70 REACTION: THE INFLUENCE OF COMMUNITY ON HABITAT 



movement of material due to wind, gravity, or water is diminished or 

 eliminated. Relations of this kind are less frequent and less evident 

 with animal forms, though burrowing rodents, earthworms, and ants 

 in particular constitute an important group. In areas of salt marsh 

 exposed to alternating land and sea conditions, it is probable that the 

 special study of reactions will disclose many animals of much 

 importance. 



Role of Reaction. As has been previously stated, the primary role 

 of reaction is seen in the process of competition. In connection with 

 this, it regularly assumes the directive function in terrestrial succes- 

 sion and in the concomitant development of the habitat, but it may 

 also lead to the production of bare areas as a requisite to succession. 

 In the development of a primary sere, plant reaction begins with the 

 establishment of the first invaders and is narrowly localized about 

 them and the resulting families and colonies. It is largely mechanical 

 at first and results in binding sand or gravel, producing finely weath- 

 ered material or building soil in water bodies, etc. In secondary 

 seres, extensive colonization may occur during the first year, and 

 reaction may at once be set up throughout the entire area. Reaction 

 then progresses with an increasing advantage to each succeeding stage 

 until the climax is attained, when the reaction of the dominants is so 

 decisive as to exclude other invaders. Thus, in one sense, succession 

 is but a series of progressive reactions by which communities are 

 sifted out in such fashion that only the one in closest harmony with 

 the climate ultimately survives. As an inevitable accompaniment, one 

 serai habitat follows another until the climax habitat becomes perma- 

 nent during the persistence of the climate concerned. In the aspec- 

 tion and annuation exhibited by the climax itself, reaction continues 

 to have an influence, but this is usually secondary to the control 

 exerted by season or climatic cycle. 



In general, animal reactions on land have not been separated from 

 the larger effect produced by plants. However, they are usually pres- 

 ent to some degree, owing to the presence of soil-moving and soil- 

 modifying animals in the climax and all serai stages, even initial ones 

 in which the plants have not yet appeared. In water, reactions are 

 evident but observation is more difficult. Reactions of the animal 

 dominants regularly play a large part, sometimes as in shallow fresh 

 waters through their influence on turbidity and sometimes through 

 disturbing the substratum (see p. 301). The microplankton, however, 

 reacts upon light and dissolved gases and salts in such a manner as 

 to produce distinct conditions, comparable in degree to reaction effects 

 on land. 



