THE ANIMAL COMMUNITY 63 



size problem, since they form organised colonies whose size 

 is entirely fluid according to circumstances. Schweitzer ^8 

 noted a column of driver ants in Angola march past for thirty- 

 six hours. They are able by the mass action of their terrible 

 battalions to destroy animals many times their own size 

 {e.g. whole litters of the hunting dog ^"^), and at the same 

 time can carry the smallest of foods. 



It must be remembered, therefore, that the idea of food- 

 chains of animals of progressively larger size is only true in a 

 general way, and that there are a number of exceptions. 

 Having considered the far-reaching effects of size on the organi- 

 sation of animal communities, we are now in a position to 

 consider the subject of 



Niches 



15. It should be pretty clear by now that although the 

 actual species of animals are different in different habitats, 

 the ground plan of every animal community is much the same. 

 In every community we should find herbivorous and carni- 

 vorous and scavenging animals. We can go further than this, 

 however : in every kind of wood in England we should find 

 some species of aphid, preyed upon by some species of lady- 

 bird. Many of the latter live exclusively on aphids. That is 

 why they make such good controllers of aphid plagues in 

 orchards. When they have eaten all the pest insects they just 

 die of starvation, instead of turning their attention to some 

 other species of animal, as so many carnivores do under similar 

 circumstances. There are many animals which have equally 

 well-defined food habits. A fox carries on the very definite 

 business of killing and eating rabbits and mice and some kinds 

 of birds. The beetles of the genus Stenus pursue and catch 

 springtails (Collembola) by means of their extensile tongues. 

 Lions feed on large ungulates — in many places almost entirely 

 zebras. Instances could be multiplied indefinitely. It is 

 therefore convenient to have some term to describe the status 

 of an animal in its community, to indicate what it is doing 

 and not merely what it looks like, and the term used is '* niche." 

 Animals have all manner of external factors acting upon them — 



