Opposite effects of nutrient enrichment and herbivory by an alien snail on growth of an invasive macrophyte and native macrophytes

Human-mediated introduction of plant and animal species into biogeographic ranges where they did not occur before has been so pervasive globally that many ecosystems are now co-invaded by multiple alien plant and animal species. Although empirical evidence of invaders modifying recipient ecosystems to the benefit of other aliens is accumulating, these interactions remain underexplored and underrepresented in heuristic models of invasion success. Many freshwater ecosystems are co-invaded by aquatic macrophytes and mollusks and at the same time experience nutrient enrichment from various sources. However, studies are lacking that test how nutrient enrichment and co-invasion by alien herbivores and plant species can interactively affect native plant communities in aquatic habitats. To test such effects, we performed a freshwater mesocosm experiment in which we grew a synthetic native macrophyte community of three species under two levels of nutrient enrichment (enrichment vs. no-enrichment) treatment and fully crossed with two levels of competition from an invasive macrophyte Myriophyllum aquaticum (competition vs. no-competition), and two levels of herbivory by an invasive snail Pomacea canaliculata (herbivory vs. no-herbivory) treatments. Results show that herbivory by the invasive snail enhanced above-ground biomass yield of the invasive macrophyte. Moreover, the invasive herbivore preferentially fed on biomass of the native macrophytes over that of the invasive macrophyte. However, nutrient enrichment reduced above-ground biomass yield of the invasive macrophyte. Our results suggest that eutrophication of aquatic habitats that are already invaded by M. aquaticum may slow down invasive spread of the invasive macrophyte. However, herbivory by the invasive snail P. canaliculata may enhance invasive spread of M. aquaticum in the same habitats. Broadly, our study underscores the significance of considering several factors and their interaction when assessing the impact of invasive species, especially considering that many habitats experience co-invasion by plants and herbivores and simultaneously undergo varous other disturbances including nutrient enrichment.