Here today, gone tomorrow: the effects of an invading host on a community of native symbionts

Abstract Invasions by new host species could potentially affect native symbionts associated with native hosts. Effects of invading hosts can be either negative or positive depending on host competence. Three native crayfish species ( Cambarus spp.) in the New River in North Carolina, USA, may host up to seven species of ectosymbiotic worms (Branchiobdellidans). Branchiobdellidans require crayfish for successful reproduction. Faxonius cristavarius , an invasive crayfish in the New River, is a noncompetent host for native branchiobdellidans and removes colonizing worms. The distributions of three native crayfish species now overlap with F. cristavarius in the New River. We collected crayfish at various locations in the New River to determine the diversity and abundance of worms on native Cambarus spp. and F. cristavarius. Sampled sites ranged from 0 to 100% invaded by F. cristavarius. There were significant reductions in branchiobdellidan abundance and richness on native Cambarus spp. in response to an increasing relative abundance of F. cristavarius . We also conducted an experiment to determine whether the presence of F. cristavarius affected the abundance of worms on the native Cambarus robustus . The control treatment (Cr/Cr) contained a single C. robustus stocked with 5 worms paired with an unstocked C. robustus; a second treatment contained a single, stocked C. robustus paired with an unstocked F. cristavarius (Cr/Fc). Significantly fewer worms remained in the Cr/Fc treatment. Our research shows that invasions by non- or low competent hosts can have significant, negative effects on native symbiont communities. Similar negative effects on native symbionts may occur in other host invasions.