The last croak | ANU Science, Health & Medicine - ANU Science

A strange fungal disease is killing frogs and other amphibians in Australia and other parts of the world. Never fear, though – ANU scientists are on the case to stop the killing spree. Will Wright and Peter O’Rourke report.

As far as deaths go, it doesn’t get much more horrible than having your skin being slowly eaten away.

This is the harsh reality for many amphibians stricken with an infectious disease caused by the chytrid fungus.

The disease – chytridiomycosis – is behind the decline of at least 40 frog species in Australia during the past 30 years, including seven extinctions.

For the first time, an international team of scientists led by ANU has quantified the global scale of impact of chytridiomycosis on amphibians. They found the disease has caused dramatic population declines in more than 500 amphibian species, including 90 extinctions, across the globe over the past 50 years. The worst affected parts of the world are Australia, Central America and South America.

Lead researcher Dr Ben Scheele from the ANU Fenner School of Environment and Society says the team found that chytridiomycosis is responsible for the greatest loss of biodiversity due to a disease.

“The unprecedented number of declines places chytrid fungus among the most damaging of invasive species worldwide – similar to rats and cats in terms of the number of species each of them endangers,” he says.

“Highly virulent wildlife diseases, including chytridiomycosis, are contributing to the Earth’s sixth mass extinction. The disease we studied has caused mass amphibian extinctions worldwide. We’ve lost some really
amazing species.”

Since the beginning of the industrial revolution about 260 years ago, the loss of animal species has been an all too common story. Earlier this year, the United Nations published the biggest-ever review of Earth’s biodiversity, finding about one million animal and plant species are at risk of extinction. Pollution, destruction of habitats, introduced species and climate change are among common causes – yet sometimes, reasons remain elusive.

Scheele says his team’s study quantifies the impact of chytridiomycosis and pieces together the timing and extent of amphibian species’ declines.

“Some species have had a relatively minor decline, while others are now extinct,” Scheele says.

The study, published in Science, involved close collaboration with Professor Frank Pasmans and Dr Stefano Canessa at the University of Ghent, Belgium, alongside 38 different amphibian and wildlife disease experts from around the world. The Threatened Species Recovery Hub of the Australian Government’s National Environmental Science Program supported the study.

Amphibians are in the firing line of this infectious disease because, throughout their lives, they either live in or breed in water, where infectious spores of the fungus are found and can survive in the absence of amphibians.

Cooler climates and regions with a high diversity of amphibian species, such as Australia’s east coast ranges, appear to provide the most favourable conditions for chytrid fungus.

Scheele says removing chytrid fungus from an ecosystem is ‘practically impossible’ at the moment, which is cause for major concern.

“If it is in an ecosystem, it’s pretty much there to stay unfortunately,” Scheele says.

“This is partly because some amphibian species aren’t killed by the disease. On the one hand, it’s lucky that some species are resistant to chytrid fungus; but on the other hand, it means that these species carry the fungus and act as a reservoir for it so there’s a constant source of the fungus in the environment.”

Scheele says globalisation and wildlife trade are the main causes of the chytridiomycosis pandemic and are enabling the spread of the disease to continue.

“Humans are moving plants and animals around the world at an increasingly rapid rate, introducing diseases into new areas,” he says.

He says improved biosecurity and wildlife trade regulation are urgently needed to prevent any more extinctions around the world. “We’ve got to do everything possible to stop future pandemics, by having better control over wildlife trade around the world,” he says.

“There’s two ways to achieve this. One is through on-ground biosecurity surveillance at points of entry and the other is through reducing the pet trade of frogs and salamanders, because it’s been identified as a problem and one of the key paths of how the pathogen is spread around the world.”

Scheele says the team’s work identified that many species were still at high risk of extinction over the next 10–20 years from chytridiomycosis due to ongoing declines. “Knowing which species are at risk can help target future research to develop conservation actions to prevent extinctions.”

Conservation programs in Australia have prevented the extinction of frog species, he says. “We don’t have a silver bullet yet, but breeding programs are buying time as we work towards a solution,” he says.

“On a local scale, we can use captive breeding programs like the one in Canberra for the Corroboree Frog. Without breeding programs, several more species would have become extinct.”

While captive breeding has been successful, many reintroduction and translocation projects have failed, Scheele says.

He is leading new research to inform the management of the Northern Corroboree Frog in the ACT and NSW – this will be a major step towards re-establishing self-sustaining wild populations in the ACT. His ANU team is working closely with the ACT Government’s Environment, Planning, and Sustainable Development Directorate.

“We need to develop, trial and evaluate new reintroduction and translocation strategies urgently if we are to establish self-sustaining wild populations of the Northern Corroboree Frog and many other chytrid-sensitive species,” Scheele says. “Our findings will be highly relevant to the conservation of other frogs affected by chytrid fungus.”

The hope is that scientists and conservationists will eventually be able to stop this killer in its tracks, but Scheele knows that much more work lies ahead.

Photo: Jonathan E. Kolby, Honduras Amphibian Rescue & Conservation Center.

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