THE NUMBERS OF ANIMALS 123 



as his example the goshawk {Accipiter atricapillus) in Canada, 

 which preys ahernately on the varying hare (Lepus americanus) 

 and upon grouse, according as one or the other is more 

 abundant. In this way, whenever one species becomes for any 

 reason scarce, the goshawk tends to eat more of the other and so 

 allows the first one a certain amount of respite.^^ This switch 

 arrangement is common enough in animal communities, and 

 is probably an important factor in preventing the complete 

 extermination of animals which happen for any reason to be 

 at a rather low ebb of numbers {e.g. after an epidemic). In just 

 the same way, the red fox in Canada preys on mice or varying 

 hares according to their relative abundance. 



33. It will already have occurred to the reader that the 

 animals which are at the end of food-chains — at the top of the 

 pyramids — are in a peculiar position, since they have no 

 further carnivorous animals present which might control their 

 numbers, although they have of course parasites. These 

 animals have in many cases evolved rather curious methods of 

 regulating their numbers, of which we can only mention a few 

 here. The Emperor Penguin is a large bird which forms the 

 end of a long chain of marine animals (it appears to live chiefly 

 on animals like fish and squids), and breeds in the heart of the 

 very cold Antarctic winter. Since it has no serious enemies 

 to control its numbers (nothing is known as to whether it has 

 epidemics), it seems to depend chiefly on climatic factors to 

 bring this about : or rather we should say that the only checks 

 against which it has to produce extra numbers are climatic 

 ones. One important thing is the cold, since the birds attempt 

 to incubate their eggs and hatch their young at a temperature 

 ranging below —70° F. and in severe blizzards. They are 

 also destroyed by avalanches of snow, which cover them and 

 cause desertion and freezing of the eggs. (Some birds were 

 seen attempting to hatch out pieces of ice, which they had 

 mistaken for their eggs.^-^) In other cases, animals at the end 

 of food-chains may control their numbers by not breeding 

 at all in some years. This appears to happen with the snowy 

 owl, and probably with skuas, in certain years when food 

 (especially lemmings) is scarcer than usual. Or again, the 



