BIOTIC CAUSES. 59 



and not an artificial one. The simplest and most convenient arrangement is 

 one based upon agents and kinds of activity (Clements 1904:116; 1905:249; 

 1907 : 279) , but this is not in fundamental relation to successional development. 



Bare areas due to destruction of vegetation alone. — The primary activities 

 by which man produces denuded areas are burns and clearings. Clearings 

 result for the most part from lumbering or from cultivation, though a host of 

 minor activities have the same result. Ant areas in arid regions are perhaps 

 the best examples of clearing by animals without soil disturbance. In all cases 

 of burning and clearing the intensity or thoroughness of the process determines 

 whether the result will be a change of vegetation or the initiation of a sere. 

 The latter occurs only when the destruction of the vegetation is complete, 

 or so nearly complete that the pioneers dominate the area. Lumbering con- 

 sequently does not initiate succession except when it is followed by fire or 

 other process which removes the undergroivth. Most fires in woodland 

 denude the burned area completely, but surface fires and top fires merely 

 destroy a part of the population. Fires in grassland practically never produce 

 bare areas for colonization. Poisonous gases from smelters, factories, etc., 

 sometimes result in complete denudation, though the action is chiefiy felt in 

 a change of vegetation. Cultivation normally results in complete destruction 

 of the original vegetation. In the broadest sense, a new sere starts with the 

 sowing or planting of the crop. In the case of annual crops, however, real 

 development begins only when cultivation is abandoned. In new or sparsely 

 settled grassland regions, the wearing of roads or trails^ results in a character- 

 istic denudation with Uttle or no soil disturbance. Complete denudation 

 by animals is only of the rarest occurrence, except where they are restricted 

 to limited areas by man. Even in striking cases of the destruction of a forest 

 by parasites, such as the repeated defoliation of aspens by caterpillars, the 

 undergrowth is little affected. Complete destruction by parasites usually 

 occurs only in the case of annual crops. A striking example of denudation by 

 a plant parasite was found on the shores of False Bay in southern California, 

 and especially on the dunes of Medanos Spit. Here families and colonies of 

 Abronia umbellata and Franseria bipinnatifida were completely covered with an 

 orange mat of Cuscuta salina. The dodder in May had already killed many of 

 the f amihes entirely, and it was obvious that many more would suffer the same 

 fate. With the gradual death of the hosts, the dodder became brown and dried 

 up with the host plants. The two were then gradually blown away by the 

 constant onshore winds and a bare sand area was left (plate 10 c). 



Bare areas with dry or drier soils. — ^These occur chiefly where there is a 

 marked disturbance of the soil. The latter affects the water-content by chang- 

 ing the texture, by changing the kind of soil, as from clay to sand or gravel, 

 or by both methods. These results may be produced by removal, by deposit, 

 or by the stirring of the soil in place. In the case of man they are produced 

 by the widest variety of construction and engineering processes, with roads 

 and railroads as universal examples. The removal and deposit of soil by 

 animals is confined to the immediate neighborhood of the burrows of rodents, 

 the homes of ants, etc. In some cases, such as densely populated prairie-dog 

 towns, the burrows are sufficiently close to produce an almost completely 

 denuded area. Insignificant as most areas of this sort are, they give rise to 

 real though minute seres of much value in communities otherwise little dis- 

 turbed (plates 2 b, 10 a). 



