Towards an integrated theory of body size distributions, life history and functional traits, and environmental flows in freshwater ecosystems

Efforts to quantify and explain individual size distributions (ISD) are prominent in ecological research and potentially for applications to management which increasingly consider ecosystem-scale objectives. We use a nation-wide database of individual size measurements for United States stream fishes paired with species’ traits and test for direct and indirect effects of traits, flow, and land use on the slope of the interspecific ISD. Modeling results show that traits have strong, direct effects on ISD whereas flow and land use primarily indirectly affect ISD via mediation by functional traits. ISDs flatten (i.e., greater abundances of larger-bodied individuals) when environmental conditions favor higher trophic levels, thermal tolerances, and periodic life histories, but steepen (i.e., greater abundances of smaller-bodied individuals) when environments benefit opportunistic life histories. Our framework that incorporates direct and indirect of traits, flow alteration, and land-use on ISD could be expanded to incorporate additional variables that interact with flow regimes (e.g., temperature, physical habitat) to understand and predict how multiple environmental stressors affect ecosystem functions.