[Species saturation and invasibility of the plant communities: a hypothesis of species pools correlation].

Within a set of plant communities of Western Caucasus (open phytogroups of mountain near-riverbed shoals, phytocoenoses of glades and stepped meadows, tree tiers of riverbed forests), analysis is given to the correlation between levels of their species completeness, and number and total abandons of the adventive species. An assessment of the species completeness is based on an assumption that its increase is directed toward maximal species diversity with minimal abundance of any particular species in the species pool. The results indicate that both the open phytogroups and closed communities with relatively high species completeness are characterized by a less number of the adventive species, as well as by their lower average and total abundance, as compared to the species-incomplete ones. A hypothesis is suggested to explain correlation between species completeness and invasiveness of communities. It is based on assumption of an approximate average equivalence of aboriginal and adventive species and of stochastic processes of their immigrations and extinctions in particular communities. In accordance to this hypothesis, a number of the adventive species in particular parts of the communities is determined by their ratio to the aboriginal ones in these communities, with their average abundance being determined by the ratio of the species number to the total individuals number in those parts of the communities. The both ratios are relatively lower in more species-complete communities, which is caused by their lower saturation by adventive species. However, the impact of the latter factor on the stability of plant communities is not especially significant, as it is decreases with increase of intensiveity of adventive species invasions and respective increase of their portion in the species pools.