THE STRUCTURE OF CLIMAXES 235 



Dominance and Influence. Dominance is the condition of control 

 of community character and composition that results from the success- 

 ful outcome of reaction, coaction, and competition, the opposite effect 

 being seen in subordinance (Clements, 1916, 1920). Naturally, no 

 sharp line can be drawn between them, since species run the entire 

 gamut from one to the other and even the same organism may some- 

 times vary widely in this respect. Cahn (1929) has pointed out that 

 fishes dominate by a combined coaction and reaction, and Petersen 

 (1918) has described dominance due to coaction alone. 



Dominance on land is commonly so much a matter of advantage in 

 terms of life form, size or height, and abundance among plants, as to 

 be readily determined when quantitative results are available. Dom- 

 inants are most potent, and major influents usually second in im- 

 portance. On land, dominance has been understood to mean the con- 

 trol of habitat through reaction, though dominance through coaction is 

 possible. It is locally evident through the exclusion of forest from the 

 Kaibab parks by deer. The exclosure shown in Fig. 53b and viewed in 

 comparison with the unprotected area (Fig. 53a) after a period of nine 

 years is significant. The three clumps of aspen stems (a, b, c) are 

 the effect of deer browsing on the unprotected area. Some exclosures 

 at the margin of the larger parks are more convincing because aspens 

 have occupied areas exclosed from the grass-covered spaces at the 

 forest edge. 



In water, by contrast, the habitat is of secondary interest, and the 

 community, which is directly and indirectly controlled, is the pri- 

 mary consideration. In marine and fresh-water climaxes animals evi- 

 dently often dominate largely through coactions by which the charac- 

 ter of the community may be almost completely changed (cf. Peter- 

 sen, 1918; Cahn, 1929; Shelf ord, 1935; Gersbacher, 1937). 



The significance of animals on land has often been questioned by 

 plant ecologists, but this cannot be taken too seriously, as the work 

 has usually been done in areas where the more significant animals have 

 been extirpated. The early efforts in animal ecology suffered from 

 the same causes, and in both fields lack of training necessary to cope 

 with animal problems was evident. This was naturally most striking 

 among plant ecologists, and until more studies such as those of Vor- 

 hies and Taylor (1922) are made, the basis for the interpretation of 

 animal relations and importance must be considered inadequate for 

 anything more than provisional evaluation. Cahn (1929), Rickcr 

 (1932), and others have credited certain fishes, such as carp, suckers, 

 and their relatives, with eating vegetation, disturbing the bottom. 



