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NATURALIST'S GUIDE TO THE AMERICAS 



meridian, and is bounded on the east by 

 the extensive grassland country just 

 discussed, while the western boundary is 

 the Rio Grande succulent desert repre- 

 sented by the valley of the Pecos River. 

 This valley pushes up a considerable 

 distance into New Mexico, but eventu- 

 ally the staked plains encompass it on 

 the north and reenter Texas again as a 

 long, slender arc extending into the 

 desert of the trans-Pecos region, then 

 recognized as the trans-Pecos plateau. 

 It is conspicuously a "short grass" coun- 

 try both in realit}^ and from a compara- 

 tive point of view. Here we find such 

 plants as Bouleloua, Hilaria, Muhlen- 

 bergia, Bulbilis, etc. The grasses are 

 dwarfed because of the xerophytic con- 

 ditions, and are of the type composed of 

 short, narrow leaves which show a 

 marked inclination to curl, due to the 

 excessive transpiration. These grasses 

 are fine as compared to the much coarser 

 grasses of the lower steppes and grass- 

 lands and, whereas the latter is so luxuri- 

 ant as to completely hide grazing cattle 

 (where over-grazing has not been prac- 

 ticed), the grasses of the staked plains 

 are rarely knee high. The landscape is 

 therefore a monotonously rolling one. 

 When this formation reenters Texas 

 in the trans-Pecos region, the same 

 unbroken fields of short grass character- 

 ize it. Thus the Davis mountains pre- 

 sent a picture of unbroken grass vegeta- 

 tion up to approximately 5000 ft., with 

 some xerophytic oak forest in some 

 places above this altitude, with occa- 

 sional western yellow pine and limber 

 pine, while the upper slopes and summits 

 — especially the upper canyons — must 

 be considered as islands of forests, 

 largely of Rocky mountain type. 



The animal life is typically steppe 

 species, and we find here extensions of 

 the prairie dog towns, and the western 

 boundary of the former range of the 

 bison. The Pronghorn antelope (Antilo- 

 capra americana) was formerly common 

 here, but is now nearly gone. Several 

 animals show an interesting distribution 

 in this area. A map of the distribution 

 of the pale ground-squirrel {Citellus 



tridecemlineaius pallidus) shows it to be 

 confined absolutely to the top of the 

 staked plains, and there is no evidence of 

 the species coming down below the gen- 

 eral topmost elevation of the region. 

 This is quite different from the distribu- 

 tion of the Richardson kangaroo-rat 

 which is characteristic of a position just 

 intermediate between the staked plains 

 and the grassland at its feet : the species 

 completely encircles the plains, giving a 

 good intermediate contour of the region. 

 Here likewise, closely confined to the 

 dwarf grasses, is the rattlesnake (CVo- 

 tahis molossus), found only in the Gua- 

 daloupe and Davis mountains at the 

 topmost margin of the plains at 6800 ft. 



7. Mesquite semi-desert 



This is the region characteristic of 

 the lower Rio Grande valley in southern 

 Texas. To the eastward it extends to 

 the coast, and to the west it follows in a 

 general way the broad valley of the Rio 

 Grande, reaching approximately the 

 mouth of the Devils river, where it 

 encroaches upon the succulent desert of 

 the Pecos and trans-Pecos region. It 

 is a low area, reaching its highest eleva- 

 tion about 1000 ft. near the mouth of 

 the Pecos. The rainfall is light over the 

 entire region, being normally about 20 

 in. Formerly this region was mostly 

 open grassland and mesquite, but now 

 it is overgrown with succulent plants of 

 the prickly pear {Opuntia) type, and 

 the grasses have been largely replaced 

 by shade-enduring species. Where the 

 open grassland prevails we find the plains 

 type of grasses represented by such 

 genera as Bulbilis, Bouteloua, Muhlen- 

 bergia and Aristida, with a relatively 

 limited assortment of sand-loving an- 

 nuals. Throughout the region the domi- 

 nant tree is the mesquite (Prosopis 

 glandulosa) and associated with it are 

 such typical species as the black per- 

 simmon {Brmjodendron texanum) hul- 

 sache, lechuquilla {Agave lechuguilla) 

 and the creosote bush, though it must 

 be recognized that both the mesquite 

 and the creosote bush extend their range 

 far beyond the limits of the semi-desert. 



