INDICATOR VEGETATION 257 



more intense grazing causes the wheat-grass to be replaced by the 

 porcupine grass-yellow brush community and excessive over-grazing 

 brings about the appearance of the earlier stages of the succession. 

 Since the porcupine grass-yellow brush community is the most 

 valuable of all, the most successful scheme of management is to 

 graze just heavily enough to prevent the development of wheat- 

 grass but not heavily enough to destroy the porcupine grass-yellow 

 brush community. Thus by staking out permanent areas on the 

 range and making observations from time to time for the appear- 

 ance of species belonging to the preceding or following stages of the 

 succession it is possible to increase or decrease the number of head 

 of stock grazed before any appreciable damage has been done, 



152. Indicator Vegetation.— We have shown in the preceding 

 paragraph how plants characteristic of certain stages of a sere may 

 be used as indicators of grazing conditions. There are many other 

 practical uses that may be made of native plants as indicators of 

 present conditions and often of past or future conditions as well. 

 Every plant is a measure of the conditions under which it grows and 

 therefore it indicates how other plants may behave in the same 

 habitat. The only thing that limits our use of native plants as indi- 

 cators of environmental conditions is our lack of accurate knowledge 

 concerning so many of the ecological relations of plants. 



Either single plants or plant communities may be used as indi- 

 cators, the communities being somewhat more significant than single 

 plants or single species. The most important species in the com- 

 munity are the dominants and they may indicate past and future as 

 well as present conditions. The native communities may be used 

 to indicate the kind of practice that will prove most successful, 

 that is, whether agriculture, grazing, or forestry should be practiced. 

 They may indicate whether ordinary humid farming, dry farming, 

 or irrigation farming should be used. For example, a tall-grass 

 community indicates humid farming conditions while a short-grass 

 community indicates dry farming and a sage-brush community indi- 

 cates irrigation farming. Even the kinds of crops that are best 

 suited to a region may be indicated by the native vegetation. Thus 

 the areas of maximum production of corn and of winter wheat 

 correspond to areas where the native grass communities are domi- 

 nated by species of Andropogon while spring wheat and durum wheat 

 reach their best development where Stipa spartra and Agropyriim 

 glaucnm. are dominant. It is undoubtedly true that a land classi- 

 fication based on a thorough study of the indicator value of the native 

 17 



