CHAPTER 3 



REACTION: THE INFLUENCE OF COMMUNITY 

 ON HABITAT 



Definition and Nature. As has been earlier emphasized, the cause- 

 and-effect cycle in the biotic community comprises the action of the 

 habitat upon the associated organisms, their response to this, and 

 the consequent effect upon the physical factors of the habitat. The 

 last process was termed reaction by Clements (1904:124, 1916:79, 

 1928; Weaver and Clements, 1929:145), and was defined as the in- 

 fluence exerted by an organism or a community upon its habitat. It 

 is entirely distinct from the response of species or group in the course 

 of adjustment or adaptation. For examjile, the physical factors cause 

 a plant to function and grow, and it then reacts upon the habitat or 

 ece, changing one or more of its factors in an appreciable or decisive 

 manner. The two processes are mutually complementary and often 

 interact in the most complex fashion. Generally, there is a primary 

 reaction with one or more secondary ones, direct or indirect, but not 

 infrequently two or more factors are affected directly and critically. 



The word interaction has long been used by zoologists to cover all 

 kinds of interrelations between organisms and habitat, but it is obvi- 

 ously too inclusive for adequate analysis, especially in the broader 

 field of bio-ecology. Although it is desirable to retain it in a compre- 

 hensive sense, the need for exactness of reference is best met by recog- 

 nizing two distinct types of interaction. The first of these is reaction, 

 the effect of organisms upon the habitat; the second, coaction, or the 

 influence of organisms upon each other. Such a distinction becomes 

 of paramount importance when the biotic community is made the 

 basis of treatment. 



The reaction of a community is always more than the sum of the 

 reactions of the component individuals and species. In the case of 

 the plant matrix, though it is the individual that produces the reac- 

 tion in the final analysis, the effect regularly becomes recognizable 

 only through the combined action of the group. In practically all 

 cases, the community accumulates or emphasizes influences that would 

 otherwise be insignificant or transient. This is strikingly illustrated 



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