On Local Species Saturation of Synanthropic Plant Communities in the Western Caucasus and the Role of Alien Species in Its Achievement

Destruction of natural landscapes in combination with their subsequent urbanization is considered as one of the main reasons for the extinction of species. This process is accompanied by the penetration of alien plants into disturbed habitats. If at the local level the disappearance of native species is fully compensated by the appearance of alien ones, then synanthropic communities should be characterized by no less saturation than natural or seminatural communities of the same area. We considered this issue using the example of plant communities of the city of Maykop, its environs, and adjacent territories (Western Caucasus, 180–700 m above sea level). For this purpose, we compared the species richness of communities with different levels of synanthropization, but similar production. Additionally, for each range of the production gradient, we considered the ratio between the number of alien species, native species, and the total number of species in communities with a high level of synanthropization. For the analysis, we used 1256 samples of dry aboveground biomass. The results showed that biomass samples collected in synanthropic communities, as expected, are characterized by a significantly higher proportion of alien plant species, their proportion among synanthropic species, and the frequency of their dominance compared to samples collected in natural and seminatural communities. At the same time, the level of their saturation is not the same in different parts of the production gradient. The lowest saturation level is found in communities (biomass samples) with a high level of synanthropization (81–100%) and average production (50–150 g per 0.25 m2). Their species richness is 15–25% lower than in natural (seminatural) communities and 20–40% lower than in slightly disturbed communities (with a synanthropization level from 1 to 20%). Synanthropic, natural (seminatural), and slightly disturbed communities of highly productive habitats (more than 200 g per 0.25 m2) differ insignificantly in this regard. The highest level of saturation is characteristic of synanthropic communities of low-productivity habitats (less than 50 g per 0.25 m2). Moreover, it can be assumed that, in areas of 0.25 m2, their species richness is close to the upper limit.