UNBALANCE IN NATURE 91 



remarkable synchrony in such widely separated 

 regions as Canada, Greenland and southern 

 Norway. When lemmings are abundant, the 

 animals feeding upon them have much food and 

 consequently increase in numbers. Elton, a 

 British naturalist much interested in these cycles 

 of animal life, has studied the records of the 

 Hudson Bay Company and finds that the times 

 when many arctic fox skins have been taken by 

 the Hudson Bay trappers follow the years of 

 great lemming abundance. 



The field mouse, Microtus, is another key- 

 industry animal of the tundra. It is a somewhat 

 close relative of the lemming and has a similar 

 cycle of abundance. When these mice are at the 

 peak of their population fluctuation, they attack 

 the vegetation in such numbers that larger ani- 

 mals like the caribou are forced to move to 

 other feeding grounds and even men are affected, 

 for the mice devour young crowberry shoots and 

 thus prevent the Eskimos from collecting their 

 annual crop of crowberries. 



The seed and bud-eating ptarmigan of the 

 tundra are protected by the increase in numbers 

 of the lemmings and field mice because their nor- 

 mal enemies are feasting on more easily caught 

 food, and thus they too tend to increase 

 when these small rodents are plentiful. Foxes, 



