tion is only about one-half what it is in the open. 

 Animals in the lower strata live in remarkably con- 

 stant environments, although by climbing into the 

 tree-tops they can enter into environments compara- 

 ble to those of open plains (Allee 1926, Moreau 

 1935a). 



Tropical America has the richest variety of epi- 

 phytes ; Africa, the poorest. These epiphytes include 

 lichens, algae, mosses, liverworts, as well as vascular 

 plants. The epiphytes are relatively small in size, 

 have high light requirements, and tolerate a precari- 

 ous water supply and lack of soil. Often they are 

 more numerous than flowering herbs on the ground. 

 They are important in the community dynamics be- 

 cause their closely overlapping leaves, especially in 

 the bromeliads, enclose large masses of humus and 

 water where mosquitoes and other aquatic animals 

 breed. 



On the mountains, the tropical evergreen forest 

 of the lowland gives way to a submontane and then 

 to a montane rain forest. The trees are still ever- 

 green but lower in stature, simpler in structure, and 

 poorer in species. In addition to strictly tropical 

 species, the forest may include many genera and 

 species both of plants and animals that are of tem- 

 perate origin (Miranda and Sharp 1950, Martin 

 and Harrell 1957). Tree ferns are common. On ex- 

 posed peaks and ridges, the trees become still more 

 dwarfed and crooked but remain covered with many 

 epiphytic ferns, mosses, and lichens ; the whole is 

 aptly, if picturesquely, described as elfin forest. 

 Lianas are scarce in the montane rain forest as com- 



FIG. 27-3 Wildebeest In the 

 tropical savanna ot Tanga 

 yiica (courtesy S. Glidden 

 Baldwin). 



pared with the lowland rain forest, the shrub layer is 

 dense, and there are only two tree strata. 



Although plant ecologists are gradually working 

 out a detailed classification of tropical vegetation 

 (Beard 1955), we will base our recognition of major 

 communities on the physiognomy of that vegetation 

 which is of importance to animals. Tropical vegetation 

 may be divided, then, into desert, tropical sai'atina, 

 and tropical jorest biomes. Deserts may be extreme 

 with little or no vegetation present or may be cov- 

 ered with scrub or thorn forest, as already discussed. 

 Savannas represent a forest-edge community in 

 which animals make use both of the trees and in- 

 tervening grassland to various extent in different 

 species. Animals may well distinguish by their segre- 

 gation into niches between park savanna and gallery 

 forests and between grassland composed of tall or 

 short bunch-grasses or sedge (Beard 1953). There 

 appears to be no critical dividing line between tropical 

 deciduous and rain forests, but together they give a 

 continuous closed forest contrasting with the open 

 forest or scattered groves of the savanna, and this 

 is of importance to animals. Geographically the trop- 

 ical forest is separable into American (Dansereau 

 1947, Leopold 1950), African, and Indo-Malayan 

 units. The family predominance, for instance, of the 

 Dipterocarpaceae in the Indo-Malayan rain forest, 

 and Leguminosae in the American rain forest, and 

 the Meliaceae in some of the west African rain for- 

 ests is noteworthy. This geographic distinction be- 

 tween the continents in plant life is reflected also in 

 the animal life. 



Tropical biomes 343 



