CHAPTER VIII 



THE NUMBERS OF ANIMALS 



The subject of this chapter is an extremely important one, (i) but at present 

 only quite a small amount is known about it. (2) Most people do not 

 realise how immense are the numbers of wild animals now, and even 

 more in the past ; but (3) in some parts of the world there are still huge 

 numbers of the larger animals, which enable us to adjust our ideas on 

 the subject. Examples among birds are : (4) aquatic birds on the 

 White Nile, (5) guillemots in the Arctic, and (6) penguins in the Antarctic 

 regions ; while examples among mammals are : (7) zebras in Africa, 

 and, comparatively recently, bison in America, passenger pigeons in 

 America, whales and walruses in the Arctic, and tortoises on the Gala- 

 pagos Islands, and (8) the animals recently protected in various countries. 

 (9) These examples, from amongst birds and mammals, enable us to form 

 some idea of the colossal numbers of smaller animals (e.g. springtails) 

 everywhere. (10) Another way of realising the large numbers capable 

 of being reached by animals is to take cases of sudden and almost 

 unchecked increase, e.g. among mice, (11) springboks, (12) insects, 

 water-fleas, or protozoa, or (13, 14) epidemic parasites. (15) Such 

 "plagues" are not uncommon under natural conditions, but are 

 (16, 17) especially striking when animals are introduced by man into 

 new countries. (18) This enormous power of multiplication is not 

 usually given full rein, owing to the fact (19, 20) that every animal 

 tends to have a certain optimum density of numbers, which (21) depends 

 among other things upon the special adaptations of each species, and 

 which (22, 23) applies both to herbivores and carnivores. (24) The 

 existence of an optimum density of numbers leads us to inquire how the 

 numbers of animals are regulated. (25) The food-cycle structure of 

 animal communities forms one of the most important regulating 

 mechanisms, (26, 27, 28) enemies being more important than food-supply 

 as a direct limiting check on numbers of most animals ; while repro- 

 ductive limitation is also of great importance but is more or less fixed 

 by heredity, whereas the other checks are very variable. (29) We 

 lack, at present, the necessary precise data for working out the dynamics 

 of a whole community, and (30, 31) it is therefore not a simple matter 

 to predict the effects of variation in numbers of one species upon those 

 of other ones in the same community. (32) The existence of alternative 

 foods for carnivores has an important influence in maintaining balanced 

 numbers in a community. (33) Animals at the end of food-chains 

 (i.e. with no carnivorous enemies) employ special means of regulating 

 their numbers, e.g. (34) by having territory, (35) a system found widely 

 among birds, and (36) probably equally widely among manimals. 

 (37) Some carnivorous animals have means of getting food without 

 destroying their prey, and may therefore encourage increase instead of 

 limiting the numbers of the latter. 



I. In the two chapters which follow we shall point out some 

 of the more important things about the numbers of animals 



