Invasive species and emerging infectious diseases are two of the greatest threats to biodiversity. American Bullfrogs (Rana [Lithobates] catesbeiana), which have been introduced to many parts of the world, are often linked with declines of native amphibians via predation and spreading emerging pathogens such as amphibian chytrid fungus (Batrachochytrium dendrobatidis [Bd]) and ranaviruses. Although many studies have investigated the potential role of bullfrogs in declines of native amphibians, analyses that account for shared habitat affinities and imperfect detection have found limited support for clear effects. Similarly, the role of bullfrogs in shaping the patch-level distribution of pathogens is unclear. We used eDNA methods to sample 233 sites in the southwestern USA and Sonora, Mexico (2016–2018) to estimate how presence of bullfrogs affects occurrence of 4 native amphibians, Bd, and ranaviruses. Based on 2-species, dominant-subordinate occupancy models fitted in a Bayesian context, federally threatened Chiricahua Leopard Frogs (R. chiricahuensis) and Western Tiger Salamanders (Ambystoma mavortium) were 8 times (32% vs. 4%) and 2 times (36% vs. 18%), respectively, less likely to occur at sites where bullfrogs occurred. Evidence for negative effects of bullfrogs on Lowland Leopard Frogs (R. yavapaiensis) and Northern Leopard Frogs (R. pipiens) was less clear, possibly because of smaller numbers of sites where these native species still occur and because bullfrogs often occur at lower densities in streams, the primary habitat for Lowland Leopard Frogs. At the community level, Bd was most likely to occur where bullfrogs co-occurred with native amphibians, which could increase risk to native species. Ranaviruses were estimated to occur at 33% of bullfrog-only sites, 10% of sites where bullfrogs and native amphibians co-occurred, and only 3% of sites where only native amphibians occurred. Of the 85 sites where we did not detect any of the 5 target amphibian species, we also did not detect Bd or ranaviruses; this suggests other hosts do not drive the distribution of these pathogens in our study area. Our results provide landscape-scale evidence that bullfrogs reduce occurrence of native amphibians and increase occurrence of pathogens, information that can clarify risks and aid the prioritization of conservation actions.
|Title||Empirical evidence for effects of invasive American Bullfrogs on occurrence of native amphibians and emerging pathogens|
|Authors||Blake R. Hossack, Emily B Oja, Audrey K Owens, David L. Hall, Cassidi Cobos, Catherine L. Crawford, Caren S. Goldberg, Shaula Hedwell, Paige E. Howell, Julio A. Lemos-Espinal, Susan K MacVean, Magnus McCaffery, Cody Mosley, Erin L. Muths, Brent H. Sigafus, Micahel J Sredl, James C. Rorabaugh|
|Publication Subtype||Journal Article|
|Series Title||Ecological Applications|
|Record Source||USGS Publications Warehouse|
|USGS Organization||Fort Collins Science Center; Northern Rocky Mountain Science Center; Southwest Biological Science Center|