ECOLOGICAL SUCCESSION. 73 



This could be discussed as in the cases of the ponds and streams. 

 The discussions already published (1. c.) are sufficient to illustrate 

 the methods and principles. Furthermore the succession of con- 

 ditions and of the tiger beetles applying to forest development 

 has already been briefly outlined (Shelford, '07). (See Adams, 

 '08, '12.) 



V. CAUSES OF ANIMAL SUCCESSION AND THE CONTROL OF 



ANIMAL COMMUNITIES. 



I . The causes of plant succession as summarized by Cowles, ' 1 1 , 

 may be divided into those related to atmosphere and those related 

 to soil. In the case of animals we recognize also difference in 

 food and materials for abode. Physical conditions are believed 

 to be most important, as indicated by the great mass of experi- 

 mental work on animal behavior. Representative literature sup- 

 porting this view is cited in the discussions which follow. 



I. Materials for Abode and Food. 



The former are of great importance. There are the greatest 

 differences between the different forest stages, in this matter. 

 The plants of the later stages are more numerous and the leaves 

 less strongly cutinized, even when the plants belong to the same 

 species. The difference in leaf structure may be a factor in 

 limiting the distribution of the phytophaga to a certain part of 

 the range of a species of plant. The leaves, fallen logs, and all 

 conditions in which the animals make their abodes, change as 

 the forest develops. Food has been but little studied and we 

 know little or nothing as to what aspects of the food factor 

 are important. Dahl ('96) has studied the relation of carrion 

 eating animals to their food supply. 



2. Soil. 



Those causes of plant succession which are due to progressive 

 changes of soil, may be briefly summarized from an inspection 

 of the description of stations given above. The chief changes 

 obvious to the eye are an increase of vegetation, of leaf covering 

 and of humus. 



(a} The last of these changes increases the water holding 

 capacity of the soil, while the other two decrease the evaporation 



