292 



Origin of Biomes and Succession 



If this latter is so, then it adds another concept to the subject 

 of siibchmax and chmax stages. It could well be true that the 

 deciduous hardwoods did not crowd out the pines to the point of 

 extinction but over at least part of the range became a new climax 

 community superimposed on the previous succession which had 

 stopped at the pine community. 



We have a little evidence here, therefore, that new types of 

 organisms may form new climax communities on top of old ones. 

 This really is not a strange concept because it is inherent in the 

 evolution of community succession in the first place. It simply em- 

 phasizes the idea that in a dynamic system even the units which 

 appear the most stable can and undoubtedly do undergo a change 

 in status. 



3. By Climatic Change 



The dominant species of a biome live together because their eco- 

 logical tolerances are similar under existing conditions. Under cer- 

 tain other conditions these tolerances might not coincide and the 

 species would not occur together. The opposite is equally possible. 

 A simple type of climatic change which would produce these results 

 is illustrated by the hypothetical example shown in Fig. 127. The 



SusToined high 

 summer 

 eitremes 



85 



Sustoined low 



winter 



extremes 



•20 



-10 



+ 10 



+ 20 



+ 30 



+ 10 



+ 20 



- +30 



+ 40 



Species A — Heat foleront species killed by winter extreme of 10 

 Species B — Cold foleront species killed by summer extreme of 100 



Fig. 127. A hypothetical cliart illusliating liow the raiities of two species, 

 A and B, would alternately separate and overlap under climatic oscillations 

 between a condition of greatei- annual extremes of temperature (left) and 

 more moderate conditions (right). 



