CHAPTER IX 



VARIATIONS IN THE NUMBERS OF ANIMALS 



The numbers of animals (i) never remain constant for very long, and 

 usually fluctuate considerably, and often rather regularly, e.g. (2) many 

 insects, (3) marine littoral animals, (4) protozoa in the soil, and elephants 

 in the tropical forests. (5) The primary cause of these fluctuations is 

 usually the unstable nature of the animals' environment, as is shown by 

 the effects of periodic bad winters on the numbers of certain small 

 birds. ((6) Fluctuations in numbers are of common occurrence among 

 birds, but their causes are not usually known.) (7) Further instances 

 of the irregular nature of the environment are cyclones in the tropics, 

 and droughts affecting small ponds, but (8) such factors are not neces- 

 sarily destructive, and may favour sudden increase of the animals. 

 (9, 10) Some of the best data about fluctuations in numbers are from 

 mammals, e.g. the lemming, whose fluctuations are partly controlled 

 by climate and partly by periodic epidemics, (11) the latter occurring 

 in a number of other small rodents, such as mice, (12) rabbits and hares, 

 (13) marmots and muskrats ; but (14) the beaver forms an interesting 

 exception to this rule. (15) These fluctuations in numbers affect the 

 other animals dependent upon the rodents, and since (16) the length 

 of the period of fluctuation depends chiefly on the sizes of the rodents 

 concerned, the final effects upon animal communities are very complex. 

 (17) Wild ungulates, at least in some cases, also have periodic epidemics. 

 (18, 19) The occurrence of " plagues " of animals results from the 

 structure of animal communities, and from the irregularities of the 

 environment, and (20) especially from the irregularities occurring over 

 wide areas, at the same time. (21) Among other results of these periodic 

 changes in numbers are changed food habits, since (22) food preferences 

 depend both on the quality and the quantity of the food. (23) The 

 other habits of a species may also change with the variations in density 

 of its numbers. 



I . So far we have been speaking as if the numbers of animals 

 remained fairly constant. We have been describing the general 

 mechanisms which assist in bringing about the optimum 

 density of numbers for each species. In the present chapter 

 we shall point out that practically no animal population remains 

 the same for any great length of time, and that the numbers of 

 most species are subject to violent fluctuations. The occa- 

 sional " plagues " of animals already referred to are extreme 



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