86 THE ECOLOGY OF ANIMALS 



England and New Zealand and Australia ; and by 

 McLagan (1932) for the springtail {Smynthurus viridis) 

 that attacks lucerne and alfalfa in Europe and 

 AustraHa. 



The second thing is that natural fluctuations in 

 numbers can be studied and in some cases forecast. 

 Thus the ten-year cycle in fur-bearing animals in the 

 North Canadian forests can be forecast with some 

 accuracy, if a large enough region is taken (Elton, 

 1931c, 1933). This is also true of the shorter four- 

 year cycle in Arctic foxes in Canada (Elton, 19316). 

 And the same methods have been developed for cock- 

 chafers in Europe (Zweigelt, 1928) . Apart from know- 

 ing production or damage in advance, the recognition 

 of natural fluctuations is essential in order that the 

 effects of control measures can be estimated. Thus 

 the results of artificial introduction of mouse-typhoid 

 cultures in the control of field-mice in Germany and 

 France are vitiated unless the possibihty of natural 

 epidemics in the mice is reahzed (Elton, 1931c). In 

 1930, introduced American grey squirrels died in 

 many parts of England (Middleton, 19316, 1932). 

 At this time national efforts at destruction were 

 being planned. Had they been in operation for 

 several years the credit for the squirrel decrease 

 might have gone to them. 



In actual control measures, only a few important 

 ecological ideas have as yet been exploited. Of 

 these the most interesting is biological control. Pests 

 that get to islands or to another continent often do 

 not have their natural parasites with them. By the 

 introduction of the appropriate parasites or similar 

 ones, control can be estabhshed. Huge sums of 

 money have been spent on this application of the 

 food- cycle organization of animal communities. 

 Sometimes the experiment has failed through the 

 existence of hyperparasites that limit the effective- 

 ness of the parasites. Good summaries of this work 

 are given by Imms (1931) and by Thompson (1930). 



