How Well Does the Flow-Trait Framework Predict Short-Term Fish Community Dynamics in Unregulated Rivers?

Natural History & Odum School of Ecology, 5USGS Virginia Cooperative Fish & Wildlife Research Unit Stream fish ecologists have recently emphasized interactions between hydrology and species’ life-history traits as useful predictors of community composition and change, particularly across large geographic areas, in regulated rivers, and at coarse spatiotemporal scales. We evaluated the applicability of such an approach at a finer spatiotemporal grain that allowed an assessment of how flow and traits affected community dynamics via demographic responses (persistence and colonization) of individual species. Specifically, we tested hypotheses that (1) greater hydrologic variation leads to greater community variation, but (2) effects of increasing hydrologic variation on demography vary by life-history, being positive for opportunistic strategists and negative for equilibrium strategists. We tested hypotheses using data from intra- and inter-annual fish surveys collected over 21 years at shoals in the Roanoke and Conasauga rivers, two unregulated ridge and valley rivers with phylogenetically comparable fish communities. We used abundance data to examine correlations between taxonomic and functional community composition and variation in flow magnitude and the frequency of extreme high and low flows. We also analyzed presence-absence data in a dynamic occupancy framework to investigate the influence of these flow metrics and species traits on colonization and persistence. In both rivers, overall community composition was highly persistent over time, and contrary to expectations, interannual variation was poorly explained by hydrologic or trait variables. As expected, the influence of extreme-flow frequency on persistence varied among life-history strategists. However, contrary to our hypothesis, extreme flows were positively related to persistence of both opportunistic and equilibrium taxa. Thus, our results suggest a relatively weak influence of hydrology and traits on fish populations and communities in these rivers, over the rang