300 GRAZING INDICATORS. 



sandy areas or in sandhills. Eriogonum jamesii is even more frequent in a 

 similar role, though it is barely shrubby (plate 76). 



Gutierrezia is also the most important ndicator of overgrazing in the eastern 

 portion of the desert plains and in the Larrea-Flourensia scruk In western 

 Mexico and Arizona it is largely or completely replaced by Isocoma coronopi- 

 folia and its varieties, which are the characteristic indicators from the lower- 

 most Prosopis valleys upward into the Bouteloua-Aristida grassland. On the 

 Parkinsonia-Cereus bajadas and hills, Franseria deltoidea is the indicator on 

 lower slopes and Encelia farinosa, or more rarely Chrysoma laricifolia, on the 

 upper, while Franseria dumosa and, to a less extent, Hilaria rigida, play a some- 

 what similar role in the Larrea plains of western Arizona and adjacent Cali- 

 fornia. In the higher desert plains, Calliandra eriophylla and Eriogonum 

 wrightii largely replace Isocoma as the most important indicator, while 

 Baccharis wrightii is more local. Other halfshrubs that occur through the 

 desert scrub in varying importance are Zinnia pumila, Psilostrophe cooperi, 

 Krameria glandulosa, Bebbia juncea, and Hymenoclea salsola. While all of 

 the halfshrubs of the desert scrub and grassland are normally indicators of 

 overgrazing, they follow the rule in that practically every one is grazed to 

 some degree when more palatable forage is lacking. This is altogether 

 exceptional in the case of Gutierrezia, Isocoma, and Franseria, but all of these 

 were found to be grazed more or less during the severe drought of 1918. 



Cacti as indicators.— Cacti owe their value as indicators of overgrazing to 

 the protection afforded by their spines. Under ordinary conditions this is 

 almost complete protection, but during drought periods in the Southwest, 

 cattle in particular make much use of cacti and often keep alive upon them as 

 an exclusive diet. At such times they are utilized by jack-rabbits and pack- 

 rats also, and the work of these rodents frequently renders the prickly pears 

 and barrel cacti available for stock. The cacti which serve to indicate over- 

 grazing belong almost wholly to the genus Opuntia. The species with flat 

 joints are commonly known as prickly pears, and those with cylindric ones as 

 chollas. In the short-grass plains and mixed prairies Opuntia polyacantha 

 and 0. mesacantha are the chief indicators, while Opuntia arborescens is often 

 the most important species from the Arkansas Valley southward. Both owe 

 their abundance as much to the great ease of propagation as to their spiny 

 protection. In the case of the chollas especially, the joints a e readily broken 

 off and carried about by cattle, and in addition they are blown off by the wind. 

 Moreover, they are well adapted to ecesis in disturbed places, owing to their 

 succulence and the shallow root-system. In the Southwest the most import- 

 ant cactus indicators in the desert scrub and savannah are Opuntia fulgida, 

 0. f. mamillata, and 0. spinosior among the chollas, and 0. engelmannii, 0. 

 discata, and 0. phaeacantha among the prickly pears. All these extend up 

 into the grassland to some degree at least, but in the foothills the most com- 

 mon species are Opuntia versicolor, 0. arbuscula, 0. bigelovii, and 0. chlorotica. 

 Nolina, Dasylirium, and Agave resemble the cacti more or less in indicator 

 value (plate 77, a). 



Shrubs as indicators.— The shrubs that indicate the overgrazing of grass- 

 land are chiefly such dominants of sagebrush, scrub, or chaparral as mix with 

 the grasses to form savannah. The most important are Artemisia, Prosopis, 



