VARIATIONS IN THE NUMBERS OF ANIMALS 143 



large areas, so that migration of enemies hardly takes place 

 at all. 



Although we have been taking mammals and birds as 

 examples, since more is known about them and they are more 

 familiar to most people, the principle outlined above applies 

 universally in animal communities and is probably the ex- 

 planation of most sudden plagues of animals. It results from 

 the variable nature of the environment on the one hand, and 

 the food- cycle mechanism of animal communities on the other. 



Having shown the fact of fluctuations in numbers among 

 many animals, and having discussed the causes of such fluctua- 

 tions, we may next turn to a consideration of some of the effects 

 which these fluctuations have upon animals in nature. 



21. There are a number of curious and interesting conse- 

 quences, of which we can only describe a few ; but it will 

 become clear that variations in numbers play a very big part 

 in the ecology of animals. 



For instance, the food habits of many species depend not 

 only upon the quality but on the quantity of their prey — a 

 fact which is often lost sight of in experiments upon food 

 preferences. As was pointed out earlier, the lower limit to 

 the size of an animal's food is partly determined by the ease 

 with which the latter can be caught ; and this in turn is tre- 

 mendously affected by its abundance ; for if the prey is too 

 scarce it takes too long to collect enough to satisfy the animal's 

 needs. Time is a vital factor in the lives of most animals, 

 and especially of carnivores. The animal must secure a certain 

 amount of food every day or month or year, in order to survive 

 and breed successfully. The seriousness of this factor varies 

 in different species and at different times and places, but it is 

 nearly always present during some part of the animal's life. 

 It is said that the sheep in Tibet have to feed at a run, since the 

 blades of grass are so scattered that only by being very active 

 and energetic can the sheep get enough in a given time to sup- 

 port life.^^^ With temperature-regulated animals the problem 

 is of course more urgent, since they cannot usually allow the 

 body-fuel to run too low, while with cold-blooded animals it 

 is more a question of accumulating reserves of food big enough 



