Processes governing riverine fish species richness are scale-independent

The ability of macroecological approaches to identify the mechanisms that underlie spatial patterns in species richness has been heavily debated in part because of continued attention to the role of scale-dependence in the mechanisms regulating biodiversity. Here we show that species abundance is a scale-independent predictor of species richness for riverine fish. We modelled local species richness of riverine fish in 976 rivers in the continental United States and used this parameterized local model to predict regional species richness at increasingly coarser scales that were defined according to U. S. Environmental Protection Agency (EPA) Ecoregion classes (Ecoregion III: fine scale, Ecoregion II: medium scale, and Ecoregion I: coarse scale). We found that the locally parameterized model predicted species richness most accurately at the coarsest scale, explaining 80% of the variability in total species richness. An inverse set of models, which used the coarsest scale data to parameterize models tested at finer scales, showed that 38% of the variability in local species richness was explained by the coarse scale model. Total abundance was the single most important predictor of species richness across all scales with a slope ranging from 0.4-0.56 on a log-log plot. Although the specific mechanisms linking total species richness and energetic constraints may be scale-dependent, total abundance may be a scale-independent predictor of the total number of species that can be supported in a given area.