Saturation and Invasion Resistance of Non-Interactive Plant Communities

Open plant assemblages in shoals of western Caucasian rivers were used as examples to analyze the relationship between the species saturation and the number and total abundance of alien species in non-interactive communities. Invasion of exotic species into highly saturated communities has been demonstrated to be, on average, less probable than their invasion into unsaturated communities. A hypothesis explaining the relationship between these parameters has been put forward. According to the hypothesis, the number of alien species in a specific locality in a community is determined by their ratio to the number of native species in the species pools of these communities; and their mean abundance, by the ratio of the total number of species to the number of individuals in the localities. Both ratios are smaller in saturated biocenoses, which determines a relatively small admixture of alien species in them.