Nutrients influence the multi-trophic impacts of an invasive species unaffected by native competitors or predators.

Non-native species often lead to undesirable ecological and environmental impacts. Two hypotheses that predict establishment of non-native species are enemy release and biotic resistance. Support for these hypotheses in freshwater invasions is mixed. Experiments combined with field observations provide a complementary approach to understanding how interactions between native and non-native species lead to enemy release or biotic resistance. We tested experimentally whether these hypotheses provided insights into the invasion of the banded mystery snail (Viviparus georgianus), which has invaded the Great Lakes region and northeastern Unites States (US) from the southeastern US. Because freshwater systems vary widely in their nutrient concentrations due to natural and anthropogenic processes, we tested whether nutrient additions altered competitive and predatory interactions that regulate mechanisms of enemy release or biotic resistance. We evaluated the status of the mystery snail invasion in a 3-year field survey of Lake George (NY, US) to identify if field observations supported any experimental conclusions. The presence of the banded mystery snail led to a 14% and 27% reduction in biomass of a native competitor under low- and high-nutrient concentrations, respectively. The mystery snail also triggered a 29% biomass loss of a native snail predator, but only in low-nutrient concentrations. Field surveys indicated that the mystery snail dominated the snail community; of seven snail species, it comprised 77% of all snails. Results from the field surveys combined with experimental results indicate that neither competitors nor predators have likely suppressed the invasion of the banded mystery snail. This conclusion is consistent with competitive- and predatory-enemy release as we found no indication of biotic resistance via competition or predation from native species. Our results further highlight that the post-establishment impacts of invasive species are altered by the trophic state of freshwater ecosystems.