138 ANIMAL ECOLOGY 



either increase, remain constant, or decrease in numbers, and 

 as there is always the danger of the numbers going down 

 steadily if the balance is not struck exactly right, it seems 

 reasonable to suppose that species would tend to be adapted to 

 a steady increase in numbers, which is terminated at a certain 

 point by disease or some other factor. 



14. It is interesting to consider at this point an animal 

 whose peculiar habits and scheme of social existence make the 

 study of its numbers of extraordinary interest. This animal 

 is the beaver {Castor fiber). The beaver has been hunted and 

 trapped and studied so intensively for the last hundred years, 

 that we possess a great deal of reliable information about its 

 ecology. The statistics of the Hudson's Bay Company show 

 that the numbers of beaver undergo no very marked short- 

 period variations ; although there was a general upward trend 

 in number of skins during the early part of the nineteenth 

 century, which was associated with the exploration and opening 

 up of new parts of Canada. ^^ This was followed by a con- 

 siderable drop, consequent upon exhaustion of the natural 

 stock of beavers through settlement and over- trapping,, The 

 reason for this comparative stability of the beaver population 

 as a whole appears to be that it is almost entirely independent 

 of short-period climatic variations. Since it uses the bark of 

 trees for food the beaver is unaffected by annual variations 

 of plant food-supply. It lives on capital and not on income — 

 an almost ideal existence. Furthermore, although it is 

 aquatic, the elaborate engineering feats which enable it to 

 construct dams and houses (not to mention long canals for 

 bringing food supplies from a distance) make it comparatively 

 immune to the effects of annual variations in water-supply — 

 unlike the muskrat, which is at the mercy of floods and droughts. 

 Each colony of beavers lives in one place until the local supply 

 of trees is exhausted, and when this takes place the animals 

 apparently move on to some other locality. One would 

 imagine that the beaver's habits, which cause it to live in 

 isolated colonies, would make it difficult or impossible for any 

 epidemic disease to spread throughout the population of a 

 whole region (unless some alternative stage of a parasite were 



