Differential saturation of Pacific Northwest and Southeast (USA) fish assemblages

Plots of local versus regional richness have been used to test whether assemblages are ‘saturated’ with species. However, the validity of these tests is limited by scale-dependence and arbitrarily defined sampling units, statistical autocorrelation between local and regional richness data, and the confounding effects of propagule pressure. In this study, local versus regional richness plots were used to test the saturation hypothesis for Pacific Northwest and Southeast (USA) fish assemblages, taking care to account for each of the above problems. Specifically, longitudinal river zones were used to ensure that the regional sampling units were not scale-dependent or biologically arbitrary. A log-ratio transformation was used to remove autocorrelation from the local and regional richness data. And a comprehensive fish stocking database was used to account for propagule pressure. Results suggest that the Pacific Northwest fish assemblages, which have low native richness, are not saturated, but the species-rich Southeast assemblages are at or approaching saturation.