20 THE ECOLOGY OF ANIMALS 



on a Greenland heath is about the same as that on 

 a heath in the south of England. We conclude, there- 

 fore, that except in very unfavourable conditions for 

 life, the number of different kinds of animals that 

 can live together in an area of uniform type rapidly 

 reaches a Saturation point. In habitats of great 

 complexity, which include important minor habitats 

 (as with vertical layering in a wood) the saturation 

 number is much higher. We do not know how far 

 this generalization applies to tropical regions. The 

 reasons for this saturation point in number of species 

 must be sought in the study of the structure of animal 

 communities and the dynamics of animal populations, 

 and the precise values given to community Usts will 

 also depend on the conventions adopted for excluding 

 casual and accidental forms. An analysis of the 

 faunas of soil as given in various surveys shows that 

 although the total number of species remains roughly 

 the same in soil in different parts of England, the 

 actual species composition varies enormously. Aber- 

 ystwyth and Cheshire surveys of pasture soil animals 

 (85 and 58 species respectively) showed nearly the same 

 number of species but only eight of them — about a 

 seventh to a tenth of the faunas — were common to 

 both areas (Thompson, 1924). The saturation figure 

 is apparently not directly dependent on the numbers 

 of individuals present, for Morris (1922) found that 

 on unmanured soil at Rothamsted there were 60 

 species and about 5,000,000 individuals per acre, 

 while on manured soil there were 72 species and about 

 15,000,000 individuals per acre. All this goes to 

 support the idea that there is some important principle 

 involved in the stability of the total number of species 

 in an animal community. Further surveys are re- 

 quired and also the standardization of methods, before 

 the problem can be carried much further. Much 

 work along these lines has been done in plant ecology, 

 and it appears that the statistical methods developed 

 in the study of the species composition and abundance 



