Selection plays an important role in the development of eco- 

 types suited to different conditions. For example, stands of 

 perennial ryegrass, Kentucky bluegrass, and orchardgrass, have 

 been grown for many years, especially in England, under various 

 treatments such as grazed or ungrazed, mowed or unmowed, and, 

 in addition, in moist or dry soils with high or low fertility. Grazed 

 pastures have produced ecotypes that are earlier in maturity and 

 more decumbent than meadows, with the result that the latter 

 tend to disappear when subjected to grazing, while pasture types 

 disappear under mowing. In this process of becoming adapted to 

 specific habitats the ecological amplitude of the population be- 

 comes narrower, at least for some conditions. 



Alteration of the habitat may thus bring about the survival of 

 certain biotypes and in time the formation of ecotypes, and the 

 loss of other biotypes. Man can, by various management opera- 

 tions such as controlling the intensity and time of grazing or 

 mowing, application of fertilizers, or the soil-water content by 

 irrigation or drainage, exercise considerable influence on the 

 evolutionary process. These manipulations change the environ- 

 mental conditions so that the requirements of some species or eco- 

 types are secured more readily, and consequently they become 

 more vigorous and stronger competitors, while other ecotypes 

 may not be able to grow at all because their ranges of ecological 

 amplitude have been surpassed. The management of one or more 

 environmental conditions may be of greater importance in cer- 

 tain places than the course of nature in determining the success 

 or failure of species, particularly in the central European grass- 

 lands. ''^•'*' Success in management operations depends upon 

 knowledge of the requirements and ecological amplitudes of 

 species and ecotypes. 



Grasses have considerable capacity to survive the combined 

 eff"ects of drought and grasshopper infestations which occur fre- 

 quently in the Great Plains (Figure 1-23). Most species of grass- 

 hoppers show selectivity, some feeding only on grasses, some only 

 on forbs or shrubs, and others on both grasses and broad-leaved 

 plants; in Montana, for example, certain species feed mainly on 

 Bouteloua gracilis, others on Agropyron smithii or Stipa comata. The 



48 • Species ai%cl Popmmlaftioxms 



