American Bullfrogs (Lithobates catesbeianus) have been widely introduced beyond their native range in North America and can negatively affect organisms in wetland environments via a suite of mechanisms including interspecific interactions and disease transmission. Bullfrogs were introduced to the Valentine National Wildlife Refuge in the Sandhill Region of Nebraska, USA, yet little is known regarding their distribution, abundance, and potential effects on other vertebrates in the Refuge. Surveys in 1991-1992 documented bullfrogs in only one lake by the Refuge headquarters, and anecdotal historical observations indicated that bullfrogs were present primarily in lakes open to public fishing. In 2012, we determined the distribution of bullfrogs across the Refuge, examined their diets, and sampled them for the occurrence of the pathogenic fungus, Batrachochytrium dendrobatidis. We documented that bullfrogs were almost ubiquitous across the Refuge lakes and wetlands. From diet analyses, we observed that adult bullfrogs consumed several vertebrate species on the Refuge including: a Blanding’s Turtle (Emydoidea blandingii), small bullfrogs, an unidentifiable frog species, and numerous invertebrates, including crayfish. The amphibian chytrid fungus was prevalent on the refuge with 73.7% of bullfrogs testing positive for the fungus in early June and 6.3% in late June-July. Preliminary data indicate that bullfrogs likely have already affected interspecific interactions with native amphibians via predation, competition, and/or disease transmission. Bullfrogs likely cannot be eradicated from the Refuge, but expanding the season of harvest of bullfrogs might reduce their abundance, which may benefit native amphibians and reptiles.
DISTRIBUTION, DIET, AND PREVALENCE OF AMPHIBIAN CHYTRID FUNGUS IN NON-NATIVE AMERICAN BULLFROGS (LITHOBATES CATESBEIANUS) AT THE VALENTINE NATIONAL WILDLIFE REFUGE, NEBRASKA, USA
