Published by Australian Academy of Science
Biomass – the growing energy resource Box 2 | Introduction to food chains

Food chains are important in biology, because they show how energy passes through all living things from one to another. This illustrates the way in which organisms depend on each other to survive. At the beginning of nearly all food chains we find plants, because plants need only air, water, sunlight and a few trace minerals to live and grow. They are able to use the energy in sunlight to convert the traces of carbon dioxide in the air (only about 300 parts per million) and water from the ground into plant material - sugars and starches – by the process called photosynthesis. The plants are the basic producers of the biological system and convert light energy from the sun into stored chemical energy in the biological molecules that make up their leaves, stems and roots. Producers occupy the first level in the hierarchy of a food chain.

Animals cannot use the sun's energy in this way. They are consumers and must get the energy they need in order to survive by eating plants and extracting the chemical energy stored in plant material. They then use the energy to keep alive – to pump their blood, to move their muscles, and to operate their nerves. These plant-eating animals are called herbivores. A high, but variable, proportion of the energy in food is not extracted at all but is passed out in an animal's wastes, which are a food source for other creatures (often microscopic ones). Of the energy that is absorbed from an animal's food some goes to build up, maintain or repair its body and some may be stored as fat that can later be used as an energy source. But much of the energy is lost as heat directly from the animal, even if it is not warm-blooded.

Because the bodies of animals are made from complex chemicals, they too represent stored chemical energy. Other animals take advantage of this available food by eating animals that feed on plants. These meat-eating animals are called carnivores. Still higher up the food chain we have other predators that may feed on these meat-eaters.

As a rough rule, each level in the food chain can support about a tenth of its own weight of animals feeding upon it and still survive. We therefore have a food and energy pyramid that looks something like this:

Other boxes

Box 1. Biomass and greenhouse

Box 3. Ways of extracting biomass energy