STRUGGLE IN NATURAL CONDITIONS 23 



About that time, however, they came in, apparently following the old 

 goldrush trail, probably attracted by the hundreds of dead horses 

 along it. The invasion of Alaska seems destined to continue until 

 coyotes have extended their range over practically all of the territory. 



It has been found in Alaska that the coyotes kill many foxes. 

 Since the coyotes have increased the foxes have decreased alarmingly. 

 In some sections practically none are believed to be left. In many 

 cases a family or entire colony of foxes are run out of their dens or 

 are both run out and killed by coyotes which then use the dens them- 

 selves. Wolves are well known to have committed similar depreda- 

 tions, but their killing is not so extensive as is that of the coyotes. 

 Serious as are the depredations of wolves throughout most of Alaska, 

 the damage done to game and fur bearing animals by coyotes is not 

 only far greater but is rapidly increasing in extent. How far the 

 coyotes will hold back the normal development in the periodical in- 

 crease of the snowshoe rabbits and the ptarmigan which are important 

 items in the food supply of fur bearers, especially the lynx and fox, it 

 is impossible even to approximate. 



(13) The examples just mentioned show that the introduction of 

 aquatic animals into waters to which they are new, or the penetration 

 of land animals into new regions, often lead to very interesting proc- 

 esses of competition. We would now like to say a few words about 

 the direct struggle for existence, in which one species devours another. 

 In this case it is very important to ascertain the exact numerical 

 relation between the population of the devoured species and that of 

 the devouring one, and this can often be attained by changing their 

 relative quantities. One of the outstanding facts is the increase of 

 the deer accompanying the destruction of wolves, foxes, etc., by 

 early settlers in Illinois which, according to Wood, has been recently 

 reported by Shelf ord ('31). Wood depicts a continuous decrease in 

 wolves and wildcats from the beginning of settlement to their practi- 

 cal extinction. When the wolf population was reduced to about one- 

 half, the deer increased rapidly for a little less than 10 years, reaching 

 a large maximum of about three times the original number. Un- 

 fortunately we have no exact data on the change of the numerical 

 relation between the wolves and the deer, but such processes are in 

 any case of great interest. 



The change of the numerical relations between the predator and 

 the prey sometimes takes place as a consequence of a mass appear- 



