HABITATS 37 



form habitats all over the surface of the earth. 

 Vegetation has two main ejBFects (apart from food and 

 shelter to animals). It modifies the natural chmatic 

 and soil conditions and to a certain extent smooths 

 out their temporal fluctuations. At the same time 

 the phenomenon of dominance, by which one or 

 two species of plants dominate the rest in the com- 

 petition for light and food, produces rather sharp 

 boundaries between different plant associations. This 

 produces comparatively sharp differences between 

 the environmental conditions, e.g., of temperature 

 and moisture in each habitat. Through the influence 

 of vegetation most of the earth is divided up into a 

 patchwork of habitats, each comparatively uniform 

 in conditions and each rather abruptly separated 

 from the next. This is usually what we mean when 

 we speak of ' major habitats ' : the area covered by 

 a particular vegetation type with its characteristic 

 dominant species, and corresponding association of 

 other subordinate species. When we speak of vege- 

 tation making conditions uniform, it should not be 

 forgotten that it also creates a variety of minor 

 habitats, partly through the variety of plant species 

 and partly through vertical layering, as in woods. 

 It is the general cUmatic variations that are toned 

 down. 



Another way in which vegetation affects animal 

 habitats is through ecological succession. Ecological 

 succession takes place also independently of vegeta- 

 tion, as when a river erodes its banks and lays down 

 sediment elsewhere, or when a sand dune advances 

 and replaces intertidal areas, or when the lime gets 

 gradually leached out of soil. But these physical 

 changes chiefly have the effect of creating new bare 

 areas on which the development of vegetation takes 

 place in a fairly orderly sequence which is character- 

 istic for any particular climate and soil and geo- 

 graphical region. This sequence of plant associations 

 is called a ' sere '. Through the effects of ecological 



