424 The Cojnmunity 



tion for communities in the free water contrasts with that for com- 

 munities on the bottom or on land in the absence of members that are 

 fixed in position. Since the pelagic plants and animals are free to 

 drift or to swim vertically as well as horizontally, subdivisions of the 

 pelagic community are less well defined and are more flexible than 

 those determined by sessile algae or rooted vegetation. Nevertheless, 

 critical changes in the physical environment, as at the lower limit of 

 the photic zone, at the thermocline, at a density discontinuity, or at 

 the margin of an oxygenless stratum bring about a definite stratifica- 

 tion within many pelagic communities. 



Many investigations have been made in marine and fresh-water 

 environments of the vertical distribution of various taxonomic groups 

 among the plankton, fishes, and benthic organisms. Summaries may 

 be found in such reference works as Welch ( 1952 ) and Sverdrup 

 et al. ( 1942 ) . However, relatively little study has been undertaken 

 of the vertical subdivisions of aquatic communities as functional 

 units. Certain zones of functional dependence are known to exist 

 at various levels along the bottom and in the free water. One such 

 unit of interdependence is formed by the plankton and the fishes of 

 the well-illuminated surface waters, another is composed of the fishes 

 and invertebrates inhabiting the bottom in deep water, and several 

 others may be distinguished at intermediate levels. More intensive 

 study will be required of the interdependencies among all the in- 

 habitants of each stratum before specific functional subdivisions of 

 pelagic communities can be clearly delineated. 



Although recognizable stratification does not exist in all communi- 

 ties, its presence is sufficiently common in aquatic situations as well as 

 in terrestrial areas for it to be considered a general characteristic of 

 community structure. Spatial organization may thus be added to 

 the other attributes of the community that have been reviewed in this 

 chapter. Each community has been shown to have a composition 

 and an integration among its members that separate it from neighbor- 

 ing communities. The degree of dominance within the community 

 and the nature of the ecotones between these units of plant and animal 

 life vary greatly from one situation to another. All these charac- 

 teristics allow the community to maintain itself as a recognizable unit 

 in a specific area for a period of time. Circumstances that sooner or 

 later may cause the replacement of one community by another or 

 that may bring about fluctuations within the community will be con- 

 sidered in the next chapter. 



