Relationship between species richness and productivity in plants: the role of sampling effect, heterogeneity and species pool

1. The relationship between environmental productivity and the number of species [species richness-productivity relationship (SRPR)] has been thoroughly studied, but the mechanisms responsible for its form are still largely unknown, possibly because the majority of studies have focused on evaluating the sole effect of a single hypothesis. 2. We tested whether variation in species richness along a productivity gradient is due to variation in (i) the number of individuals, (ii) the number of species in the species pool or (iii) habitat heterogeneity. 3. We measured species richness (S), individual abundance (N) and productivity (P) estimated as standing biomass in different herbaceous communities in the Czech Republic at two spatial scales. Species pool (Spool) was obtained from a database concerning individual habitats, and habitat heterogeneity (H) was measured using the community dissimilarity index. 4. The SRPR was scale-dependent: at the smaller spatial scale of individual plots, there was a significant curvilinearly negative relationship between S and P, whereas at the larger site scale it turned into a non-significant relationship. 5. Species richness was significantly affected by a combined effect of N and Spool at the plot scale and by a combined effect of Spool and H at the site scale. None of these variables was sufficient to explain the SRPR by itself. 6. Synthesis. Our findings indicate that there is no universal form of the species-productivity relationship, and the SRPR is driven by multiple scale-dependent mechanisms. It is important to consider the joint effect of different factors in explaining species richness patterns rather than to focus on the sole effect of productivity.