preferred species and the interspecific association of many species, 

 but may also damage some of the most favorable habitats. Ex- 

 cessive trampHng of nonsandy soils causes packing, and reduces 

 the accumulation of mulch and organic matter and the infiltra- 

 tion of water. Droppings and liquid manure may also modify the 

 soil and, where they are excessive, permit invasion of certain 

 weeds. Reduction in height and density of the vegetation increases 

 soil temperature and evaporation rate, so that the changed 

 habitat conditions may no longer be within the ranges of ecolog- 

 ical amplitudes of preferred species. Severe overgrazing in the 

 Wasatch Plateau in Utah has resulted in eroded habitats and dis- 

 placement of herbs and shrubs by an open stand of species such 

 as Festuca ovina, Poa spp., and Sedum stenopetalum, capable of grow- 

 ing on erosion pavement.'^ It appears, therefore, that plants 

 have a high degree of specificity for various durations and inten- 

 sities of overgrazing. This specificity is also present in various 

 stages of succession on abandoned farmlands. In Idaho, for ex- 

 ample, populations of Russian thistle, mustards, and downy 

 bromegrass have optimal time and space requirements for their 

 appearance. ^^2 



Preferred species that grow sparsely are most susceptible to 

 elimination by overgrazing, for they are closely grazed while less 

 palatable and more abundant kinds remain untouched. In the 

 Front Range in central Colorado, bunches of grasses such as 

 Muhlenbergia montana under close grazing tend to break into small 

 parts, becoming sod-like, while other grasses such as Festuca 

 arizonica are more resistant to disintegration. However, in both, 

 plant vigor is reduced and Bouteloua gracilis replaces them, while 

 forage yields decrease."^ Heavy grazing may also cause clumps 

 of the last-mentioned grass to separate into smaller, closer-spaced 

 tufts, and bunches of Stipa comata to become smaller and more 

 decumbent, with fewer stems and finer, shorter leaves. The 

 capacity of grasses to endure grazing is related to characteristics 

 such as a decumbent growth-form, rhizome production, large 

 food reserves, rapid growth renewal and production of new shoots, 

 long season of vegetative growth, location of growth-renewal 

 tissue below the lowest level of grazing, and resistance to un- 

 favorable environmental conditions, especially drought. 



Ecological Characteristics of Species & Populations • 47 



