27 



these changes are dependent upon physical conditions which are 

 equally potent in their influence upon animals. Thus physical and 

 vegetational changes in combination influence animals directly and in- 

 directly, and in the conditions due to this fact we find the basis for the 

 important control which vegetation exerts upon animals. 



Animals themselves f(^rm an important part of their own environ- 

 ment, not only in their relation to their own kind, as mates or as prog- 

 eny, but also as members of an animal community whose members 

 must adjust their activities to one another through symbiotic, competi- 

 tive, or predatory relations. If any animal becomes abnormally abun- 

 dant, that is, more numer(nis than the conditions can support, this 

 number in itself becomes a weakness, through the positive attraction 

 of the organisms (plant and animal) which are able to prey upon it, 

 and soon the normal al^undance is restored. For example, in a conif- 

 erous forest, bark-beetles (Scolyfoidca) may increase to such an extent 

 that the forest is largely destroyed, and a succession is produced in 

 the vegetation as the conifers are replaced by a growth of aspen and 

 birch. As a result of this destruction of the kind of food and habitat 

 essential for the next generation of beetles, a proper habitat is lacking, 

 and the restoration of the normal number of beetles is hastened. This 

 same example also shows how one kind of animal may influence the 

 character of a whole community by its control over the vegetation. 



The influence of man must be looked on from the same standpoint 

 as one views the activitv of any other animal ; as that of a member of 

 an animal communitv. He hastens and retards the changes in his en- 

 vironment as do other animals. In general his early methods are pred- 

 atory ; he reaps where he does not sow ; but later the milder competi- 

 tive and symbiotic relations and the constructive or productive aspect 

 become more prominent. Civilization is an attempt to make the en- 

 vironment "to order," but as yet man has not learned how to produce 

 a permanent "optimum" along the lines of an ecological community. 

 As has already been said, to understand man we should view him as 

 an integral part of an ecological communitv, as one member of a biotic 

 community of plants and animals, or at least of an animal community 

 which includes all animals that are influenced by man — and not con- 

 sider him, as some students do, as a distinct entity with little regard 

 to his animal and j^lant associates. 



The main features of the preceding discussion may be summarized 

 as in the following table. 



