STRUCTURE AND UNITY 257 



It is manifest that these are the dominants that bespeak the unity 

 of the gi-assland as a cHmax formation and constitute the primary pat- 

 tern in which the differentiation into associations has been carried out. 

 Other grasses have similar extensive areas, such as Andropogon sac- 

 charoides and Sporobolus air aides, but the former is chmax in but a 

 single association and the latter is generally a subclimax dominant of 

 the halosere. 



Binding Influents. Among the influents which give unity to the 

 biome are the bison {Bison bison), the pronghorn antelope {Antilo- 

 capra americana and variety), and the badger {Taxidea taxus and 

 varieties) which covers all the climatic grasslands and is almost re- 

 stricted to them, merely extending into some savanna areas. It is 

 found in all the grassland associations from Indiana to the San Joa- 

 quin valley and from the bunch grass of British Columbia to the desert 

 plains of the Southwest. It invades other biomes only locally in serai 

 stages, as in the saltbush subclimax of the Larrea desert, though maps 

 suggest its presence in the desert generally (Figs. 55-57) . The buffalo 

 wolf [Canis nubilus) , the horned lark (represented by a different va- 

 riety in several of the associations), and a few insects, of which the 

 orthopteran (Mermiria neomexicana) is an example, occur in nearly 

 all the associations. The differences between the associations as to 

 influents are given on pages 264, 273, etc. 



The genus Geomys (eastern pocket gopher) is largely confined to 

 the moister grasslands likewise, and the smaller Thomomys (western 

 pocket gopher) , is also a grassland influent. Certain species are char- 

 acteristic of climax grassland ; others extend into mountain meadows 

 and into serai communities dominated by a mixture of grasses and 

 trees. The same is true of the several species of Onychomys, the 

 grasshopper mouse, and Citcllus, the ground squirrel. 



The genus Lepus includes the varying hares and jack rabbits which 

 occupy open and brush-covered areas in grassland and tundra. Jack 

 rabbits comprise two quite distinct groups: (a) the northern white- 

 tailed jack rabbit closely related to the varying hare, and (6) the 

 southern black-tailed jack rabbit. Certain species or varieties of these 

 two groups range through the climatic grassland and into the park- 

 like types of vegetation and grassy serai stages in low altitudes. 



Some genera of birds divide their time or range between tundra 

 and grassland, such as Calcarius (longspurs), Otocoris (horned lark), 

 and Anthus (pipits). Other genera range between prairie and 

 meadow. Among them are Dolichonyx (bobolink) , Sturnella (meadow- 

 lark) , and Ammodramus (grasshopper sparrow) (Pearson 1923). Of 

 the reptiles two species of bullsnake or gopher snake (Pituophis) 



