Figure 1-12. Competition may be severe between plants of 

 different life-form, such as grasses and mesquite (Prosopsis). 

 When the latter is killed the grasses increase in vigor. In 

 this late-summer aspect, grasses are dominant in the fore- 

 ground, mesquite in the background. September, Santa Rita 

 Experimental Range, southern Arizona. (U. S. Forest 

 Service.) 



time. The requirements of species in various genera usually differ 

 more in time and space than species in the same genus, but this 

 is not invariably true (Figure 1-12); differences in either case 

 may be so great that they cannot grow in the same habitat. On 

 the other hand, species in different genera such as Bouteloua 

 gracilis and Buchloe dactyloides in parts of the central Great Plains, 

 undergo competition similar to that between individuals of the 

 same species; and in some habitats species in the same genus, such 

 as Bouteloua gracilis and B. curtipendula, are associated apparently 

 because the resources of the environment are adequate to supply 

 the somewhat different needs of each and because the genetic in- 

 dividuality of each is not lost in hybridization. Much more field 

 and experimental research is needed for understanding the asso- 

 ciation or lack of it between similar and dissimilar species. 



A species can be expected to have greatest competitive capa- 



Ecological CHaracteiristics of Species &. Popmalations 



29 



