120 ANIMAL ECOLOGY 



is usually also kept down in numbers by factors which aflPect 

 breeding, e.g. lack of breeding-sites, etc. It is further controlled 

 by its enemies, and if these fail, by starvation. But the latter 

 condition is seldom reached. There are a few species which 

 seem to regulate their numbers almost entirely by limiting re- 

 production, although they belong to groups which are normally 

 controlled by carnivores. There is a species of desert mouse 

 (Dipodomys merriani) which only has two young per year, 

 that is to say, probably very few more than would be necessary 

 to replace deaths in the population caused by old age or 

 accident.-^^^ This, however, is unusual except in the case of 

 animals at the end of a food-chain, with which we shall deal 

 later. 



It has been necessary to speak in generalities, since so little 

 is known at present about the rules governing the regulation 

 of animal numbers. There are, however, a number of special 

 separate phenomena which we shall pick out, and which will 

 serve to illustrate the importance of animal interrelations, and 

 the study of animal communities along the lines of food-cycles. 



29. When we are dealing with a simple food-chain it is 

 clear enough that each animal to some extent controls the 

 numbers of the one below it. The arrangement we have 

 called the pyramid of numbers is a necessary consequence of 

 the relative sizes of the animals in the community. The smaller 

 species increase faster than the large ones, so that they produce 

 a sufficient margin upon which the latter subsist. These in 

 turn increase faster than the larger animals which prey upon 

 them, and which they help to support ; and so on, until a stage 

 is reached with no carnivorous enemy at all. Ultimately it 

 may be possible to work out the dynamics of this system in 

 terms of the amount of organic matter produced and consumed 

 and wasted in a given time, but at present we lack the accurate 

 data for such calculations, and must be content with a general 

 survey of the process. The effect of each stage in a food- 

 chain on its successor is easy to understand, but when we try 

 to estimate the effect of, say, the last species in the chain upon 

 the first, or upon some other species several stages away, the 

 matter becomes complicated. If A keeps down B, and B 



