ANIMAL CYCLES 177 



ANIMAL CYCLES 



The question of animal cycles has been a subject of interest for 

 more than a half-century, though the locust plague, as the most serious 

 expression, has been a matter of concern for hundreds of years. Scien- 

 tific activity in this field has been much stimulated by the work of 

 Collett with the lemming (1895) and the observations of Seton in 

 Arctic America (1911) , and today it constitutes one of the most signifi- 

 cant, as well as most difficult, lines of research in bio-ecology. How- 

 ever, as a field in which quantitative methods are paramount, the 

 study of animal populations in nature still lingers on the threshold. 

 It is manifest that many species have already dwindled beyond the 

 point at which their fluctuations can be profitably investigated, a con- 

 sequence that rules out practically all settled districts. So far as 

 mammals are concerned, only regions in high latitudes hold much 

 promise, but these are the very ones, as a rule, in which the difficulty 

 of resident study over a long period all but eliminates adequate quanti- 

 tative determinations. In the succinct account that follows, it should 

 be constantly borne in mind that numbers and cycles are still based, 

 for the most part, upon general and incidental observations and that 

 discrepancies and contradictions are frequently to be encountered. 

 Moreover, the only records that approximate accuracy are those of the 

 fur returns of the Hudson Bay Company, and it is obvious that even 

 such data as to populations fail in complete accuracy. 



Nature of Animal Cycles. Cycles are characterized by alternating 

 phases of plus and minus departures that pass more or less gradually 

 into each other, the sunspot cycle being the best-known example. 

 Cycles in populations are rather more variable, the rise to a maximum 

 or the fall to a minimum occasionally occurring in a single year, 

 while a high or low level may be maintained for two or more years. 

 Low levels are the rule with desert annuals, a "flush" appearing only 

 at intervals of several years. Low levels are likewise frequent in 

 animal cycles, but maxima nearly always take the form of sharp 

 peaks, as illustrated by the rabbit curve (Fig. 37). Fig. 38 shows 

 seasonal variation in insect larvae and suggests an unusual abundance 

 for Chironomus bathophilus for 1935. 



In mammal cycles, the fall to the minimum has been regarded as a 

 catastrophe that signalizes but one or, at most, two seasons, and 

 hence has been commonly known as the "crash." However, an inspec- 

 tion of the curves of furs taken makes it clear that the rise is rapid 

 quite as often as the fall and that the idea of a crash is in part due to 

 the necessarily local, brief or discontinuous and superficial observa- 



