180 AGGREGATION, COMPETITION AND CYCLES 



clearer view of the intervals than the curves do, and also makes it 

 possible to determine the average length of rise and fall as approxi- 

 mately 3.3 years. These figures would seem to dispose of the "crash" 

 as anything more than a rare catastrophe. They constitute a direct 

 contradiction of the statements that "in a few weeks usually, the 

 rabbits are wiped out," and that "to explain the variations we must 

 seek not the reason for the increase — that is normal — but for the 

 destructive agency that ended the increase" (Seton, 1911; cf. Leopold, 

 1931, 1934). 



CAUSES OF ANIMAL CYCLES 



In some cases the rise in abundance is important; in others, the 

 decline. There are cases in which the decline affords the key to the 

 explanation, while in others some special factor brings about an 

 increase and the reverse causes a decline. 



It is customary to view cycles as produced by conditions favor- 

 ing reproduction and survival; however, all plants and animals pro- 

 duce more spores, seeds, eggs, or young than can normally develop to 

 maturity. Under ideal conditions the tendency for any species is in 

 the direction of an almost unlimited population. Such large numbers 

 are unattainable because of various forces which tend to reduce the 

 number of individuals. Small reproductive populations, though fre- 

 quently a contributing factor, are responsible for minima, if at all, 

 only in special types of life histories, such as that of the salmon. The 

 maximum may appear important in one case, the minimum may be 

 equally so in another. For convenience, the problem of fluctuations 

 in abundance, whether these be cyclic or irregular, may be approached 

 more often from the standpoint of the causes of failure to produce 

 large populations of mature individuals or, in other words, causes of 

 small populations. 



There are eight causes of failure to produce a large number of late 

 juvenile or adult offspring, which means the decrease in numbers of 

 any abundant species. They are (1) decrease in the number of eggs 

 produced or fertilized, owing to various causes, or destruction of eggs 

 and very young stages; (2) death of adults and late juvenile stages 

 from adverse physical conditions; (3) destruction by enemies and dis- 

 ease; (4) quantitative or qualitative insufficiency of food; (5) unsuc- 

 cessful competition for space, shelter, or food; (6) physiological changes 

 in reproductive vigor; (7) initial shortage in reproducing population; 

 (8) cannibalism. To these may be added certain assumptions such 

 as loss of immunity, etc., but of these little is definitely known. 



Loss of Eggs and Early Stages. The codfish affords a noteworthy 

 example of loss of eggs after fertilization as well as before. Though 



