bioaccumulation

Build up of concentration of a pollutant in the animals at the end of a food chain. When one organism in a food chain eats another not only food chemicals are transferred, but any of the chemicals it contains. Some pollutants released as a result of human activity are very stable - or non-biodegradable. They do not break down nor can micro-organisms break them down. Once released they stay in the environment for hundreds and possibly thousands of years. These chemicals dissolve in fat deposits in the body and remain there. When the fat is eaten by another animal, all these chemicals are transferred with it. So the further down a food chain an organism is, the greater the concentration of chemicals. Animals at the end of food chains may be poisoned by the final concentrations of chemicals. The first chemical shown to be affecting animals in this way was DDT.

There are usually more prey than predators in a habitat. This is because there is not enough energy available in the bodies of the prey to sustain more predators. The drop in numbers of organisms along many food chains is shown using a pyramid of numbers.