ii8 ANIMAL ECOLOGY 



general way, the food-cycle mechanism is in itself a fairly good 

 arrangement for regulating the numbers of animals, and it 

 works efficiently as long as the environment remains fairly 

 uniform, or at any rate as long as its periodic pulsations con- 

 tinue fairly steadily and regularly. If the balance of numbers 

 in a community is upset by some sudden and unusual occur- 

 rence, then the ordinary relations of carnivores and parasites 

 to their prey are no longer effective in controlling numbers, and 

 various results of a curious nature ensue. With the effects of 

 irregularities in the surroundings of animals we shall deal more 

 fully in the next chapter. We are here concerned chiefly with 

 the ways in which the general system of food-cycles and food- 

 chains in animal communities acts as a method of regulating 

 numbers. 



26. It is plain enough that the amount of food available 

 sets an ultimate limit to the increase of any animal ; but in 

 practice, starvation seldom acts as a direct check upon numbers, 

 although the possibility of it is always present. Instead we 

 find that other factors, such as enemies of all kinds, usually 

 keep numbers down well below the point which would bring 

 the population in sight of starvation. There appear to be 

 several good reasons for this. First of all, food, whether of 

 an animal or plant nature, is not always available ; or, what 

 comes to the same thing in the end, is not always increasing to 

 keep pace with the needs of the animals requiring it ; so that 

 the maximum numbers feasible for an animal at any moment 

 are not only determined by the food-supply at that moment, 

 but must be adjusted to the needs of the future. It would 

 be an unworkable system for animals to live all the time up to 

 the extreme limits of their food-supply, since no margin would 

 be left for the times of scarcity which are always liable to 

 occur. This can be well seen in the example of deer in 

 Arizona quoted previously, where increase during the summer 

 imperilled the food-supply for the following winter. It is one 

 of the most obvious ideas to all stock-farmers that the number 

 of cattle which can be kept on a given acreage is determined 

 by the margin of food left over for the winter (when plant 

 growth ceases), as well as by the immediate requirements of the 



