lO] ENVIRONMENTAL FACTORS 307 



which have a similar effect upon the vegetation. Thus the forest 

 generally tends to advance on the grassland with the removal of the 

 heavy pasturing which ordinarily keeps it in check. Commonly, 

 browsing cattle kill all the young trees, so preventing forest regenera- 

 tion, and by their trampling, grazing, and other activities lead to 

 the replacement of the characteristic forest-floor litter and vegeta- 

 tion by Grasses (Fig. 86). For the Grasses are hemicryptophytes, 

 having their buds protected within (or at least lying at) the surface 

 of the soil, and far from being killed by grazing, may actually be 

 stimulated by it. Certainly they are encouraged by the removal of 

 their broad-leafed and woody competitors, so that they will normally 

 extend their area if the factor or factors suppressing these competitors 

 are increased in intensity. Overgrazing may, however, lead to 

 replacement of the Grasses by unpalatable herbs such as the weeds 

 of open soils — perhaps followed bv erosion if the rainfall is heavy, 

 so that a barren waste may result (Fig. 87). 



These last biotic effects are commonly regulated by Man and tend 

 to be destructive, often getting far out of hand, as in bad cases of 

 erosion. However, animals are still often helpful to vegetation — 

 for example in distributing seeds, fruits, and spores, etc., in effecting 

 pollination, in loosening or compacting soil as well as in manuring 

 it, in trampling seeds into the soil (for example, of range Grasses) 

 and favouring regeneration, in keeping down injurious Rodents etc. 

 (in the case of carnivorous Mammals and predatory Birds), and, 

 in the instance of Man, in such activities as irrigation, the construc- 

 tion of wind-breaks, soil-improvement and cultivation of many sorts, 

 and the transplantation of useful plants and even the creation of 

 new ones. 



Human activity is, indeed, the outstanding biotic factor in the 

 world today, at least if we include consideration of Man's domestic 

 animals. Especially is Man the enemy of forests, whether he 

 realizes it or not. His ' shifting cultivation ' in the tropics is 

 particularly damaging to vegetation, trees being girdled and the 

 forest burnt, after which the accumulated fertility of the soil is 

 exhausted in a very few years by cultivation ; thereupon a move 

 is made, similarly to cultivate and desecrate a fresh area. Con- 

 sequently some secondary and usually inferior type of forest now 

 replaces the original one over vast areas. In other cases still more 

 devastating erosion may result from forest clearance as indicated 

 above, from over-pasturing, or from ' exsiccation ' leading to exten- 

 sion of desert areas. In such instances of penetrating and serious 



