298 GRAZING INDICATORS. 



This leads to greater storage in the propagative organs as well as to larger 

 seed-production. At the same time, the grazed species are correspondingly 

 handicapped in all these respects, and the gap between herbs and grasses, for 

 example, constantly widens. With the increase of the less palatable species, 

 especially when they are bushy, the grasses are further weakened by tramp- 

 ling. This soon produces small bare spots which are colonized by annual 

 weeds or weed-like plants. The latter set up a new and intense competition 

 with the grass survivors, and these are still further decreased as a result. The 

 weed areas widen, and sooner or later come to occupy most or all of the space 

 between the relict herbs or half-shrubs. Before this condition is reached, 

 however, the latter are brought into requisition for grazing and they then 

 begin to yield to the competition of the annuals. In the case of the severest 

 overgrazing, they too finally disappear, unless they are woody, wholly un- 

 palatable, as in Gutierrezia, or thoroughly protected by spines, as in Opuntia. 

 In the grassland climax, where the effects of overgrazing have been most 

 studied, it is possible to recognize three or four stages. The first is marked 

 by the decrease or disappearance of Stipa or Agropyrum, or of both of them, 

 and the corresponding increase of the short-grasses wherever these are associ- 

 ated; the second stage is characterized by the greater vigor and abundance 

 of the normal societies, as well as by the increased importance of some; the 

 third stage begins with the replacement of the grasses by annuals, while the 

 fourth is marked by the spread of annuals and of introduced weeds generally 

 over the area. Not all of these necessarily occur in the same spot, especially 

 when the process of overgrazing takes place rapidly. Destructive over- 

 grazing may result in a few years, or even in a single year, and in such in- 

 stances the native vegetation may disappear completely or nearly so. It is 

 replaced by a pioneer associes of native and introduced weeds, whose persist- 

 ence will depend upon the continuance of the disturbance. These four stages 

 indicate so many primary degrees of overgrazing, while minor degrees are 

 denoted by the dropping out of particular dominants or subdominants. Thus 

 in the mixed prairie, Stipa drops out before Agropyrum, because it is grazed 

 more heavily in spring, and Bouteloua disappears from the desert plains before 

 Aristida, owing to its greater palatability. Palatability is the chief factor in 

 determining the successive disappearance of species, and hence the indicators 

 of the corresponding degrees of overgrazing, though the sequence is often 

 disturbed by the vigor of certain dominants. Since there are few species 

 that are wholly unpalatable or inedible, it becomes possible to construct for 

 a particular community a complete sequence of indicators, reflecting each 

 appreciable degree in the process of overgrazing. In severe periods of drought, 

 overgrazing may reach the point where even the annuals are eat^n out and the 

 plant covering vanishes completely. This happens regularly in pastures, 

 corrals, and bedding-grounds where animals are kept in masses. It has 

 even been found in desert scrub and savannah where the effects of over- 

 grazing are supplemented by the work of kangaroo-rats (plate 74). 



Societies as indicators. The number of overgrazing indicators for the sev- 

 eral climaxes is legion, and it is possible to consider only the most widespread 

 and important. With the perennial grasses as a background, it is convenient 

 to distinguish several groups of such indicators, namely, herbs, subdominant 

 halfshrubs, cacti, serai annuals, introduced weeds, and shrubs. The first 



