
to compare the
productivity of different ecosystems. It can be divided into gross primary
productivity (GPP) and net primary productivity (NPP). Gross primary
productivity of an ecosystem is the rate of production of organic matter
during photosynthesis. A considerable amount of GPP is utilised by plants
in respiration. Gross primary productivity minus respiration losses (R),
is the net primary productivity (NPP).
GPP – R = NPP
Net primary productivity is the available biomass for the consumption
to heterotrophs (herbiviores and decomposers). Secondary productivity
is defined as the rate of formation of new organic matter by
consumers.
Primary productivity depends on the plant species inhabiting a
particular area. It also depends on a variety of environmental factors,
availability of nutrients and photosynthetic capacity of plants. Therefore,
it varies in different types of ecosystems. The annual net primary
productivity of the whole biosphere is approximately 170 billion tons
(dry weight) of organic matter. Of this, despite occupying about 70 per
cent of the surface, the productivity of the oceans are only 55 billion tons.
Rest of course, is on land. Discuss the main reason for the low
productivity of ocean with your teacher.
14.3 DECOMPOSITION
You may have heard of the earthworm being referred to as the farmer’s
‘friend’. This is so because they help in the breakdown of complex organic
matter as well as in loosening of the soil. Similarly, decomposers break
down complex organic matter into inorganic substances like carbon
dioxide, water and nutrients and the process is called
decomposition.
Dead plant remains such as leaves, bark, flowers and dead remains of
animals, including fecal matter, constitute detritus, which is the raw
material for decomposition. The important steps in the process of
decomposition are fragmentation, leaching, catabolism, humification and
mineralisation.
Detritivores (e.g., earthworm) break down detritus into smaller particles.
This process is called fragmentation. By the process of leaching, water-
soluble inorganic nutrients go down into the soil horizon and get precipitated
as unavailable salts. Bacterial and fungal enzymes degrade detritus into
simpler inorganic substances. This process is called as catabolism.
It is important to note that all the above steps in decomposition operate
simultaneously on the detritus (Figure 14.1). Humification and
mineralisation occur during decomposition in the soil. Humification leads
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