CHAPTER 8 



THE NORTH AISIERICAN GRASSLAND: STIPA-ANTILO- 

 CAPRA BIOTIC FOR^IATION (BIOME) 



Introduction. The grassland is well suited to illustrate the charac- 

 tertistic features of the biome and the interdependence of the constit- 

 uents and their relation to chmate. This results not merely from its 

 wide extent and exceptional differentiation structurally, but likewise 

 from the unusual number of striking coactions. 



From the ecological viewpoint there is no essential distinction be- 

 tween so-called prairie and plains, just as there is also no consistent 

 diversity in topography. Both are uniformly characterized by a 

 cover of perennial grasses in close harmony with the climate and like- 

 wise by a former population of gi-azing animals. The usual concep- 

 tion of a prairie is that of a rolling landscape by contrast with the 

 level expanse of plains, a view that receives much support from the 

 so-called high plains of the West, but over the entire area of the grass- 

 land the exceptions are so numerous as to obscure the rule. The plant 

 life form (grass form) is further to be regarded as the decisive criterion 

 in many districts of sandhills and foothills in which the relief is much 

 bolder than in traditional prairie, though the cover exhibits the usual 

 dominants (Pound and Clements, 1898). 



The peculiar physiognomy of grassland is well known to all who 

 have visited it. The vegetation itself offers little or no obstruction 

 to vision. Large areas of the central portion of our North American 

 grasslands, green in summer and brown in autumn and winter, stretch 

 away as far as the eye can reach. Generally, however, the uniformity 

 is relieved by slopes, rolling ground, small hills or ranges, and ravines 

 and valleys often marked by relict or serai communities of trees or 

 shrubs. Consequently, strong contrast and sharp delimitation of val- 

 ley and plain are among the striking features. Near the grassland 

 margins where the contacts are with tree or shrub communities, there 

 are small groups of shrubs that break the monotony and afford habi- 

 tations, shelter, or perching places for various animals. 



In conformity with the biotic concept, animals have a distinct role 

 in the physiognomy of grassland, but this was naturally much more 

 evident before the period of settlement. At that time, an ecologist 



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