Cyclic Fluctuation 457 



parts of the ocean have revealed great variations in abundance from 

 year to year. As an example, the drastic differences in the seasonal 

 growth of a population of a planktonic copepod during 1929 to 1932 

 are shown in Fig. 12.14. Presumably these great annual changes in 

 the copepods influenced other members of the oceanic community, 

 but no adequate explanation for these fluctuations in the plankton is 

 known. Possibly the survival of young fish is dependent upon a 

 simultaneous reduction in number of predators and an increase in 

 availability of food. The relation between conditions favoring plank- 

 ton growth and the abundance of young fish in the Enghsh Channel 

 is discussed by Harvey ( 1950 ) . Search for the factor or factors that 

 control year-class success in specific instances is one of the many fasci- 

 nating problems in ecology awaiting solution— and obviously one of 

 great practical significance. 



Cyclic Fluctuation 



The fluctuations of the population in some communities appear to 

 be regular or cyclic. The phenomenon is particularly noticeable in 

 high latitudes of the northern hemisphere among small mammals, cer- 

 tain predators, gallinaceous birds, and some fish such as the salmon. 

 The abundance of certain insect species and the rate of tree growth 

 also exhibit cycles in various parts of the world. In addition, the 

 occurrence of disease epidemics and other biological events are be- 

 lieved by some to be definitely cyclic. 



Periodicity among northern mammals may be studied from the 

 records of the Hudson Bay Company and other fur-trading agencies. 

 As with commercial fisheries we must realize that the variations in the 

 take by man may not represent the actual fluctuations of the entire 

 population. Nevertheless the mass of evidence from fur traders is 

 corroborated by other observations and leaves no doubt that oscil- 

 lations of approximately uniform periods take place over long periods 

 of time (Elton, 1942). Cycles of abundance with 3 to 4 years be- 

 tween peaks are exhibited by mice, voles, and lemmings, and also by 

 arctic foxes, martens, and snowy owls. Many of these cycles can be 

 traced back in the records for a hundred years or more. The snow- 

 shoe rabbit, the grouse, and certain insects are commonly reported to 

 fluctuate with a 9- to 10-year cycle. Data for the Canadian lynx are 

 available from 1735, but difference of opinion exists as to the size of 

 the peaks that constitute maxima. If only the "big" peaks are counted, 

 the average period of fluctuation is 9.6 years. If every year in which 

 the population is greater than the preceding and the following year 



