Assessment of Bottom River Communities in Considering the “Dark and Gray Diversity” of Species: Approaches to a Solution

The species diversity of the entire meta-community is made up of local pools of species of individual communities, including both the totality of actually observed taxonomic groups and species of “dark” diversity, which, by their ecological properties, are theoretically suitable for living in existing conditions and are potentially found during repeated studies. To assess the dark diversity, quantitative indices of suitability of the surveyed areas of the study area are calculated in relation to various taxa of the community under consideration. The study of the species composition of benthic communities was carried out on small and medium lowland rivers, tributaries of the Kuibyshev, Saratov and Volgograd reservoirs, including the rivers of the arid region of the Lake Elton basin. A total of 132 local communities were surveyed, from which 147 taxa of macrozoobenthos were selected for calculations. To highlight the dark diversity, an analysis of the joint occurrence of species was carried out and, using hypergeometric distribution formulas, estimates of the conditional probabilities of the statistical relationship of each of these species with individual sections of the rivers were calculated. Relationships between observed species richness and dark diversity are considered; the occurrence of latent species was tested in sections of rivers of a similar type. The concept of “gray” diversity is introduced, i.e., a subset of observed species, in which the indices of correspondence to the focal area are less than a given threshold.