130 ANIMAL ECOLOGY 



Rothamsted shows that there are similar fluctuations in 

 numbers among these animals, but upon a much smaller scale. 

 These variations are of the order of two days instead of two 

 years, but the principle is the same.^^^ If we go to the 

 other extreme it can be shown that animals as big as the 

 Indian elephant are also subject to fluctuations in numbers, 

 caused by epidemics ; but at very long intervals, of seventy to 

 a hundred years. ^i^' i^*^ Whenever a group of animals or any one 

 species is studied carefully over a series of years, it is found to 

 vary in numbers in a more or less marked way. There is not 

 space here to give all the evidence for this statement, but it 

 seems to be true in practically all cases where there are any 

 accurate data. Even when a species does not vary very much 

 from year to year, it has in the vast majority of cases a marked 

 \^ariation within each year, caused by its cycle of reproduction. 

 Every year there is an annual increase in numbers from com- 

 paratively few individuals, among such animals as protozoa, 

 rotifers and water-fleas, many of which possess some well- 

 defined means of increasing rapidly — by parthenogenesis, for 

 instance. In fact, the numbers of very few animals remain 

 constant for any great length of time, and our ideas of the 

 workings of an animal community must therefore be adjusted 

 to include this fact. It is natural to inquire why the numbers 

 vary so much, and why, with all the delicate regulating 

 mechanisms described in the last chapter, the community does 

 not more successfully retain its balance of numbers. 



5. The chief cause of fluctuations in animal numbers is 

 the instability of the environment. The climate of most 

 countries is always varying, in some cases regularly — as in the 

 case of the eleven-year cycles in temperature and the frequency 

 of tropical cyclones associated with the sun-spot cycle ; or 

 of snowfall in Norway, which has a very marked short perio- 

 dicity of three or four years .23, 24 Qn the other hand, there 

 are a number of irregular and so far unpredictable cycles, such 

 as that of rainfall in England. The variations in climate 

 affect animals and plants enormously, and since these latter 

 are in intimate contact with other species, there are produced 

 further disturbances which may radiate outwards to a great 



