CHAPTER II 



THE DISTRIBUTION OF ANIMAL COMMUNITIES 



Each habitat (i) has living in it a characteristic community of animals ; 

 (2) these can be classilEied in various ways and (3) their great variety 

 and richness is due to the comparative specialisation of most species of 

 animals. (4) It is convenient to study the zonation of such com- 

 munities along the various big gradients in environmental conditions, 

 such as that from the poles to the equator, which (5) shows the dominating 

 influence of plants upon the distribution of animals, in forming special 

 local conditions and (6) by producing sharp boundaries to the habitats 

 so that (7) animal communities are more sharply separated than they 

 would otherwise be. This is clearly shown by (8) the vertical zones 

 of communities on a mountain-side, which also illustrate the principle 

 that (9) the members of each community can be divided into those 

 *' exclusive " to and (10) into those *' characteristic " of it, while the 

 remaining species, which form the bulk of the community, occur in 

 more than one association. (11) Other vertical gradients are that of 

 light in the sea and (12) that of salts in water. (13) Each large zone 

 can be subdivided into smaller gradients of habitat, e.g. water-content 

 of the soil, and (14) these again into still smaller ones, until we reach 

 single species of animals, which in turn can be shown to contain gradients 

 in internal conditions supporting characteristic communities of parasites. 

 (15) In such ways the differences between communities can be classified 

 and studied as a preliminary to studying the fundamental resemblances 

 amongst them. 



I. One of the first things with which an ecologist has to 

 deal is the fact that each different kind of habitat contains a 

 characteristic set of animals. We call these animal associations, 

 or better, animal communities, for we shall see later on that 

 they are not mere assemblages of species living together, but 

 form closely-knit communities or societies comparable to our 

 own. Up to the present time animal ecologists have been 

 very largely occupied with a general description and classifica- 

 tion of the various animal habitats and of the fauna living in 

 them. Preliminary biological surveys have been undertaken 

 in most civilised countries except England and China, where 

 animal ecology lags behind in a pecuKar way. In particular 



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