of the disrupted community requires many years or even decades. It is not 

 yet clear whether massive development of A canthaster occurs periodically, 

 or whether this is an unprecedented phenomenon, related to anthropogenic 

 influence on the community (Endean, 1973). This sort of disruption of a 

 mature community plays a significant role in the evolution of the reefs, 

 creating a temporary mosaic of various stages of succession and allowing 

 the existence of opportunistic and fugitive species (Grassle, 1973). 



Human activity has the same sort of influence on a community. Oppor- 

 tunistic species occupy the primary position in the list of organisms 

 indicating pollution. They are the lest to disappear when pollution becomes 

 fatal for all life, and the first to appear in the course of self-purification 

 (Grassle, Grassle, 1974). Man's influence may also cause brief disruption 

 of mature communities. For example, on the shores of Puget Sound 

 (Washington), the climax community consists of ubiquitous colonies of 

 mussels. During stormy weather, the mussels die due to the impacts of 

 logs lost in timber floatage, which strike the rocks quite forcefully. 

 The empty spaces thus formed are rapidly covered with a bacterial -algal 

 film, after which these spots are inhabited by Chthalamus dall i , followed by 

 Balanus glandula , then B. cariosus and finally young mussels which, 

 with time, completely displace the algae, as well as the limpets, which 

 feed on them, barnacles, etc. (Dayton, 1971; Payne, 1974). 



The activity of "key species" can come to the same process--preventing 

 full colonization of a space and of all available food resources by dominant 

 species and exclusion of opportunistic species, except that they do this 

 constantly and gradually, not catastrophical ly rapidly. 



Studies of such processes have established the extremely important 

 role of the indirect, "weak" (MacArthur, 1972) relationships between 

 species, which may change the tolerance of species to abiotic and biotic 

 environmental factors and shift the equilibrium of the direct relationships 

 (Turpaeva, 1969, 1972; Turpaeva, Maksimov, 1971; Darnell, 1970; Dayton, 

 1971 ; and others) . 



The idea of the selection of communities from species which interact 

 minimally with each other has been applied in works dedicated to the 

 analysis of the structure of plankton ecosystems. In studying the phyto- 

 plankton of Pamlico Sound (North Carolina), Hulburt and Horton (1973) stated 

 the hypothesis that conditions favoring the creation of a high biomass 

 of phytoplankton might be dual: In one case, the growth of phytoplankton 

 as a whole is accelerated, while in another, the interactions between 

 species are minimized. In this latter case, one might observe either a 

 stop in the growth of one species, allowing rapid multiplication of another, 

 usually competing, species, or both species, multiplying rapidly and 

 simultaneously, no longer hindering each other or, finally, the development 

 of one species begins after the other, but their maxima overlap. The 

 conditions of minimization of interaction, therefore, are manifested in 

 that the influence of the law of competitive exclusion of species occupying 

 the same niche, limiting the growth of the biomass, is removed; the "either- 

 or" principle (either one species or another with which it competes) is 

 replaced by the "and" principle. 



