Productivity and Populations 319 



Ontario. Green (1935 et ante) has long held that disease plays a 

 dominant part in wild-life cycles, particularly documenting evi- 

 dence in tlie cases of snowshoe hares and ruffed grouse. Elton 

 (1942) pointed out the occurrence of disease in foxes and sledge 

 dogs in the North associated with cycles. He stated that ". . . the 

 cycle rests on mice or lemmings, . . . and is also marked by out- 

 breaks of epidemic nervous disease that devastate fox populations 

 and appear also among sledge dog teams . . ." 



If cycles are a function of population density, then disease is 

 likely to play a large part. There is no question but that the in- 

 cidence of infestations increases with increasing grouse abundance. 

 This was clearly observed in the specimens taken annually on a por- 

 tion of the Connecticut Hill area. Not only does the incidence of 

 infestations of all sorts increase with growing populations but the 

 severity of the average case increases. For four years, from 1929 

 to 1933, tlie density of grouse increased on Connecticut Hill, par- 

 alleled by increasing incidence of infestation, before a case of 

 death from disease was found. That year several were noted. This 

 is significant considering the difficulty of finding grouse bodies be- 

 fore the scavengers get them. Likewise, there is no doubt but that 

 disease (lesions caused by Disphartjnx spiralis) was an important 

 cause, if not tire most important cause, of an abrupt loss of grouse 

 observed by Allen near Ithaca, N. Y., in the fall of 1927. The evi- 

 dence surely does not permit us to discard the disease factor in 

 major grouse declines. 



9. Predator-Buffer Species Relations are not Primarily Correlated 

 with Grouse Die-off s. 



Practically all authors of recent years have supported this con- 

 clusion.' Nevertheless, when an agency normally accounts for a 

 large proportion of a population, as predators do with grouse, a 

 delicate balance exists that may be seriously upset if conditions 

 temporarily favor an increase in predation. This occurs most com- 

 monly in winter and spring and is usually induced by climatic con- 

 ditions. The possibihties of increased predation on grouse result- 

 ing from losses in rodent prey species may be of considerable im- 



1 It may be inferred from Elton ( 1942 ) that grouse may well fall victim to buffer- 

 predator population changes. He notes: "After the lemming crash, the foxes were seen 

 to have turned in the emergency to other supplies." Then: 'The vole cycle profoundly 

 influences some predatory birds. . . . These changes are reflected in the southv^'ard 

 migrations of several species. . . ." 



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