Populations of a Susceptible Amphibian Species Can Grow despite the Presence of a Pathogenic Chytrid Fungus

Disease can be an important driver of host population dynamics and epizootics can cause severe host population declines. Batrachochytrium dendrobatidis (Bd), the pathogen causing amphibian chytridiomycosis, may occur epizootically or enzootically and can harm amphibian populations in many ways. While effects of Bd epizootics are well documented, the effects of enzootic Bd have rarely been described. We used a state-space model that accounts for observation error to test whether population trends of a species highly susceptible to Bd, the midwife toad Alytes obstetricans, are negatively affected by the enzootic presence of the pathogen. Unexpectedly, Bd had no negative effect on population growth rates from 2002–2008. This suggests that negative effects of disease on individuals do not necessarily translate into negative effects at the population level. Populations of amphibian species that are susceptible to the emerging disease chytridiomycosis can persist despite the enzootic presence of the pathogen under current environmental conditions

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Katedra občanského právaDepartment of Civil LawFaculty of LawPrávnická fakult

Estimates of observed infection prevalence and intensity in <i>Alytes</i> tadpoles and in all other species (pooled) for all study ponds.

<p>Prevalence is the proportion of infected individuals, infection intensity are mean zoospore equivalents (genomic equivalents) among infected individuals ±1 SD.</p

Population growth rates as predicted by the state-space model.

<p> is the average population growth rate. is the observation variance and is the process variance. All estimates are given as means ± standard deviation with the 95% CRI in brackets. Bd status 0 means that Bd was not detected while Bd status 1 means that Bd was detected.</p

Observed counts and model-estimated trends per population and year.

<p>The circles show the observed counts of calling males, with sites where we did not detect <i>Bd</i> in grey and <i>Bd</i>-positive sites in black. The solid line shows the predicted population trend (with 95% CRI (dotted lines)).</p

Parameter estimates (equation 4), standard deviations and 95% CRI for the effects of the presence of <i>Bd</i> and the number of years in which tadpoles were observed on population growth rates.

<p>Parameter estimates (equation 4), standard deviations and 95% CRI for the effects of the presence of <i>Bd</i> and the number of years in which tadpoles were observed on population growth rates.</p