VARIATIONS IN THE NUMBERS OF ANIMALS 139 



carried by its enemies). This is actually the case. Epidemics 

 are unknown among these animals, and, as a matter of fact, 

 they have no very serious carnivorous enemies.''^ But local 

 outbreaks of disease are probably an important check in 

 numbers, since MacFarlane says,'^^ " j^ is not an uncommon 

 occurrence for hunters to find one or more beavers dead of 

 disease in their houses or ' washes.' " In the beaver we find 

 a rodent which apparently limits its numbers mainly by disease 

 in the form of local outbreaks, and not by widespread epi- 

 demics ; and which is unaflFected by cycles of climate with a 

 short period, so that the numbers of the population as a whole 

 — apart from the effects of trapping by man — remain the same 

 from year to year. 



15. Fluctuations in the numbers of any one species in- 

 evitably cause changes in those of others associated with it, 

 especially of its immediate enemies. This is well shown in 

 rodent fluctuations. In Canada the arctic fox fluctuates with 

 the lemmings, the red fox with mice and rabbits, the fisher 

 with mice and rabbits and also fish, the lynx with rabbits, 

 and so on. All these carnivorous mammals, which prey 

 largely or partly on rodents, have periodic fluctuations of 

 great regularity. This is true of weasels, ermine, martens, 

 foxes, wolverene, skunks, mink, lynx, and others. The sable 

 of Siberia depends to a great extent upon squirrels, and 

 fluctuates accordingly. In all cases there is a lag in the increase 

 of the carnivores owing to their larger size. Many birds of 

 prey show similar fluctuations due to the variations in their 

 food-supply, but here the question of numbers is complicated 

 by migration, and it is rather hard to get at the real facts about 

 changes in the whole population. But it can at any rate be 

 said that the numbers of hawks and owls often vary regularly 

 in any one place. One of the best descriptions of the enormous 

 changes wrought in the abundance and habits of carnivorous 

 birds and mammals by rodent cycles is that given by Cabot 11 ^ 

 at the end of his book on Labrador. 



There is little doubt that insectivorous mammals and 

 birds, such as shrews, moles, and warblers, are subject to 

 considerable fluctuations dependent upon the abundance of 



