UNBALANCE IN NATURE 93 



It is interesting to note, however, that many of 

 the observed biological cycles do not run with an 

 eleven-year period. The lemming-mouse-ptar- 

 migan-fox cycle is less than four years in average 

 length. A study of the size of rings of growth in 

 the fossil trees of southwestern United States 

 and of living forest trees in Germany do apparently 

 indicate an eleven-year cycle but otherwise the 

 most commonly indicated biological cycle is one 

 of some nine or ten years. Such cycles have been 

 found in the population numbers of grouse and 

 rabbits in Wisconsin, in grouse and other non- 

 migratory birds, rabbits and coyotes of Alberta, 

 Canada; in hares, muskrats, grouse, lynxes, foxes, 

 martins, wolves, minks and goshawks of Canada 

 in general. 



An astonishing feature of all this lies not only 

 in the fact that the same cycle occurs in so many 

 different animals which are not closely related in 

 an evolutionary sense although they are related 

 ecologically by food webs or habitat relationships, 

 but also that the same period occurs in the far 

 northwestern part of Canada and south well into 

 the United States. The increase or decrease of 

 population appears to begin in northern Canada 

 and works its way slowly southward and east- 

 ward; it reaches southern Canada in about three 

 years but in each region the period of somewhat 



