Non-native fish facilitate non-native snails and alter food web structure in experimental pond communities

1. Contemporary ecosystems commonly support multiple non-native species, which can alter community structure and directly or indirectly interact with one another. 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 have been introduced widely through the aquarium trade and co-occur in their non-native ranges. We used an outdoor mesocosm experiment to assess separate and combined effects of goldfish and mystery snails on urban 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 filamentous algae, zooplankton, juvenile native snails and survival of amphibians via consumptive effects. Goldfish indirectly increased 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 grey tree frogs Hyla versicolor, likely through competitive interactions.4. With goldfish present, mystery snails showed increased reproductive success (higher 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.

Automated summary

Contemporary ecosystems are often populated by multiple non-native species that can interact with and change community structure. In this study, the effects of goldfish and Chinese mystery snails on urban pond community structure were examined. Goldfish had direct negative effects on various species, while mystery snails had more subtle effects, especially on the growth rates of grey tree frogs. The results indicate unidirectional facilitation between these invaders through indirect changes in the food web.