1. Contemporary ecosystems commonly support multiple nonnative species, which can alter community structure and interact with one another directly and indirectly. Novel interactions between invasive species can result in the facilitation or suppression of one or both invaders, making it important to understand the underlying mechanisms that affect invasion outcomes.   

2. Goldfish (Carassius auratus) and Chinese mystery snails (Cipangopaludina chinensis) are freshwater species that have been introduced widely through the aquarium trade and co-occur in their nonnative ranges. We used an outdoor mesocosm experiment to assess separate and combined effects of goldfish and mystery snails on pond community structure and to investigate whether goldfish and mystery snails affected one another positively, neutrally, or negatively. We predicted that the distinct trophic levels of each species and the anti-predator traits of mystery snails would lead to positive interactions between invaders and synergistic effects on the aquatic community. 

3. Goldfish directly increased turbidity and decreased floating filamentous algae, zooplankton, juvenile native snails, and survival of amphibians via consumptive effects. Goldfish indirectly increased attached periphyton, phytoplankton, and body size of surviving native green frogs (Lithobates clamitans) through trophic cascades and/or release from competition. Mystery snails had more subtle effects, including reducing growth rates of native gray tree frogs (Hyla versicolor), likely through competitive interactions. 

4. With goldfish present, mystery snails showed increased reproductive success (higher total biomass of juveniles), likely due to indirect competitive release from other herbivores. In contrast to their effects on native snails, goldfish did not consume juvenile mystery snails, likely due to their unique traits (e.g., large size and shell thickness). Mystery snails slightly reduced goldfish growth but did not affect their survival.  

5. These results suggest unidirectional facilitation between invaders via indirect changes in the food web. Overall, our study found strong direct and indirect effects of invasive aquarium species, particularly goldfish, and provides experimental support for the invasional meltdown hypothesis within urban waterbodies, which are especially prone to invasions.