464 



THE COMMUNITY 



local weather, and pertinent literature have 

 been previously summarized (Hesse, Allee, 

 and Schmidt, 1937, pp. 355-357). This 

 fauna includes free-living protozoans, 

 acarinid mites, tardigrades, and certain 

 herbivorous beetles. They are preyed upon 

 by other mites and stray carnivorous bee- 

 tles. 



The short food chain and the single ver- 

 tical and horizontal stratum indicate that 

 this rock assemblage is primitive. There is 

 almost no community control over the 

 physical environment, and the assemblage 

 may be small in area and volume. Never- 

 theless, given air, sunlight and water, this 

 is a self-sustaining and self-reproducing 

 complex. This aspect of community organi- 

 zation gives a criterion for community 

 validity at the level of survival through 

 cooperation and clearly shows why the 

 shelf-fungus on a forest tree is not a self- 

 sustaining community, although the fungus 

 is microstratified from cortex to medulla, 

 and although both shelf-fungus and granitic 

 rock community may be similar in area or 

 volume. 



The granitic rock community is one of 

 the few instances in which there is but a 

 single stratum. At this point in its develop- 

 ment this unstratified condition may be 

 merely a lack of human appreciation, 

 since for bacteria, protozoans, and nema- 

 todes the crustose lichens and mosses 

 might be physically microstratified. Such 

 an apparent lack of stratification soon 

 vanishes. With the accumulation of moss, 

 humus, and rock particles above the rate 

 at which they can be carried oflf by wind 

 and water, foliose lichens and vascular 

 plants invade the mat, and stratification be- 

 comes evident. 



This process of invasion follows a gen- 

 erally reliable pattern that results in con- 

 centric horizontal stratification. This has 

 been well summarized by Costing and 

 Anderson (1939): 



"These invaders advance centrifugally over 

 the mat at about the same rate that the 

 pioneers spread upon the rock, and they may 

 themselves be superseded by other species [of 

 plants] which again invade the central area. 

 This results in a series of more or less con- 

 centric zones or girdles, each representing a 

 stage in mat development. The pioneer stage is 

 invariably at the periphery, the most mature at 

 the center." 



It is interesting to observe that even 

 such a restricted community soon passes 

 from the unstratified to a stratified condi- 

 tion, increasing the amount and kind of 

 food and shelter for the concurrently in- 

 vading animals, and increasing the biolog- 

 ical impact upon the primary physical gra- 

 dients. It is also of interest to note the 

 parallel between the zonation of such a 

 granitic community and the major features 

 of the suppositional dispersal pattern of 

 the world biota (Matthew, 1915). In 

 both, the primitive types are peripheral, 

 and the more highly evolved or adjusted 

 ones are at or near the center. This is 

 evidently no more than an analogy. In 

 the rock community the pioneer plants are 

 unable to compete centrally where their 

 humus has made possible the existence of 

 more highly evolved plants. Such pioneer 

 species do not move to the periphery; 

 rather, their place is taken wherever they 

 have sufficiently ameliorated their habitat. 



The extensive and numerous communi- 

 ties of desert, semidesert, grassland, and 

 forest, together with their intergrades, 

 have not been fully studied as regards ver- 

 tical and horizontal organization. When at- 

 tention has been directed to a single com- 

 munity—for example, semidesert— the inher- 

 ent pecuHarities of such a community often 

 obscure the fundamental organization 

 which it shares with all other communi- 

 ties. When this is the case, the fundamen- 

 tal stratification, well shown in the forest 

 or sea, is not stressed, since its particular 

 development in the community under ex- 

 amination may be either reduced or ob- 

 scured by diurnal and nocturnal popula- 

 tion shifts. 



All terrestrial communities, at the level 

 of self-sustenance, are stratified in the 

 broad meaning of the term. In general, the 

 vertical gradients are relatively much bet- 

 ter developed than the horizontal gradi- 

 ents; the latter may or may not be broken 

 into more or less discrete zones. Such com- 

 munities have a subterranean stratum, fre- 

 quently further stratified, above which is 

 a floor stratum, followed by at least one 

 stratum of vegetation. While the subter- 

 ranean stratum of soil and the floor stratum 

 of organic debris may be diversely consti- 

 tuted, and qualitatively and quantitatively 

 variable within a given community, they 

 are structurally continuous. On the other 



