genuine prairie must also contain the high-fidelity, exclusive 

 species. For example, ground plum {Astragalus crassicarpus), prairie 

 violet {Viola pedatifida), rough pennyroyal {Hedeoma hispida), and 

 lousewort {Pediculans canadensis) are all fairly common on upland 

 prairie in Iowa; but the first two are the most exclusive. The other 

 two rate much lower in fidelity, Hedeoma growing readily as a 

 weed in dry soil, and Pedicularis occurring in woodlands farther 

 eastward.' In western North Dakota, Andropogon gerardi and Stipa 

 spartea were restricted to one of nine types of vegetation, thus 

 rating in class 4; Distichlis stricta was restricted to two types, and 

 so rated in class 3; Bouteloua gracilis and Agropyron smithii grew in 

 all the types present, but having greatest abundance and vitality 

 in only one type, they also rated in class 3.'°^ A species growing 

 near the limits of its geographical range, such as Andropogon 

 gerardi and Stipa spartea in western North Dakota, can be expected 

 to occur in fewer kinds of communities than when it is growing 

 near the center (Figure 2-6). 



Dominance 



Dominance is the characteristic of vegetation which expresses 

 the predominating influence of one or more species in a stand so 

 that populations of other species are more or less suppressed or 

 reduced in number or vitality. Dominants are those species which 

 are so highly successful ecologically in their relations to the en- 

 vironment and with other species that they determine to a con- 

 siderable extent the conditions under which associated species 

 must grow. In a mature grassland (Figure 3-3), dominant species 

 are few, in tropical rain forest, many. Cover and population 

 density are the chief qualities determining dominance, but fre- 

 quency, height, life-form, and vitality are also important. Height 

 alone is insufficient, e.g., the tall goldenrods and asters in the 

 prairie are not sufficiently numerous nor are the root systems well 

 enough adapted to enable them to dominate the densely grow- 

 ing grasses. In savanna (Figure 1-12) the scattered trees or tall 

 shrubs may be considered as physiognomic dominants together 

 with the grasses, but the ecologic dominants are the grasses, for 

 they exert more inffuence on the habitat and on other plants. 



128 • The ComMnunity 



