Reintroduction programs are used widely in conservation to reduce a species' risk of extinction and amphibians are considered suitable candidates for such programs because of their behavioural simplicity and high reproductive output. The Green and Golden Bell Frog Litoria aurea is an endangered species that has been reintroduced into several areas within its natural range, but the outcome of these programs remain unknown. This paper presents the results from the first release of the bell frog in the Hunter Region of New South Wales. This reintroduction released 850 tadpoles into a closed system of three ponds and rehabilitated habitat. Tadpole survival was high but following metamorphosis a decline in numbers began that continued for 13 months and resulted in the disappearance of all released bell frogs. The cause of this decline was investigated and eventually attributed to infection by the Amphibian Chytrid Fungus Batrachochytrium dendrobatidis. These results emphasize the importance of including regular chytrid testing in the monitoring of both natural populations and reintroduction programs, particularly as few sick and dead animals were found to indicate its presence.

With the incidence of emerging infectious diseases on the rise, it is becoming increasingly important to identify refuge areas that protect hosts from pathogens and therefore prevent population declines. For the chytrid fungus Batrachochytrium dendrobatidis, temperature and humidity refuge areas for amphibian hosts exist but are difficult to manipulate. Other environmental features that may affect the outcome of infection include water quality, drying regimes, abundance of alternate hosts and isolation from other hosts. We identified relationships between water bodies with these features and infection levels in the free-living hosts inhabiting them. Where significant relationships were identified, we used a series of controlled experiments to test for causation. Infection loads were negatively correlated with the salt concentration of the aquatic habitat and the degree of water level fluctuation and positively correlated with fish abundance. However, only the relationship with salt was confirmed experimentally. Free-living hosts inhabiting water bodies with mean salinities of up to 3.5 ppt had lower infection loads than those exposed to less salt. The experiment confirmed that exposure to sodium chloride concentrations >2 ppt significantly reduced host infection loads compared to no exposure (0 ppt). These results suggest that the exposure of amphibians to salt concentrations found naturally in lentic habitats may be responsible for the persistence of some susceptible species in the presence of B. dendrobatidis. By manipulating the salinity of water bodies, it may be possible to create refuges for declining amphibians, thus allowing them to be reintroduced to their former ranges.

No abstract

Introduced pathogens are increasingly being implicated in population declines and their effects are difficult to manage. In the absence of methods to eradicate pathogens acting as threatening processes, intervention before population decline is necessary. Such an intervention requires an ability to predict when population declines will occur, and therefore, an understanding of when exposure will lead to infection, disease, death and population decline. This study investigates when pathogen exposure leads to disease for the amphibian chytrid fungus Batrachochytrium dendrobatidis, which has been implicated as a causal agent in the global amphibian decline. Susceptibility studies were conducted on two anuran species, the green and golden bell frog Litoria aurea and the striped marsh frog Limnodynastes peronii, when exposed to the fungus as either tadpoles or juveniles. Host species was found to significantly affect the outcome of exposure, with infection loads in L. aurea increasing over time and resulting in significantly lower survival rates than unexposed. By comparison, infection loads in L. peronii remained the same or decreased over time following the initial infection, and survival rates were no different whether exposed to B. dendrobatidis or not. These outcomes were independent of the life stage at exposure. Individuals with higher infection loads were not found to have lower survival rates; rather, an infection load threshold was identified where individuals with infection loads that crossed this threshold had high likelihoods of showing terminal signs of chytridiomycosis. Therefore, host species determined whether infection load crossed this threshold and the crossing of the threshold determined the incidence of disease and survival. The quantification of infection load thresholds for survival, along with the time it takes to reach them, will enable infection loads in wild populations to be related to the likelihood of disease and is the first step in the understanding and prediction of when exposure will result in population decline.

Emerging infectious diseases are one of the greatest threats to global biodiversity. Chytridiomycosis in amphibians is perhaps the most extreme example of this phenomenon known to science. Translocations are increasingly used to fight disease‐induced extinctions. However, many programmes fail because disease is still present or subsequently establishes in the translocation environment. There is a need for studies in real‐world scenarios to test whether environmental manipulation could improve survival in populations by generating unfavourable environmental conditions for pathogens. Reintroductions of amphibians impacted by chytridiomycosis into environments where the disease persists provide a scenario where this paradigm can be tested. We tested the hypothesis that manipulating environmental salinity in outdoor mesocosms under near‐identical environmental conditions, present in a nearby translocation programme for an endangered amphibian, would improve survival and determine the mechanisms involved. One hundred and sixty infected and 288 uninfected, captive‐bred, juvenile frogs were released into 16 outdoor mesocosms in which salinity was controlled (high‐ or low‐salinity treatment). The experiment was run for 25 weeks from the mid‐austral winter to the mid‐austral summer of 2013 in a temperate coastal environment, Australia. Increasing salinity from c. 0.5 ppt to 3.5–4.5 ppt reduced pathogen transmission between infected and uninfected animals, resulting in significantly reduced mortality in elevated salt mesocosms (0.13, high‐salt vs. 0.23, low‐salt survival at 23 weeks). Increasing water temperature associated with season (from mean 13 to 25°C) eventually cleared all surviving animals of the pathogen. Synthesis and applications. We identified a mechanism by which environmental salinity can protect amphibian hosts from chytridomycosis by reducing disease transmission rates. We conclude that manipulating environmental salinity in landscapes where chytrid‐affected amphibians are currently translocated could improve the probability of population persistence for hundreds of species. More broadly, we provide support for the paradigm that environmental manipulation can be used to mitigate the impact of emerging infectious diseases.