Species pools and the "hump-back" model of plant species diversity: an empirical analysis at a relevant spatial scale

We investigated the relative roles of productivity, the species pool, and spatial habitat structure in determining local species richness (alpha diversity) of plant communities within a single, well-defined landscape unit, at spatial and ecological scales where the relationship between community productivity and species diversity often assumes a unimodal or "hump-back" form. At high levels of productivity, the decrease-phase of the unimodal model of the diversity-productivity relationship is typically explained as the dynamic outcome of increased competitive exclusion, but it may also be the passive consequence of a small pool of species possessing attributes necessary to competitively survive in high-fertility environments. We conducted statistical analyses of previously collected data to determine whether variations in local richness in the herbaceous vegetation of a Slovakian mountain valley were best explained by habitat productivity itself (which presumably leads to more intense competition) or by the sizes of the relevant community species pools. We also used measures of spatial habitat structure to investigate the extent to which habitat patchiness influenced patterns of species diversity. In the study system, both community biomass and size of the species pools contributed significantly to local species richness, but the positive effect of the species pools was about twice as important as the negative effect of biomass. The combined area of related associations (alliance area), association perimeter, and habitat patch geometry were all closely related to species pool size.