280 Origin of Biomes and Succession 



cession in a given area, including the climax, may form a recurring 

 cycle of communities, although the cycle may be highly irregular 

 because of the many variables which affect it; and (4) under spe- 

 cial ecological conditions new communities, short-lived in a geo- 

 logic sense, may be formed from species living in other commu- 

 nities. 



From these considerations of succession it is clear that any one 

 area is occupied not by one community alone but recurrently by 

 a system of successive communities. The same idea is expressed by 

 calling the entire system one community, as is implicit in the life 

 zone terminology of some authors, but because climax and sub- 

 climax communities of the same successional series may be radically 

 different it is simpler to consider each distinctive subclimax stage 

 as a separate community. Because the subclimax communities are 

 distinctive for a given biome and merge imperceptibly with the 

 climax types, it seems most feasible to consider these successional 

 communities as a part of the biome in which they occur. 



Evolution of Subclimax Species 



A great many subclimax species occur in more than one biome and 

 consequently have ranges quite different from those of species 

 which are restricted to a single biome. In the species of three- 

 awned grass Aristida, which are common subclimax species in the 

 Nearctic eastern deciduous forest biome, some extend considerably 

 beyond the biome and are also components of subclimax commu- 

 nities in central and western grassland areas. Five of these Aristida 

 species are illustrated in Fig. 122; A. dichotoma and A. ramosissima 

 occur only in the deciduous forest biome, but A. hiramosa, A. cur- 

 tissii, and A. oligontha occur considerably west of it (Hitchcock 

 and Chase, 1950). 



More extensive ranges of subclimax organisms occurring in the 

 eastern deciduous forest include the side-oats grama grass Bouteloua 

 curtipendida and the sandbur Cenchrus pauciflorus, both distrib- 

 uted from the Atlantic to the Pacific and from southern Canada 

 to Argentina, and the wild cherries Prunus americana and P. vir- 

 giniana (Fig. 123), which extend through other biomes to the 

 north and west. 



Many species of animals have ranges which likewise extend over 

 subclimax areas of several biomes and, as Slielford and Olsen 

 (1935) pointed out, may range over both subclimax and climax 

 communities. The black bear Ursiis arnericanus, the cottontail rab- 



