THE PHENOMENON OF PREDATION — ERRINGTON 515 



Breeding resilience may also compensate for high juvenile mortality 

 in some of the more prolific mammals. This, too, should not be con- 

 fused with the mere production of immense numbers of yomig to allow 

 for or to compensate in advance for heavy losses. Rather, the popula- 

 tion adjusts to the social tolerances of the species and the status of 

 the habitat. Extraordinary losses of yomig may stimulate reproduc- 

 tion. For the muskrats of north-central United States, averages 

 approaching four litters during a breeding season may be born to 

 uncrowded adult females living under favorable conditions. Averages 

 as low as a litter to a litter and a half may satisfy crowded popula- 

 tions in the same kind of place. But, if the early-bom young suffer 

 very high rates of mortality — as through the agencies of floods and 

 epizootic disease on the north-central study areas — even crowded pop- 

 ulations may give birth to many additional litters that plainly would 

 not have come into existence had it not been for the severity of the 

 earlier losses. After the young of these resilient breeders are hatx^hed 

 or born, compensatory trends in loss rates go into a substitution phase. 

 While a minimal loss of young during the rearing season is inevitable 

 under the best of conditions, a lot of the postbreeding shaking down 

 of overproduced young depends upon the extent that their environ- 

 ment is already filled up with their own species. The net population 

 increases often tend to be according to definite curves or to reach 

 certain density levels, often in conformity to year-to-year mathemat- 

 ical patterns that look unaffected by changes in kinds and numbers 

 of predatory enemies, the impacts of the less sweeping deadly emer- 

 gencies, and so on. We can thus see evidence of balancing and coun- 

 terbalancing that make meaningless any calculations as to population 

 effect based solely upon the numbers or percentages of individuals that 

 may die through this agency or that. 



Muskrat populations comfortably situated in rich environment may 

 give birth to many young and rear most of the young born; those 

 populations that are beset by endless stress may give birth to few young 

 and rear comparatively few of them. When the social squeeze is on 

 and life is hard, there are bound to be heavy losses from various agen- 

 cies, including predation from different kinds of predators. Still, 

 I cannot see that such predation actually operates as a limiting 

 factor — at any rate insofar as something else is doing the real limiting. 

 Particularly do I find it difficult to see why some predators, for ex- 

 ample the mink, may be considered a limiting factor on the basis of 

 the large numbers of muskrats the mink as a species may kill, as long 

 as in the absence of minks the muskrats may neither reach nor maintain 

 their numbers at perceptibly higher density levels than they do in the 

 presence of the minks. The Iowa case histories of mink-muskrat rela- 

 tionship repeatedly support this view. 



