THE NUMBERS OF ANIMALS 121 



keeps down C, while A also preys on C, what is the exact 

 effect of A upon C ? Two examples will show the sort of way 

 in which this process works. 



30. For some years the great bearded seal or storkobbe 

 {Erignathus harhatus) has been very intensively hunted and 

 killed by Norwegians who go up every summer into the outer 

 fringes of the ice-pack round about Spitsbergen. They seek 

 the seals for the sake of their skins and blubber. The serious 

 toll taken of their numbers can be gauged by the fact that one 

 small sealing-sloop may bring back five thousand skins and 

 sometimes many more in the course of a single summer. In 

 spite of this steady drain on the numbers of seals the animals 

 are, if an)rthing, more abundant than ever. This appears to 

 be due to the fact that the Norwegian sealers also hunt and 

 kill large numbers of polar bears, whose staple article of diet 

 is the bearded seal, which they stalk and kill as they lie out on 

 the pack-ice. By reducing the numbers of bears the sealers 

 make up for their destruction of seals, since there are so many 

 extra seals which would otherwise have been eaten by bears. 

 The diagram in Fig. 9 sums up the situation which has just 



3£AL )-BEAR > 



I 



MAN 



Fig. 9. 



been described. In this case we know the results of man's 

 interference, but they might very well have been different. 

 For instance, if fewer bears had been killed, the seal numbers 

 might have gone down considerably. On the other hand, if 

 the same number of bears had been killed and more seals 

 destroyed by the Norwegians, the seal numbers might also 

 have gone down. The final result, as far as seals are con- 

 cerned, depends entirely upon the relative numbers and de- 

 structive powers of the species concerned. In this case a 

 balance happens to have been struck. 



31. The second example illustrates the same point. The 

 tsetse fly {Glossina palpalis) is preyed on in part of the Lake 

 Victoria district by a small species of dragonfly {Cacergates 

 leucosticta), and the latter is preyed on in turn by a larger 



