et|iiivak'nt serai coniiinmity. \\ itli the biome, we 

 recognize tlie primary importance of the life-form of 

 the principal climax organisms for establishing the 

 major units into which the geographical distribution 

 of organisms are naturally divided. The biociation 

 concept does not regard differences in the species com- 

 position of plant dominants, such as are recognized 

 in the plant associations, as establishing the funda- 

 mental subdivisions of the biome for animals. So long 

 as they are of the same life-form, variations in the 

 plant species composition produce only minor differ- 

 ences in dominance, insufficient, for the most part, 

 to induce striking changes in the species composition 

 of the rest of the community. The relation between 

 biocies, associes, and changes in vegetation that occur 

 with succession (Fig. 8-4) obtain also between bio- 

 ciations. associations, and changes in vegetation that 

 occur geographically. Actually, the animal ecologist 

 has no absolute need to identify the species compo- 

 sition of the plant dominants if he can describe the 

 vegetation accurately in other ways (Dansereau 

 1951 ). If one wishes not to define plant communities 

 on the basis of the taxonomic composition of the 

 dominants alone, the concept of biociations may be 

 equally useful for the analysis of distribution of plant 

 species. The so-called natural areas of Cain (1947) 

 are a step in this direction. 



A biociation may originally derive its species com- 

 ponents from several faunal elements (Table 10-1), 

 but once the community constitutes a unit, it may 

 thereafter serve as an element itself, and a geographic 

 area may be described in terms of the biociations and 

 biocies it contains. Biociations differ from biotic 

 provinces in that the latter are geographic areas, 

 rather than communities, and in mountainous areas 

 may contain several life-belts or different types of 

 community. Furthermore, biotic provinces may be 

 characterized in part by the presence of particular 

 subspecies. Subspecies are not used in defining bio- 

 ciations. 



The faunal system of Allen bears some resem- 

 blance to the biome system. Allen's barren-ground 

 fauna is equivalent to tundra : the cold temperate sub- 

 region together with the Pacific Coast district equates 

 with coniferous forest ; the warm temperate sub- 

 region with deciduous forest ; the Great Plains dis- 

 trict with semi-arid grassland ; the Great Basin dis- 

 trict with arid grassland and sagebrush ; and the 

 Sonoran sub-province with desert. The different 

 faunas within these major divisions may correspond 

 to biociations. Although the terminology is different 

 and the refinement of details is greater, the system of 

 biomes and biociations is manifestly built upon the 

 basic foundation laid by a long line of zoogeographers 

 (Kendeigh 1948, 1954). 



It is clear, then, that two sets of factors control 



local and geogra])hic distribution of organisms. The 

 first set is ecological, including the physical factors of 

 nature of the substratum and climate for terrestrial 

 organisms and the composition of the water for aqua- 

 tic forms ; the biotic factors, especially of food, cover, 

 reproductive requirements, competition, and preda- 

 tors : and the psychological factors of behavior ad- 

 justments, inherited mores, and siiecific niche re- 

 (juirements. On the other hand, zoogeographic factors 

 include the considerations of place of origin ; dispersal 

 pathways, rates, and barriers ; and evolutional acqui- 

 sition of new structural, functional, or behavioral 

 adjustments that permit invasion of new areas, sur- 

 mounting of old barriers, or incorporation into new 

 communities. The present-day distribution of ani- 

 mals and ])lants into different community units is the 

 result of all these forces at work, both at the present 

 time and for untold generations in the past. 



SUMMARY 



There are three zoological realms and five 

 zoological regions into which the world is divided. 

 Free dispersal between regions is prevented by the 

 major barriers of oceans, mountain ranges, and des- 

 erts ; each region is thus characterized by distinct 

 orders and families of organisms. 



Within each region secondary barriers separate 

 divisions of lesser rank characterized by genera, 

 species, and subspecies. Of the Nearctic region of 

 North America, such differentiation has prompted 

 the recognition of faunal areas, life-zones, and biotic 

 provinces. 



In contrast to the analysis of fauna in terms of 

 geographic units is analysis in terms of the elements 

 that it possesses. A faunal element is a group of 

 species coming from the same center of origin and 

 having similar geological histories. 



Zoogeography is the study of animal distribution 

 in terms of centers of origin, dispersal routes, and 

 evolution. The ecological system emphasizes envi- 

 ronmental relations and community dynamics. 



The principal unit in the ecological system is the 

 biome. A biome is a major biotic community char- 

 acterized by distinctiveness in the life-forms of the 

 important climax species. Serai communities are de- 

 velopmental stages of the biome. The biome is di- 

 vided into plant associations, distinguished by uni- 

 formity in the species composition of the climax plant 

 dominants, and into biociations, identified by uni- 

 formity and distinctiveness in the species composi- 

 tion of the climax community, particularly of the ani- 

 mal predominants. Some nine terrestrial and four 

 marine biomes are recognized, to the study of which 

 we now proceed. 



Distributional units 279 



