Species and Species Change 



129 



25- 







o 



3: 



1850 



I860 



1870 



1880 



1890 



1900 



1910 



Fig. 52. Fluctuations in the estimated total populations of the arctic fox and 

 wolf in Canada. (Adapted from Hewitt.) 



ranges, such as the Ilhnois chinch bugs ( Shelford and Fhnt, 1943 ) , 

 show similar oscillations. Attempts have been made to resolve these 

 into regular cycles of three and one-half, four, six, eight, ten, and 

 up to twenty-five years between peaks of abundance. Portions of 

 most charts do fit this cyclic concept. Cole (1951) suggested that 

 these natural population oscillations are random rather than regular. 

 Other analysts believe that beneath many of the seemingly incon- 

 sistent gross oscillations there is some sort of regular cyclic popula- 

 tion rhythm affecting many kinds of organisms synchronously (Er- 

 rington, 1945, 1954; Mills, 1953). As a possible factor responsible 

 for this basic cycle, Shelford (1951) and others have suggested 

 ultraviolet light correlated with the ten or eleven-year sunspot 

 cycles. As Shelford and Mills have pointed out, however, oscilla- 

 tions of other factors in the environment such as humidity or tem- 

 perature (which are irregular and not synchronous with sunspots) 

 would reduce or augment, or lengthen or shorten, any basic regular 

 cycle. If the gross population oscillations are the result of many 

 independent factors, some regularly and others irregularly cyclic 

 (aptly termed polyvalent by Mills), then we would expect both 



