A food chain represents the flow of food (or energy) in a given set of organisms or living beings. The various steps in a food chain at which the transfer of food (or energy) takes place are called trophic levels. In fact, in a food chain, each step representing an organism forms a trophic level. In most simple terms, ‘trophic level’ means ‘feeding level’ of the organism.
(i) The plants are producers (or autotrophs) and constitute the first trophic level. They fix up the sun’s energy and make it available for consumers (or heterotrophs).
(ii) Herbivores (which feed upon plants) constitute the second trophic level.
(iii) Carnivores (that feed upon herbivores) constitute the third trophic level.
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(iv) Large carnivores or Top carnivores (which feed upon small carnivores), constitute the fourth trophic level.
Herbivores are called primary consumers, small carnivores are called secondary consumers whereas top carnivores or large carnivores are called tertiary consumers. So, we can draw another diagram to represent various trophic levels by using the terms producers, primary consumers, secondary consumers and tertiary consumers.
Please note that both, secondary consumers and tertiary consumers are carnivores, the only difference being that secondary consumers are small carnivores (though we usually do not write the word small), whereas tertiary consumers are large carnivores which are usually called top carnivores. Please note that the diagram shown in Figure 18 is tapering upwards because as we go up towards higher trophic levels, the number of organisms in them decreases gradually.
The simplest food chain that we have already studied is:
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Grass –> Deer –> Lion
Now, this food chain involves three trophic levels. Grass (being producer) represents the 1st trophic level. Deer (being herbivore) represents the 2nd trophic level, and lion (being carnivore) represents the third trophic level.
We have also studied another food chain operating in the grassland, which is:
Grass –> Insect –> Frog –> Bird
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In this food chain, grass represents the 1st trophic level; insects represent the 2nd trophic level; frog represents the 3rd trophic level, whereas birds represent 4th trophic level. This is shown more clearly.
We will now consider some of the food chains involving man (human beings). Now, when man eats plants (or plant products), then the food chain involves only producer and consumer:
Plants –> Man
(Producer) (Consumer)
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This food chain has only two trophic levels. Plants being the first
tropliic level and the man representing second trophic level. But in the case of man who also eats meat (of animals like goat), the food chain involves producer and two consumers, the primary consumer and the secondary consumer:
(Producer) (Herbivore) (Omnivore)
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(Primary consumer) (Secondary consumer)
This food chain involving man has three trophic levels. Plants represent 1st trophic level, goat represents 2nd trophic level whereas man represents the 3rd trophic level.
The trophic levels in a food chain can also be represented by pyramid of numbers. Thus, if we count the number of species (or organisms) living at each trophic level in a food chain, then we can represent the food chain by a pyramid of numbers.
A pyramid of numbers showing the various trophic levels in the grassland food chain: Plants -> Mice -> Snakes -> Hawks. The base of this pyramid is formed by producers (plants) and the top of this pyramid is formed by the highest order consumers (or top carnivores).
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Please note that the same ecosystem may be supporting many different pyramids of organisms, each starting with plants at the base but ending in a different organism at the top.
Another point to be noted is that there is a greater number of organisms at the lower trophic levels of an ecosystem (the greatest number being at the producer level). As we go to higher and higher trophic levels, the number of organisms in each trophic level goes on decreasing.