Il] MAIN HABITATS, SUCCESSIONS, AND CLIMAXES 315 



when they are prepared for, and further altered by, various forms of 

 husbandry. Here all manner of changes in climatic and edaphic 

 conditions from place to place give rise to a great variety of different 

 habitats — usually supporting different types of forest if undis- 

 turbed, or of arable or pasture land if used for agriculture. More- 

 over, in these agriculturally favourable regions more than most others, 

 Man is constantly changing old habitats or opening up new ones. 

 There is no need to dwell upon examples, which are familiar in 

 almost all areas ranging from the Subarctic to the tropics. With 

 drastic reduction in rainfall, the resulting habitats favour diverse 

 forms of ' parkland ', scrub, heath, or grassland vegetation. 



In truly arctic regions various types of treeless ' tundra ' and 

 ' barrens ' take the place of forests, etc., in most of the flatter areas, 

 with a tendency for less and less of the vegetation to be continuous 

 over the surface {i.e. ' closed ') as we go farther north. The growing- 

 season is greatly reduced and frost-heaving and allied influences are 

 active and, frequently, disruptive. These conditions are partly, but 

 by no means wholly, simulated on mountains elsewhere — in general, 

 at higher and higher levels as we travel towards the Equator — so 

 that even in the tropics we may get, at very high altitudes, whole 

 series of habitats and attendant communities reminiscent of those 

 of the Arctic {cf. Fig. 137, B, and Fig. 138). 



Concavities and convexities in the general surface of the earth 

 may lead to all manner of local variations — including marked changes 

 in water conditions — resulting in what amounts to a wide range of 

 different habitats. So may local cliffs and talus or gravel slides or 

 wave-washed banks, blow-out or wash-out or other erosion effects, 

 and many other types of surface phenomena, profoundlv affect con- 

 ditions locally. In arctic and alpine regions, features leading to 

 deep drifting of snow in winter {see Chapter XIII) are also very 

 important in changing local conditions and resulting in whole series 

 of special habitats. Here the dynamic or other inimical forces of 

 nature may prevent vegetation from taking a hold and changing the 

 habitat and whole aspect as it usually does elsewhere. In general, 

 however, vegetation rings many changes before coming to a state 

 of relative equilibrium with the environment, by which time various 

 factors of the habitat have usually become altered quite drastically. 

 The connection with landscapes is treated in Chapter XVII. Change 

 in habitat conditions through the activity of vegetation is particularly 

 marked in forested areas ; but it may also be considerable where the 

 vegetation is less prolific, and e\en where it is entirely dwarfed. It 



