48 BASES AND CRITERIA. 



The distinction between the dominance and the mere presence of a species is 

 vital, from the standpoint of the structure of vegetation as well as from that of 

 indicators. It is this which makes catalogues, lists of species, and general 

 descriptions of the flora of a region of little value to the ecologist. In fact, such 

 materials are trustworthy only in associations already known, where they are 

 superseded. This is exemplified by a number of grass dominants. Bouteloua 

 gracilis is found from Manitoba to Wisconsin and Mississippi, west to Texas, 

 central Mexico, and California, and northward to Alberta and Saskatchewan, 

 It occurs as the characteristic climax dominant of the short-grass association 

 only in eastern Colorado, southwestern Nebraska, western Kansas and Okla- 

 homa, northeastern Arizona, northern and eastern New Mexico, and in the 

 Panhandle and Staked Plains of Texas. Usually with Bulbilis, it is more or 

 less regularly associated with Stipa and Agropyrum from northwestern 

 Nebraska and northern Wyoming through the Dakotas and Montana, into 

 Saskatchewan. Altogether it is a climax dominant over perhaps a quarter 

 of its range and a serai dominant over another quarter. Stipa comata is a 

 climax dominant to-day only in Nebraska, northern Colorado, Wyoming, the 

 Dakotas, Montana, and Saskatchewan, though it ranges from the latter to 

 Nebraska, New Mexico, California, and northward to Alaska. As a conse- 

 quence, the vegetational and indicator importance of any dominant species can 

 be determined only by field studies of its abundance and r61e. Maps and con- 

 clusions based upon the distributional area alone are both misleading and 

 erroneous (plate 8.) 



Equivalence of dominants. — The dominants of a formation owe their associa- 

 tion to the generally similar responses which they make to the climax habitat. 

 This fact is further attested by the identity of life-forms and, to a small degree 

 as yet, by actual measurement of the controlling factor. As the sum of similar 

 responses, the formation is thus the largest and most distinctive of all indicator 

 communities. Within the formation the dominants fall into associations by 

 virtue of still closer similarity in response. Thus Stipa, Agropyrum, and 

 Koeleria constitute the climax prairies. By their height and general turf 

 habit they indicate a rainfall of 20 to 30 inches. Bouteloua gracilis and Bul- 

 bilis dactyloides form the short-grass plains. Their short stature and mat habit 

 are responsive to a smaller rainfall of 12 to 22 inches, which in effect is much 

 reduced by evaporation. The Aristidas and Boutelouas of the desert plains 

 from Arizona to western Texas are somewhat taller, but their bunch habit is 

 an index of a smaller water efficiency, largely the result of excessive evaporation . 

 This relation is further indicated by the presence of Bouteloua gracilis in the 

 moister valleys, and by the fact that Stipa and Agropyrum regularly mix with 

 the short-grasses as indicated above, but have never yet been found mixed 

 with the species of Aristida and Bouteloua characteristic of the desert plains. 

 So far as our present knowledge goes, dominants of the same association or of 

 the same associes are never exactly equivalent. Actually, they may seem to 

 be since the annual variations of the climatic cycle are often much greater than 

 the difference in conditions. Even here, however, they tend to maintain their 

 position or abundance, relative to the controlling factor. As a consequence, 

 each consociation has its own indicator value, which, so far as its presence is 

 concerned, necessarily varies somewhat from wet to dry phases of the cycle, 

 but is checked by corresponding variations in growth, reproduction, and abun- 



