Food chain can be defined as a sequence of transfer of energy from organisms in one trophic level to those in another trophic level.
Sun is the major source of energy.
It helps in the growth of plants on the soil and water bodies.
Plants form the basis of food for large number of herbivores.
These herbivores are used as food substances for carnivores.
Besides, there are omnivores who feed on plants as well as animal flesh.
The solar energy absorbed by the soil is reflected in the form of plants and animals.
These organisms have a limited cycle and die after some time.
Once these organisms die, another group of organism starts their functioning as they feed on dead material.
They help in decomposing the dead bodies of plants and animals on releasing the energy which is again absorbed by the soil to enrich its production of plants.
Thus cycle completes.
The above said food chains are very simple food chain.
But food chains are not always so simple and isolated sequences.
Several inter-connected and overlapping food chains present complicated patterns.
Such patterns are called food web.
Various trophic levels –
As we have discussed earlier sun or solar energy is the source for all the plants for preparation of their food.
The energy which is stored by the plants is known as trophic level I.
It becomes the source of energy for the herbivores.
Therefore, transfer of energy from trophic level I to trophic level II takes place when the plant eating animals consume these plants.
Again this chemical energy (through foods) consumed by herbivores gets stored at trophic level II and becomes source of energy for the carnivores at trophic level III.
Carnivores are flesh eating animals and depend upon other animals for food.
These animals require a lot of energy for building their tissues.
They receive their energy from trophic level II through food consumption.
A part of the chemical energy from this level III of the food chain is transferred to omnivores at trophic level IV.
Therefore, omnivores are at the top level of the food chain which receives their energy from all the three levels.
So, in a food chain the members at the successive higher levels become smaller in number.
When the numbers at successive levels are plotted, they assume the shape of a pyramid; hence it is called food pyramid or pyramid of numbers.
The number of organisms at any trophic level depends upon the availability of food at its lower level.
With an increase in availability of food at the lower level, there is a consequent increase in the number and variety of organisms at its higher trophic level.
Thus, availability of food is the main factor which maintains the grand balance of nature.