There is an opinion that the pressure of competition in the plant communities of stable and productive habitats leads to the decrease in evenness of structure of species abundance and species richness up to the monopolization of plots by the most successful competitor (Huston, 1979; Bengtsson et al., 1994). Hence, between the species richness of phytocoenoses and relative density of individuals of dominant species (RDD), negative dependence should be observed. However, according to another view, the number of species and structure of their abundance are determined by the different processes. The number of species is determined by the species pool size and the rate of species immigration, while the abundance structure — by the competition (Stirling, Wilsey, 2001; Ma, 2005; Wilsey, Stirling, 2007). In particular, in some communities a decrease of RDD leads to an increase in abundance of subdominant species and to oligodominant structure. These changes in relative abundance could occur without changes in species richness. Therefore these variables could be weakly related (Ma, 2005; Wilsey, Stirling, 2007; Munson, Lauenroth, 2009). However, it is unclear how widely these scenarios are realized in the plant cover. The aim of our study was to examine the relationship between RDD, the dominance structure as a whole (mono- or oligodominant), the species richness and the size of the species pool in the forest stands of the Western Caucasus.
Species-pool hypothesis: Limits to its testing
