Origin of Biomes and Succession 



281 



Fig. 122. Ranges of five species of the grass genus Aristida. (Adapted from 

 Hitchcock and Chase.) 



bit Sylvilagiis -ftoridamis, the coyote Canis latrans, and insects such 

 as wide-ranging mosquitoes and sawflies are but a small number 

 of many examples in North America. 



Certain of the plants in this category are of unusual interest 

 because they are subclimax species in one biome and dominants 

 of some climax communities in another biome. Thus the two Amer- 

 ican grasses Andropogon scopariiis and A. gerardii are subclimax 

 plants in the eastern deciduous forest biome but are dominant 

 species in the long-grass prairie biome immediately to the west and 

 subclimax components again in the swale communities of more 

 xeric biomes even farther west. The American Aristida oligantha 

 is a grass of early stages of succession in Illinois and eastward but 

 a co-dominant with Andropogon gerardii in prairie communities of 

 Oklahoma (Ray, 1959). 



These wide-ranging subclimax organisms differ markedly in evo- 

 lutionary potential from species whose ranges more or less concide 

 with a single biome. In the first place, the selection pressures in- 

 fluencing genetic change in species with a wider ecological range 

 will come from a wider spectrum of ecological situations and thus 



