Origin of Biomes and Succession 



249 



throneura leafhoppers feeding on Platanus has six species (Ross, 

 1957). Similarly in the same forest the tree genus Gleditsia con- 

 tains only a single species which is host to another monophyletic 

 cluster of four species of Erythronetira leafhoppers. 



In contrast, the eastern Nearctic species of the sawfly genus Neo- 

 diprion are more nearly equal in number to the species of their 

 host genus Pimis, perhaps indicating a more nearly comparable in- 

 crease in number of species of insect and host. 



At various times changes involving only slight ecological magni- 

 tude undoubtedly fragmented the range of only a portion of the 

 species in the community. Different species have inherent differ- 

 ences in ecological tolerence. For this reason, it is quite within rea- 

 son to suppose that climatic changes of only a small magnitude 

 which might occur across the range of a community would effec- 

 tively split the range of only those species with certain narrow 

 limits to their ecological tolerance. However, the same change would 

 have no such dividing effect on species having wide ecological toler- 

 ances. Figure 109 depicts hypothetically the effect of a small change 



Wetter 

 period 



'■7-^////////////l>'>^.V////////.^^ 



Species A (persists between 10 ond 40)- 

 Species B (persists between 20 ond 40)- 



Drier 

 period 



)^ ///////// T77^^. V////////. }^ 



-Species A 

 Species B - 



Ronge undivided 



[-<— Species B^*-l 



Range division 



Fig. 109. Differential effect of assumed changes in rainfall on the contiguity 

 of two hypothetical species differing in rainfall requirements. Note that in the 

 assumed drier period (below) the range of Species A remains intact; that of 

 Species B is divided. 



in rainfall on two species, one having a narrow and the other a 

 wide tolerance for this factor. The indicated change in rainfall di- 

 vides the range of the species having the narrower tolerance but 

 not that of the other. 



These examples are the results of two mechanisms acting inde- 



