A food chain represents a simple, linear one way relationship between one trophic level and the other. But in nature trophic relationships do not operate is such a simple manner, because if for some reason the link is broken between one trophic level and the other in a food chain it disturbs the whole ecosystem. In order to assure the stability of relation between different trophic levels, several links are established both in a linear and horizontal fashion and different food chains are interlinked.
A food Web may be defined as a net work representing the multiple inter links between various trophic levels is an ecosystem.
The trophic relationship in an ecosystem is more a web (network) than a chain. For instance in a grazing food chain the grass is consumed by rabbit as well as mouse. The mouse is turn may be eaten either by hawk or by snake which in turn is eaten by hawk. In other words these are always alternate sources of energy at different trophic levels.
In a grass land ecosystem there are as many as five linear food chains which are inter linked. These are –
(i) Grass – grasshopper – hawk.
(ii) Grass – grasshopper – lizard – hawk
(iii) Grass – Rabbit – hawk
(iv) Grass – mouse – hawk
(v) Grass – mouse – snake – hawk.
These five food chains are not isolated instead they are interlinked at different points to form a network or food web.
In a pond also a food web is formed with different food chains as follows–
(i) Phytoplanktons – Zooplanktons – fish
(ii) Phytoplanktons – Insects
(iii) Phytoplanktons – Small fish
(iv) Phytoplanktons – small fish – large fish (large plants)
(v) Macrophytes – Insects large crustaceous – insects- fish molluscus etc.
(vi) Zooplanktons – fish – large fish
The purpose of food web is twofold – one to make a alternate sources of food available at energy point and two to keep in natural check the number of organisms at any one given level by linking them with several predators. The complexity of the food web depends on two main aspects, (i) the length or (levels) of a food chain and (ii) the number of alternative links at every trophic level.