Origin of Biomes and Succession 251 



europeus starts earlier than that of L. timidus, it seems probable 

 that, as L. europeus spreads into the range of L. timidus, more 

 females of the latter would be cross-mated and hence leave no 

 productive progeny. In this way the gradual range extension of L. 

 europeus is causing a progressive extinction of L. timidus (Udvardy, 

 1951). 



From the standpoint of community evolution, the most far- 

 reaching effects of community mixing are the resulting patterns of 

 distribution and abundance of community dominants. Dominant spe- 

 cies producing communities of different types ( such as forest versus 

 grasslands) may outcompete each other in different areas, or domi- 

 nant species producing communities of the same type may pre- 

 dominate in different local or regional areas. Examples of these 

 patterns have been illustrated in detail for many plant commu- 

 nities. 



Local predominance of dominant species is illustrated in studies 

 of 56 hill prairies in Illinois containing over 40 species of grasses. 

 Evers (1955) found that the dominant plants were tall grasses of 

 the genera Andropogon, Bouteloua, and Sorgastrum. Andropogon 

 scoparius was predominant in all but a few prairies; Bouteloua 

 curtipendula was predominant only in part of one prairie although 

 it grew in 47 others; Andropogon gerardii was predominant in 

 scattered patches of several prairies and grew in a total of 37; and 

 Sorgastrum nutans was predominant in two prairies and grew in 

 21 others. The interesting point in these figures is that although 

 in ahnost every instance one species of grass predominated in a 

 particular prairie, this species never excluded all other grasses, 

 and in many instances the two most abundant species approached 

 numerical equality. 



An example of the regional predominance of different dominant 

 species occurs in the forests of Ohio. Sears (in Shelford, 1926) 

 (Fig. 110) indicated that five major combinations of dominants, 

 all deciduous hardwood trees, form an intermingled, crazy-quilt 

 pattern over the state. Similar patterns of local differences in domi- 

 nants have been given by many authors. 



The total ranges of the important dominants mentioned by 

 Sears and of a few hardwood dominants in adjacent areas (Fig. 

 Ill) show that species which predominate in certain areas nor- 

 mally occur as less numerous members of the dominant class far 

 beyond where they comprise the bulk of the vegetation. 



Whittaker (1956), analysing the forests of the Great Smoky 

 Mountains of Tennessee and North Carolina, emphasized the fact 



