462 



THE COMMUNITY 



the organization of a biome or of one of its 

 self-sustaining assemblages, is a further 

 demonstration of the eflFect of these physical 

 and biological gradients in the distribution 

 of animal and plant life. 



Within each of the major biogeographic 

 girdles, more local factors, such as physical 

 or physiological availability of water or 

 edaphic factors of the soil cover, further 

 restrict vegetation and animal Hfe. These 

 secondary influences serve to dissect the 

 available space into numerous habitat types, 

 each with its own ecological potential. This 

 process is carried much further on land 

 than in water, where the more stable and 

 uniform aquatic medium resists isolation 

 and augments greater interdependence. 



Within each of these broad habitat 

 types there is a tendency for organisms to 

 aggregate within the toleration limits of the 

 species populations, and consequently to 

 realize more or less the ecological potential 

 of the given area. This results in a larger 

 variety of self-sustaining assemblages than 

 is possible in sea or fresh water. For ex- 

 ample, a cross country trip over land takes 

 one through many desert, semidesert, grass- 

 land, and forest communities, of large or 

 small area, often geographically in close 

 proximity, yet all essentially independent 

 of one another. The residents of a meadow 

 are closely interdependent within their 

 own grassland community, as are the in- 

 habitants of an adjacent forest. Both grass- 

 land and forest can exist alone; hence both 

 are communities in the major sense used 

 at this time. A large lake bordering both 

 grassland and forest, or for that matter, 

 an ocean, may not be so subdivided. Their 

 aquatic subdivisions are not self-sustaining. 

 Thus the bottom stratum of either lake or 

 sea is dependent upon the uppermost stra- 

 tum for essential nourishment, and neither 

 the epilimnion nor photic zone could long 

 exist without the upwelling of inorganic 

 nutriments. Hence, size is not a criterion of 

 the community. 



This brings to attention the major com- 

 munities or communities at the self-sustain- 

 ing level of integration. From this point 

 of view the ocean is a vast major com- 

 munity, geographically divisible, but not 

 separable into numerous self-sustaining as- 

 ere^ations. The lake, pond, or river is 

 similarly a single major community. The 



land, on the other hand, supports a 

 large number of major communities, 

 which are not to be confused with the 

 older concept of the biome. The salt-water 

 major community is coextensive with the 

 salt-water biome. No biome has ever been 

 proposed for fresh waters in general; in 

 fact, rivers have been regarded as edaphic 

 or local (Clements and Shelf ord, 1939). 

 From our point of view, the marine photic 

 zone, the lacustrine epilimnion, and the 

 forest canopy are analogous strata of three 

 major communities of variable size. Simi- 

 larly, each permanent deep pond is as much 

 a major community as one of the Great 

 Lakes, and of similar fundamental struc- 

 ture. The evolution of communities in se- 

 quence \\'ith the evolution of life, i.e., the 

 notable increase of major communities of 

 terrestrial and presumably more recent 

 constituents, is an interesting field for spec- 

 ulation. 



Within one of these terrestrial communi- 

 ties, stratification, especially along the ver- 

 tical gradient, is obvious. It is usually a 

 matter of visual comprehension, in contrast 

 with aquatic stratification, in which the or- 

 ganizational pattern must be pieced to- 

 gether from samplings at various depths. 

 Only in recent years has it been possible to 

 examine organismal stratification in lakes, or 

 along the seashore, directly, by diving ap- 

 paratus (Rickett, 1920, 1922, 1924; Beebe. 

 1928). The better-documented picture of 

 lacustrine and marine littoral stratification 

 may be partially a consequence of the 

 necessity for using refined and quantita- 

 tive, though indirect, methods in order to 

 examine the organization of the commu- 

 nity. 



Among terrestrial communities, at least, 

 there appears to be a positive correlation 

 between increasing maturity and intensifi- 

 cation of stratification. The primary biolog- 

 ical gradient, consisting of vegetation, re- 

 acts to the initial physical gradients, both 

 horizontal and vertical. Such gradients are 

 apt to be so limital in character that they 

 can be tolerated only by one or a few 

 pioneer species. As soil is formed, and 

 sufficient moisture and shade are pro- 

 vided, shade-tolerant plants can take their 

 place in the biological gradient, and by 

 their presence further differentiate the 

 physical influences operating. With each 

 change or augmentation of the vegetation. 



