Host-parasite interactions are indirectly influenced by trophic and competitive interactions in the broader ecological community. Invasive species often exert strong effects on ecological communities, which may include altering disease dynamics. We tested the potential for invasive macrophytes to influence the dynamics of freshwater snails and the schistosomes they transmit to humans. Schistosome production by infected snails increases with resource availability, and snails can scrape detritus, biofilms, and algae from plant surfaces, but cannot eat live plant tissue. Therefore, we hypothesized that inedible plants could disrupt schistosome transmission if they outcompete edible resources for light and nutrients. We established 100 L mesocosms inoculated with snails, phytoplankton, and zooplankton and manipulated the presence of four invasive, inedible plants common to schistosome-endemic regions: water hyacinth, water lettuce, duckweed, and hornwort, with a macrophyte-free control. We dosed in schistosome eggs four times and estimated algal production, host growth, reproduction, and total parasite production over 16 weeks. We predicted plant-specific effects on schistosome dynamics based on individual traits like nutrient uptake rates and shading potential. We predicted duckweed would strongly decrease algal production, host growth, reproduction, and total parasite production, water lettuce and water hyacinth would cause a decrease, and hornwort would have no effect.
Results/Conclusions
As expected, the inedible invasive plants reduced the growth of edible algal food. Snail population density and reproduction also depended on invasive plant treatment. We observed two initially counterintuitive phenomena. First, despite suppressing algal resources, water lettuce invasions lead to large increases in snail abundance but not parasite production. We speculate that this was caused by the plant’s high rate of detritus production subsidizing snail populations. We did not detect differences in total parasite production, but infections were rare, which limited our power. Second, water hyacinth invasion caused snail abundance to remain low, but individuals grew significantly larger. This effect may have been driven by poor hatching or recruitment of juveniles. Thus, species-level traits of invasive plants related to light and nutrient composition as well as tissue turnover could drive predictable changes in the transmission of parasites of aquatic consumers. Scaling these effects up could enable managers to manage invasive species without unintentionally increasing transmission potential to humans, livestock, or other wildlife.