Presence of invasive American bullfrogs may reduce infectious disease in a native frog species

Amphibians breeding in aquatic environments may encounter a myriad of threats during their life cycle. One species known to prey on native amphibians in aquatic habitats is the invasive North American bullfrog Lithobates catesbeianus, which, besides being a voracious predator and competitor, often acts as a pathogen carrier and disease superspreader because it tolerates high infection loads of the frog-killing fungus Batrachochytrium dendrobatidis (Bd). Here, we hypothesized that the presence of the bullfrogs in microcosms should either (1) decrease Bd disease severity in native frog species by discouraging them from using the aquatic environment, or (2) increase the mortality of the native species. We tested these 2 mutually exclusive hypotheses by co-housing the snouted treefrog Scinax x-signatus (native to our study area) with L. catesbeianus in the laboratory, exposing them to Bd, and using qPCR analysis to quantify the resulting Bd infection loads in the native frogs. Our experiment had the following replicated treatments: (1) native-only treatment (3 individuals of S. x-signatus), (2) native-predominant treatment (2 S. x-signatus + 1 L. catesbeianus), and (3) exotic-predominant treatment (1 S. x-signatus + 2 L. catesbeianus). We found that Bd infection loads in the native S. x-signatus were highest in the native-only treatment, and lowest in the exotic-predominant treatment, indicating that bullfrogs may discourage native frogs from occupying the aquatic habitat, thus reducing encounter rates between native frogs and the waterborne pathogen. This effect could be driven by the bullfrogs' predatory behavior and their high philopatry to aquatic habitats. Our results highlight that predation risk adds to the complexity of host-species interactions in Bd epidemiology.