Community Concept 405 



velop on flooded soil along the coast of Maine, Argentina, or else- 

 where are of the salt-marsh community type but the particular species 

 of plants and animals present will vary. Differences in species com- 

 position will similarly be found in the alpine type community occupy- 

 ing mountain peaks in various parts of the world. 



To a certain extent community types can be based on the occur- 

 rence of characteristic species (or other taxonomic groups) of plants 

 and animals. The classification of communities on a taxonomic basis 

 is difficult because of the variation in species composition from one 

 community to another of the same general type and because of many 

 irregularities in occurrence. Furthermore, the composition of a com- 

 munity depends not only upon which species will grow under the 

 conditions of the habitat but also upon which species happen to have 

 been successfully distributed to the area. Designation of commun- 

 ity type on a taxonomic basis has been undertaken principally in 

 regions of the middle latitudes where the contrasting biotopes are 

 often populated by a few characteristic species represented by large 

 numbers of individuals, so that the communities are relatively distinct. 

 Nevertheless so many exceptions exist in any of the systems of classifi- 

 cation suggested that many ecologists doubt the validity of any 

 classification and prefer to consider communities on an individualistic 

 basis, as discussed by Gleason ( 1926 ) and Cain ( 1947 ) . In the 

 tropics it is extremely difficult, if not impossible, to characterize 

 communities on a taxonomic basis because the fauna and flora are 

 made up of a large number of species many of which appear in the 

 role of dominants (Richards, 1952). In the arctic relatively few 

 species are present, but over wide areas changes in the vegetation are 

 not due to the occurrence of different recognizable taxonomic com- 

 munities but merely to different combinations of the same species 

 (Raup, 1951). 



Where the physical features of the biotope are sharply delineated 

 and where these features exert primary control over the occurrence 

 of the organisms, a correspondingly abrupt change in the fauna and 

 flora takes place. An outcropping of rock in a forested region, for 

 example, will result in an abrupt shift in the vegetation from trees to 

 mosses or lichens. In the Florida everglades hammock communities 

 are sharply delineated on slight elevations of the ground from the 

 surrounding sawgrass community (Fig. 11.2). Sharp demarcation 

 among the animals of a community due to physical causes is also 

 frequently shown by the marine benthos, as for example, in localities 

 where the bottom material changes abruptly from sand to mud. 



Sharp limits to a biocenose or to its subdivisions may sometimes be 



