Invaded habitats often contain multiple exotic species which may reflect positive invader-invader interactions. The invasional meltdown hypothesis proposes that exotic species increase the success of other exotic species, often those on different trophic levels, resulting in a highly invaded habitat. To fully understand the causes and effects of multi-trophic invasions, invader-invader interactions must be considered. In a 16-week mesocosm experiment we investigated whether there was mutual facilitation between an exotic plant (Alternanthera philoxeroides – alligator weed) and an exotic herbivore (Pomacea insularum – island apple snail) consistent with an invasional meltdown. Forty freshwater wetland communities were subjected to single (snails or plants), successive (snails then plants or plants then snails), or simultaneous invasions (snails and plants), or were left as uninvaded controls. We collected data on native plant biomass and diversity, alligator weed biomass, native snail abundance, and apple snail size and abundance.
Results/Conclusions
Reductions in native plant biomass and diversity versus controls were greater for communities only invaded by apple snails compared to those only invaded by alligator weed. This suggests that exotic herbivores have greater impacts on these native wetland plant communities than do exotic plants. Apple snails fed preferentially on native plants so that even though they significantly reduced alligator weed biomass, they significantly increased the proportion of plant mass that was alligator weed. This suggests that these exotic herbivores may facilitate invasions by exotic plants in these ecosystems. Apple snail growth, survival, and fitness were all independent of the presence of alligator weed. This study shows that apple snails directly damage wetland ecosystems by consuming native plants and indirectly damage such ecosystems by providing opportunities for exotic plants to invade. However, we did not find mutual facilitation between these exotic plant and herbivore species as predicted by the invasional meltdown hypothesis.