COMMUNITY ORGANIZATION: STRATIFICATION 



463 



each lengthening of the physical gradient, 

 corresponding changes occur in the fauna. 

 Once this process is begun, its intensity 

 is progressive and usually not reversible, 

 so that stratification leads to further micro- 

 stratification until the maximal condition 

 is approached in the mature community. It 

 follows that as vegetational strata develop, 

 there is an increase in potential food and 

 shelter niches for animals. In a well-ap- 

 propriated stratum the resident animals 

 tend to be adapted structurally to the exi- 

 gencies of the habitat. The numerous, well- 

 described adjustments of many species— 

 for example, the arboreal or fossorial ad- 

 justments of many forest-dwelHng animals 

 (Hesse, Allee, and Schmidt, 1937, pp. 

 421-441)— are visible evidences of such 

 stratal selection. 



One of the most apparent differences 

 between aquatic and terrestrial stratifica- 

 tion lies in the gravitational difference. 

 Since organisms are heavier than air or 

 water, position must be maintained against 

 gravity in any stratum through which they 

 would otherwise fall. In aquatic communi- 

 ties all strata above the floor are composed 

 of water, and there is a sustained effort, 

 variously ameUorated by structural adjust- 

 ments and changes in behavior to bring 

 about flotation to maintain position. In ter- 

 restrial communities the strata above the 

 floor are based fundamentally on vegeta- 

 tion, and this series of "false bottoms," as it 

 were, places less survival value on such 

 mechanisms for resident animals with re- 

 spect to maintenance of position, although 

 this does nbt necessarily imply a lack of 

 survival value for maintenance of foothold. 

 Rather, selection is towards maintaining a 

 foothold on the vegetational stratum— e.g., 

 prehensile appendages— and movement is 

 thus freed for reaction to local stimuli con- 

 nected with food, shelter, and reproduc- 

 tion. 



Just as the shorter food chains of the 

 tundra are more easily understood than 

 the longer food chains of the equatorial 

 rain forest, so is the study of stratification 

 facihtated by examination of a relatively 

 barren area rather than a rich woods. Such 

 short food chains or pioneer surfaces are 

 not necessarily as easily appreciated; in 

 fact, the wealth of detail in more luxuriant 

 regions makes initial recognition much 



simpler, but also compUcates any serious 

 study. 



Among such barren surfaces none could 

 be more stark than the exposed surfaces 

 of flat rocks. On such surfaces the air-rock 

 interphase is sharply defined. On un- 

 weathered rock the initial inhabitants have 

 no soil, rock particles or humus; hence the 

 community of which they form a part has 

 no subterranean stratum. Total lack of a 

 subterranean stratum is rare, and its ab- 

 sence at once simpHfies the dynamics of 

 the community. The initial stages in the 

 colonization of such surfaces are similar, 

 so that it would appear that the specific 

 chemical influence of the rock substrate is 

 of less weight than the toleration of pioneer 

 organisms to the exposure (Cowles, 1901). 

 Granite flat-rocks of the southeastern 

 United States, studied by McVaugh 

 (1943), present a typical picture of early 

 capture of such a barren surface by or- 

 ganisms. It is apparent that upon such ex- 

 posed rocks physical conditions for exist- 

 ence are extremely adverse. Organisms are 

 directly exposed to the full strength of 

 sunhght, and through insolation, indirectly 

 to extensive reradiation, to the daily and 

 seasonal range of temperature, to precipi- 

 tation and subsequent water erosion 

 through flooding and runoff, and to wind 

 and strong evaporation. 



Despite these, often violent, variations in 

 weather, such rocks are colonized success- 

 fully. The first inhabitants are crustose 

 Uchens and mosses, the former of httle 

 effect on soil formation, the latter relatively 

 good humus accumulators. Corrosive action 

 of the Hchens on the granitic surface, 

 which would tend to soften the rock, is 

 nulHfied by the transportation of such 

 loosened rock particles through wind and 

 water action; hence a new surface is often 

 exposed (Whitehouse, 1933). CUnging to 

 the rock, these mosses and Hchens form the 

 only vertical as well as the only horizontal 

 stratum of the community. 



Associated with these pioneer plants, es- 

 pecially with the mosses, is a characteristic 

 fauna of hardy species, often cosmopolitan 

 in mosses over the world, and especially 

 signahzed by their abihty to withstand 

 otherwise limital temperature and evapora- 

 tion, through sundry adjustments, such as 

 the formation of cysts. Enumeration of 

 these animals, their adjustment to adverse 



