l6 ECONOMIC MAMMALOGY 



for some of the native animals. In many other ways we disturb the 

 balance that has been established. Such rapid transformations in the 

 environment introduce many complex and difficult problems, the cor- 

 rect solution of which requires an accurate and comprehensive knowl- 

 edge of all nature, and taxes the wisdom of the best-informed natural- 

 ists; yet men with very little knowledge of nature consider themselves 

 quite competent to decide all such problems very promptly. The tend- 

 ency of nature is not always to restore the former balance, but some- 

 times to establish a new one. 



One species, unless a very rare one, cannot be destroyed or greatly 

 lessened in numbers without affecting others. One species cannot be 

 introduced into a new region and become abundant there, or become 

 much more abundant than formerly in its native habitat, without 

 affecting others. Small rodents form the favorite food of certain 

 species of hawks, as well as of certain fur-bearing mammals. If such 

 rodents become scarce their enemies are likely to visit the poultry 

 yards, or attack wild birds or find some other food not usually taken 

 by them. Because of improved living conditions, many species of 

 mammals are known to increase in numbers when their food supply is 

 abundant and easy to obtain, and to decrease when the supply becomes 

 scarce or hard to obtain. 



Each species is to some extent, sometimes to a great extent, kept in 

 check by competition in the search for food and by the depredations of 

 natural enemies. If removed to a region where it is free from such 

 competition and depredations, or relieved therefrom by the destruction 

 of its competitors and enemies, it is likely to increase to such an extent 

 as to make it a menace to other species or even so to overpopulate the 

 region that its own demand for food exceeds the supply, and thus 

 limits further increase. This is particularly true of species that are 

 naturally very abundant and prolific, such as mice and rabbits. There 

 are many instances known of lemmings, field mice and even house 

 mice eating up the supply of food in one area, then swarming out over 

 adjacent territory and destroying all vegetation . in the line of their 

 march, a subject more fully discussed in another chapter. A great in- 

 crease in the number of small rodents has often followed wholesale 

 destruction of their chief bird enemies, hawks and owls. 



On our western plains and in the mountains, coyotes, wolves and 

 other predatory mammals had long lived in proximity to deer, rabbits 

 and other game mammals, in such equilibrium as neither to exterminate 



