Small-scale plant species richness in calcareous grasslands determined by the species pool, community age and shoot density

Variation in small-scale plant species richness was examined between twenty discrete sites of calcareous alvar grasslands in the western Estonian coastal area. This community type is very species rich at the small-scale. The pattern of diversity has often been explained by variation in ecological conditions which makes it possible to avoid competitive exclusion. Here we test an alternative species pool hypothesis. The species pool was defined as the set of species present in a community, but excluding all those species whose presence may be connected with exceptional environmental conditions for this community. Monte Carlo modelling was used to determine the strengths of the relationships between the non-independent variables: richness vs species pool and average shoot density vs richness. Site age was characterized by its elevation above sea level, since there is a continuous land uplift in the study area. Species richness was positively correlated to the size of the species pool, community age and vegetation density. We conclude that historical processes on both the regional and local levels, which determine the arrival of any particular species to a target community, and not environmental heterogeneity, are responsible for the variations in species richness between communities of this type.