324 



POPXJLATIONS 



MacLulich also discusses the cyclic 

 character of fluctuations of lynx popula- 

 tions in the same area in Canada (Fig. 

 118), He concludes that "the abundance of 

 the lynx was shown to be definitely corre- 

 lated with that of varying hares," and that 

 these fluctuations were not associated with 

 sunspot activity. The trends are clearly reg- 

 ular and cyclic, and the mean period of 

 time between peaks is 9.7 years. It should 



plotted against months of the year for 

 nearly three years. The figure also records 

 the percentage of adult females pregnant 

 in each month. Elton has this to say 

 about the mice: 



"The diagrams are most easily understood 

 if one looks at the three winter seasons, say 

 November-February. During the first winter 

 the mice were not breeding at all (actually not 

 from October to March). The numbers showed 



X 8,000 



S 4,000 



1780 1790 1800 



TIME IN YEARS 



1810 



1820 1830 1840 



1850 



1870 



1880 



1890 



1900 



1910 



1920 



1930 



TIME ID YEARS 



Fig. 118. Population trends of the lynx (solid line) graphed against sunspot numbers (dotted 



lines). (From MacLulich.) 



be noted that the lynx populations do not 

 fall so low during the periods of depression 

 as do those of the rabbits. Apart from this 

 fact the general correspondence between 

 the two species is remarkably close. (See 

 discussions of predation, page 370, and of 

 cycles, page 366. ) 



Elton (1942) reports some data on fluc- 

 tuations of the EngHsh field mouse, Apode- 

 mus sylvaticus, a form ecologically not un- 

 like the American deer mouse, Peromyscus. 

 These mice were trapped in a woods not 

 far from Oxford University. Figure 119 

 shows the number of mice caught in traps, 



a general falling in trend: the mice were not 

 balancing their budget of population. But when 

 summer breeding began the numbers in the 

 traps went up again. In the second winter 

 breeding practically stopped, but it went on 

 further into the autumn and began sooner in 

 the spring. The numbers began to drop in 

 November, but there was an extraordinary in- 

 crease in the number caught early in 1927, 

 which we attributed to abnormal movements. 

 The usual summer increase occurred that year. 

 The following winter was remarkable, for 

 breeding practically never stopped, and the 

 population never fell to the low level it had 

 reached in 1926" (pp. 165-166). 



