COMMUNITY ORGANIZATION: STRATIFICATION 



485 



their lives in the soil and include certain 

 reptiles, birds, mammals, and a host of in- 

 sects (Bryson, 1931, 1933; McColloch and 

 Hayes, 1922). Such organisms aid in soil 

 formation, soil transfer, aeration, and drain- 

 age. They use the stratum for oviposition, 

 pupation, hibernation (Weese, 1924), 

 aestivation, or as a sheltering niche during 

 their diurnal or nocturnal periods of relative 

 inactivity. 



Finally, there are the geoxenes. These are 

 stray or accidental animals in the soil that 

 have little permanent eflFect upon the 

 stratum. 



Forest geobionts generally are adjusted 

 to the moist, cool, dark, relatively stable 

 microclimate and relatively dense matrix 

 of the subterranean stratum. In contrast to 

 the relatively progressive instability of 

 higher strata, and their contained epigean 

 residents, the forest geobionts are in less 

 danger from extreme temperature changes, 

 excessive radiation, and desiccation. Cor- 

 related with these influences is a low 

 toleration for high temperatures and light 

 intensities. Many are white or bleached in 

 color, have no eyes or have rudimentary 

 or vestigial vision, and are structurally ad- 

 justed for digging. In a very real sense, soil 

 animals are important in the construction 

 of their stratum. 



In quantitative work it is difficult to 

 separate the fauna of the H-layer of the 

 floor (usually 2 to 3 mm. deep) from the 

 Ai-layer of the soil (Fig. 54). 



The floor is a complex stratum. It is inter- 

 mediate between subterranean and epigean 

 levels in microclimate, matrix continuity, 

 foods, habitats, and taxonomic composition. 

 It is the recipient of diverse organic incre- 

 ments from higher strata (flower parts, 

 fruits, seeds, twigs, leaves, feces, dead 

 animals). These organic materials are de- 

 posited regularly in part and irregularly, 

 seasonally, and continuously, depending 

 upon the latitude, altitude, and total com- 

 position of a given forest. Such a rain of 

 debris parallels the building up of the pro- 

 fundal or abyssal strata of aquatic com- 

 munities. 



The floor is diverse in aspect. Mush- 

 rooms, the bodies of dead and decomposing 

 animals, fallen logs, and broken stumps are 

 apparently isolated, yet in reality are in- 

 tegral parts of the stratum, each with its 

 own closely knit food chain (Chap. 27), 



activity pattern (Chap. 28), and succes- 

 sional pattern (Chap. 29). These micro- 

 cosms or biocoenoses are surrounded and 

 progressively covered by leaf mold and 

 litter. 



Ignition by fire or transportation by 

 storm winds and floods may remove parts 

 of the floor. Barring destruction by civilized 

 man, all these diverse parts are eventually 

 reduced and transported in large measure 

 to the subterranean stratum of the com- 

 munity involved. 



In addition to these obvious, apparently 

 detached, floor parts, there are other por- 

 tions less readily discerned. For example, 

 the standing dead tree is a vertical, colum- 

 nar extension of floor log mold. It comes to 

 be inhabited by typical log mold animals 

 and in addition is used as a shelter or nest- 

 ing site by woodpeckers and squirrels 

 Where falling trees and limbs are prevented 

 from reaching the ground level as a con- 

 sequence of the density of higher strata, de- 

 composition of their tissues and mechanical 

 interception of dead and decomposing 

 organic fragments from the canopy pro- 

 duce, for a time, a "second floor." Such 

 accumulations of litter are not uncommon 

 in rain forest, may be of small to moderate 

 size, and during the time they exist apart 

 from the floor proper, support a large popu- 

 lation of floor animals (Onychophora. ori- 

 batid mites, ants, and numerous spiders). 

 There are also the tree holes. These are 

 small to moderate-sized, more or less 

 hemispherical cavities in tree trunks, usually 

 where limbs or branches have been re- 

 moved. If such holes are not repaired by 

 the tree, fungus decay sets in, and rain 

 water is trapped in summer and snow in 

 winter. As the holes age, they enlarge by 

 decay of their walls and may pass from an 

 above-ground aquatic communitv to a filled- 

 in, moist log mold habitat above the floor. 

 Such a process is parallel with the fillint^ 

 up of ponds and lakes. Some animals are 

 tree-hole specific. For example, the tree- 

 hole mosquito (Anopheles harberi) breeds 

 exclusivelv in the water of tree holes, and 

 the adults seldom get far from their breed- 

 injj grounds (Headlee, 1921a). The tree 

 hole reservoirs of water sufficiently perma- 

 nent to maintain a distinctive biocoenosis in 

 the tropical forest are familiar to biologists 

 who have visited the Barro Colorado Lab- 

 oratory in Panama. Tadpoles conspicuous in 



