404 The Community 



a higher level of integration than the individual plants and animals 

 that make it up. For this reason some ecologists refer to the com- 

 munity as a "superorganism." But it is doubtful if the integration of 

 any community is so closely knit as to justify an analogy with an 

 organism. The composition of each community is determined by 

 the selection of plants and animals which happened to reach and to 

 survive in the area; the activities of the members of the community 

 depend upon the adjustment of each individual to the physical and 

 biological factors operating in the area. According to this "indi- 

 vidualistic concept" no community necessarily reaches any prescribed 

 composition or steady state, but each is a law unto itself (Gleason, 

 1926; Cain, 1947). 



Turning from the functional aspect of the community to the de- 

 scriptive aspect, we find that the community is characterized by 

 having a more or less definite species composition. Often the change 

 in species from one community to another is sharp, and if the principal 

 species of the two communities are conspicuously different, the line 

 of demarcation will be clearly apparent. In other situations one 

 community grades slowly or irregularly into another. Where the 

 species composition definitely changes— signifying the change from 

 one community to another— some controlling environmental change 

 has taken place. This may be an obvious topographic or edaphic 

 change, or it may be a less apparent difference in local climate. From 

 the descriptive view point a community may be recognized ( 1 ) on 

 the basis of the habitat it occupies or (2) on the basis of the species 

 of plants and animals present. Both of these bases have been used 

 for the classification of communities into types. 



A clearly distinguished unit of the environment showing uniformity 

 of principal habitat conditions is known as a biotope. The term may 

 be used to describe an individual area or a type of area. A mud flat, 

 a sandy beach, a sand desert, a mountain brook, and a unit of the 

 ocean are examples of biotopes, and each supports a characteristic 

 type of community. The foregoing biotopes are recognized primarily 

 by the physical features. Other biotopes are determined primarily by 

 living elements, as is illustrated by a spruce forest, a marsh, or a 

 prairie. The spruce forest and the organisms living in it comprise a 

 biocenose, and at the same time the spruce forest serves as the biotope 

 for subordinate plants and animals present. 



Although each clearly distinct type of biotope will be inhabited 

 by a characteristic type of biocenose, the species composition may 

 differ widely, depending upon the source of animals and plants avail- 

 able to populate the area. For example, all the communities that de- 



