Low impact of chytridiomycosis on frog recruitment enables persistence in refuges despite high adult mortality

Abstract

The global chytridiomycosis pandemic caused by the pathogen Batrachochytrium dendrobatidis (Bd) is implicated in the apparent extinction or severe decline of over 200 amphibian species. Many declined species now only persist in isolated remnant populations. In this study we examine how remnant populations coexist with Bd, focusing on disease impact on adult survival and recruitment potential in the chytridiomycosis-threatened frog Litoria verreauxii alpina. Using skeletochronology we found that the adult male population in both 2011 and 2012 was dominated by a two year old age cohort. The lack of recruitment into the three year old cohort in 2012 indicates that annual adult survival is very low. Combined with high Bd prevalence and heavy infection burdens, the pathogen appears to drive almost complete mortality of breeding adults over their first breeding season. However, adults successfully mate prior to large increases in disease prevalence that occurs during the breeding season. Infection prevalence among tadpoles and juveniles is low. Exposure to warm water could provide a mechanism for avoiding or clearing Bd infection. Relatively low Bd prevalence in juveniles prior to dispersal into terrestrial habitat indicates that Bd has minimal impact on early life history stages. As such, recruitment is probably high, allowing populations to persist despite low adult survival. This dependence on reliable annual recruitment may explain why remnant populations persist in permanent ponds rather than ephemeral ponds that were historically occupied. New management strategies that focus on increasing recruitment may provide a way forward for the management of disease-threatened amphibian species.

Population dynamics of the critically endangered toad Atelopus cruciger and the fungal disease chytridiomycosis

“…A previous study suggested that adult toads at this location had high mortality rates but these losses were compensated by the rapid recruitment of new adults [17]. The estimated total number of adult toads and new recruits indicate an almost complete turnover of the reproductive population every year at the study site, a demographic process also observed in other frog species infected with Bd [58–60]. For adult recruitment to be high, however, metamorphs and juveniles must show some resistance to chytridiomycosis or have a low exposure to infection.…”

Mitigating amphibian chytridiomycoses in nature

“…There is some empirical evidence that this might work, and existing theory of harvested and exploited populations might guide such a strategy [5,85].…”

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

“…Secondly, the pathogen may be parasitic but have compensatory, rather than additive, effects on the vital rates of individual hosts; that is, the individuals that die of disease would also have had higher mortality rates in the absence of the pathogen [13]. Alternatively, the parasitic pathogen may inflict additive mortality, but this is compensated at the host population level, for example through increased recruitment and a shift in age structure [4,14,15]. Understanding whether, and by what mechanisms, host populations truly coexist with a pathogen is crucial for predicting and eventually managing the potential impacts of a disease.…”