Food webs

Although we have looked at food chains as a way of examining the passage of energy from the Sun into higher organisms, the inter-relationship between different organisms is never that simple. Frequently a number of food chains have species in common producing a 'food web'. For example, looking back at our food chain - other organisms, such as ladybirds, feed on aphids; sparrowhawks prey on more than just blue tits. Some food webs can be extremely complex, but all of them have three groups of organism - 'producers' (some sort of green plant), 'primary consumers' (those organisms which feed on green plants) and 'secondary consumers' (those organisms which feed on other animals).

Sparrowhawks prey on other birds, such as chaffinches and thrushes as well as blue tits. This expands a simple food chain into a food web, with many different organisms involved.

Accumulation of toxic materials in a food chain

The inter-relationships between different organisms through food chains and webs can sometimes have dramatic and unexpected results. The use of the insecticide DDT to control insect pests in crops caused a fall in the populations of birds of prey such as sparrowhawks and buzzards. The natural predators of the insect pests (small birds such as blue tits) took in DDT residues when they ate affected insects, this concentrated the chemical in their bodies. When birds of prey ate the blue tits the pesticide levels were high enough to either poison them, or to make their eggs infertile. Other organisms which suffered from pesticide poisoning were less expected. Fish-eating birds, such as herons, and even mammals, such as otters, have been poisoned by pesticides which get washed into water courses and enter food chains involving the fish present.

Pesticides can enter the food chain and have a devastating effect on those organisms at the top, such as birds of prey.

Factors affecting populations size

For a given organism in its natural habitat, there are several factors which prevent the population from continuing to increase. Firstly, there is usually a limit to the food supply available. Once this runs out, some of the population will starve. Secondly, predators may take more of the organisms, because with so many they're easier to catch. Finally, overcrowded conditions help the spread of any disease, and cause stress which can affect successful reproduction. Thus for all types of organism there are natural controls on population growth.

The populations of predator and prey (foxes and rabbits in this case) are closely linked.