320 Relations within the Species 



monly break clusters of young oysters apart to insure that the anhnals 

 develop in the well-rounded form characteristic of solitary oysters. 



The harmful effect of excessive numbers of animals is sometimes 

 manifested as an interference with breeding. The progressive re- 

 duction in the reproductive rate of DrosopJiila with increased density 

 of the population is shown in Figure 9.4. As the flies become more 

 crowded, interference with feeding and possibly also with oviposition 

 is experienced. Frequent collision and interruption of feeding re- 

 sult in inadequate nourishment and lowered fecundity although the 

 supply of food may be ample (cf. Robertson and Sang, 1944). In 

 some kinds of animals the ratio of males to females is also significant 

 in controlling success of reproduction. Although many animals do 

 best when the two sexes are present in equal numbers, a one-to-one 

 ratio is detrimental for some polygamous species. Among pheasants, 

 for example, each cock normally maintains a harem of about five hens, 

 and, if more males are present in the population, the consequent fight- 

 ing and disturbance of the incubating females greatly reduces the 

 success of the breeding. With species requiring highly specialized 

 places for breeding or nesting, the first effect of an enlarged popula- 

 tion may be a critical shortage of breeding sites. 



I'holo by Swem 



Fig. 9.5. "Deer-line" formed by denuded branches of Douglas fir and Ponderosa 

 pine, caused by browsing of overabundant deer in Oregon. 



