THE ASSOCIATIONAL BASIS. 49 



dance. Thus, Stipa spartea and Agropyrum glaucum show climatic differences 

 from S. comata and A. spicatum, while Stipa comata and Agropyrum glaucum 

 occur together over thousands of square miles, but are differentiated by water 

 relations determined by soil and slope. The actual physical differences in equiva- 

 lence are slight, and hence the dominants of an association tend to mix or to 

 alternate intimately instead of being pure over wide areas. However, this is 

 necessarily truer of an association with several to many dominants than of one 

 with but a few (cf. Zon, 1914 : 124). 



Each dominant will grow in a fairly wide range of conditions, but will thrive 

 only in a much narrower range. The field optimum for each is not a single 

 point but an area. The areas of the dominants of the same association or 

 associes overlap to such an extent that they coincide except at the extremes. 

 If the ranges of normal adjustment of Stipa comata and Agropyrum glaucum be 

 represented in each case by a rectangle, the two rectangles will coincide for 

 three-fourths of their lengths approximately. This indicates the degree of 

 equivalence, the projections of each rectangle representing the actual difference 

 in water-response for each species. This overlapping has its real counterpart 

 in communities where the dominants are zoned. The mixed area between two 

 zones represents the range of factors for which the two dominants are equiva- 

 lent, and the pure zone on either side indicates the range peculiar to each. 

 There is no necessary correspondence between the width of the zones and the 

 mixed area, and the range of factor coincidence for the two dominants, owing 

 to the varying rate at which such a factor as depth of water or amount of 

 water-content may change. In the lakes of Nebraska, the two successive 

 dominants, Scirpus and Typha, occupy the same depths from a few inches to 

 several feet. Over most of this range they are mixed or alternating, but 

 beyond 4 to 5 feet Typha drops out, while Scirpus may persist to a depth of 

 6 to 7 feet. Except where shores slope rapidly, the mixed zone is many times 

 wider than the zone of pure Scirpus. 



In this connection it should be recognized that dominants show a wider 

 margin between the normal range and better conditions than between it and 

 worse conditions. In other words, a species is quickly and definitely limited 

 by unfavorable factors, while those generally favorable to growth exert little 

 limiting effect, the real effect being due to competition. This is the obvious 

 explanation of the number of dominants and the abundance of species in sunny 

 well-watered habitats, such as prairies, open woods, alpine meadows, etc., and 

 their paucity in deserts and saline wastes. In short, abundance is itself an 

 indicator, whether it concerns the individuals of one species or the species of a 

 community. 



Absence of dominants. — The absence of a dominant from its particular com- 

 munity is often of indicator significance. A dominant may be lacking as a 

 result of several different causes. Its absence may be due to unfavorable 

 controlling factors, to very uniform conditions, to competition, destruction, or 

 to the failure of invasion for any reason. In all of these cases except the last, 

 absence has a definite indicator value, though it is practically always supple- 

 mentary to the presence of its associates. This is perhaps its chief value, in 

 that it enables us to check the positive indications obtained from presence. 

 Absence due to unfavorable conditions or to competition is the rule. Uni- 

 formity of conditions, however, is a more frequent cause than has generally 



