OVERGRAZING. 301 



Acacia, Yucca, Quercus, and Adenostoma. They resemble each other in that 

 grazing gives them the advantage in competition with the grasses, partly by 

 decreasing the hold of the latter through eating and trampling, and partly 

 by disseminating the seeds and rendering their germination more certain. 

 This advantage is largely or completely lost in the case of browsing animals, 

 such as goats, since all of these are readily browsed, with the exception of Yucca 

 and Arctostaphylus. Species of Artemisia are the chief shrub indicators of 

 overgrazing in the mixed prairies, short-grass plains, and Agropyrum bunch- 

 grass prairie, though various dominants of the chaparral not infrequently assume 

 this role also. The most widespread and important is Artemisia tridentata, 

 while A. cana is perhaps the most common in the mixed prairies and A. 

 filifolia in sandy areas and sandhills. The lower forms, such as A. trifida, A. 

 arbuscula, A. rigida, and A. spinescens, might well be regarded as half shrubs. 

 They are more or less widely distributed, but their contact with grassland is 

 more local. In California, fragments of savannah composed of Artemisia 

 californica and Stipa indicate a similar relation between sagebrush and grass- 

 land. This appears to have been true formerly of Adenostoma as well, but the 

 observed contacts with Stipa grassland are as yet too few for certainty. In 

 the desert plains, Prosopis, often with Acacia or Celtis, is the characteristic 

 shrub indicator of overgrazing. It also extends northward in the short-grass 

 plains to southern Colorado and Kansas. It is perhaps the most typical of 

 all such indicators, owing to its height and the ready dissemination of its seeds 

 by cattle. Quercus virens, Q. breviloba, and Q. undulata, as well as other mem- 

 bers of the chaparral, take similar parts in the grassland of southwestern Texas 

 and adjacent New Mexico. The role of Yucca radiosa and macrocarpa as 

 indicators of overgrazing is somewhat less clear, but their constant occurrence 

 in the sandy grasslands of the Southwest and the connection between their 

 propagation and disturbance by cattle seem to leave little doubt of a similar 

 correlation (plate 77, b). 



Annuals as indicators. — Annuals are typically indicators of serious disturb- 

 ance, and hence serve to mark the existence of serious overgrazing when 

 abundant. They are the universal pioneers of secondary successions, and 

 they regularly disappear in the course of development. When the disturb- 

 ance is continuous or recurrent, they may persist for years, but their serai 

 nature is readily disclosed by protecting an area. In a few cases they become 

 suppressed by the perennials and continue as a dwarfed ground layer. In the 

 Southwest the winter rains permit a characteristic development of annuals, 

 which complete their growth and mature their seeds before the perennial 

 communities of the summer become controlling. Annuals usually first 

 appear in spots denuded by trampling and extend from these throughout the 

 community in proportion to the degree of overgrazing. Their mobility is 

 often very great and they may take more or less complete control of a badly 

 overgrazed range in a few years. Indications of varying degrees of over- 

 grazing are given by differences in species as well as in density and vigor. The 

 first annuals to appear are native species, or subruderals, which are given a 

 chance to spread or develop because of the trampling and overcropping of the 

 climax dominants. These often give way to more vigorous subruderals, or 

 they become mixed with introduced weeds or ruderals, and are sometimes 



