Community Corn-position 417 



organisms. The complex ecosystems of temperate and tropical ter- 

 restrial areas are particularly difficult to analyze, but some idea of the 

 number and diversity of the living components may be obtained from 

 existing studies of portions of the biota. A comparison of the num- 

 bers of species that may be involved in communities of different kinds 

 has been made by Elton ( 1946 ) ; a few examples follow. 



The biocenoses of certain British rivers include representatives 

 of as many as 131 species of invertebrates in addition to fishes, 

 amphibians, rooted aquatic plants, and vast populations of algae, 

 flagellates, and bacteria that were not enumerated. A meadow on clay 

 near Oxford, England, served as the biotope for 93 species of inverte- 

 brate animals in the soil and surface vegetation, in addition to the 

 plants and the microorganisms. At the other numerical extreme, the 

 biocenose on the sandy shore of Amerdloq Fjord, west Greenland, con- 

 sisted of only 5 species of invertebrates. In the Betiila odorata forest 

 in Finmark, Norway, only 29 species of plants were reported, whereas 

 in an ash forest in Yorkshire, England, 72 species were found, and in 

 the red fir forest of the Sierra Nevada, California, 93 species of plants 

 were recorded. The animal components of these forest communities 

 were not recorded, but they would include birds, mammals, am- 

 phibians, and a great many species of insects and other invertebrates; 

 the total number of species would be further swelled by a wide 

 variety, of protozoans and cryptogamic plants. Even within a single 

 microhabitat a surprising number of animals may be present as evi- 

 denced by counts of 111 species of invertebrates found in pine logs 

 and 136 species found in oak logs during investigations in the Duke 

 Forest, North Carolina. 



Another observation in regard to community composition is that 

 the species present for the most part belong to different genera. In 

 many biocenoses each genus is represented by only one species; in 

 others certain genera are represented by several species, as is often 

 true, for example, of the sedges, willows, and oaks in communities 

 of temperate and boreal North America. Rarely are more than a few 

 species of a genus found in the same biocenose. Quite exceptional 

 is the occurrence of "species flocks," such as those in Lake Baikal, 

 where, for example, 300 species of gammarid crustaceans are found 

 and these belong to only 30 genera (Brooks, 1950). The typical 

 biocenose is composed of species that are separated by at least generic 

 differences, whereas in the biota of a country or a region many species 

 may be recorded for each genus. Comprehensive surveys of the 

 animals in 55 communities and of the plants in 27 communities from 

 a wide range of habitats showed that 86 per cent of the animal genera 



