Community Dominance 409 



example in an alpine region, or is largely unmodifiable as in the open 

 ocean. This type of condition is also seen in communities of marine- 

 bottom organisms in which the chief controlling influences are the 

 mud of the bottom and the water currents. These factors act on the 

 species present more or less directly, as indicated by the accompany- 

 ing scheme (case 1). 



Mud-Bottom Biocenose 



Water currents 



Case (1) A B C B 



t t t t 



Substratum 



Case (2) A -^B ^C-«- 



Case (3) A -^B *►€-<- B 



(C) (C) (C) 



In addition to the major effect of the physical features of the en- 

 vironment a certain amount of interdependency may also exist among 

 the members of biocenoses of this sort. In communities living on 

 isolated rocks the interrelations between the species may be quite 

 minor or entirely absent. When interdependencies do occur, the 

 relations may be specific or non-specific. In case (2) species C is 

 primarily controlled by the physical features of the environment, but 

 it is also influenced by the presence of A and B specifically. An illus- 

 tration of this situation is furnished by the fresh-water mussels that 

 make use of fish swimming past as transport agents for their larvae. 

 During most of their lives the primary dependencies of the mussels 

 are concerned with the bottom, the water, and the plankton of their 

 biotope, but at the time of reproduction this particular relation with 

 the fish of their community becomes significant. 



In a third type of situation the presence of the other species exerts a 

 definite effect but not in a specific way. In other words, as far as C 

 is concerned, A and B may merely be occupying space, and individ- 

 uals of C could be substituted for them without significant conse- 

 quence to C as is suggested in case ( 3 ) above. Illustrations are found 

 in the competition for space among attached animals on submerged 

 rocks. Similarly, in the competition for light among plants the 

 proximity of other individuals often has its chief effect in cutting off 



