i6 ANIMAL ECOLOGY 



are important (particularly temperature), but the salt content 

 itself is undoubtedly very important as a controlling factor, 

 since it acts not only directly but also by affecting the hydrogen 

 ion concentration of the water. 



13. Within each of the big zones which owe their existence 

 to major differences in climate there are numerous smaller 

 gradients in outer conditions, each of which gives rise to a 

 series of more or less well-marked associations of plants and 

 animals. These gradients are caused by local variations in 

 soil and climate, or by biotic factors such as grazing by animals. 

 One obvious example is the gradient in the amount of water in 

 the soil. At one end we may find the animal community of a 

 dry heather moor, and at the other the community of free- 

 floating and free-swimming animals which form the plankton 

 of a lake. Between these two extremes there would be zones 

 of marsh, reed swamp, and so on, each with a distinctive set of 

 animals. These various zones are due to the fact that at one 

 end of the gradient there is much soil and practically no water 

 (at any rate in summer), while at the other there is much water 

 and very little soil, the proportion of soil and water gradually 

 changing in between. 



14. We can carry the subdivision of animal communities 

 further and split up one ordinary plant association, like an oak 

 wood, into several animal habitats, e.g. tree-tops, tree-trunks, 

 lower vegetation, ground surface, and underground, and we 

 should find that each of these habitats contained an animal 

 community which could be treated to some extent at least as a 

 self-contained unit. Again, each species of plant has a number 

 of animals dependent upon it, and one way of studying the 

 ecology of the animals would be to take each plant separately 

 and work out its fauna. Finally, each animal may contain 

 within its own body a small fauna of parasites, and these again 

 can be split up into associations according to the part of the 

 body which they inhabit. If we examine the parasites of a 

 mouse, for instance, we find that the upper part of the in- 

 testine, the lower part, the caecum, the skin, the ears, each have 

 their peculiar fauna. 



It is obviously impossible to enumerate all the different 



