Many ecosystems are now co-invaded by alien plant and herbivore species. The evolutionary naivety of native plants to alien herbivores can make the plants more susceptible to detrimental effects of herbivory than co-occurring invasive plants, in accordance with the apparent competition hypothesis. Moreover, the invasional meltdown hypothesis predicts that in multiply invaded ecosystems, invasive species can facilitate each other’s impacts on native communities. Although there is growing empirical support for these hypotheses, facilitative interactions between invasive plants and herbivores remain underexplored in aquatic ecosystems. Many freshwater ecosystems are co-invaded by aquatic macrophytes and mollusks and simultaneously experience nutrient enrichment. However, the interactive effects of these ecological processes on native macrophyte communities remain an underexplored area. To test these effects, we performed a freshwater mesocosm experiment in which we grew a synthetic native community of three macrophyte species under two levels of invasion by an alien macrophyte Myriophyllum aquaticum (invasion vs. no-invasion) and fully crossed with two levels of nutrient enrichment (enrichment vs. no-enrichment) and herbivory by an invasive snail Pomacea canaliculata (herbivory vs. no-herbivory). In line with the invasional meltdown and apparent competition hypotheses, we found that the proportional above-ground biomass yield of the invasive macrophyte, relative to that of the native macrophyte community, was significantly greater in the presence of the invasive herbivore. Evidence of a reciprocal facilitative effect of the invasive macrophyte on the invasive herbivore is provided by the results showing that the herbivore produced greater egg biomass in the presence than in the absence of M. aquaticum. However, nutrient enrichment reduced the mean proportional above-ground biomass yield of the invasive macrophyte. Our results suggested that herbivory by invader P. canaliculata may enhance invasiveness of M. aquaticum. However, nutrient enrichment of habitats that already harbor M. aquaticum may slow down invasive spread of the macrophyte. Broadly, our study underscores the significance of considering several factors and their interactions when assessing the impact of invasive species, especially considering that many habitats experience co-invasion by plants and herbivores and simultaneously undergo various other disturbances, including nutrient enrichment.

We performed a freshwater mesocosm experiment, and collected all data from this experiment. More details please see the Methods Section in the paper.

Details please see the ReadMe file.