62 THE ECOLOGY OF ANIMALS 



the result of developments of human civiHzation in 

 which man has interfered with natural conditions, as 

 has happened with the introduction of pests into 

 islands or the destruction of predatory animals. It 

 is known, for instance, that animal life in Labrador 

 is subject to violent periodic fluctuations of popula- 

 tions which affect a great many different kinds of 

 animals — field-mice, foxes, ptarmigan, hawks, owls, 

 caribou, and wolves, to name only a few. In this 

 fluctuating nexus of animal communities man (Indians 

 and Eskimos), far from being the interfering factor, 

 is the unwilling victim of his environment, and in 

 turn fluctuates in numbers through the ravages of 

 starvation and disease (Elton, 1930, 19316). 



Some of these fluctuations are rather unexpectedly 

 regular in their periodicity, others are irregular both 

 in the period and the amphtude. Some examples 

 may be given. Short-tailed field-mice or voles have 

 periodic fluctuations in which disease plays an impor- 

 tant part, both in Labrador, Great Britain, and 

 Norway, and with a periodicity of about four years. 

 Voles in Germany and France also fluctuate, but 

 partly in connexion with agricultural conditions 

 (Elton, 1931c). In the forests of northern Canada 

 and also on the prairies of the Middle West, many 

 of the smaller fur-bearing animals and rodents and 

 game birds fluctuate in a rhythmical way, reaching 

 abundance about every ten years, with slight varia- 

 tion in the exact periodicity. Johnstone (1928) 

 showed that the marine flshes of the Mersey Estuary 

 region between 1893 and 1927 (when records were 

 avaflable) fluctuated in a manner characteristic for 

 each species. Thus plaice had maxima about 1895, 

 1910, and 1919 ; soles about 1898 and 1905 ; dabs 

 about 1897, 1902-3, 1910, and 1924 ; and whiting 

 about 1902, 1910, 1918, and 1925. Bottom-Hving 

 marine animals of other kinds also suffer periodic 

 fluctuations, as in the echinoderm {Echinus miliaris), 

 which was destroyed in numbers on parts of the 



