II 



The Community 



In previous chapters we have considered separately the chief physical, 

 chemical, and biological influences of the environment. In all natural 

 situations many physical and chemical factors are in operation simul- 

 taneously, and each organism is surrounded by other plants and 

 animals that affect it continuously as a part of its environment. The 

 analytical approach which has been followed is useful in disen- 

 tangling the complex web of relationships within which the real life 

 of plants and animals is carried on. Furthermore, even though our 

 ultimate interest may be centered in the population or in the com- 

 munity rather than in the individual organism, we realize that the 

 population or community exists only because of sufficient adjustment 

 of each individual member. Having obtained some understanding 

 of each of the important relations between organism and environment, 

 we shall now consider certain of the more significant effects of the 

 concurrent operation of these influences in natural situations. 



The interplay of physical and biological forces has resulted in the 

 establishment of characteristic groups of plants and animals in all 

 habitable areas of the world. Each community maintains itself by 

 adaptations that allow it to withstand adverse influences and to 

 obtain its needs by exchange with the environment. The community 

 comprises the living portion of the ecosystem, as explained in Chapter 

 1. In the remaining chapters we shall discuss the nature of the com- 

 munity, its changes, and the role of the various components of the 

 community in the dynamics of the entire ecosystem, 



COMMUNITY CONCEPT 



The plants and animals living in any natural area form an assem- 

 blage in which each individual finds the environment to be tolerable 

 and to provide at least the minimum requirements. The presence of 

 many of the organisms is necessary for the continued life of other 

 members of the group, and, although antagonisms occur, obviously 

 the individuals that continue to exist in the area have survived any 



401 



