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13.1.1 Food Chains and Webs

In Activity 13.4 we have formed a series of organisms feeding
on one another. This series or organisms taking part at
various biotic levels form a food chain (Fig. 13.1).
Each step or level of the food chain forms a trophic level.
The autotrophs or the producers are at the first trophic level.
They fix up the solar energy and make it available for
heterotrophs or the consumers. The herbivores or the primary
consumers come at the second, small carnivores or the
secondary consumers at the third and larger carnivores or
the tertiary consumers form the fourth trophic level (Fig. 13.2).
We know that the food we eat acts as a fuel to provide us
energy to do work. Thus the interactions among various
components of the environment involves flow of energy from
one component of the system to another. As we have studied,
the autotrophs capture the energy present in sunlight and
convert it into chemical energy. This energy supports all the
activities of the living world. From autotrophs, the energy goes
Figure 13.1
Figure 13.1 to the heterotrophs and decomposers. However, as we saw in
Figure 13.113.1
Figure
Figure 13.1
Food chain in nature
(a) in forest, (b) in the previous Chapter on ‘Sources of Energy’, when one form
grassland and (c) in a of energy is changed to another, some energy is lost to the
pond environment in forms which cannot be used again. The flow
of energy between various components of the environment
has been extensively studied and it has been found that –
n The green plants in a terrestrial ecosystem capture about
1% of the energy of sunlight that falls on their leaves and convert it
into food energy.
n When green plants are eaten by primary consumers, a great deal
of energy is lost as heat to the environment, some amount goes
into digestion and in doing work and the rest goes towards growth
and reproduction. An average of 10% of the food eaten is turned
into its own body and made available for the next level of
consumers.
n Therefore, 10% can be taken as the average value for the amount
of organic matter that is present at each step and reaches the next
level of consumers.
13.2
Figure 13.213.2
Figure
Figure 13.2 n Since so little energy is available for the next level of consumers,
Figure 13.2
Figure
Trophic levels food chains generally consist of only three or four steps. The loss
of energy at each step is so great that very little usable energy
remains after four trophic levels.
n There are generally a greater number of individuals at the lower
trophic levels of an ecosystem, the greatest number is of the
producers.
n The length and complexity of food chains vary greatly. Each organism
is generally eaten by two or more other kinds of organisms which in
turn are eaten by several other organisms. So instead of a straight
line food chain, the relationship can be shown as a series of branching
lines called a food web (Fig. 13.3).

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