ECOLOGICAL SUCCESSION 21 



colonised by mosses or algae or lichens ; these are driven out 

 by low herbs, which kill the pioneer mosses by their shade ; 

 these again may be followed by a shrub stage ; and finally 

 a woodland community is formed, with some of the earlier 

 pioneers still living in the shade of the trees. 



This woodland may form a comparatively stable phase, and 

 is then called a climax association, or it may give way to one or 

 more further forest stages dominated by different species of 

 trees in the manner described above. It is not really possible 

 to separate development of communities from succession 

 caused by extrinsic changes, such as the gradual leaching out 

 of salts from the soil or other such factors unconnected with 

 the plants themselves. The important idea to grasp is that 

 plants react on their surroundings and in many cases drive 

 themselves out. In the early stages of colonisation of bare 

 areas the succession is to a large extent a matter of the time 

 taken for the different plants to get there and grow up ; for 

 obviously mosses can colonise more quickly than trees. 



5. In any one region the kind of climax reached depends 

 primarily upon the climate. In high Arctic regions succession 

 may never get beyond a closed association of lichens, contain- 

 ing no animals whatsoever. In milder Arctic regions a low 

 shrub climax is attained, while farther south the natural 

 climax is forest or in some cases heath, according to whether 

 the climate is of a continental or an oceanic type. Sometimes 

 ecological succession is held up by other agencies than climate 

 and prevented from reaching its natural climax. In such 

 cases it is a common custom to refer to the stage at which it 

 stops as a sub-climax. A great deal of grassland and heath 

 comes under this heading, for further development is pre- 

 vented by grazing animals, which destroy the seedlings of the 

 stage next in succession. An area of typical heather moor 

 in the New Forest was fenced off for several years from grazing 

 ponies and cattle by its owner, with the immediate result 

 that birches and pines appeared by natural colonisation, and 

 the young pines, although slower in growth than the birches, 

 will ultimately replace them and form a pine wood. Here 

 grazing was the sole factor preventing normal ecological 



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