The Factor Complex
Every organism is exposed to a multitude of environmental influences which vary in time and space.. These factors can be loosely classified into physical and chemical on the one hand, and biotic on the other. Each individual has a limited tolerance range to physical and chemical factors. Beyond these limits it will die. Physical and chemical factors therefore determine the geographical range within which a species can possibly occur. Many species, however, only occur in a small proportion of the possible range. Biotic factors, such as the uneven distribution of a food plant, predation, inefficient dispersal, or competition from other species, influence the presence or absence of each species on a small scale.
(Taken from Nuffield Advanced Biology, Pub. Longmans) (HOME)