As you know from the discussion of the previous subsection, one of the ways in which the organisms are related is through food, that is, one organism becomes food for the other. A sequence of organisms that feed on one another constitutes a food chain.
The arrows in the figure show the direction of passage of nutrients and energy from the producer, a plant, to caterpillar, to the sparrow and finally to the eagle. The number of links or steps in a food &.in are usually limited to four or five. You may wonder why it is so. You would find the answer to this later. Coming back to the links or steps in a food chain, each of the link is also referred to as a trophic level. The trophic leve! to which an organism belongs, indicates how far it is away from plants in the food chain.
Green plants or producers make up the first trophic level. The second trophic level contains the plant eating animals, the herbivores, and higher trophic levels are made up of carnivores. Another point, that we wish to make here is, that an organism cannot always be assigned to one specific trophic level. Take the example of man who is an omnivore, i.e., he is both a herbivore and a carnivore, so he may belong to the second and/or the third trophic level.