80 REACTIONS. 



comes to cover the whole area as vegetation becomes closed. It is often felt 

 for a considerable space around the individual or group, especially when exerted 

 against the eroding action of wind or water, or the slipping consequent upon 

 gravity. In most secondary areas and serai stages the reaction is the combined 

 effect of the total population. In it the preponderant r61e is played by success- 

 ful competitors and particularly by the dominants. These determine the 

 major or primary reactions, in which the part of the secondary species is slight 

 or neghgible. 



Role in succession. — ^In the development of a primary sere, reaction begins 

 only after the ecesis of the first pioneers, and is narrowly localized about them 

 and the resulting families and colonies. It is necessarily mechanical at first, 

 at least in large degree, and results in binding sand or gravel, producing finely 

 weathered material, or building soil in water areas, etc. In secondary seres, 

 extensive colonization often occurs during the first year and reaction may 

 at once be set up throughout the entire area. The reactions of the pioneer 

 stage may be unfavorable to the pioneers themselves, or they may merely 

 produce conditions favorable for new invaders which succeed gradually in the 

 course of competition, or become dominant and produce a new reaction unfa- 

 vorable to the pioneers. Naturally, both causes may and often do operate at 

 the same time. The general procedure is essentially the same for each suc- 

 cessive stage. Ultimately, however, a time comes when the reactions are 

 more favorable to occupants than to invaders, and the existing community 

 becomes more or less permanent, constituting a climax or subclimax. In short, 

 a climax vegetation is completely dominant, its reactions being such as to 

 exclude aU other species. In one sense, succession is only a series of progres- 

 sive reactions by which communities are selected out in such a way that only, 

 that one survives which is in entire harmony with the cHmate. Reaction is 

 thus the keynote to all succession, for it furnishes the explanation of the 

 orderly progression by stages and the increasing stabiUzation which produces 

 a final climax. 



Previous analyses of reaction. — ^The essential nature of reaction has been 

 Kttle recognized in the past, and there have been but two attempts to analyze 

 and group the various reactions. Clements (1904:124; 1905:257; 1907:282) 

 pointed out that the direction of movement in succession was the immediate 

 result of its reaction, and that the latter is expressed chiefly in terms of water- 

 content. He further stated that the initial causes of succession must be sought 

 in the physical changes of the habitat, but that the continuance of succession 

 depended upon the reaction which each stage exerted upon the habitat. 

 The general reactions of vegetation were classified as follows: (1) preventing 

 weathering, (2) binding aeoKan soils, (3) reducing run-off and preventing 

 erosion, (4) filling with silt and plant remains, (5) enriching the soil, (6) exhaust- 

 ing the soil, (7) accmnulating humus, (8) modifying atmospheric factors, Ught, 

 humidity, etc. Cowles (1911 : 173) has classified plant and animal agencies in 

 succession in five groups: (1) hmnus complex, (a) water, (6) soil organisms, 

 (c) toxicity, (d) food, (e) temperature and aeration; (2) shade; (3) plant 

 invasion; (4) man; (5) plant plasticity. The factors of the humus complex 

 and shade are reactions, as the term is understood here. Invasion is the basic 

 process of which succession is but the continuance or recurrence; man is an 

 initial cause, and plasticity a response to the habitat as modified by reaction. 



