Impacts of Invasion on a Freshwater Cleaning Symbiosis

Abstract Invasions are a driving force behind the loss of biodiversity, but the effect of invasion on native symbiont communities has received scant attention. While invasive hosts may present an opportunity to native symbionts through providing a novel host environment, invasive hosts could also be noncompetent host, resulting in negative impacts on native symbionts. Two invasive crayfish hosts in the genus Faxonius have been introduced in three watersheds in western Virginia, USA. Previous studies suggest that the invasive host F. cristavarius is a less competent host for native branchiobdellidan symbionts when compared to native hosts in the genus Cambarus. We carried out an extensive survey in these watersheds to determine impacts of varying degrees of invasion on symbiont abundance and diversity. We also conducted an experiment to investigate how increases in the relative abundance of invasive hosts contributes to observed patterns of symbiont abundance and diversity in the field. In our survey, as the proportion of invasive hosts at a site increased branchiobdellidan abundance and diversity declined significantly. In the experiment, the worms dispersed onto both native and invasive hosts. As the percentage of invasive hosts increased, the survival of symbionts declined. Both symbiont survival and opportunities for successful dispersal were reduced as invasive hosts displaced native hosts. Given that many native hosts accrue significant fitness benefits from their relationships with native symbionts, losses of beneficial symbionts may produce a feedback loop that decreases invasion resistance of native species, exacerbates the effects of invasions, and presents a major conservation issue.