Simple food chains, such as those already described, seldom occur by themselves. In most habitats there are many different plants and animals; some creatures have several sources of food and may be eaten by any number of different predators. Food chains therefore join one another to form a food web.

Thus, in a freshwater pool or pond there are usually large numbrs of minute plants, some so small that they can only be seen with a microscope. These form the food of tiny animals such as water fleas. Water fleas are eaten by small fish like minnows which in their turn are taken by larger fish, for example, perch. Fish may also be eaten by birds like the kingfisher and heron. Water spiders and water beetles are examples of smaller predators. Other food chains are based on water weeds - duck, for instance, feed on water plants, and pike (predatory fish up to a metre (3 ft) in length) often take ducklings as well as the young of other water-birds and quite large fish.

Frogs eat many of the smaller pond animals. They and the plant- eating watervoles also form part of the diet of pike, and in addition may occasionally be taken by a hungry heron.

Complex sets of food linkages such as these exist in almost every natural community.  There are some 230 types of insects that live on oak trees. This number includes over 80 moths which have caterpillars that feed on oak leaves, as well as many types of flies and beetles. Winter-moth caterpillars may become particularly abundant with perhaps as many as 60,000 feeding on the leaves of one oak tree. Sometimes they destroy almost the entire leaf canopy of an oak.

The leaf-feeders of the oak canopy provide food for many predators. These include small spiders, some of which are seldom found away from oak trees, as well as birds such as blue tits and wood warblers.

Other small creatures live only in acorns. Another group of insects that dwell on the oak tree is the gall-formers. There are several dozen gall-wasps which have grubs that live within the leaves and twigs of oak trees and cause the growth of oak-galls (the marble-like oak-apple is a typical example). Galls in their turn provide homes for several other insects, some of which feed on the gall- makers!

An oak tree thus supports a most elaborate food web. And it may also be the home of animals such as squirrels that build their dreys (nests) within its branches, and of owls that nest in hollow cavities within its trunk, but which seek some of their food elsewhere.