similar to be considered as one kind of community — the blue 

 gramagrass-needlegrass-sedge community type — which occupied 

 more of the region than any of the other types. Other groupings 

 occurred on valley-fill deposits on long, gradual slopes of clay- 

 loam soil with a shallow layer of dark soil; on steep slopes of clay- 

 loam to loam with a shallow layer of dark soil; and on sandy soil. 

 Many species possessed wide ecological amplitude, as indicated 

 by their occurrence in different habitats, such as Bouteloua gracilis, 

 Agropyron smithii, Stipa comata, Koeleria cristata, Chenopodium leptophyl- 

 lum, Artemisia frigida, and Gaura coccinea. The first three were much 

 more successful ecologically than the others, largely, it appears, 

 because of their superior competitive capacity. On the other 

 hand, a few species had a narrow range of ecological amplitude, 

 or had little capacity to utilize the resources of any of the habitats 

 of this region. Two of these, Stipa spartea and Sporobolus heterolepis, 

 are better adapted to conditions farther east, i.e., those which 

 the prairies provide as an environment more nearly within the 

 range of their optimum ecological amplitudes. The big bluestem, 

 or turkey-foot, Andropogon gerardi, grew in only one kind of 

 habitat, which was restricted to the lower parts of steep slopes 

 where the loam to sandy-loam soil was deep and moist; and 

 here it was successful with A. scopanus and Sporobolus heterolepis as 

 characteristic associates. Puccinellia airoides, because of its toler- 

 ance of poorly drained soil with a high content of soluble salts, 

 utilized resources of low stream terraces where the conditions 

 were outside the amplitude range of most of the species occurring 

 in this region. Distichlis stricta was a common associate but, prob- 

 ably because of possessing greater competitive capacity, it grew 

 also in association with Agropyron smithii and other species on 

 better-drained terraces. 



The number of species per community type in this region varied 

 from about 20 to 29 in the less favorable habitats to 41 to 86 in 

 the more favorable ones. Apparently many species were excluded 

 from some habitats because of one or more limiting factors, and 

 possibly because of insufficient competitive capacity when grow- 

 ing in environments near the limits of their ecological amplitude. 

 Although grasses were usually dominant, 66 to 80 per cent of the 



Groixpinci of Species • 71 



