Northern Baffin Islond 



Southern Baffin Island 



Northern Quebec 



Northern Lobrodor 



Victorio Island (East) 



Victoria Island (West) 



FIG. 17-4 Relation be- 

 tween peaVs and lowj of 

 lemming cycle at differen 

 localities in eastern and 

 northern Canada (Chitty 

 1950). 



eastern N'orth America have occurred in 1923, 1926, 

 1930, 1934, 1938. 1941-12, 1945-te, 194S-49, and 

 1953, with deviations of one or more years for par- 

 ticular regions (Siivonen 1954). A peak occurred in 

 Alaska in 1956. 



The predator cycle is dependent on the cycle of 

 the herbivorous mammal or bird prey species. The 

 correspondence in the cycles of predator and prey is 

 usually close, although that of the predator soine- 

 times lags a year behind that of its prey (Chitty 

 1950, Butler 1953). The snowy owl emigrates in 

 large numbers from Canada into the United States 

 within a year after the decline in the lemming popu- 

 lation (Shelford 1945). In those parts of Greenland 

 where the fox population lives largely on lemmings, 

 the 3-4 year fox cycle is very pronounced, but this 

 is not true for the coastal areas where the fox de- 

 pends on a variety of food other than lemmings 

 (Braestrup 1941). 



Inl 



rinsic causes 



According to early mathematical theories of 

 Lotka and Volterra (D'Ancona 1954) and of Nichol- 

 son and Bailey (1935), a population consisting of a 

 single prey species and a singe predator or parasitoid 

 species occurring together in a limited area, with all 

 external factors constant, automatically displays pe- 

 riodic oscillations or cycles in the numbers of both 

 species. As the predator population increases, it will 

 consume a progressively larger number of prey until 

 the prey population begins to decrease. As the num- 

 ber of prey diminishes, there will be less food for the 

 predator, and they will thus decline. After a time 

 the number of predators will be so reduced that the 

 high reproductive rate of the remaining prey will 

 more than compensate for the loss from predation, 

 and the numbers of the prey species will again in- 

 crease. This will be followed shortly by an increase 



<— ^ Varying hare 

 — -► Canada lynx 



FIG. 17-5 Population 

 cycles of the snowshoe 

 rabbit and one of its chief 

 predators, the Canada 

 lynx, in northern Canada 

 (data adiusted for years 

 1912 to 1920), based on 

 number of pelts handled 

 by the Hudson Bay Com- 

 pany (from MacLulich 

 1937). 



Irruptions, catastrophes, and cycles 239 



