PARASITES 8i 



be a great many more stages in the history of the food than 

 when parasites alone or carnivores alone are considered. 

 In this example, in which mutton is converted, amongst other 

 things, into flea-protozoa, there are six stages involved, whereas 

 the simple carnivore chain from the sheep to the wolf has only 

 two stages, and the simple parasite chains only two or three 

 at the most. The reason for the greater number of stages 

 possible in the former case is simply that the size-relationships 

 of the animals make it possible for the chain to continue longer 

 without reaching either the upper or lower size limits of 

 animals. 



13. A great many ectoparasites have no very important 

 direct effects upon the food-cycle in general, since they are 

 eaten either by their hosts {e.g. birds and bird-lice) or else they 

 are eaten together with their host by the enemy of the latter 

 {e.g. copepods containing worm-larvae by fish). In such cases 

 the parasite and its host act as one unit for food purposes. 

 (Of course the circulation of the parasite in this way may 

 ultimately have very important effects upon the numbers of 

 animals in the community.) It is for this reason that 

 parasites can very often be ignored in practice, when one 

 is making out the first rough scheme of food-relations in 

 an animal community, although the control of numbers can 

 only be understood by bringing in the parasites too. For 

 instance, in the arctic tundra food-cycle shown in Fig. 4, 

 on p. 58, the parasites do not play a very important part, 

 and probably it is the exception rather than the rule for 

 parasites to form a large independent food-supply for any 

 other animal. There are, however, a number of ectopara- 

 sites {e.g. ticks) occurring upon mammals, which form an 

 important article of diet for certain species of birds. There 

 is one rather interesting instance of tliis sort of thing, recorded 

 by Wilkins ^4 from the antarctic regions. On Elephant 

 Island there is in summer a colony of nesting Gentoo Penguins 

 {Pygoscelis papiid) and haunting these colonies are a certain 

 number of birds called Paddies {Chionis alba). The Paddies 

 hve largely upon parasitic nematode worms which pass out 

 from the intestine of the penguins with their excreta. In 



G 



