Connectivity, scale-dependence, and the productivity-diversity relationship

We surveyed freshwater ponds (localities) nested within watersheds (regions) to evaluate the relationship between productivity and animal species richness at different spatial scales. In watersheds where the ponds were relatively distant from one another (likely reducing the level of interpond dispersal of many organisms), we found a scale-dependent productivity-diversity relationship; at local scales (among ponds), diversity was a hump-shaped function of productivity, whereas at regional scales (among watersheds), diversity monotonically increased with productivity. Furthermore, this relationship emerged because there was a strong relationship between productivity and pond-to-pond species compositional differences. Alternatively, in watersheds where ponds were relatively close together (likely leading to higher rates of dispersal of many organisms), we found no scale-dependence; diversity was a hump-shaped function of productivity at both local and regional scales. Here, the relationship between species compositional dissimilarity and productivity was much weaker. We conclude that whether or not scale-dependence is observed in productivity-diversity relationships will depend, at least in part, on the degree of connectivity among localities within regions.