The Evolution of Communities 



229 





70 



6 0' 



5 0- 



40 



30 



2 



Eurytomo tephrilidis 

 Eupelmus cushmani 

 ^ Bracon terryi 



10 





■^■Mv 



^ 



^ 





?!=!??^ 



Z 



Keanae A Keanae B 



Hana Kipohulu Waihou A Woihou B lao Valley Kanoio 



LOCALITY 



Fig. 99. Parasitization of the gall fly Piocecidochares utilis by three para- 

 sites at different localities representing different ecological communities in 

 Hawaii. (After Bess and Haramoto. ) 



Considering the dynamic aspects of population densities, espe- 

 cially the irregularity and ephemeral nature of unusually high den- 

 sities (Fig. 50), it is highly likely that non-competitive conditions 

 prevail in far more cases than now suspected. This mechanism may 

 be the reason for the survival of many closely related phylogenetic 

 lines for long periods of time as is the case in the insects, molluscs, 

 and some other invertebrates. 



THE COMPLEX PREDATOR-PREY SYSTEM 



Increase in the number of prey species in the community makes 

 it possible for more species of competing predators to coexist. Be- 

 cause each species has different ecological responses to a dynamic 

 weather pattern, population oscillations of different prey species 

 do not rise and fall synchronously. As a result, if one prey species 

 is at a low ebb, another probably will be abundant, and the gross 

 food supply for the predators may change only slightly. Thus in 

 years when prey rodent populations are low, red foxes eat a larger 



