ECOSYSTEM–STRUCTURE AND FUNCTION
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.
Ecosystem – Structure and Function
In earlier classes, 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:
Productivity;
Decomposition;
Energy flow; and
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.
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 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.
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 12.1). Humification and mineralisation occur during
decomposition in the soil. Humification leads to
accumulation of a dark coloured amorphous substance
called humus that is highly resistant to microbial action and
undergoes decomposition at an extremely slow rate. Being
colloidal in nature it serves as a reservoir of nutrients. The
humus is further degraded by some microbes and release of
inorganic nutrients occur by the process known as
mineralisation.
Decomposition is largely an oxygen-requiring process. The
rate of decomposition is controlled by chemical composition
of detritus and climatic factors. In a particular climatic
condition, decomposition rate is slower if detritus is rich in
lignin and chitin, and quicker, if detritus is rich in nitrogen
and water-soluble substances like sugars. Temperature and
soil moisture are the most important climatic factors that
regulate decomposition through their effects on the activities
of soil microbes. Warm and moist environment favour
decomposition whereas low temperature and anaerobiosis
inhibit decomposition resulting in build up of organic
materials.
Energy Flow
Except for the deep sea hydro-thermal ecosystem, sun is the
only source of energy for all ecosystems on Earth. Of the
incident solar radiation less than 50 per cent of it is
photosynthetically active radiation (PAR). We know that
plants and photosynthetic bacteria (autotrophs), fix Sun’s
radiant energy to make food from simple inorganic
materials. Plants capture only 2-10 per cent of the PAR and
this small amount of energy sustains the entire living world.
So, it is very important to know how the solar energy
captured by plants flows through different organisms of an
ecosystem. All organisms are dependent for their food on
producers, either directly or indirectly. So you find
unidirectional flow of energy from the sun to producers and
then to consumers. Is this in keeping with the first law of
thermodynamics?
Further, ecosystems are not exempt from the Second Law of
thermodynamics. They need a constant supply of energy to
synthesise the molecules they require, to counteract the
universal tendency toward increasing disorderliness.
The green plant in the ecosystem are called producers. In a
terrestrial ecosystem, major producers are herbaceous and
woody plants. Likewise, producers in an aquatic ecosystem
are various species like phytoplankton, algae and higher
plants.
You have read about the food chains and webs that exist in
nature. Starting from the plants (or producers) food chains or
rather webs are formed such that an animal feeds on a plant
or on another animal and in turn is food for another. The
chain or web is formed because of this interdependency. No
energy that is trapped into an organism remains in it for
ever. The energy trapped by the producer, hence, is either
passed on to a consumer or the organism dies. Death of
organism is the beginning of the detritus food chain/web.
All animals depend on plants (directly or indirectly) for their
food needs. They are hence called consumers and also
heterotrophs. If they feed on the producers, the plants, they
are called primary consumers, and if the animals eat other
animals which in turn eat the plants (or their produce) they
are called secondary consumers. Likewise, you could have
tertiary consumers too. Obviously the primary consumers
will be herbivores. Some common herbivores are insects,
birds and mammals in terrestrial ecosystem and molluscs in
aquatic ecosystem.
The consumers that feed on these herbivores are carnivores,
or more correctly primary carnivores (though secondary
consumers). Those animals that depend on the primary
carnivores for food are labelled secondary carnivores.
The detritus food chain (DFC) begins with dead organic
matter. It is made up of decomposers which are
heterotrophic organisms, mainly fungi and bacteria. They
meet their energy and nutrient requirements by degrading
dead organic matter or detritus. These are also known as
saprotrophs (sapro: to decompose). Decomposers secrete
digestive enzymes that breakdown dead and waste
materials into simple, inorganic materials, which are
subsequently absorbed by them.
In an aquatic ecosystem, GFC is the major conduit for energy
flow. As against this, in a terrestrial ecosystem, a much
larger fraction of energy flows through the detritus food
chain than through the GFC. Detritus food chain may be
connected with the grazing food chain at some levels: some
of the organisms of DFC are prey to the GFC animals, and in
a natural ecosystem, some animals like cockroaches, crows,
etc., are omnivores. These natural interconnection of food
chains make it a food web. How would you classify human
beings!
Organisms occupy a place in the natural surroundings or in a
community according to their feeding relationship with other
organisms. Based on the source of their nutrition or food,
organisms occupy a specific place in the food chain that is
known as their trophic level. Producers belong to the first
trophic level, herbivores (primary consumer) to the second
and carnivores (secondary consumer) to the third.
The important point to note is that the amount of energy
decreases at successive trophic levels. When any organism
dies it is converted to detritus or dead biomass that serves
as an energy source for decomposers. Organisms at each
trophic level depend on those at the lower trophic level for
their energy demands.
Each trophic level has a certain mass of living material at a
particular time called as the standing crop. The standing
crop is measured as the mass of living organisms (biomass)
or the number in a unit area. The biomass of a species is
expressed in terms of fresh or dry weight. Measurement of
biomass in terms of dry weight is more accurate. Why?
The number of trophic levels in the grazing food chain is
restricted as the transfer of energy follows 10 per cent law –
only 10 per cent of the energy is transferred to each trophic
level from the lower trophic level. In nature, it is possible to
have so many levels – producer, herbivore, primary
carnivore, secondary carnivore in the grazing food chain
(Figure 12.3) . Do you think there is any such limitation in a
detritus food chain?
Ecological Pyramids
You must be familiar with the shape of a pyramid. The base
of a pyramid is broad and it narrows towards the apex. One
gets a similar shape, whether you express the food or
energy relationship between organisms at different trophic
levels. This, relationship is expressed in terms of number,
biomass or energy. The base of each pyramid represents the
producers or the first trophic level while the apex represents
tertiary or top level consumer. The three types of ecological
pyramids that are usually studied are (a) pyramid of number;
(b) pyramid of biomass and (c) pyramid of energy. For detail
(see Figure 12.4 a, b, c and d).
Any calculations of energy content, biomass or numbers, has
to include all organisms at that trophic level. No
generalisations we make will be true if we take only a few
individuals at any trophic level into account. Also a given
organism may occupy more than one trophic level
simultaneously. One must remember that the trophic level
represents a functional level, not a species as such. A given
species may occupy more than one trophic level in the same
ecosystem at the same time; for example, a sparrow is a
primary consumer when it eats seeds, fruits, peas, and a
secondary consumer when it eats insects and worms. Can
you work out how many trophic levels human beings
function at in a food chain?
In most ecosystems, all the pyramids, of number, of energy
and biomass are upright, i.e., producers are more in number
and biomass than the herbivores, and herbivores are more in
number and biomass than the carnivores. Also energy at a
lower trophic level is always more than at a higher level.
There are exceptions to this generalisation: If you were to
count the number of insects feeding on a big tree what kind
of pyramid would you get? Now add an estimate of the
number of small birds depending on the insects, as also the
number of larger birds eating the smaller. Draw the shape
you would get.
The pyramid of biomass in sea is generally inverted because
the biomass of fishes far exceeds that of phytoplankton. Isn’t
that a paradox? How would you explain this?
Pyramid of energy is always upright, can never be inverted,
because when energy flows from a particular trophic level to
the next trophic level, some energy is always lost as heat at
each step. Each bar in the energy pyramid indicates the
amount of energy present at each trophic level in a given
time or annually per unit area.
However, there are certain limitations of ecological pyramids
such as it does not take into account the same species
belonging to two or more trophic levels. It assumes a simple
food chain, something that almost never exists in nature; it
does not accommodate a food web. Moreover, saprophytes
are not given any place in ecological pyramids even though
they play a vital role in the ecosystem.
An ecosystem is a structural and functional unit of nature
and it comprises abiotic and biotic components. Abiotic
components are inorganic materials- air, water and soil,
whereas biotic components are producers, consumers and
decomposers. Each ecosystem has characteristic physical
structure resulting from interaction amongst abiotic and
biotic components. Species composition and stratification
are the two main structural features of an ecosystem. Based
on source of nutrition every organism occupies a place in an
ecosystem.
Productivity, decomposition, energy flow, and nutrient
cycling are the four important components of an ecosystem.
Primary productivity is the rate of capture of solar energy or
biomass production of the producers. It is divided into two
types: gross primary productivity (GPP) and net primary
productivity (NPP). Rate of capture of solar energy or total
production of organic matter is called as GPP. NPP is the
remaining biomass or the energy left after utilisation of
producers. Secondary productivity is the rate of assimilation
of food energy by the consumers. In decomposition, complex
organic compounds of detritus are converted to carbon
dioxide, water and inorganic nutrients by the decomposers.
Decomposition involves three processes, namely
fragmentation of detritus, leaching and catabolism.
Energy flow is unidirectional. First, plants capture solar
energy and then, food is transferred from the producers to
decomposers. Organisms of different trophic levels in nature
are connected to each other for food or energy relationship
forming a food chain. The storage and movement of nutrient
elements through the various components of the ecosystem
is called nutrient cycling; nutrients are repeatedly used
through this process. Nutrient cycling is of two types—
gaseous and sedimentary. Atmosphere or hydrosphere is the
reservoir for the gaseous type of cycle (carbon), whereas
Earth’s crust is the reservoir for sedimentary type
(phosphorus). Products of ecosystem processes are named
as ecosystem services, e.g., purification of air and water by
forests.
