8 ANIMAL ECOLOGY 



subject which will be followed up separately in the next 

 chapter. 



4. One way of giving some idea of the range of different 

 animal habitats, and of the communities living in them, is to 

 take some of the big gradients in environment and show how 

 the communities change as we pass from one end to the other. 

 The biggest of these is the gradient in temperature and light 

 intensity between the poles and the equator, which owes its 

 existence to the globular shape of the world. At the one 

 extreme there are the regions of polar ice-pack, with their 

 peculiar animal communities living in continuous dayhght 

 during the summer and continuous darkness in the winter, and 

 with corresponding abrupt seasonal changes in temperature. 

 At the other end of the scale we have equatorial rain forest 

 with a totally different set of animals adapted to life in a 

 continuously hot climate, and in many cases in continuous 

 semi- darkness, in the shade of the tropical trees. An animal 

 like Bosman's Potto sees less light throughout the year than the 

 Arctic fox. In between these extremes we have animal com- 

 munities accustomed to a moderate amount of heat and light. 



5. These examples serve to introduce a very important 

 idea, namely, the effect of vegetation upon the habitats and 

 distribution of animals. 



Although a tropical rain forest partly owes its existence to 

 the intense sunlight of the tropics, yet inside the forest it is 

 quite dark, and it is clear that plants have the effect of trans- 

 lating one climate into another, and that an animal living in 

 or under a plant community and dependent on it, may be 

 living under totally different climatic conditions from those 

 existing outside. Each plant association therefore carries 

 with it, or rather in it, a special local " climate " which is 

 peculiar to itself. Broadly speaking, plants have a blanketing 

 effect, since they cut off rain, and radiant energy like light 

 and heat. Their general effect is therefore to tone down the 

 intensity of any natural climate. At the same time they 

 reduce the amount of evaporation from the soil surface, and 

 so make the air damper than it otherwise would be. 



Looking at the matter very broadly, in the far north there 



