10 ANIMAL ECOLOGY 



of optimum conditions for maximum growth, and as con- 

 ditions depart from this optimum, growth becomes less efficient. 

 Since each dominant has different optimum conditions, there 

 is always a certain point in an environmental gradient 

 where one dominant, and therefore one community, changes 

 over fairly abruptly into another. There may be originally 

 a regular and gradual gradient in, say, water-content of the 

 soil, as from the edge of a lake up on to a dry moor; but the 

 existence of dominance in plants causes this to be transformed 

 into a series of sharply marked zones of vegetation, which to 

 some extent mask the original gradient, and may even react 

 on the surroundings so as to convert the conditions themselves 

 into a step-like series. 



7. It is clear, then, that because green plants feed by means 

 of sunlight, the boundaries of their communities tend to be 

 rather sharply defined ; and since we have seen that each plant 

 community carries with it a special set of " climatic " con- 

 ditions for the animals living in it, the rather sudden difference 

 in conditions at the edges of the plant communities will be 

 reflected in the animals. This means that the species of animals 

 will tend to be subdivided into separate ones adapted to different 

 plant zones, instead of graded series showing no sudden 

 differences. It also means that animal communities are made 

 much more distinct from one another than would be the case 

 if they were all living in one continuous gradient in conditions, 

 or in a series of open associations of plants like arctic fjaeldmark 

 (stony desert). It would be infinitely more difficult to study 

 animal associations if this were not the case, for we should 

 not have those convenient divisions of the whole fauna into 

 communities which are so useful for working purposes. It is 

 sometimes assumed in discussions on the origin of species 

 that the environmental conditions affecting animals are always 

 in the form of gradients. It is clear that such is by no means 

 always the case. 



8. As has been mentioned above, the abrupt transitions 

 between plant communities are particularly well seen on the 

 sides of mountains, where there are vertical zones of vegetation 

 corresponding on a small scale to the big zones of latitude. 



