starting with tropical rain forest at the base and ending with perpetual snow and ice at the summit.

    The main climatic influences which determine these ecosystems are rainfall, temperature and the availability of light from the sun. For instance, forests are usually associated with high rainfall, but the type of forest is influenced by temperature and light. The same applies to deserts which occur in regions where rainfall is extremely low.

    But these links are never as simple or rigid as the word ‘chain’ suggests. For example, aphids are eaten by many insectivorous birds in addition to warblers, and also ladybirds and other insects; hawks, on the other hand, prey upon a considerable variety of birds and small mammal-So the term food web is often a better one to use when being precise, as it suggests a far greater number of possible links and reflects the fact that the whole community is a complex inter-connected unit. Thus the original energy from the sun flows through the whole ecosystem from one tropic level to another.

    Let us observe the diagram (fig-2) which shows some of the feeding relationships amongst organisms living in deciduous woodland. You will see from the diagram that animals fit into special positions within the food web; each is described as its niche. For instance, there is a niche for insects such as aphids which suck up the juices of leaves. Another niche for insects such as caterpillars which have strong jaws for biting off pieces of a leaf and a niche for relatively large animals such as deer which browse on the vegetation. All these animals feed on leaves but they differ both in size and in the manner in which they feed. So the term ‘niche’ denotes not only the animal’s position in the food web and what it eats, but also its mode of life. Just as a habitat is the place where an animal lives, so a niche describes its occupation the way it ‘goes about its business and earns its livings.

Discuss one more example of niche with your teacher.

Food Web
    In nature food chains are usually not simple and linear. They are often branched, because at every stage or trophic level consumers have several alternative forms of food to choose from. For example, in the fig.-3 snakes prey upon frogs, mice and small birds. Similarly, cranes prey upon fish