Environmental Determinants of Recent Endemism of Batrachochytrium dendrobatidis Infections in Amphibian Assemblages in the Absence of Disease Outbreaks

Abstract

The inconsistent distribution of large-scale infection mediated die-offs and the subsequent population declines of several animal species, urges us to understand how, when, and why species are affected by disease. It is often unclear when or under what conditions a pathogen constitutes a threat to a host. Often, variation of environmental conditions plays a role. Globally Batrachochytrium dendrobatidis (Bd) causes amphibian declines; however, host responses are inconsistent and this fungus appears equally capable of reaching a state of endemism and subsequent co-existence with native amphibian assemblages. We sought to identify environmental and temporal factors that facilitate host-pathogen coexistence in northern Europe. To do this, we used molecular diagnostics to examine archived and wild amphibians for infection and general linear mixed models to explore relationships between environmental variables and prevalence of infection in 5 well-sampled amphibian species. We first detected infection in archived animals collected in 1999, and infection was ubiquitous, but rare, throughout the study period (2008-2010). Prevalence of infection exhibited significant annual fluctuations. Despite extremely rare cases of lethal chytridiomycosis in A. obstetricans, Bd prevalence was uncorrelated with this species' population growth. Our results suggest context dependent and species-specific host susceptibility. Thus, we believe recent endemism of Bd coincides with environmentally driven Bd prevalence fluctuations that preclude the build-up of Bd infection beyond the critical threshold for large-scale mortality and host population crashes.

Fragile coexistence of a global chytrid pathogen with amphibian populations is mediated by environment and demography

“…Our results reinforce the findings of previous studies about the role of environmental conditions and demography in regulating Bd infection in amphibian hosts [4,11,14,15]. Furthermore, by explicitly modelling both individual-and population-level effects, we could use our results to support the initial hypothesis of coexistence between Bd and B. variegata at population level, despite a potential cost of Bd infection for individual toads, and to suggest this equilibrium is most probably maintained by environmental mediation.…”

“…knmi.nl). For each sampling occasion we calculated daily mean temperature, the minimum temperature and the maximum temperature (all in 0.18C) estimated over two periods: 7 and 30 days prior to each site visit [11]. We measured pond pH using HI 2211 pH/ORP Meter (Hanna w instruments, Temse, Belgium), and water temperature at each visit 1 m from the shore-or in the centre of the pond if the pond had a diameter smaller than 1 m-using an HI 98311 DiSTt5 EC/TDS/temperature tester (Hanna Instruments, Nieuwegein, The Netherlands) to the nearest 0.18C repeatedly at the same position and depth (10 cm) at each site.…”

“…In Europe, Bd outbreaks have been limited in number and are variable in extent [8][9][10]. In particular, in northern Europe, Bd was shown to have been present at least since the late 1990s and is widespread, but mass mortalities and consistent negative effects on population trends have not been observed [11]. In northern Europe, Bd seems to have reached a state of endemism and the build-up of lethal Bd infection-loads is hypothesized to be precluded by unfavourable environmental conditions [11].…”

Canopy cover and drought influence the landscape epidemiology of an amphibian chytrid fungus

“…During both years, Bullfrogs had higher naïve prevalence than any other species, a result consistent with other studies (Sanchez et al 2008, Schloegel et al 2010, Spitzen-Van Der Sluijs et al 2014). Occupancy models for 2012 common species reflected the influence of Bullfrogs as all supported models contained 'species' as a covariate (Bullfrog or not), and estimated Bd prevalence was greater for Bullfrogs than for other species.…”

“…0.5). 'Species' was used as a covariate when modeling 2012 and 2013 common species and it identified whether or not a sample was from a Bullfrog, a species considered an important Bd carrier because it often has a higher naïve Bd prevalence than other species (Sanchez et al 2008, Schloegel et al 2010, Spitzen-Van Der Sluijs et al 2014. SVL was only used for Spring Peeper models.…”