POPULATION FACTORS AND SELECTED POPULATION PROBLEMS 



367 



Despite the ecological significance of the 

 problem and the volume of work reported 

 in the literature, there is some confusion 

 about population cycles. The confusion 

 arises largely from the attempt to identify 

 the causes of cycHc phenomena. Some par- 

 dcular cases are well established and ade- 

 quately interpreted, but many explanations 

 of cyclic phenomena are not yet definitive 

 (p. 326). 



The case is excellently stated by Hutch- 

 inson (1942) in a thoughtful review of 

 Elton's Voles, Mice and Lemmings (1942), 

 a book largely devoted to population 

 cycles. We quote from Hutchinson as fol- 

 lows: 



"... [A real periodicity] . . . may presum- 

 ably be due to cyclical events outside the 

 ecosystem or to tlie generation of cycles by 

 mechanisms inside the biological system, the 

 former condition corresponding to forced vibra- 

 tions in mechanics, the latter to free vibration. 

 In the case of two isolated but climatically and 

 geographically close populations of an animal, 

 if the periodicity is externally determined or 

 forced, the fluctuations in the two populations 

 may reasonably be expected to be synchronous. 

 In more remote regions, the oscillations might 

 show a constant phase difference, but any irreg- 

 ularities in one region should correspond to 

 irregularities in another. If the oscillation is 

 free, there is no reason why the two popula- 

 tions should fluctuate in phase or with cor- 

 respondence in their irregularities. The exact 

 number of possible ways in which free fluctua- 

 tions may occur is hard to ascertain a priori, 

 but most cases probably reduce formally either 

 to die symmetrical, so-called classical oscillations 

 involved in the predator-prey relationship 

 ( Lotka-Volterra periodicity, developed in a 

 different form by Nicholson and Bailey) or to 

 the relaxation oscillations of Gause, Cause's 

 theory involves the building up of the popula- 

 tion to a certain critical level, after which it 

 becomes unstable and declines suddenly, the 

 most obvious causes of the decline being ex- 

 haustion of food, or abnormal opportunities for 

 the spread of epidemics, when the critical 

 density is reached. A final word of warning is 

 needed; it is tempting to suppose a multiple 

 causality, and this is no doubt correct, but it is 

 clear that one must not postulate interaction 

 of periodic causes with different periods, not 

 multiples of each other, and then hope to 

 obtain a highly regular set of maxima. This has 

 been done in the past by authorities who 

 should have known better" (p. 355). 



An illustration of a population cycle is 

 afforded by MacLulich's (1937) study of 

 the varying hare, Lepus americanus, in 



Canada. On page 270 the census tech- 

 niques used by MacLuhch in the estima- 

 tion of hare abundance were enumerated; 

 on page 323 the cycle over a many-year 

 period was figured (Fig. 117); on page 

 324 the relation of lynx abundance to hare 

 abundance was mentioned and graphed in 

 Figure 118. 



With this background we need only re- 

 mind the reader that the hares studied by 

 MacLuhch exhibited a cychc growth form 

 with a mean span between maxima of 9.7 

 years. After rather hitensively investigating 

 various possibilities, MacLulich concluded 

 that the cycle was of the "relaxation oscilla- 

 tion" type resulting chiefly from operations 

 within the population system; that the 

 population "crashes," which started during 

 the eighth year and continued on a re- 

 duced level during the ninth year, were 

 primarily the result of nonspecific, explo- 

 sive epidemics; that the lynx cycle is def- 

 initely correlated with the hare cycle (p. 

 324), and that the "fluctuations in num- 

 bers of neither lynx nor varying hares are 

 correlated with sunspots." 



In connection with sunspots it is of in- 

 terest to note that Elton, in Voles, Mice 

 and Lemmings, observes: "I do not intend 

 to go very deeply here into this theory 

 about the sunspots. There can be httle 

 doubt that it is wrong: the arguments 

 against it have been reviewed by Mac- 

 Luhch, and they agree with my own un- 

 pubhshed evidence. The chief point is 

 that the biological rhythm is sfightly 

 shorter than that of the sun-spots, and 

 long series of fur returns shows that the 

 two cycles pass right out of phase . . . 

 The sun-spot theory is mentioned chiefly 

 because I also suggested [Elton, 1924] a 

 correlation between the early records of 

 mouse plagues in Great Britain and the 

 sun-spots, which can also no longer be 

 seriously upheld" (p. 160).* 



Although some authors (for example, 

 MacLagan, 1940) apparently are still im- 

 pressed by apparent correlations with the 

 sunspot cycles, our general position is that 

 sunspot activity has been too readily in- 

 voked as an explanation of population 

 cycles; that in any investigation the more 

 immediate environmental factors, whether 



* For further introduction to the literature on 

 population cycles several other citations 

 are recommended: Hamilton (1937), Cross 

 (1940), and Stoddard (1932). 



