Host–pathogen metapopulation dynamics suggest high elevation refugia for boreal toads

Abstract

Emerging infectious diseases are an increasingly common threat to wildlife. Chytridiomycosis, caused by the fungal pathogen Batrachochytrium dendrobatidis (Bd), is an emerging infectious disease that has been linked to amphibian declines around the world. Few studies exist that explore amphibian-Bd dynamics at the landscape scale, limiting our ability to identify which factors are associated with variation in population susceptibility and to develop effective in situ disease management. Declines of boreal toads (Anaxyrus boreas boreas) in the southern Rocky Mountains are largely attributed to chytridiomycosis but variation exists in local extinction of boreal toads across this metapopulation. Using a large-scale historic data set, we explored several potential factors influencing disease dynamics in the boreal toad-Bd system: geographic isolation of populations, amphibian community richness, elevational differences, and habitat permanence. We found evidence that boreal toad extinction risk was lowest at high elevations where temperatures may be suboptimal for Bd growth and where small boreal toad populations may be below the threshold needed for efficient pathogen transmission. In addition, boreal toads were more likely to recolonize high elevation sites after local extinction, again suggesting that high elevations may provide refuge from disease for boreal toads. We illustrate a modeling framework that will be useful to natural resource managers striving to make decisions in amphibian-Bd systems. Our data suggest that in the southern Rocky Mountains high elevation sites should be prioritized for conservation initiatives like reintroductions.

Body size mediates latitudinal population differences in response to Bd infection in two amphibian species

“…It is currently unclear how well the present results translate to natural conditions, but we note that Bd causes sublethal effects in terms of reduced movements and body condition in wild Scandinavian amphibians (Kärvemo et al 2019(Kärvemo et al , 2020. Furthermore, the lethality of Bd is highly dependent on environmental conditions, including temperature (e.g., Novakowski et al 2106, Mosher et al 2018, Cohen et al 2019, and relatively minor elevations in mortality may risk long-term survival of Bdinfected amphibian populations (Muths et al 2011;Spitzen-van der Sluijs et al 2017). Two important conclusions can be drawn.…”

Highly variable rates of survival to metamorphosis in wild boreal toads (Anaxyrus boreas boreas)

“…These locations vary in current Bd status, permanence and presence of introduced trout. Spruce lake is Bd negative, while the other sites are Bd positive; Bd status was determined based on annual sampling (skin swabs) of up to 20 individuals at breeding or reintroduction locations (unpublished data, also see Mosher et al, 2018;Muths, Bailey, & Watry, 2014). The Fay Lakes breeding site is ephemeral, while the other sites are permanent.…”

Few Impacts of Introduced Cutthroat Trout (Oncorhynchus clarki) on Aquatic Stages of Boreal Toads (Anaxyrus boreas boreas)

“…Our study suggests that this negative effect does not persist in native Boreal Toad systems. Boreal Toads use a variety of ephemeral and permanent wetlands for reproduction (Holland et al, 2006;Dodd, 2013), but currently persist in high-elevation sites within their historic distribution in the southern Rocky Mountains (Mosher et al, 2018, Crockett, 2019. These same wetlands and lakes are also targeted for restoration efforts for the threatened greenback Cutthroat Trout, because barriers can be placed downstream to avoid colonization by nonnative salmonids.…”

“…The Boreal Toad (Anaxyrus boreas boreas) has declined throughout the southern Rocky Mountains (Carey, 1993;Muths et al, 2003;Mosher et al, 2018) and is listed as an endangered, or tier 1, species in Colorado, Wyoming, and New Mexico. Regional declines are primarily attributed to chytridiomycosis, a disease caused by the fungal pathogen Batrachochytrium dendrobatidis (Muths et al, 2003;Scherer et al, 2005); however, some populations have declined despite low prevalence of the pathogen (Muths and Scherer, 2011).…”