272 PLANT SOCIOLOGY 



climax communities; it may cause profound disturbances and, in 

 extreme cases, the destruction of vegetation. It is consequently of 

 major importance to investigate the influence of this easily controllable 

 factor in order to reconcile economic interests and exigencies with the 

 duty incumbent upon citizens of maintaining intact the order of nature. 

 Causes of injury are: 



1. The withdrawal of large quantities of plant matter. 



2. Mechanical harm to plants by grazing, gnawing, brushing against, and 

 trampling. 



3. Selective destruction by pasturing animals. The desired plants are at a 

 disadvantage, or entirely destroyed, while the undesirable ones increase 

 abundantly. 



4. Disappearance of dung-avoiding species from much frequented feeding 

 places, and introduction of strongly nitrophilous communities (typical vegetation 

 of cattle yards). 



5. Direct effect upon soil formation by stirring of the fine earth and changing 

 the microrelief (hummocks and paths). 



The almost treeless plains of the Dark Continent are still inhabited 

 by herds of wild game which number tens of thousands. One can 

 hardly estimate the vast effect which such herds exert upon the original 

 vegetation. Large quantities of dust are raised by hoofed animals so 

 that the top soil is robbed of its finer particles. Before the advent of 

 man the European steppes and grasslands must have been overrun from 

 time to time by huge herds of game, for only thus can we interpret 

 the great Tertiary fossil beds of Pikermi near Athens and of the Mont 

 Luberon in Provence. 



Nowadays the wild grazing quadrupeds are everywhere replaced by 

 more or less domesticated herds of cattle. But the influence of the 

 latter far exceeds that of the former, especially as it is continually 

 augmented by man through the cutting of trees and bushes which 

 interfere with pasture. 



Intensive pasturing leads directly to a selective reduction in the 

 number of species and also to the breaking of sod and the gradual 

 erosion of the soil. Extended biotic grasslands due to excessive 

 pasturing have sometimes arisen in the midst of a wooded region, and 

 the bare, plantless soil appears where, within the memory of living men, 

 trees have stood (Fig. 139). 



Arid regions, subjected to excessive pasturing, have even assumed a 

 desertlike character, as shown by vast expanses of north and south 

 Africa, Australia, and western Asia. 



The ancient centers of civilization on the shores of the ]\Iedi- 

 terranean Sea and in the Orient owe their most striking types of vegeta- 

 tion to excessive pasturing. The Spanish tomillares, the southern 



