In nature, food chain relationships are very complex. They never operate as isolated sequences, as one organism may form the food source of many organisms and so on. Thus, instead of a food chain, a number of food chains are interconnected with each other and form a web-like structure known as 'food web'.
For example, grass may be grazed by grasshoppers as well as cattle, rabbits and each of these may be eaten by different type of carnivores, such as birds, toads, snakes, foxes, depending on their food habit.
Thus, a particular organism may not occupy the same tropic level in every food chain; it may simultaneously behave as secondary, tertiary or a top consumer.
Organisms, whose food is obtained from plants by the same number of steps are said to belong to the same tropic level. Thus, green plants occupy the first tropic level or the producer level. The plant grazers occupy the second tropic level or primary consumer or herbivore level (all plant-grazing insects, cattle, deer, rabbits, etc.).
Flesh-eaters, that eat herbivores, form the third tropic level or the secondary consumer or carnivore level-1 (frogs, small fish, etc.).
The third tropic level is the tertiary consumer or carnivore level-2, which eats the flesh of herbivores and secondary consumers. In a similar fashion, tropic levels can be expanded based on the food habits of organisms.
Charles Elton, a British ecologist, however, concluded that the number of links in a food chain rarely exceeds five, because in the process of energy transfer there is always the loss of energy to the environment. It is the energy transfer mechanism which determines the number of links in a food chain.
Man and many other animals who are omnivores occupy different tropic levels in food chains in relation to pure carnivores.
The food web maintains the stability of the eco-system. For example, green land can be grazed by different organisms like insects, rabbits, rodents, etc.
The insects then can be eaten by frogs which can be eaten by snakes. Snakes can either be eaten by hawks or can take rodents or rabbits as their praise which only consumes grass. Thus, nature has sufficient alternatives and greater the number of alternative pathways, the more stable will be the community of living things.