38 THE ECOLOGY OF ANIMALS 



succession, which in later stages is usually brought 

 about by the plants changing their own soil condi- 

 tions through accumulation of humus, decreased hght 

 kilhng subordinate species or preventing seed germ- 

 ination, etc., many animal habitats are more or less 

 temporary, or at any rate tend to move about and 

 change their position. A burnt heather moor re- 

 generates new heather : if grazing is stopped it may 

 turn into a birch plantation, and later to a pine 

 wood, from which most of the previous stages of 

 succession have disappeared. 



The subject of succession is dealt with by Leach 

 (1933) in another volume of this series. Its impor- 

 tance in animal ecology Hes in two things : the 

 concept of shifting habitats which introduces a 

 dynamic motif into surveys of animal communities, 

 and the basis it gives for classifying vegetation 

 into a logical sequence of types related through 

 succession. Animal ecology itself is of great im- 

 portance in the investigation of plant succession, 

 through such influences as grazing, destruction of 

 seed by rodents, and so on. It is partly for this 

 last reason that Phillips (1931) has stressed the idea 

 of biotic communities, since in the study of succession 

 it is often difficult to disentangle the effects of 

 plants and animals in causing changes in the soil 

 and vegetation. 



We come now to the consideration of physical 

 factors. On land these can be divided roughly into 

 climatic and edaphic, the latter including the soil 

 and shelter, etc. In water, there are also chemical 

 factors to be considered. This phase of animal 

 ecology can be conveniently discussed from several 

 different angles. First, the study of physical habitats 

 themselves. Secondly, the co-ordination of such 

 observations with ecological surveys. Thirdly, the 

 manner in which these factors influence animals. 

 To these a fourth subject should be added, which 

 also brings in considerations of vegetation, and 



