With ecosystems increasingly supporting multiple invasive species, interactions among invaders could magnify or ameliorate the undesired consequences for native communities and ecosystems.  We evaluated the individual and combined effects of Chinese mystery snails (Bellamya chinensis) and rusty crayfish (Orconectes rusticus) on native snail communities and ecosystem attributes.  In many regions of the US, Chinese mystery snails appear to be widespread in freshwater habitats but, as of yet, remain poorly understood with respect to their ecological importance.  The large size (up to 60 mm), unique life history, and high density (~10 m^-2) of these snails suggest that their invasion may lead to important ecological changes in lake communities.  We combined intensive surveys/re-surveys of 40 north temperate lakes, extensive surveys of >200 lakes, and a mesocosm-based experiment to evaluate the causes and consequences of both invaders and their interactions. 

Results/Conclusions

Survey data from 242 Wisconsin lakes revealed that these species co-occurred 50% more often than expected by chance, indicating that their co-occurrence is common.  Intensive field sampling suggested that Chinese mystery snails were more common in lakes with a higher density of human habitation, higher levels of nutrient enrichment, and a lower native snail diversity.  A factorial experiment revealed that while the two invaders had only weakly negative effects upon one another, both negatively affected the abundance and biomass of native snails, and their combined presence drove one native species to extinction and reduced a second by >95%.  Crayfish predation also had a cascading effect by reducing native snail biomass, leading to increased periphyton growth.  Bellamya, in contrast, reduced periphyton biomass, likely causing a reduction in growth by native lymnaeid snails.  Bellamya also increased water column N:P, possibly because of low P excretion rate relative to native snail species.  Together, these findings suggest that interactions among invasive species at different trophic levels can cause important community and ecosystem level effects.