Animal Ecology

Every animal is (1,2) closely linked with a number of other animals living round it, and these relations in an animal community are largely food relations. (3) Man himself is in the centre of such an animal community, as is shown by his relations to plague-carrying rats and (4) to malaria or the diseases of his domestic animals, e.g. liver-rot in sheep. (5) The dependence of man upon other animals is best shown when he invades and upsets the animal communities of a new country, e.g. the white man in Hawaii. (6) These interrelations between animals appear fearfully complex at first sight, but are less difficult to study if the following four principles are realised: (7) The first is that of Food-chains and the Food-cycle. Food is one of the most important factors in the life of animals, and in most communities (8) the species are arranged in food-chains which (9) combine to form a whole food-cycle. This is closely bound up with the second principle, (10) the Size of Food. Although animals vary much in size, anyone species of animal only eats food between certain limits of size, both lower and (11) upper, which (12) are illustrated by examples of a toad, a fly, and a bird. (13) This principle applied to primitive man, but no longer holds for civilised man, and (14) although there are certain exceptions to it in nature, it is a principle of great importance. (15) The third principle is that of Niches. By a niche, is meant the animal's place in its community, its relations to food and enemies, and to some extent to other factors also. (16), (17), (18), (19), (20) A number of examples of niches can be given, many of which show that the same niche may be filled by entirely different animals in different parts of the world. (21) The fourth idea is that of the Pyramid of Numbers in a community, by which is meant the greater abundance of animals at the base of food-chains, and the comparative scarcity of animals at the end of such chains. (22) Examples of this principle are given, but, as is the case with all work upon animal communities, good data are very scarce at present.