THE GROWTH FORM OF POPULATIONS 



323 



1929 the yield fell regardless of effort and 

 techniques. (6) During the postwar period 

 of 1919 to 1933 the general trend in den- 

 sity was downward. We are particularly in- 

 terested in these curves because they de- 

 scribe population fluctuations that are the 

 result primarily of a specific type of mortal- 

 ity, that owing to exploitation by man. 



In the discussion of equiUbrium we de- 

 scribed the trend of populations of the dia- 

 tom, Rhizosolenia styliformis, based on the 

 work of Lucas (loc. cit.), and pointed out 

 that this species existed in a state of rela- 

 tive constancy during a number of spring 

 and summer months for several years. The 

 graph (Fig. 110) also illustrates fluctua- 



varying hare (Lepus americanus) in the 

 Hudson Bay watershed area of Canada. We 

 discussed MacLulich's methods of estimat- 

 ing hare abundance in Chapter 18 (p. 270) 

 and pointed out that by using four sorts of 

 criteria (fur returns, statements in the 

 literature, questionnaires, and field work) 

 he was apparently able to obtain a rela- 

 tively accurate picture of the population 

 growth form of this species over many 

 years. Figure 117 describes, with an unre- 

 ported gap from 1825 to 1844, these fluc- 

 tuations from 1786 to 1903 on the basis 

 of records released by the Hudson's Bay 

 Company and from 1903 to 1935 on the 

 basis of questionnaires. 



150,000- 



^ 100,000- 

 LjJ 



cr 50,oooF 



X 20.000 



1790 '95 1800 '05 '10 '15 '20 '25 '30 '35 40 "45 '50 '55 '60 



50,000 

 20,000 



1865 '70 '75 '80 '85 '90 '95 1900 '05 '10 '15 '20 '25 '30 '35 



TIME IN YEARS 



Fig. 117. Population trends of the varying hare in the Hudson's Bay watershed. (From 



MacLulich. ) 



tions since it can be seen that there is a rise 

 in numbers of the form in the autumns of 

 1932, 1933, and 1934. It seems probable 

 that these "autumnal patches " are related 

 in some way with the seasonal changes in- 

 volved. 



Natural population studies of mammals 

 afford many illustrations of fluctuations. 

 Certain of these are well summarized in 

 Elton's comprehensive book (1942) as 

 well as in numerous research reports. We 

 wish to present briefly some data on the 

 varying hare and the lynx taken from Mac- 

 Lulich (1937), and the mouse and the fox 

 from Elton. 



MacLulich's investigation, summarizing 

 and extending the hterature beginning with 

 Seton, is a detailed discussion of the cyclic 

 nature of fluctuations in populations of the 



It is worthwhile to examine Figure 117 

 carefully since it is an excellent illustration 

 of a long-time fluctuation pattern. The 

 striking point about the trend is its regular- 

 ity coupled with the fact that in low years 

 the curve always returns to a point near 

 the baseline, and in peak years the max 

 imal population sizes are rather similar with 

 occasional exceptions. The elapsed time be- 

 tween peaks varies somewhat, but usually 

 is of the magnitude of nine to ten years. It 

 is clear merely from inspection that these 

 are not random, haphazard fluctuations, but 

 are instead regular, and even predictable, 

 cycles. In a later chapter, when considering 

 population cycles in general, we extend 

 MacLulich's analysis and discuss causal 

 factors (p. 367). 



