Hr.BE?-" 



Figure 3-1. The kinds of species and life-forms are numerous 

 and varied in some desert communities, growing in the 

 summer or the winter rainy periods in the southwestern 

 United States. The saguaro (Carnegiea gigantea) has fair 

 frequency. Superstition Mountains in background. Arizona. 

 (U.S.D.A. Soil Conservation Service.) 



ing that a decrease in the abundance of dominant prairie plants 

 and a lessening of their competitive capacity had permitted many 

 invaders to become established. In 60 sq m of a western wheat- 

 grass range under deferred and rotation grazing in Colorado, 75 

 species were present, while in the adjoining continuously grazed 

 pasture 76 were found in the same number of quadrats. The dif- 

 ference in grazing intensity evidently was not as great between 

 the two pastures in Colorado as between the grazed and non- 

 grazed prairie areas in Oklahoma. Although all the species in a 

 stand are significant, a single species is often used in naming a 

 vegetation type because of its abundance or dominance — such 



Analytic Ckairacteristics of the Commimimity • 79 



