The relationship between regional species richness, local hydrologic characteristics, and local species richness in North American freshwater fishes

Background/Question/Methods

The positive relationship between local and regional species richness is considered a general pattern in numerous taxonomic groups, including aquatic species.  However, instream flow regimes are important factors shaping local aquatic assemblages.  Local assemblage structure, therefore, is ultimately constrained by the regional species pool and further filtered by local hydrologic regimes.  We tested three hypotheses using North American fishes: 1) discharge volume and variability explains a significant component of variation in the relationship between regional and local richness; 2) the relationship between hydrology, regional species richness, and local species richness varies among families; and 3) within each family, species richness is also correlated with local richness of other species (OTrich).  Total local richness and richness estimates within the four most diverse families were used to assess the degree that historical constraints within lineages may influence relationships among variables.  We compiled data from 41 stream fish assemblages, daily discharge measurements, and regional species distributions.  Mean annual discharge and flow variability were quantified based on 13 hydrological indices summarized with principal component analyses (PCA).  We conducted multiple regressions upon local richness, with regional species pool measures, mean annual discharge, principal component scores (PC1–PC3), and OTrich (for families only) as independent variables.  

Results/Conclusions

Total local richness was positively correlated with regional richness and mean annual discharge.  Regional richness was an important explanatory variable for cyprinids, centrarchids, and catostomids, although the amount of variation explained by this variable differed among families.  Local richness in Cyprinidae was also correlated with PC1 due to a positive correlation with one-day maximum flow.  Local richness in Centrarchidae was positively correlated with OTrich and PC1, which was positively correlated with high flow events.  Local richness in Catostomidae was positively correlated with mean discharge.  Regional species richness was almost always correlated with local richness.  Discharge volume, indicating larger stream size and potentially more habitat area, was frequently correlated with local species richness, while flow variability appeared to be important for cyprinids and centrachids, suggesting taxon-specific responses to flow regimes.