264 THE NORTH AMERICAN GRASSLAND 



regularly typical of it, together with some Panicum virgatum and 

 Elymus canadensis, but in sandhills the leading roles are usually taken 

 by Calamovilfa longifolia and Andropogon halli. 



The most interesting feature of the mixed prairie today is its con- 

 version over large 'areas into a disclimax caused by overgrazing. Such 

 disturbance is rarely so thorough as to eliminate the mid grasses com- 

 pletely, but under extreme conditions they persist only in reduced form 

 and number or in sites with more or less protection from grazing. 

 While the original composition of the climax can be readily inferred 

 from these, and especially from fenced railways, it can actually be 

 restored by means of exclosures, such as have been employed in the 

 several faciations. The characteristic dominants of the short-grass 

 community are Bouteloua gracilis and Buchloe dactyloides, though 

 several other gi^asses and sedges of this life form play some part. The 

 reduction or suppression of the mid grasses depends primarily upon 

 the intensity and duration of grazing, but it is also related to rainfall 

 and hence increases to the southward as a rule. It is likewise subject 

 to marked annuation, the mixed composition being evident during sea- 

 sons of high rainfall and being correspondingly obscured during 

 drought periods. 



Influents. The bison occurred through practically all the extent of 

 mixed prairie except western New Mexico and Arizona, the number 

 formerly present being estimated at 30 million. Though generally 

 harassed by the buffalo wolf, Canis nubihis, herds of a million or more 

 bison were reported by the early explorers. These aggregations were 

 of the greatest magnitude during migration, as the herds moved north 

 a distance of several hundred miles in early summer and south again 

 in late autumn. The pronghorn antelope occurred in great numbers 

 in this association, the population on the mixed prairie being variously 

 estimated at 4 to 8 millions. It preferred the rolling portions and 

 sought shelter in ravines and cottonwood valleys during storms, or 

 shifted to areas without snow during winter; otherwise the bands re- 

 mained in the same general locality (cf. Watson, 1911). 



The prairie dog ranged over the entire area, except the portion 

 north and east of the Missouri River in North Dakota, where its place 

 was taken by the Richardson ground squirrel. The former occurred 

 in large aggregations, often covering 20 to 640 acres, but also scat- 

 tered about. The northern white-tailed jack rabbit reached southward 

 over about two-thirds of the north-to-south extent, where it overlapped 

 the black-tailed jack rabbit which occupied the area north into Ne- 

 braska. Grasshopper mice, as well as a series of species of pocket 

 mice, supplemented each other in covering the plains. The pocket 



