Origin of Biomes and Succession 277 



bogs, compared with the long geologic life of such types as the 

 tropical rain forest or the marine organic reef. 



Other long-term geomorphic events produce new denuded areas 

 which support simply the successional communities normal for the 

 area. Receding glaciers leave in their wake gravel and boulder beds 

 which are colonized by the same plants which colonize nearby 

 areas denuded by stream scouring or landslides, A retreating or 

 advancing ocean shore creates either new areas for open beach 

 communities or new areas for marine communities. In themselves 

 such actions do not provide entirely new types of habitats that set 

 the stage for the evolution of new types of life, as maintained by 

 Axelrod ( 1958 ) ; such actions simply contribute to local reversals of 

 the succession which is normal for the area. The chief difference 

 is that the geomorphic event would likely create much larger areas 

 for subclimax colonization than would local events. 



BIOTIC FACTORS 



In a number of communities the unusual increase of plant-feeding 

 species leads to sufficient local or regional modification of the vege- 

 tation that earlier stages of succession become established on the 

 area. One of the best examples is overgrazing by livestock in which 

 the climax grasses of the prairie areas may be cropped too severely 

 and perish; the entire community of climax plants and undoubtedly 

 the obligate climax animals would disappear with the dominants. 

 Subclimax communities then colonize the areas. Weaver (1954) 

 described the mechanics of this destruction of climax plants which 

 leads to greatly reduced shade on the soil surface and thus effects 

 a reversal of succession (Fig. 121). 



Destruction of vegetation likewise occurs naturally during grass- 

 hopper and locust outbreaks in both the Old and New Worlds. In 

 the short grass region of North America these insect outbreaks have 

 commonly reduced the grass cover to such an extent that wind 

 has been able to loosen and drift the soil, uprooting some plants 

 and burying others. These changes have resulted in large areas 

 becoming available for colonization by organisms belonging to the 

 earliest successional stage. 



It is almost certain that periodically through geologic time grazing 

 species of vertebrates have produced overgrazing with its conse- 

 quent reversal of succession just as commercial livestock do now. 

 Normal concentration of grazing around watering places would 

 produce small, almost continuously overgrazed local areas, and 

 any coincidence of abundant grazing populations with a drought- 



