Disease

Western palearctic salamander susceptibility to the skin disease caused by the amphibian chytrid fungus Batrachochytrium salamandrivorans (Bsal) was recognized in 2014, eliciting concerns for a potential novel wave of amphibian declines following the B. dendrobatidis (Bd) chytridiomycosis global pandemic. Although Bsal had not been detected in North America, initial experimental trials supported the heightened susceptibility of caudate amphibians to Bsal chytridiomycosis, recognizing the critical threat this pathogen poses to the North American salamander biodiversity hotspot. Here, we take stock of 10 years of research, collaboration, engagement, and outreach by the North American Bsal Task Force. We summarize main knowledge and conservation actions to both forestall and respond to Bsal invasion into North America. We address the questions: what have we learned; what are current challenges; and are we ready for a more effective reaction to Bsal’s eventual detection? We expect that the many contributions to preemptive planning accrued over the past decade will pay dividends in amphibian conservation effectiveness and can inform future responses to other novel wildlife diseases and extreme threats.

Species with extensive geographical ranges pose special challenges to assessing drivers of wildlife disease, necessitating collaborative and large-scale analyses. The imperilled foothill yellow-legged frog (Rana boylii) inhabits a wide geographical range and variable conditions in rivers of California and Oregon (USA), and is considered threatened by the pathogen Batrachochytrium dendrobatidis (Bd). To assess drivers of Bd infections over time and space, we compiled over 2000 datapoints from R. boylii museum specimens (collected 1897–2005) and field samples (2005–2021) spanning 9° of latitude. We observed a south-to-north spread of Bd detections beginning in the 1940s and increase in prevalence from the 1940s to 1970s, coinciding with extirpation from southern latitudes. We detected eight high-prevalence geographical clusters through time that span the species' geographical range. Field-sampled male R. boylii exhibited the highest prevalence, and juveniles sampled in autumn exhibited the highest loads. Bd infection risk was highest in lower elevation rain-dominated watersheds, and with cool temperatures and low stream-flow conditions at the end of the dry season. Through a holistic assessment of relationships between infection risk, geographical context and time, we identify the locations and time periods where Bd mitigation and monitoring will be critical for conservation of this imperilled species.

Ephemeral streams (hereafter, creeks) along the sandstone canyons of the Colorado and Uncompahgre Plateau provide habitat and breeding sites for native amphibians, although little is known about the diversity and distribution of amphibians that live in these harsh, dynamic environments. In addition, rivers that border these canyon tributaries serve as corridors for non-native species and disease. The American bullfrog (Lithobates catesbeianus) is a non-native species in western Colorado known to prey on native amphibians and act as a reservoir for pathogens such as Batrachochytrium dendrobatidis (Bd). From 2019-2022, we surveyed for amphibians using visual encounter surveys (VES) and eDNA surveys throughout the McInnis Canyon National Conservation Area (MCNCA), the Dominguez-Escalante National Conservation Area, and the Dolores River Canyon Wildlife Study Area. Our primary goals were to document the diversity and distribution of native amphibians in the canyonlands and evaluate possible threats to these species from bullfrogs and Bd. We found direct (VES) and indirect (eDNA) evidence that sensitive species, such as the Great Basin spadefoot (Spea intermontana) and the Northern leopard frog (Lithobates pipiens), inhabit these protected canyons. Most of the ephemeral tributaries did not support bullfrog populations, although we often detected them where the tributaries joined the rivers. In Mee Canyon (MCNCA), however, bullfrogs appear to migrate upstream into the canyon tributary in some years. A bullfrog individual also tested positive for Bd from Mee Canyon in 2019 and diet contents indicated that bullfrogs prey on native amphibians in this system. While non-native predators and disease are a concern for these ephemeral desert tributaries, they are likely minor relative to other threats such as drought and hydrological changes associated with ongoing climate change.