Experimental test of the Invasional Meltdown Hypothesis: an exotic herbivore facilitates an exotic plant, but the plant does not reciprocally facilitate the herbivore

Ecosystems with multiple exotic species may be affected by facilitative invader interactions, which could lead to additional invasions (invasional meltdown hypothesis). Experiments show that one‐way facilitation favours exotic species and observational studies suggest that reciprocal facilitation among exotic species may lead to an invasional meltdown. We conducted a mesocosm experiment to determine whether reciprocal facilitation occurs in wetland communities. We established communities with native wetland plants and aquatic snails. Communities were assigned to treatments: control (only natives), exotic snail (Pomacea maculata) invasion, exotic plant (Alternanthera philoxeroides) invasion, sequential invasion (snails then plants or plants then snails) or simultaneous invasion (snails and plants). Pomacea maculata preferentially consumed native plants, so A. philoxeroides comprised a larger percentage of plant mass and native plant mass was lowest in sequential (snail then plant) invasion treatments. Even though P. maculata may indirectly facilitate A. philoxeroides, A. philoxeroides did not reciprocally facilitate P. maculata. Rather, ecosystems invaded by multiple exotic species may be affected by one‐way facilitation or reflect exotic species’ common responses to abiotic factors or common paths of introduction.