Regional and local impact on species diversity - from pattern to processes

The impact of regional factors (such as speciation or dispersal) on the species richness in local communities (S-L) has received increasing attention. A prominent method to infer the impact of regional factors is the comparison of species richness in local assemblages (S-L) with the total number of species in the region (S-R). Linear relations between S-R and S-L have been interpreted as an indication of strong regional influence and weak influence of interactions within local communities. We propose that two aspects bias the outcome of such comparisons: (1) the spatial scale of local and regional sampling, and (2) the body size of the organisms. The impact of the local area reflects the scales of ecological interactions, whereas the ratio between local and regional area reflects the inherent moment of autocorrelation. A proposed impact of body size on the relation is based on the high dispersal and high abundance of small organisms. We predict strongest linearity between S-R and S-L for large organisms, for large local areas (less important ecological interactions) and for sampling designs where the local habitat area covers a high proportion of the regional area (more important autocorrelation). We conducted a meta-analysis on 63 relations obtained from the literature. As predicted, the linearity of the relationship between S-L and S-R increased with the proportion of local to regional sampling area. In contrast, neither the body size of the organisms nor the local area itself was significantly related to the relation between S-L and S-R. This indicated that ecological interactions played a minor role in the shape of local to regional richness plots, which instead was mainly influenced by the sampling design. We found that the studies published so far were highly biased towards larger organisms and towards high similarity between the local and regional area. The proposed prevalence of linear relationships may thus be an artefact and plots of S-L to S-R are not a suitable tool with which to infer the strength of local interactions.