Globally, amphibians have suffered serious declines and extinctions over the last 30 years. But our research, published today, shows that at least one subtropical rainforest frog is recovering.
The most recent IUCN assessment suggests a third of all known amphibian species are now considered threatened. More than 400 species have rapidly declined since 1980.
Habitat loss and degradation have played a major role in many of the reported declines. But habitat loss cannot explain the decline of frogs living in or near streams in the protected rainforests of eastern Australia, and Central and South America. The “wave-like” spread of the Australian declines suggested that disease was a likely factor, although this hypothesis has been debated.
The discovery in 1998 of the pathogenic and highly transmissible amphibian chytrid fungus Batrachochytrium dendrobatides (Bd) offered a plausible explanation. This fungus gives rise to the disease chytridiomycosis, and has been detected in sick and dying frogs that have been observed periodically in some frog populations.
The symptoms in advanced cases of the disease include lethargy, abnormal sitting position, sitting exposed during the day, excessive shedding of the skin, and in some species redness in the thighs and belly. The impact of chytridiomycosis has since been described as “the most spectacular loss of vertebrate biodiversity due to disease in recorded history”.
The rainforests of eastern Australia have been a focal point for research into the declining frog problem. More than 10 species have declined or disappeared from these ecosystems since 1979. Bd has been detected in sick and dying frogs from the region and shown to cause death in experimentally infected species. Bd appears to be widespread, with infection rates varying with season. Recent evidence suggests that some populations are now able to persist with endemic levels of infection.
A number of species that were reported to have declined have since been rediscovered in remnant populations. The ability of these populations to persist is of considerable interest to global amphibian conservation efforts. While the mechanisms are not yet understood, this could be the result of habitats that offer thermal environments (the fungus cannot survive higher temperatures) or antifungal properties that reduce infection prevalence. It could also be due to the absence of Bd, or perhaps the rise of resistant individuals through natural selection.
Understanding the fate of species that previously suffered declines may provide considerable insights for future conservation efforts.
Assessing population dynamics in frogs is inherently difficult. Most species are only active for short periods and activity is linked to environmental cues associated with breeding. These are often not well understood. Because of this, measures of abundance can vary widely between one count and the next, unless researchers can factor in the probability of detection. Surprisingly, few long-term studies of population dynamics appear in the literature for Australian frogs despite the high number of at risk species.
We have just published the results of one such study, which provides unambiguous evidence of population recovery in an endangered Australian frog. We have documented the population dynamics of the endangered Fleay’s barred frog (Mixophyes fleayi) over a period of seven years, at two independent rainforest sites in northern New South Wales (NSW).
This frog had suffered serious declines across its narrow geographic range (essentially the rainforests straddling the NSW-Queensland border). Surveys for this species during the 1990s suggested this frog had become uncommon, which was at odds with earlier anecdotal evidence of relative abundance.
In our study, we used transponder tags to permanently mark frogs captured during repeated surveys. This enabled us to follow their fate through time. From this we could describe the detection and survival probabilities of this species over a seven-year period and objectively estimate the population size at each site over this period. This revealed that at both locations the local population began at low levels, but increased three to ten-fold over time.
Our research did not examine levels of Bd infection over time because we did not have funds to do so. We do know that Bd was present at both locations at the start of the study. We know from the work of other researchers that Bd is widespread in rainforest streams throughout our region, so it is unlikely it has disappeared from our sites.
This raises some important questions for global amphibian conservation efforts. Is it possible that a shift in the host-pathogen relationship may have occurred, or that Bd has always existed in these ecosystems and only reduces populations when other factors are stressing frogs?
Further studies are now required to test hypotheses regarding possible mechanisms of acquired immunity and disease dynamics in wild populations if the tragedy of declining frogs is to be resolved.