Widespread Occurrence of Batrachochytrium dendrobatidis in Contemporary and Historical Samples of the Endangered Bombina pachypus along the Italian Peninsula

Abstract

Batrachochytrium dendrobatidis is considered a main driver of the worldwide declines and extinctions of amphibian populations. Nonetheless, fundamental questions about its epidemiology, including whether it acts mainly as a “lone killer” or in conjunction with other factors, remain largely open. In this paper we analysed contemporary and historical samples of the endangered Apennine yellow-bellied toad (Bombina pachypus) along the Italian peninsula, in order to assess the presence of the pathogen and its spreading dynamics. Once common throughout its range, B. pachypus started to decline after the mid-1990s in the northern and central regions, whereas no declines have been observed so far in the southern region. We show that Batrachochytrium dendrobatidis is currently widespread along the entire peninsula, and that this was already so at least as early as the late 1970s, that is, well before the beginning of the observed declines. This temporal mismatch between pathogen occurrence and host decline, as well as the spatial pattern of the declines, suggests that the pathogen has not acted as a “lone killer”, but in conjunction with other factors. Among the potentially interacting factors, we identified two as the most probable, genetic diversity of host populations and recent climate changes. We discuss the plausibility of this scenario and its implications on the conservation of B. pachypus populations.

No evidence for effects of infection with the amphibian chytrid fungus on populations of yellow-bellied toads

“…Thus, our observation in the yellowbellied toad population Baumbach marl pit/Hergerhausen stone pit with high prevalence of Bd but population growth confirms that of Tobler et al (2012), who also concluded that observed negative effects of Bd on survival of individuals do not necessarily translate into negative effects at the population level. In yet another study, Canestrelli et al (2013) showed that Bd has been present in Apennine yellow-bellied toad populations Bombina pachypus at least since the late 1970s, but strong declines only started after the mid-1990s. From this, the authors concluded that Bd cannot be the only reason for declines but possibly interacts with other stressors.…”

“…However, deadly chytridiomycosis in captive toads has been previously observed and was thought to be responsible for declines (Stagni et al 2004). Hence, a role for Bd in the widespread decline of the Apennine yellow-bellied toad still appears plausible (Canestrelli et al 2013), and it could be hypothesized that the closely related B. variegata is also suceptible under certain conditions.…”

Designing screening protocols for amphibian disease that account for imperfect and variable capture rates of individuals

“…For our study species, this assumption is justified by the relative tolerance to the pathogen observed in B. variegata, where populations are known to have persisted in the presence of chytrid for several years (Canestrelli et al 2013), suggesting that infected and uninfected individuals will not exhibit important differences in behavior (hence our reference to pathogen prevalence, rather than disease prevalence). However, this assumption may be violated for other species, and lead to different recapture rates: for example, altered behavior may render infected individuals more likely to be captured.…”

“…Populations in the north and center of the species' range have declined in both number and abundance since the mid-1990s (Barbieri et al 2004). A recent study has found evidence of infection by B. dendrobatidis on specimens across the Italian peninsula (Canestrelli et al 2013). Other subspecies of Bombina variegata are prone to infection by B. dendrobatidis (Spitzen-van der Sluijs et al 2011, Sztatecsny andGlaser 2011), and mortality resulting from chytridiomycosis has been recorded for B. v. pachypus originating from two populations in the northeastern Apennines (Stagni et al 2004).…”

“…1-3; Supplement), to predict the estimate of prevalence that would be obtained for every day in the 2012 season, in the event that all individuals swabbed returned negative results. We ran the model with three different priors for prevalence: (1) the previously used uninformative prior (uniform 0, 1), (2) an informative prior with a ¼ 3.5 and b ¼ 31.5, corresponding to a mean expected prevalence of 0.1 6 0.05 (mean 6 SD), on the basis of expert judgment and existing information on the prevalence of chytrid in B. variegata across Europe (Spitzen-van der Sluijs et al 2010, Sztatecsny and Glaser 2011, Tu¨nde et al 2012, Canestrelli et al 2013, Vo¨ro¨s et al 2013, and (3) an informative prior based on the parameter distribution for prevalence obtained from analyzing the 2011 survey results. To account for stochasticity in the determination of node n, we ran each model 1000 times and calculated average results.…”