The extent of the area occupied by each kind of grouping de- 

 pends chiefly upon the similarity or homogeneity of the environ- 

 mental conditions (see Figures 1-1 and 1-22). "As widely in 

 space as a uniform physical environment and a uniform physio- 

 logical interference [similar to relations between species] are 

 maintained, just so widely will the vegetation remain similarly 

 uniform, modified only by the factor of time, which is necessary 

 to the attainment of uniformity." ^^P'*'^'' "Uniformity, area, 

 boundary, and duration are the essentials of a plant community. 

 ... A community is uniform, either in space or in time, only to 

 a reasonable degree. This uniformity is sufficient to enable us to 

 recognize the community and to accept it as a unit of vegeta- 

 tion. . . ."90.pp.l03.104 



The grouping cannot be separated functionally from the en- 

 vironment. The community with all its plants and animals forms 

 the living part of the ecosystem, while the environment forms the 

 nonliving part. In most ecosystems the kinds of organisms are 

 numerous and diverse and include producers, consumers, and 

 decomposers. The relations between the organisms themselves, 

 and between the organisms and the environment are also num- 

 erous and complex. Because of this complexity and the basic 

 function of plants as producers, and also because they usually 

 form the structural habitat within which animals live, plant 

 groupings are treated primarily here, even though they form only 

 one part of the living component of the ecosystem. The plant 

 grouping is an advantageous starting point because it is more 

 easily analyzed than the animal life or the environment; more- 

 over, an understanding of the plant community facilitates analysis 

 of the other components of the ecosystem. Many plant-animal 

 relations are, of course, included in the study of plant communi- 

 ties, but less exhaustively than when the ecosystem is the primary 

 object of study. 



FACTORS INVOLVED IN THE GROUPING OF SPECIES 



Groupings of plants occur because the requirements and eco- 

 logical amplitudes of several to many species are adapted to the 



OO • Species amttd Popimlatioims 



