72 ANIMAL ECOLOGY 



hawks, owls, foxes, and weasels on the one hand, and by lice, 

 fleas, ticks, mites, tapeworms, and protozoa on the other. 

 Small oligochaete worms in the arctic soil are eaten by purple 

 sandpipers and parasitised by protozoa, while earthworms 

 in England are eaten by moles and shrews and thrushes and 

 toads, and parasitised by protozoa and nematodes. In fact, 

 most animals have a set of carnivorous animals much larger 

 than themselves, and a set of parasitic enemies much smaller 

 than themselves, and usually there are very few enemies 

 of intermediate size. In all these cases we are, of course, 

 speaking of the size relative to their prey or host. There are 

 perfectly good reasons for this size- distribution. Most car- 

 nivores are able to overpower and eat their prey by virtue of 

 their larger size and greater strength (with exceptions noted in 

 the last chapter). On the other hand, parasites must necessarily 

 be much smaller than their hosts, since their existence depends, 

 either temporarily or permanently, upon the survival of the 

 host, and for this reason the parasite cannot exceed a certain 

 size without harming its host too much. 



2. It is very important to realise quite clearly that most 

 parasites are in their feeding habits doing essentially the same 

 thing as carnivores, except that while the carnivore destroys 

 its prey, the parasite does not do so, or at any rate does not do 

 so immediately or completely. A parasite's existence is usually 

 an elaborate compromise between extracting sufficient nourish- 

 ment to maintain and propagate itself, and not impairing too 

 much the vitality, or reducing the numbers of its host, which 

 is providing it with a home and a free ride. In consequence 

 of this compromise, a parasite usually destroys only small 

 portions of its host at a time, portions which can often be 

 replaced fairly quickly by regeneration of the tissues attacked. 

 Or it may exploit the energies of its host in more subtle ways, 

 as when it subsists on the food which the host has collected 

 with great expenditure of time and energy. The difference 

 between the methods of a carnivore and a parasite is simply 

 the difference between living upon capital and upon income ; 

 between the habits of the beaver, which cuts down a whole 

 tree a hundred years old, and the bark-beetle, which levies a 



