68 THE ECOLOGY OF ANIMALS 



interaction of the internal and external oscillations. 

 This leads on to the second generaUzation that can 

 be made about animal populations : the close inter- 

 dependence of the numbers of one form upon those 

 of another. No animal can fluctuate without affect- 

 ing the numbers of some other animal. In many- 

 parts of Great Britain areas of pasture have been 

 fenced off from sheep grazing and turned into young 

 plantations. Within these fenced areas voles {Mi- 

 crotus) multiply greatly, and probably exist (in terms 

 of total weight of animal) in quantities as great as 

 the sheep were before enclosure. In 1891 and 1892 

 voles multiplied on grazing areas in the Border 

 counties of Scotland, with the result that lambs 

 starved and a Parliamentary Committee was set up 

 to see what should be done about it (MaxweU and 

 others, 1893). After the Committee had sat, the 

 voles disappeared. 



When lemmings multiply in Scandinavia or in the 

 Arctic regions, there are usually great increases in 

 predatory birds and animals such as skuas, snowy 

 owls, hawks, ravens, foxes, ermines, etc. When the 

 lemmings disappear, as they do after a year or two, 

 these predatory animals starve, or do not breed for 

 a year or two, or migrate elsewhere (Manniche, 1910). 

 In tropical countries, multipUcation of rats together 

 with multiplication of rat-fleas (which depends on a 

 combination of certain cHmatic conditions of tem- 

 perature and humidity) causes an unusual multipli- 

 cation of Bacillus pestis, and when the rats die, as 

 well as before this, the fleas may migrate on to 

 human beings and cause their population to fluctuate 

 also, through bubonic -plague deaths. 



We see, then, that the structure of an animal com- 

 munity, together with the variable environment that 

 it Hves in, leads both to fluctuations in individual 

 species, and to far-reaching effects upon other species. 

 It is frequently suggested that animal ecology can be 

 divided into autecology (the study of single species 



