All animals and plants require food. Green plants are able to take in water and mineral salts from the soil, and a gas called carbon dioxide from the air. They use sunlight to build these into substances such as carbohydrates and proteins. This process is called photosynthesis which means `building up using light'.

Animals are unable to make proteins and carbohydrates themselves and obtain their food direct from plants (such animals are called herbivores - 'plant eaters') or from other animals (carnivores - 'flesh eaters') that have themselves eaten plants. The various plants and animals linked in this way make up a food chain.

For example, heather is one of the commonest plants of the uplands of northern England and Scotland. The young shoots, flowers and seeds of the heather form important food sources for the red grouse, which is common in many moorland areas. Grouse in their turn may occasionally be taken by a bird of prey such as a peregrine falcon.