122 ANIMAL ECOLOGY 



dragonfly. Both dragon-flies are eaten by various species of 

 bee-eaters {Merops and Melittophagus)A^ Problem : what is 

 the effect of the bee-eaters upon the numbers of tsetse flies ? 

 The diagram in Fig. lo sums up the food-relations which we 



1 ^ 



TSETSE > DRAGONFLY >DRAGONFLy >BEE-EATER 



Fig. io. 



have described. It is clear that in this case we cannot say at 

 a glance whether the birds are having a beneficial influence 

 by helping to reduce tsetse flies, or the reverse. The result 

 depends entirely upon the relative numbers of the species 

 concerned in the matter, and upon a number of other things, 

 such as the food preferences of the bee-eaters, the number of 

 individuals eaten in a given time by the dragonflies, the rate 

 of increase of the different species, and so on. But the example 

 does show that each species will have some effect upon the 

 numbers of the others, even though we cannot precisely define 

 it without further investigation. In fact, no species in a 

 community, unless it happens to live a very isolated life or be 

 very rare, is without its effect upon numbers of the rest of the 

 community, and that is why it is practically hopeless to reach 

 any complete knowledge of the natural methods of regulation 

 of numbers of an animal without doing a general biological 

 survey, backed up later by some investigation of the food- 

 cycle. 



32. It might be thought that there would be some danger 

 of enemies doing too much in the way of controlling the 

 numbers of their prey, so that the carnivore would run a 

 risk of eating out its food-supply, and the prey of being 

 exterminated or reduced below its lower limit of safety. There 

 is, however, a natural method by which such a contingency is 

 usually avoided, depending upon the fact that most carnivores 

 do not confine themselves rigidly to one kind of prey ; so that 

 when their food of the moment becomes scarcer than a certain 

 amount, the enemy no longer finds it worth while to pursue 

 this particular one and turns its attention to some other species 

 instead. This process was pointed out by Hewitt, who gave 



