What Drives Chytrid Infections in Newt Populations? Associations with Substrate, Temperature, and Shade

Abstract

The pathogenic chytrid fungus Batrachochytrium dendrobatidis (Bd) is considered responsible for the population declines and extinctions of hundreds of amphibian species worldwide. The panzootic was likely triggered by human-assisted spread, but once the pathogen becomes established in a given region, its distribution is probably determined by local drivers. To assess the relative importance of potential drivers of infection in red-spotted newts (Notophthalmus viridescens), we measured Bd levels in 16 populations throughout central Pennsylvania. Infected individuals were detected in all but four populations, indicating that Bd is widespread in this region. We quantified local factors hypothesized to influence Bd, and found that infection levels were best predicted by the proportion of the pond substrate consisting of leaf litter or vegetation, along with a significant effect of water temperature. Bd infection in amphibians is temperature-dependent, and one possible explanation of the apparent substrate effect is that tree cover and vegetation provide shade, reducing the availability of shallow, warm-water patches in which newts might reduce or clear Bd infections. Alternatively, leaf litter and emergent vegetation might increase Bd infection more directly, perhaps by providing substrates for environmental growth of the fungus. We also observed a curvilinear relationship between Bd load and snout-vent length (a proxy for age), hinting that newts might develop acquired resistance to Bd infection. Though correlational, these results add to a growing body of evidence suggesting that environmental temperature is an important driver of Bd infection dynamics.

Climate, vegetation, introduced hosts and trade shape a global wildlife pandemic

“…Bd was also positively associated with vegetation (mean NDVI), consistent with previous studies on Bd [22,23]. The NDVI is often a surrogate for the dynamics of ground moisture and temperature, which might affect the survival of many pathogens, including Bd, which is known to be sensitive to moisture and temperature [19].…”

“…The NDVI is often a surrogate for the dynamics of ground moisture and temperature, which might affect the survival of many pathogens, including Bd, which is known to be sensitive to moisture and temperature [19]. Vegetation might increase moisture by reducing evaporation and might reduce temperature extremes preventing high-temperature clearance of Bd [23]. We did not find a significant contribution of precipitation or water habitat to the global distribution of Bd.…”

Potential influence of plant chemicals on infectivity of Batrachochytrium dendrobatidis

“…More work needs to be done to assess what the natural levels of leaf extracts are and how these concentrations can influence Bd dynamics. In contrast to our results, leaf litter and vegetation were found to be related to higher Bd infection in Pennsylvania newts Notophthalmus viridescens which was also related to cooler water temperature (Raffel et al 2010). This demonstrates that in natural conditions the effect of plants may be more indirect by providing shade or thermal refugia for Bd growth.…”

Tropical amphibian populations experience higher disease risk in natural habitats

“…Multiple studies have investigated the role of regional climate in Bd-induced amphibian declines (24,46,47). However, few studies to date have focused on how microclimate mediates host-pathogen interactions (48,49), and more broadly, how large-scale climate relates to microclimates experienced by individual frogs. A recent study found that ponds with fewer surrounding trees reached higher temperatures, significantly reducing Bd infections in salamanders (49).…”

“…However, few studies to date have focused on how microclimate mediates host-pathogen interactions (48,49), and more broadly, how large-scale climate relates to microclimates experienced by individual frogs. A recent study found that ponds with fewer surrounding trees reached higher temperatures, significantly reducing Bd infections in salamanders (49). Given the known temperature-dependence of host-pathogen interactions, another hypothesis is that temperature variability, rather than average temperature, is an important predictor of disease risk (47).…”