CHAPTER 4 

 COACTION: THE INTERRELATIONS OF ORGANISMS 



Nature and Significance. As has been indicated in Chapter 2, all 

 the activities of the biutic community may be summed up in the action 

 of the habitat upon the organisms, the reaction of these upon the 

 physical factors, and the coaction of the organisms upon each other. 

 In a general sense, the word interaction has been applied to some of 

 these (Forbes, 1880, a) , but the need for exact analysis renders the use 

 of reaction and coaction all but indispensable (cf. Clements, 1916, 

 1926) . The latter is made peculiarly necessary by the concept of the 

 biome and the consequent dropping of the term biotic factor. The 

 word coaction is especially fitted to designate the enormous range of 

 interactions among plants, plants and animals, and animals alone, 

 since it involves not only the idea of acting together, but also that of 

 urging or compelling. 



Although the three processes of response, reaction, and coaction 

 are usually sharply delimited, the last two in particular are often 

 closely associated and hence may seem confused. Such a food coac- 

 tion as that of the mole brings about reaction through disturbance, 

 and this may modify the plant matrix, which is usually not directly 

 concerned in the coaction. Similarly, the reactions of a prairie dog in 

 digging its burrow may destroy, stimulate, or change the plant cover, 

 ciuite apart from its consumption as food. The material-shelter coac- 

 tion of the beaver involves a minor reaction in the use of mud, a major 

 one when canals are dug to transport logs, and a combination of 

 coaction and reaction in the destruction of vegetation and modification 

 of the habitat as the consequences of flooding. 



It has previously been pointed out that the control exerted by 

 land plants is primarily a matter of reaction and related competition, 

 leading to various degrees of dominance and subordination, and that 

 animals enact a somewhat similar role in river, lake, and ocean. By 

 contrast, terrestrial animals usually exert their major effects through 

 coaction, with the resulting gradations in influence. In consequence, it 

 is coaction that constitutes the chief bond in the biotic connnunity, 

 both on land and in water, but on land especially, while reaction is 



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