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Each link in a food chain is known as a trophic level. For example, in each food chain, plants always form the first trophic level. The plant eating animals called herbivores like insects, rodents, rabbits, deer, cattle, etc. form the second trophic level. The animals like frog, small fish, small birds which feed on the second trophic level organism form the third trophic level. These are eaten by still longer carnivores like lion or tiger, who constitute the fourth trophic level.
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