A second example of an analogous structure of a community is to be 

 found in the benthic communities of the intertidal and phytal zones. 

 BoudouresQue (1971a) demonstrated that the species composition of algae 

 in the Mediterranean sea near Marseille changes continually with depth, 

 the distribution of each species of algae changing along a transect 

 independently of the others, although in some places there is a sudden 

 change in species composition. A similar conclusion was reached by 0. G. 

 Kussakin et al . (197^, p. 23). At times, the fauna is distributed 

 less discretely than the flora (Denisov, 1974; Boudouresque, 1971a, b). 

 Of course, such cases--and the number of examples may be quite great-- 

 fit well into the system of Lindroth. 



The empirical conclusions of Lindroth were subjected to theoretical 

 analysis by Fager (1963), Mills (1959), G. F. Jones (1969), Boudouresque 

 (1971a) and other authors (see the reviews of Longhurst, 1964, and 

 W. Stephenson, 1973). It follows from these works that the species of 

 animals and plants, both in the water, and on the bottom, are distributed 

 so that each of them reacts to gradients of environmental factors independ- 

 ently of the others. The environmental factors may be abiotic or biotic, 

 the latter including most commonly the influence of the dominant species, 

 the edifier species. Communities of species are, strictly speaking, 

 recurrent groups, i.e., groups of species which usually (in the statistical 

 sense of the word) are encountered together. This simultaneous encountering 

 of members of the group results from the similarity of their response to 

 environmental factors. Under conditions of extreme variability of abiotic 

 environmental factors, these factors may have a decisive influence on the 

 composition of a community--such biotopes are occupied by "physically 

 controlled communities." As the variability of the physical (abiotic) 

 factors decreases, biotic relationships take over, and in biotopes with 

 relatively stable abiotic conditions, "biologically accommodated" or 

 "biologically fitted" communities arise. They differ from physically 

 controlled communities in that the former are composed of species which 

 react identically to certain gradients in abiotic conditions, while the 

 latter consist of species which are "fitted" to each other, mutually 

 adapted to coexistence. Thus, a community can be defined as a "group of 

 organisms occurring in a particular environment, presumably interacting 

 with each other and with the environment, and separable by means of 

 ecological survey from other groups" (Mills, 1969, p. 1427). Each community 

 is, therefore, a "relative continuum between relative discontinuities" 

 (Boudouresque, 1971a, p. 132). To describe communities of this type, 

 Boudouresque (1971b) used the concept of the nodum. A nodum is an 

 accumulation ("constellation") of points in a certain hyperspace, completely 

 or partially isolated from other such accumulations. Groups of species 

 which react identically to environmental factors are nodes within the 

 multidimensional continuum. A community is also a nodum, i.e., a sector 

 of the continuum limited by a space of interrupted continuum, an ecotone. 

 A biocenosis (parallel to a community in the sense of Thorson) is a nodum 

 consisting of a certain number of elementary nodes--ecologic or recurrent 

 groups. 



It is not difficult to see that the concept of the community as a 

 continuum limited by sectors of interrupted continuity corresponds best to 

 the model of the biocenosis as a structure with negative interspecific 

 correlations, i.e., with the least competitive interactions (Ivlev, 1954; 



