Using stochastic epidemiological models to evaluate conservation strategies for endangered amphibians

Abstract

Recent outbreaks of chytridiomycosis, the disease of amphibians caused by the fungal pathogen Batrachochytrium dendrobatidis (Bd), have contributed to population declines of numerous amphibian species worldwide. The devastating impacts of this disease have led researchers to attempt drastic conservation measures to prevent further extinctions and loss of biodiversity. The conservation measures can be labour-intensive or expensive, and in many cases have been unsuccessful. We developed a mathematical model of Bd outbreaks that includes the effects of demographic stochasticity and within-host fungal load dynamics. We investigated the impacts of one-time treatment conservation strategies during the disease outbreak that occurs following the initial arrival of Bd into a previously uninfected frog population. We found that for all versions of the model, for a large fraction of parameter space, none of the one-time treatment strategies are effective at preventing disease-induced extinction of the amphibian population. Of the strategies considered, treating frogs with antifungal agents to reduce their fungal load had the greatest likelihood of a beneficial outcome and the lowest risk of decreasing the persistence of the frog population, suggesting that this disease mitigation strategy should be prioritized over disinfecting the environment or reducing host density.

Individual and temporal variation in pathogen load predicts long‐term impacts of an emerging infectious disease

“…For estimating the sensitivity of the three possible longterm outcomes (devil extirpation, DFTD extirpation, coexistence) to variation in the posterior estimates of key parameters (i.e., the likely parameter values obtained through the ABC approach), we used boosted regression trees using the gbm.step routine (binomial error structure, learning rate of 0.001, tree complexity of 5, k-fold crossvalidation procedure) in the R package dismo (Elith et al 2008). Similar approaches to global sensitivity analysis were recently applied to eco-epidemiological models (Wells et al 2015, Drawert et al 2017.…”

Evolutionary rescue in a host–pathogen system results in coexistence not clearance

“…However, we captured some variation in pathogen loads in our model because (i) pathogen loads and Bd ‐induced mortality in the field are often greatest in the winter (Grogan et al., ; Phillott et al., ), simulated with Bd ‐induced winter mortality in our model and (ii) newly infected individuals only transmitted Bd after they had been infected for a year, which would be expected if pathogen loads must accumulate before transmission. We chose to model transmission in a density‐dependent manner rather than through the environment because (i) previous studies have found evidence for density‐dependent transmission of Bd (Rachowicz & Briggs, ) and that altering the amount of Bd in the environment has little to no impact on host population outcomes (Drawert et al., ), (ii) in most conditions, Bd zoospores remain motile for only short periods of time outside the host (Piotrowski, Annis, & Longcore, ; Voyles et al., ; Woodhams, Alford, Briggs, Johnson, & Rollins‐Smith, ) and are unlikely to survive long periods of freezing temperatures in the winter (Boyle et al., ), and (iii) transmission and mortality are decoupled in this system, such that allowing for build‐up of zoospores within the environment would only influence the number of infected individuals within a given year, which we modelled by changing the transmission rate (see below). Once infected with Bd , individuals remained infected through metamorphosis and across years.…”

“…Critically, however, it remains unknown whether the evolution of amphibian hosts in response to Bd can lead to evolutionary rescue. Several previous models of amphibian‐ Bd dynamics have been developed (Briggs, Knapp, & Vredenburg, ; Briggs, Vredenburg, Knapp, & Rachowicz, ; Converse et al., ; Drawert, Griesemer, Petzold, & Briggs, ; Grogan et al., ; Wilber, Langwig, Kilpatrick, McCallum, & Briggs, ), but none focuses on evolution of the amphibian host. The goals of our study were to concentrate on the amphibian‐ Bd system to (i) identify how variation in host susceptibility (range and distribution of host susceptibilities) and host genetic diversity (number of host genotypes) affects the ability of populations to rapidly evolve in response to a pathogen and (ii) quantify how this host evolution affects population recovery (i.e., evolutionary rescue).…”

Risk of survival, establishment and spread of Batrachochytrium salamandrivorans (Bsal) in the EU

“…There are also some measures that have been trialled for Bd in the wild and have exhibited some promise but only in very specific circumstances ( in situ treatment, site disinfection; Bosch et al. ; Hardy et al., ; Hudson et al., ; Drawert et al., ).…”