Worldwide decline of amphibians

Around the world, amphibians have suffered serious declines and even large-scale extinctions over the past 30 years, with a deadly fungus the likely cause. The most recent assessment suggests that a third of all known amphibian species are considered threatened.

But now new research has revealed that at least one subtropical rainforest frog is recovering, holding out hopes for other species – without bringing them back from the dead.

Dr David Newell and Dr Ross Goldingay from Southern Cross University in New South Wales (NSW) documented the population dynamics of the endangered Fleay’s barred frog (Mixophyes fleayi) over the course of seven years at two independent rainforest sites in northern New South Wales.

Like its counterparts in other countries, the frog had suffered serious declines across its narrow geographic range – essentially the rainforests straddling the NSW-Queensland border. Surveys of the species during the 1990s suggested that the frog had become uncommon, which was at odds with earlier anecdotal evidence of its relative abundance.

The discovery in 1998 of a pathogenic and highly transmissible amphibian chytrid fungus called Batrachochytrium dendrobatides, or Bd, offered a plausible explanation for the global fall in frog populations.

The fungus gives rise to the disease chytridiomycosis and had been detected in sick and dying frogs whose symptoms in advanced cases included lethargy, abnormal sitting position, sitting exposed during the day, excessive shedding of the skin, and in some species redness of the thighs and belly.

The impact of chytridiomycosis has 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 populations problem and more than 10 species have declined or disappeared since 1979.

The Bd fungus has been detected in sick and dying frogs from the region and shown to cause death in experimentally infected species. It appears to be widespread, with infection rates varying with the season, although recent evidence suggests that some populations are now able to survive even with endemic levels of infection.

Also, a number of species that were reported to have declined or disappeared have since been rediscovered in remnant populations. The researchers say the ability of these populations to persist is of considerable interest to scientists involved with global amphibian conservation efforts.

While the mechanisms of survival are not yet understood, the researchers believe survival 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.

In the Australian study, the researchers used transponder tags to permanently mark frogs captured during repeated surveys. This enabled them to follow their fate over time, and from this they could describe the detection and survival probabilities of the species over the seven-year period and objectively estimate the population size at each site.

The results showed that at both locations, the local population began at low levels but increased three- to 10-fold over time. The researchers did not examine levels of Bd infection because they lacked the funding but they knew the fungus was present in the two locations at the start and was widespread in rainforest streams throughout the region.

The fact that it was unlikely to have disappeared from the two sites raised some important questions. Was it possible, the scientists asked, that a shift in the host-pathogen relationship might have occurred, or that Bd had always existed in these ecosystems and only affected the frog populations when other factors were stressing them?

They say 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.

A report of the study was published in the open access, peer reviewed journal PLOS One.