In the age of the Anthropocene, the loss of a species is an all too unfortunately common story. Reductions of habitats, pollution, introduced species or climate change are common causes – yet sometimes, the answers are less obvious.

For frog populations worldwide, an unusual fungal pathogen – spread globally over the last 40 years by humans – is the cause of many species declines and extinctions.  

For the first time, a global group of scientists have quantified the scale of impact of the chytrid fungus on amphibians. Directed by lead author Dr Ben Scheele, with support from Professor David Lindenmayer and Dr Claire Foster from the Fenner School at the ANU, the group found that at least 501 species have declined as a result of chytrid fungus, and 90 of these are confirmed or believed extinct.

Published in Science, their paper expands on earlier estimates of the fungus’ impact, and identifies chytrid fungus as one of the most damaging invasive species in terms of threats to biodiversity. “It’s unprecedented in terms of scale,” Ben said.

“There’s no other wildlife pathogen that has caused a similar number of declines. Its impact is on par with the worst invasive species like cats or rats.”

For biologists, the decline of species from seemingly pristine habitats was a mystery that had them stumped for years.

The fungus was first identified in 1998 by a team of Australian and international scientists led by Lee Berger, who demonstrated the pathogen was the cause of amphibian declines in the rainforests of Australia and Central America. In 2009, researchers discovered how chytrid fungus kills frogs, while in 2018 the Korean peninsula was identified as the likely origin of the most deadly lineage of the fungus.

This new work not only quantifies the impact of the pathogen, but also pieced together the timing of declines, finding that declines peaked globally in the 1980s, about 15 years prior to the discovery of the disease.

“The paper assesses how many species have been affected and the severity of their loss. Some species have had a relatively minor decline, while others are now extinct. It also looks at the timing of declines, and the environmental conditions that have allowed the pathogen to thrive.”

Cooler climates and those with high frog diversity appear to provide the most favorable conditions for chytrid fungus, and in Australia the biggest impacts have been felt on the ranges along the east coast. Severe declines and extinctions have been most common in species with small distributions and restricted elevational ranges.

“No one had assessed the global impact before, with this study bringing together many different researchers from across the world,” Ben said.

“It was a complex process, because we needed on-ground, species-specific information, requiring collaborators to invest substantial effort in contributing to this project.”

“So the first step was getting in touch with the right people and building trust. This is because biodiverse countries such as Ecuador, Bolivia and Columbia had many species to assess, requiring a substantial time commitment for collaborators to access and compile data”.

“It required weeks of people’s time, and they are often trusting you with unpublished data, which has taken many years to collect. Building up that relationship has been critical.”

The data set will be published alongside the scientific paper, so other scientists can use it as a baseline of where research is at 2019.

So what will become of the frogs?

Ben says the fungus has proved difficult to control, and there’s no solution for it at the moment.

“Biosecurity is key. We just need better biosecurity full-stop,” Ben said.

“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, because it’s been identified as a problem and one of the key paths of how the pathogen is spread around the world.”

“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.”

“We don’t have a silver bullet yet, but breeding programs are essentially buying time as we work towards a solution.”