Great Basin, Sonoran, and authochthonous elements 

 (Miller 1951) (see also Table 10-1). 



THE ECOLOGICAL SYSTEM 



A still different approach for the study of 

 animal distribution is the ecological one, involving 

 the biome concept (Clements and Shelford 1939). 

 The other systems described are zoogeographical in 

 their attention to centers of origin, dispersal routes, 

 and evolution. The ecological system emphasizes en- 

 vironmental relations and community dynamics. This 

 does not mean that zoogeographers do not consider 

 the relations of climate and terrain in controlling 

 dispersal and evolution, nor that ecologists are not 

 concerned with the geological history of the forms 

 with which they deal, but the viewpoints and objec- 

 tives are different. 



Basic to the understanding of the ecological sys- 

 tem is the recognition of communities composed of 

 characteristic combinations of animal and plant spe- 

 cies, of successional relations between communities, 

 and that succession in all local habitats eventually 

 ends in a climax community pattern, the most im- 

 portant ecological characteristic of a geographic area 

 (Cain 1939, Whittaker 1953). The development of 

 an understanding of these ideas has been a major 

 concern of this book, especially Chapters 5 to 9. 



The major unit in the ecological system is the 

 biome. A biome is a biotic community, characterized 

 by distinctiveness in life-forms of the important cli- 

 max species. On land, the most important climax 

 species are usually plant dominants that occur in dis- 

 tinctive vegetation and landscape types ; in the ocean, 

 the important organisms that define biomes are usu- 

 ally the predominant animals, which are sometimes 

 also dominants. 



Serai communities are developmental stages. 

 They are as much a part of the biome as develop- 

 mental instars are a part of a species of insect. The 

 animal and plant constituents of serai stages are, 

 however, more widely distributed than are species 

 belonging to climax communities, since the habitats 

 in which they belong are more nearly alike in dif- 

 ferent parts of the world than are the habitats that 

 contain the climax. Serai species are not generally 

 useful, therefore, to defining the limits of biomes. 

 The majority of animal constituents of the climax 

 community, however, are characteristic only of the 

 climax vegetation or habitat and therefore of re- 

 stricted distribution. 



The principal biomes of the world, insofar as they 

 have been identified, are the following: 



Marine 

 Oceanic plankton 



and nekton 

 Balanoid-gastropod- 



thallophyte 

 Pelecypod-annelid 

 Coral reef 



Terrestrial 

 Temperate decidu- 

 ous forest 

 Coniferous forest 

 Woodland 

 Chaparral 

 Tundra 

 Grassland 

 Desert 



Tropical savanna 

 Tropical forest 



The vegetational portion of the biome is some- 

 times called the plant formation (Weaver and Clem- 

 ents 1938). A plant association is a subdivision of 

 a biome or formation, distinguished by uniformity in 

 the species composition of the climax plant dominants. 

 The associes is the equivalent serai plant community, 

 regardless of whether it belongs to the same or a 

 different type of vegetation than the climax. The im- 

 portant point to remember here is that the biome is 

 distinguished by the life-forms of the climax dom- 

 inants, but subdivisions of the biome are recognized 

 principally by taxonomic composition. 



The type of climax in a terrestrial area is deter- 

 mined mainly by the conditions of climate, although 

 secondary correlation also occurs with major soil 

 groups. Were climate the only factor involved and 

 the terrain uniform, the climax community would be 

 monotonous in its composition and structure, except 

 as one community graded into another with change 

 in climate. Where the composition and character of 

 the prevailing vegetation varies more or less perma- 

 nently with changes in physiography, soil, or fire fac- 

 tors, we may speak of physiographic, edaphic, or fire 

 subclimaxes or faciations. 



This is in agreement with the monoclimax view- 

 point ; that is, that there is only one true climax in 

 an area, determined by the prevailing climate. An 

 opposing concept is that of polyclimaxes. Proponents 

 of the latter viewpoint give nearly equal significance 

 to climate, soil, topography, and other factors, and 

 believe that each major variation in composition or 

 structure of the mature vegetation should be con- 

 sidered as equally important. Hence, several cli- 

 maxes, or at least a complex community pattern that 

 varies in structure and species composition from one 

 site to another, may be claimed for an area (Whit- 

 taker 1957). The controversy is largely one of em- 

 phasis and semantics. 



A biociation is a subdivision of a biome distin- 

 guished by uniformity and distinctiveness in the spe- 

 cies composition of the climax community, particu- 

 larly of the animal predominants. The biocies is the 



276 Geographic distribution of communities 



