430 Succession and Fluctuation 



could not live there as far as the climatic and edaphic conditions are 

 concerned. Finally the newly established species will be forced to 

 defend themselves against additional invaders that continue to arrive. 

 The success with which a species can extend into a variety of new 

 regions depends on its ability to tolerate widely different ecological 

 influences— both physical and biological. 



After the invading species becomes established in a new biotope, 

 it may undergo genetic changes. This will further complicate or 

 modify the situation and the effects of such alterations in the inherent 

 nature of the members of the community must be taken into account, 

 especially in any study of an area extending over many generations of 

 the inhabitants. The consequences of mutation and hybridization 

 and the effects of isolation are topics beyond our present scope and 

 the reader is referred to specific treatments such as those of Mayr 

 (1942), Cain (1944), or Anderson (1949). 



In summary we observe that from the point of view of the species 

 a tendency for dispersal is always present. When pioneers reach a 

 new area, a struggle for establishment takes place, and, if the struggle 

 is successful, genetic modification or evolutionary change may ensue. 

 Looking at the same process from the point of view of a specific area 

 we may observe the change of inhabitants as time passes. If we 

 watched one spot for a long period of time, we would witness a suc- 

 cession of communities— each one formed from new arrivals, allowed 

 to flourish for a while, and then replaced by a new community. 



Succession and Climax 



The species that have successfully invaded a biotope dominate the 

 scene for a period and form a closed community; further arrivals 

 cannot at first establish themselves. However, in the course of time 

 conditions become altered with the result that the members of the 

 existing community no longer compete successfully with the invaders. 

 A new dominant type gains a foothold, and a new community suc- 

 ceeds the old. By the modification of the environment one com- 

 munity puts itself at a disadvantage and gives way to another; com- 

 munities appearing at later stages of ecological succession are estab- 

 lished partly or chiefly because of the modifying action of earlier 

 communities. 



One community continues to follow another until in many situa- 

 tions a type of community is reached that cannot be displaced under 

 the prevailing conditions. The community that can maintain itself 



