Effects of invasive species on native herpetofauna and pond communities in urban environments

2022 Spring.Includes bibliographical references.Urban environments can provide high quality habitat for native species, yet these environments are also prone to species invasions via numerous introduction pathways. Invasive species may affect communities through multiple mechanisms including ecosystem engineering, predator-prey interactions, and resource competition, so isolating individual effects can be challenging. Further, urban environments commonly host multiple invasive species, which may interact with one another and amplify effects on native species. Despite their potential effects, however, the roles of invasive species in urban communities are often poorly understood. For example, the city of Madison, Wisconsin, USA is home to multiple invasive species, including jumping worms (Amynthas spp.), goldfish (Carassius auratus), and Chinese mystery snails (Cipangopaludina (=Bellamya) chinensis), but little is known about the community effects of these species. Jumping worms are non-native invasive earthworms that may act as ecosystem engineers in leaf litter habitat and potentially provide an abundant novel prey resource to native predators. Invasive goldfish and mystery snails have been widely introduced through aquarium trading and are known to co-occur in urban ponds. In the first study, we used a mesocosm experiment, laboratory feeding trials, and nonlethal stomach contents surveys to assess effects of invasive jumping worms on microhabitat and trophic interactions of native herpetofauna predators. We found that jumping worms reduced leaf litter biomass but did not influence soil conditions or survival of American toad (Anaxyrus americanus) metamorphs. Our results revealed that jumping worms are viable prey for native herpetofauna, especially common garter snakes (Thamnophis sirtalis), but may be challenging to capture due to defensive behaviors. In the second study, we used a mesocosm experiment to test for facilitative or antagonistic interactions between goldfish and mystery snails, and to examine individual and combined effects of goldfish and mystery snails in urban pond communities. We found evidence that goldfish facilitate invasive mystery snails, and that goldfish drive shifts in community composition and food web structure through trophic cascades. Comparatively, mystery snails had limited effects in the experimental community. Overall, our studies emphasize the diversity of roles invasive species can play in communities as ecosystem engineers, prey, predators, or competitors, and that their effects on native species are dependent on these roles. Broadly, our findings highlight the value of understanding invasive species effects in urban environments to direct management and support native species conservation