8o ANIMAL ECOLOGY 



cycle provides a means of dispersal for parasites, a means 

 which is very commonly employed by them. The most 

 striking effects, however, are concerned with the numbers 

 of animals ; but this subject must be reserved for another 

 chapter. 



12. When the food- relations of parasites and carnivores 

 to other species are combined into a common food-cycle 

 scheme, the amazingly complex nature of animal interrela- 

 tions is seen. The diagram in Fig. 8 is intended to give some 

 idea of the sort of thing which is met with ; but it is to be under- 

 stood that the diagram is only illustrative and does not claim 

 to have anything but a general basis of truth, i.e. it does not 



I FLEA I > I PROTOZOA I 



I 

 „ I 



I GRASS I 



LAMB 



Placental 



» |WOLF[ 



*- ' |BrRDLICE] J^^ 



^■^ 1 TICK h » I STARLING \ ^- >{cH} 



I PFtPTOZOA I ^v 



ISPIROCHAETESI 



mm 



I WARBLE FLyI |PR0T0Z0A| 



I FLATWORM | | ROUNDWORM! 



sk 



I roundwormI 



Fig. 8. 



represent any particular community in real life, and ** all the 

 characters in the story are entirely fictitious." If there were 

 plenty of real examples to choose from, it would be much 

 better and more satisfactory ; but there are not. It is this 

 very fact — the lack of properly worked-out examples — ^which 

 makes it important to try and point out the type of problem 

 which requires solution. The diagram illustrates several 

 points. For one thing it shows that the food-chains do not 

 always go on from carnivore to carnivore, or parasite to para- 

 site. In this case the tick which is parasitic upon the sheep 

 is eaten by the starling, which is a carnivore, the latter in turn 

 by an animal larger than itself (the cat), and the cat in turn by 

 fleas, and the fleas by protozoa. In this way, when parasites 

 and carnivores are considered together it is seen that there may 



