tained by the parent plant during the time that new roots and 

 leaves are forming, and thus are able to carry on competition 

 advantageously with plants already present in the area. For ex- 

 ample, the rhizomes of Agropyron smithii were undoubtedly a great 

 aid in the invasions of the Nebraska prairies during the great 

 1930-1940 drought, when nearly pure, large populations, rating 

 class 5, were formed. Certain species such as Bouteloua gracilis, 

 Aristida longiseta, Artemisia frigida, and Astragalus missounensis have 

 a higher class of sociability under continuous grazing than under 

 deferred and rotation grazing, while the opposite is true for 

 Agropyron smithii, Schedonnardus paniculatus, Eurotia lanata, and 

 Seneao perplexus^^^ Under some conditions such as severe grazing, 

 shallow-rooted plants may invade and become very dense, replac- 

 ing the former deep-rooted ones. 



Association of Species 



Association of species, or interspecific association, is the 



growing together of two or more species in close proximity to 

 one another as a rather regular occurrence; for example, Ken- 

 tucky bluegrass and white clover in many pastures, Stipa comata 

 and Bouteloua gracilis in some grasslands in the Great Plains, and 

 Agropyron spicatum and Poa secunda in the lower grassland zone in 

 British Columbia^^^ (Figures 1-14 and 1-15). Association of 

 species may be brought about by the similarity in ecological 

 amplitudes of two or more species; similarity in geographic 

 ranges; differences in life-form (such as shallow and deep root 

 systems) so that excessive competition can be avoided; depend- 

 ence of one species upon another for shade (Figure 3-2), or for 

 food as in parasites; or dependence for protection from grazing, 

 as in grasses growing in dense clumps of cactus. Association may 

 be so pronounced that a certain species may indicate the presence 

 of other species in the stand, so that prediction is possible to some 

 extent, i.e., if species A is found in a certain area then species B 

 can also be expected there. 



When environmental conditions change, the species that are 

 associated will vary. A species growing as a dominant in one 

 stand usually has different associates when growing as a sub- 



98 • Tke Comxnunity 



