All living organisms require nutrients, space, and the opportunity to reproduce themselves. These demands must be met by the environment which provides radiant energy (fixed by green plants in photosynthesis), inorganic salts, carbon dioxide and water. The properties of the environment that permit organisms to survive are not distributed evenly, however, and this results in the non-uniform distribution of living things. Resources also vary greatly in quality as well as in quantity, and there are, therefore, great evolutionary pressures towards specialisation in requirements. These pressures derive from competition between individuals, predation, and from the heterogeneous nature of the environment: they result in the profusion of species seen on earth today (Healey, 1972).