86 I The Process of Evolution 



COMMUNITIES 



Even if a single interbreeding population were found in an environ- 

 mentally diverse area, it would be expected that, in time, genetic 

 processes would lead to diversification (Chap. 6). The processes in- 

 volved in the formation of complex communities of organisms are 

 virtually unknown. It is clear that a denuded area will become re- 

 populated. During the early stages of repopulation, the aggregations 

 of plants and animals are short-lived. Several different aggregations 

 can be distinguished over a period of time before a relatively stable 

 community develops. These stages make up what is referred to eco- 

 logically as succession, and the terminal stage often is called a 

 climax community. 



Ecologists do not agree on the best methods of studying succession 

 or terminal communities. Modern workers feel that communities are 

 part of a continuum and that they can be distinguished as units only 

 artificially. Indeed, since there are no biological functions operating 

 between reproductively isolated populations, one would not expect 

 communities to be discrete units. Each interbreeding population 

 behaves according to its own cytogenetic processes, producing indi- 

 viduals whose genotypes determine ranges of tolerance that enable 

 them to function. Each entity has had an evolutionary history de- 

 pendent on, among other things, its own genetic processes. Because 

 of these diflFerences one would not expect any two populations to 

 follow the same historical pattern for even a short period of time. 

 The community, however defined, results from the overlapping 

 ranges of tolerance of the individual organisms for the various fac- 

 tors of the environment at a particular place. 



If studied for relatively short periods, the terminal communities 

 in a successional series usually appear to be stable or in a steady- 

 state equilibrium. Within such communities, cycling of energy and 

 matter is constant and regulated by feedback mechanisms. Much 

 energy is stored in organic materials (plants, animals, humus, etc.). 

 Such a community is disturbed by outside influences only with 

 difficulty. It is thought that organisms from other environments find 

 it difficult to migrate into the community. Energy relations appear 

 to be clearly established, and primary producers (green plants), 

 primary consumers ( herbivores ) , secondary consumers ( carnivores ) , 

 and decomposers (microorganisms) can be distinguished. Natural 

 selection has resulted in an ecological unit of great complexity. 

 Organisms have evolved with respect to their position in this com- 

 plex (or in successional stages) as the environment of the earth has 

 changed through time. 



