Phenotypic variability of rusty crayfish (Faxonius rusticus) at the leading edge of its riverine invasion

Species around the globe are undergoing phenotypic shifts at ecologically relevant timescales as they invade new ecosystems and respond to changing environments. Disentangling the contribution of environmental gradients from the process of range expansion in shaping these changes, and identifying the specific traits undergoing selection, are both critical to anticipate the secondary spread and impact of invasive species. Here, we investigate phenotypic changes in rusty crayfish (Faxonius rusticus), a nuisance invasive species, through an extensive survey of their invasion gradient in multiple tributaries of the John Day River (JDR, Oregon, U.S.A.), a major tributary of the Columbia River. Rusty crayfish in the JDR have developed better physiological condition (intrinsic growth and/or reproductive potential measured as RNA/DNA ratio) but less competitive morphology (lighter body and smaller claws) as they spread upstream and downstream from their location of initial introduction. In addition, rusty crayfish in invasion front populations are at a lower trophic level than conspecifics closer to core areas. By accounting for variations in temperature, primary productivity, and prey (macroinvertebrates) biomass throughout the invasion extent of rusty crayfish, our findings suggest that low conspecific densities at the invasion edge and spatial sorting primarily drive these phenotypic changes. The trends observed here are thus likely to intensify over time as rusty crayfish continues to rapidly spread throughout the JDR and reach the mainstem Columbia River. Our study shows that phenotypic shifts can manifest rapidly over environmental gradients experienced during the range expansion of aquatic invasive species. Patterns in both morphological and functional traits documented in the JDR demonstrate the importance of both environmental factors and dispersal processes in shaping these responses in riverine networks.