484 



THE COMMUNITY 



ume of the community and hence increases 

 kind and amount of shelter and foods, di- 

 rectly for herbivores and indirectly for car- 

 nivores and carrion-feeders and dung-feed- 

 ers. It follows that the taxonomic complex- 

 ity of the community increases with its in- 

 crease in stratification. The process of strat- 

 ification is intensified as the ontogenetic 

 age of the forest increases. Stratification 

 usually increases in direct proportion to the 

 serai age of the forest. Finally, it follows 

 that stratification becomes a criterion of 

 both actual and relative maturity. 



The subterranean stratum has been de- 

 fined in general terms. This layer has been 

 contrasted with the corresponding layer in 

 aquatic communities, and this stratum 

 has been compared in grassland and 

 forest. A dynamic view of soil is essential 

 for a better understanding of soil prob- 

 lems. Rommell (1930, p. 843; 1935) 

 emphasized this concept: "It is more 

 and more generally recognized that a 

 natural soil, hke a living organism, must be 

 studied as a whole to get a correct idea of 

 its responses." The subterranean portions of 

 plants of the higher strata, the resident sub- 

 terranean plants and animals, and the 

 soil jointly compose this lowermost level in 

 the vertical forest gradient. 



In general, forest soils have a rich, black, 

 porous humus component that may reach 

 a depth of 9 feet in some equatorial 

 forests (Warming, 1909). Usually forest 

 soil is inhabited by animals for at least 

 1 foot. This animate substratum varies 

 with the development of the natural 

 vegetation, i.e., the development of root 

 systems. As the community matures onto- 

 genetically, the heavier vegetation of the 

 higher strata is matched by a corresponding 

 increase in the volume and complexity of 

 the root mat. Since rootlets are always dying 

 in some parts of the mat, even in forest 

 perennials, fungi of the soil feed on these 

 dead portions, reducing them to a soft 

 matrix. Saprophytic soil animals, especially 

 oribatid mites, eat away this soft core, 

 leaving a tubule, varying from 1 to 20 

 mm. in diameter, with relatively indigestible 

 bark walls. In old forest soils such a system 

 of tubules aids in aeration and drainage, 

 as well as in the formation of potential mi- 

 crohabitats. The saprophytic arthropods 

 leave their feces within the tubules, add- 

 ing to the soil fertility. 



A logical classification of soil animals is 

 that of Jacot (1936), whose system is ex- 

 tended here. First, there are the geobionts, 

 true soil organisms that normally spend 

 all their lives in the soil. This is a large and 

 important category. Here would be in- 

 cluded a notable population of bacteria 

 (Dubos, 1928; Ramann, Schellhorn, and 

 Krause, 1899; Skinner and Mellem, 1944; 

 Waksman, 1932); soil algae (Transeau, 

 Sampson, and Tiffany, 1940, Chap. 47); 

 fungi (Ramann, Schellhorn, and Krause, 

 1899; Waksman, 1932); and soil protozoans 

 (Sandon, 1927; Woodruff, 1938). These 

 are all basic inhabitants, responsible for 

 certain phases of soil formation and struc- 

 ture, and in the formation of much raw 

 produce vital to the Ufe of the whole 

 forest community. The point is worth re- 

 emphasizing that, although there must be 

 a necessary preUminary physical formation 

 of soil from rock, such soil would be 

 incapable of supporting life as we know 

 it. The transition from mineral soil to rich 

 humus takes place through the action of 

 organic agencies; through the slow working 

 over and adding to the preliminary rock 

 particles (Nikiforoff, 1942). 



Besides these minute organisms, many 

 other geobionts are important in soil forma- 

 tion or structure. For example, certain 

 species of the following groups are im- 

 portant: nematode worms (Cobb, 1915), 

 enchytraeid and lumbricid worms (Darwin, 

 1881; Olson, 1928), micromyriapods [Pau- 

 ropoda (StarHng, 1944) and scutigerelHds], 

 tardigrades, oribatid mites, CoUembola, 

 Protura, Thysanura, many tropical termites, 

 mole crickets ( Gryllotalpidae ) , many ants 

 in the genera Ponera, Myrmecina, Brachy- 

 myrmex, Solenopsis (Jacot, 1936), and the 

 leaf-cutting, fungus-gardening, and other 

 ants (Weber, 1941; Wheeler, 1926), moles, 

 and possibly shrews, (Hamilton, 1939; 

 Hamilton and Cook, 1940; Taylor, 1935). 

 Others, chiefly insects, although neither 

 feeding on decayed plant parts nor excavat- 

 ing, are continually using the minute sub- 

 terranean channels and hence aid in their 

 maintenance. Such insects include thrips 

 and many predaceous beetles of the families 

 Staphylinidae and Pselaphidae (Jacot, 

 1936; Pearse, 1943, 1946). 



In the second category are the soil 

 transients (geocoles, = geophiles of Jacot). 

 They spend a regular or irregular portion of 



