448 Succession and Fluctuation 



area is a serai shrub formation, is of the shrub Hfe form, or is of the 

 shrub community type. 



The Biome 



Certain communities whose dominant species have a distinctive hfe 

 form have become more or less permanently established in certain 

 climatic regions of the earth and are believed to be in the climax 

 condition. Associated with these climax communities are communi- 

 ties in earlier stages of ecological succession and also communities of 

 a type controlled by special local conditions different from the general 

 nature of the region. Such a complex of communities, characterized 

 by a distinctive type of climax community and maintained under the 

 climatic conditions of the region is known as a biome. All animal 

 and plant components of each of the included communities are mem- 

 bers of the biome. The biomes constitute the great regions of the 

 world distinguished on an ecological basis, such as the tundra, the 

 desert, the grasslands, and the various forests. 



The biome consists of a special combination or complex of com- 

 munities. The essential matrix of the biome is composed of climax 

 communities with dominants of a certain life form that give the biome 

 its particular character. Communities of different life form are pres- 

 ent as minor constituents of the biome. The major climax communi- 

 ties in the biome are of the same type but differ in species composition 

 in different parts of the biome. The deciduous forest biome that 

 extends across the eastern part of the United States, for example, is 

 characterized by a community type in which deciduous trees are 

 dominant. Major communities of equal rank in this deciduous forest 

 biome are the oak-hickory forest of the central Atlantic states, the 

 beech-maple forest of the Middle West, the hardwood forest of 

 northern New England, and others of the same life form. These 

 major climax communities that form the essential matrix of the biome 

 have been termed associations by Clements and his followers. The 

 life form characteristic of the major climax communities— and hence 

 the general character of the biome— is determined primarily by the 

 nature of the regional climate. Which of several major communities 

 of the same type will be present in each part of the biome is de- 

 termined by local variations in both climatic and edaphic conditions. 



Within each community Clementsian ecologists recognize certain 

 subdivisions. Geographical differences in abundance or in relation- 

 ship of the dominants in the community are called faciations. Within 

 a region occupied by a grass community, for example, variations in 



