Origin of Cycles 461 



As a species becomes more abundant, its metabolites tend to accu- 

 mulate and other physical and chemical changes are brought about 

 that may curtail the reproductive rate of the population. With in- 

 creased density of the population predators are attracted and disease 

 parasites can spread more easily. As crowding continues, food and 

 other necessities become scarcer, and the animals tend to wander 

 more extensively. This latter fact causes the further distribution of 

 infectious disease and exposes the animals more frequently to detec- 

 tion by their predatory enemies. Furthermore, with progressive re- 

 duction of the vegetation by herbivores less cover for protection 

 against their predators is available. Thus increase in abundance may 

 automatically cause a curtailment of the population in ways that might 

 tend to bring about regular self-induced oscillations. However, the 

 inevitable occurrence of other changes in the environment would be 

 expected to impose irregularities upon any intrinsic cycle. 



Approximately regular fluctuations in populations caused by in- 

 dependent changes in the environment— either animate or inanimate- 

 are known as extrinsic cycles. Obviously intrinsic and extrinsic 

 types of cycles are not sharply separated since many populations 

 respond both to self-induced and independent variations in their en- 

 vironment. Irregularities in the environment might set in motion 

 regular internal oscillations by a sort of resonance, just as irregular 

 waves can cause a boat to roll in its natural period. 



Scrutiny of the data on populations with apparently cyclic fluctua- 

 tions has led Cole ( 1951 ) to conclude that "the oscillations of any 

 hypothetical factor determining population size need only be about as 

 regular as would be expected of a random variable." Since we know 

 that the abundance of every species is affected by many environmental 

 factors, it may be that chance combination of favorable and unfavor- 

 able influences in any year will produce apparently cyclic fluctuations. 

 Support for this simple explanation is found in a comparison of certain 

 fluctuations in nature with those of a series of random sampling num- 

 bers in which the former are no more convincingly "cyclic" than the 

 latter (Fig. 12.15). Cole also urges that here, as elsewhere, the so- 

 called "law of scientific parsimony" be applied, that is, the avoidance 

 of a more complicated hypothesis than is actually required to explain 

 the observed facts. However, it is unlikely that random fluctuation 

 alone would produce cycles of the same period in widely different 

 species. 



In some instances variation in abundance or growth has been defi- 

 nitely related to the variation of some extrinsic factor in the environ- 

 ment, and no doubt many more such relationships will be found in the 



