AUTHORS: Cheyenne E. Stratton, Missouri Department of Conservation; Donald C. Behringer, University of Florida; Jamie Bojko, Teesside University; Margaret L. Taylor, University of Florida; Lindsey S. Reisinger, University of Florida
ABSTRACT: Parasites can affect their hosts’ ecological impacts by reducing host density and altering host traits such as behavioral traits. Most research focuses on density-mediated impacts, but trait-mediated impacts may be equally or more important as they control the interactions among organisms. Parasites can act as top-down forces in ecosystems and can trigger trophic cascades through density- or trait-mediated effects. We investigated how a parasite alters the well-documented ecological impacts of the invasive rusty crayfish (Faxonius rusticus). In 2019, we discovered a significant decline in the F. rusticus population in a Wisconsin lake corresponding to an outbreak of a microsporidian parasite, Nosema rusticus, which reduced infected crayfish activity levels. We hypothesized that infected crayfish, due to reduced activity and lethargy, would have lesser impacts on lower trophic levels including macrophytes and leaf litter (i.e., a trait-mediated trophic cascade). We also predicted that N. rusticus would increase crayfish mortality. To test this, we conducted a 4-week mesocosm experiment in which we varied parasite prevalence to test the trait-mediated indirect effects of the microsporidium on resource biomass and abundance. We also varied crayfish density to compare the strength of trait-mediated effects to a reduction in crayfish density. In support of our hypothesis, we found that less macrophytes and leaf litter were consumed in mesocosms with infected crayfish. Our results also suggest that this parasite is likely to cause density-mediated effects because crayfish infected with N. rusticus had higher mortality rates. These results suggest that parasites can mitigate the impacts of invasive species through both trait- and density-mediated trophic cascades.
