THE BIOTIC ENVIRONMENT OF ORGANISMS 



585 



the habitat of a number of successive microcommunities, each dependent 

 upon some particular stage in the decay of the log and each tending to 

 destroy its own environment and make conditions favorable for its 

 successors. 



Fig. 33.19. An undersea community. Portion of a Bermuda coral reef. Here most of the food 

 chains start with the floating plankton, which is the food of the sponges (bottom and right 

 rear), of the stony corals, the fingerlike horny corals and sea fans (all coelenterates), and 

 of the simple chordates called "sea squirts" or tunicates, of which two groups can be seen 

 attached to dead coral branches. Coral snails and small fishes form other links in the 

 chain, and these are in turn fed upon by larger vertebrates such as the fierce moray (shown 

 with head projecting from a crevice). (Courtesy University of Michigan Museums.) 



Since all succession is the result of the dynamic relations among popula- 

 tions and many kinds of intricately interrelated organisms, it exhibits 

 many details, modifications, and local variations that cannot be con- 

 sidered here. It should be apparent, however, that the activities of 

 civilized man, with his crop plants and his herds, his abandoned fields, 

 cutover forests and roadsides, his drainage and irrigation projects, and 

 his damming and pollution of streams, intrude on a huge scale into 



