RELATIONS BETWEEN PLANTS AND ENVIRONMENT 75 



vegetation temporarily. Succession may follow the disappearance of 

 the water. 



Biotic Causes. Under some conditions plants destroy themselves 

 and their specialized habitat, as when they create conditions favoring 

 the invasion of their home by other species. Animals, particularly 

 man, do much damage, often exiling themselves from an area or 

 region by destroying their own food supply. Ants create bare spots, 

 but their contribution to soil formation makes the damage picayune. 

 Insect plagues can and do destroy all annual vegetation and its seed 

 from areas of various sizes, so that invasion is necessary to replace 

 these annuals. The persistent roots and underground stems of peren- 

 nials may survive. Plant-eating animals confined by man or by natural 

 boundaries to a limited space will, if crowded, denude the landscape. 

 Elk, snowbound in small valleys, may do this. Hogs will make a 

 desert of a small, fenced lot in short order. Prairie-dog towns may 

 present extensive bare areas to view. Beavers build dams and flood 

 small areas, destroying the land vegetation by submersion, and creat- 

 ing a pond temporarily barren, barren at least in comparison to the 

 rich variety of life it may later contain. 



Man's principal denuding activities are clearing (whether by ax 

 in the forest or plow on the plains) and burning (whether deliberate 

 or accidental). Lumbering, even the clear cutting so opposed by sci- 

 entific foresters, need not destroy the climax, unless followed by fire, 

 overgrazing, or erosion. If the operation is conducted with considera- 

 tion for the seedlings and saplings, the climax is retained. 



Succession does not occur unless the climax species are destroyed 

 and the environment set back one or more phases. Some ecologists 

 insist that true succession begins only with complete denudation of an 

 area, and the entrance of pioneer plants from surrounding areas. A 

 severe forest fire followed by erosion usually causes this. A light 

 ground fire usually does not. Even the spectacular crown fire does 

 not necessarily do it in all instances, unless joined by equally severe 

 ground fire. The rapid rise of heat from a crown fire may leave the 

 soil level comparatively cool. 



The preparation of a farm field for seeding usually involves the 

 complete destruction of all original vegetation. Certain fumes and 

 gases from smelters, factories, coke ovens, or smouldering coal mine 

 gob piles may result in killing or maiming all vegetation within radii 

 up to several miles. Strip mining creates not only bare areas, but 

 turns over the soil to great depths and leaves a rugged terrain. Dredg- 

 ing and draining create barrens. Canals and ditches, constructed 

 ponds and lakes, create aquatic barrens. 



This Changing World. Bare areas as listed in this chapter, are 

 constantly being created on the world landscape. Wherever climate 

 is favorable, migration or invasion begins at once. The outdoor air 

 is never without many forms of life spores, bacteria, and seeds. 

 These are carried by wind. They are carried, too, by water, birds, 

 mammals, man. Some plant species have migrated across oceans and 



