Small-scale species richness of plant communities with similar biomass: the influence of habitat types in the context of historical hypothesis (Western Caucasus and Ciscaucasia, Black and Azov Seas, Russia)

We compared the species richness of the plant communities of different habitats on different sites of the biomass gradient. The analysis was based on data on 1089 above-ground biomass samples taken from the sites 0.25 m2 of terrestrial herbaceous communities of the Western Caucasus and Ciscaucasia (30–2800 m a.s.l.) and 317 samples taken from the same sites of the macrophytobenthos of the Black and Azov Seas (0.5–10 m depth). Results showed that a certain amount of biomass, sampled in communities of different types, includes, on average, a significantly different number of species. Thus, among terrestrial plant communities, highmountain communities of the Western Caucasus are characterized by the highest species richness in most ranges of the biomass gradient; communities of herb layer of shaded forests – on the contrary, the lowest richness. The macrophytobenthos of the Black Sea is characterized by lower species richness than most of the terrestrial communities, but higher species richness than the benthic communities of the Azov Sea. We showed that significant difference in the small-scale species richness of these communities can be plausibly explained by the peculiarities of their history.