Stratification of the Couuminity 421 



spiders, worms, and other invertebrates. In addition, it provides a 

 part-time abode for many other animals inckiding other species of 

 insects, rodents, reptiles, mammals, and a few burrowing birds. The 

 main activity in the soil for both plants and animals occurs in the 

 upper layers, but the longer roots of prairie grasses extend to depths 

 of about 2 meters and the roots of other prairie plants occasionally 

 reach levels of 5 or 6 meters below the surface. In an extensive 

 discussion of the relation of underground plant parts Weaver and 

 Clements ( 1938 ) report that 65 per cent of a group of true prairie 

 species studied have root systems that penetrate to depths between 

 1.5 and 6 m. Prairie ants are active to 3 m, and prairie dogs are 

 known to burrow to more than 4 m. 



The floor subdivision of the grassland community contains basal 

 portions of the vegetation, including particularly the rhizomes of the 

 grass plants partially covered by litter and debris of both animal and 

 plant origin. A characteristic group of animals in which insects, 

 spiders, reptiles, and rodents are prominent join with the plants to 

 form this subdivision of the community. Most of the animals inhabit- 

 ing the grassland floor also usually invade one or both of the other 

 subdivisions. The herbaceous stratum of this biocenose consists of 

 the upper parts of the grasses and herbs and a characteristic assem- 

 blage of animals. The vertical dimension of this subdivision is vari- 

 able, according to the plant species and the local conditions. The 

 area may be covered by sparse, rosetted grass only a few centimeters 

 in height or by coarse tough species growing 2 or even 3 m high. 

 The animals of the herbaceous subdivision include a wide variety of 

 insects, birds, and ruminants. 



Stratification of land communities reaches its greatest complexity in 

 the forest. Five vertical subdivisions of the forest biocenose are 

 typically present, namely: the (1) subterranean, (2) forest-floor, 

 (3) herbaceous, (4) shrub, and (5) tree strata (Fig. 11.8). Each 

 of these may exhibit certain further subdivisions in particular situa- 

 tions, and the air above the forest canopy is sometimes considered to 

 comprise a recognizable division of the community. Allee (1926) 

 distinguished 8 strata in the tropical rain forest of Barro Colorado 

 Island. The subterranean layer of the forest is typically damp and 

 contains a large amount of humus that in extreme cases may be 

 prominent to a depth of 2 or 3 m. The effective depth of the sub- 

 terranean division of the forest community is difficult to determine 

 but in wet forests it appears to be definitely shallower than in dry 

 prairies. The adequate supply of water near the surface, the avail- 

 ability of nutrients, and the inability of roots to penetrate into poorly 



