Dominants in plant communities: the nature of impact on biomass determines the thresholds of impact on local species richness

In accordance with existing ideas, a state of dominance in plant communities by some species (including alien and expansive ones) can be achieved through the use of resources of other species (1), also and additionally through the use of previously unused resources (2), also and additionally by allelopathy or changes in environmental conditions (3). It is believed that in the first case this process does not affect the total biomass of communities, in the second it is accompanied by its increase, in the third it is mainly reduced. It can be assumed that the mechanism of increasing the degree of dominance of individual species also determines the nature of their influence on species richness. To test this hypothesis, we compared the participation of dominants, biomass, and the number of accompanying species in a series of biomass samples taken from 67 sites of terrestrial plant communities in the Western Caucasus and Ciscaucasia (high and low mountain meadows and steppes, communities of wastelands, old fallows, etc.). The results showed that 1) in these communities different variants of the relationship between the participation of dominants and biomass are observed, which means that, presumably, different mechanisms of influence of dominants on accompanying species are realized; 2) the distribution of these mechanisms differs in natural (semi-natural) and synanthropic communities, with the dominance of native and alien species; 3) the nature of the impact of dominants on biomass determines the thresholds for their impact on local species richness; 4) in synanthropic communities with the dominance of alien species, these thresholds are more pronounced than in communities of other types. At the same time, our earlier obtained results showed that native and alien species do not often reach the degree of dominance, the excess of which poses a significant threat to the species richness of plant communities.