and succession wliicli arc beyond the cliaracteristics 

 of the individual organisms of wliich the coninninity 

 is composed. Tlie community behaves as a unit in its 

 competitional and successional relations with other 

 connnunities, in its local and geographic distribution, 

 in its seasonal activities and res]X5nse to climate, and 

 in its evolution. Although the community varies in its 

 taxonomic comijosition and structure in different en- 

 vironmental situations, this variation is proportion- 

 ally no greater than occurs in different cells of the 

 same type or between individuals belonging to the 

 same species. 



The individualistic concept places emphasis on the 

 species, rather than the community, as the essential 

 unit for analysis of interrelations, activities, distribu- 

 tion, and evolution. ICach species responds independ- 

 ently to the integrated influence of the various factors 

 of the physical environment and biotic coactions. 

 The environment may be conceived as a pattern of 

 gradients with the intensity of the various factors 

 changing gradually in space from one extreme to the 

 other. The gradient may be a short one, as from the 

 subterranean to the tree stratum in a forest or from 

 the open water of a pond to a nearby swamp or cli- 

 max forest, or it may be longer, as from the bottom 

 to the top of a mountain or even from the tropical 

 to the arctic zones of a continent. The population 

 density of each species is distributed in a form re- 

 sembling a normal curve when plotted along the 

 gradient of a given factor, and the curves of many 

 species in relation to various environmental gradients 

 overlap in a heterogeneous, and apparently almost 

 random, manner. There is seldom agreement be- 

 tween the limits of the distribution curves of any two 

 species : species are not in general bound together 

 into groups of associates which must occur together. 

 Furthermore, the vegetation and its associated animal 

 life very often form a continuum of gradually chang- 

 ing composition and complexity from one extreme of 

 the environmental gradient to the other. A vegeta- 

 tional continuum has no sharp boundaries between 

 individual communities of different types, and the 

 ecologist must choose the manner in which he dis- 

 tinguishes these units so as best to suit his interests 

 and objectives. 



These two points-of-view are not necessarily in- 

 compatible. There is no doubt that each species is 

 distributed according to its own physiology, its own 

 complex interrelations with other species, and its own 

 tolerances, and that no two species are exactly alike 

 in their responses to the environment. On the other 

 hand, one can be convinced that the community and 

 its habitat, collectively the ecosystem, is a functional 

 system and that every species of necessity occurs in 

 and as a part of such a system so that its distribution 

 is importantly modified by these interactions and 

 community relations. 



The community is usually recognized and identi- 

 fied by its most imiwrtant org.inisms, the dominants 

 and ])redominants (Shelford 1932). Subdominants 

 and member species, however, are not usually de- 

 jjendent on the dominant species directly : rather, on 

 the environmental conditions that the dominants 

 establish. DifTerent sjjecies of dominants in adjacent 

 or related communities may react on the environment 

 in a manner so nearly the same that subordinate spe- 

 cies find suitable conditions for existence in each, 

 although they are usually more characteristic of one 

 than the other. 



The niche that a species occupies is a finite unit 

 of distribution that can be measured in an absolute 

 and objective manner. The community-stand is an 

 actual aggregation of organisms occurring in a par- 

 ticular locality. In a sense, it is a collection of niches 

 occupied by a particular set of species, but it is some- 

 thing that one can see and study in the field. Because 

 of the great variation in composition and character of 

 community-stands in different habitats and parts of 

 the world, they need to be evaluated and classified 

 in some logical manner for reference purposes. Com- 

 munity-types are abstract groupings of individual 

 community-stands which resemble one another and 

 consequently must be defined rather arbitrarily. Dif- 

 ferent systems of community-types have been pro- 

 posed, each designed to emphasize a particular point- 



15 20 25 30 35 40 45 50 55 60 

 ELEVATION IN FEET, lOO's 



FIG. 3-3 Continuum of tree (e,b,c.d) and foliage Insect (e,f, 

 g,h,i,i,k) species in an elevation gradient in the Smoky Mountains, 

 Tennessee, (a) 7sugo canademii; (b) Haleiio Carolina: (c) Acer 

 spicatum: [d) Fagui grartdilolia: (e) GropAocep/io/o coccineo; (f) 

 Coec/7/us sp.; (g) Agalliopsis ryovella; (h) Polypioeui corrupiui; 

 (i) Anospii rula; (j) Cicadella HaYOiCufo; (k) Oncops/s $p. (after 

 Whittaker 1952). 



The biotic community 27 



