lished as the dominant. The more vigorous root growth of 

 Bothriochloa, as compared to Danthonia, gives the former an initial 

 advantage in competition. But in native pastures already occupied 

 by Danthonia, invasion by Bothriochloa is not likely to occur unless 

 the former have been weakened by overgrazing.^"*^ Plants prop- 

 agating vegetatively have an advantage in invading bare areas 

 because the shoots during establishment secure water and nutri- 

 ents, and possibly also synthesized substances, from the parent 

 plants. 



Competition and adaptation have played important roles in the 

 survival of species in various environments, and the space and 

 time in which a species grows are often decisive. Shallow-rooted 

 plants, by rapid growth when soil moisture is adequate, can 

 escape competition with deep-rooted, slow-growing plants. A 

 number of single-stalked grasses, forbs, mosses, and lichens can 

 grow in the large spaces between bunchgrasses but cannot endure 

 the competition in the small spaces between densely growing 

 rhizome grasses. Plants of many species often grow together at 

 the same time in habitats where the environmental resources are 

 ample, but when the resources are limited in quantity or dura- 

 tion, rigorous natural selection of adapted biotypes has occurred. 

 In desert areas of the southwestern United States species of an- 

 nual life-form are numerous, partly because competition has been 

 reduced by growing in different seasons, e.g., Streptanthus anzonicus 

 completes its Hfe-cycle in the cool, late winter while Amaranthus 

 palmeri thrives in the warmth of August and September.''^ 



Competition between species and strains of forage plants may 

 be severe for soil moisture, nutrients, and light. For example, in 

 an experiment in Iowa the relative capacity of two strains and 

 four species of legumes was determined by growing them in asso- 

 ciation at the same time, with the result that Madrid sweet clover 

 rated first, followed in order by African alfalfa. Ranger alfalfa, 

 Kenland red clover, and Ladino white clover. The ranking was 

 the same as for relative yields when grown singly, so apparently 

 a close relationship exists between competitive and yielding 

 capacities in this case^^^ (Figure 1-14). The competitive ability 

 of one strain or species as compared to that of another may be 

 controlled to some extent by measures such as the application of 



32 • Species aamd Popmilatioxus 



