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Chytrid and Frogs in Australia’s High Country

By Ben Scheele

Science is helping conservation managers deal with the curse of chytrid fungus. While the threat has devastated many frog species, there is reason to be hopeful.

Frogs are in trouble. A devastating disease called chytridio­mycosis has been wiping them out, often from pristine habitats. The disease is caused by amphibian chytrid fungus, which disrupts the skin function of infected frogs, leading to cardiac arrest.

Since the identification of chytrid fungus by Australian researchers in 1998, the pathogen has been documented in more than 500 amphibian species around the world. Fortunately the pathogen is not universally deadly, with some species demonstrating high resistance. However, many species are highly susceptible and the pathogen has been identified as the primary driver of decline for more than 200 species of frog.

It’s believed the pathogen may have originated from Brazil. The earliest record of chytrid in Australia is from a preserved frog specimen collected in 1978 in south-east Queensland. From its potential introduction in Brisbane, chytrid appears to have spread rapidly both north and south.

Over the past 3 years we have focused on the long-term impacts of chytrid on frogs in the Australian high country – a region that is home to several endemic species. In the mid-1980s mysterious frog declines were reported from the region. We can now be confident that these declines were caused by chytrid. In conjunction with David Hunter from the NSW Office of Environment and Heritage, we have examined how these species are faring three decades later.

Some species are in a continued state of decline, and are being pushed closer to extinction. These declining species include some of our most iconic fauna, the corroboree frogs. The northern corroboree frog was once highly abundant in the Kosciuszko and Namadgi national parks. Populations were decimated by chytrid in the 1980s, and only a handful of remnant populations survived.

Unfortunately for the northern corroboree frog, the common eastern froglet – a highly resistant species – also remains abundant at these sites. In ongoing research, we have shown that the froglet acts as a reservoir host that carries infection but is rarely killed by the disease. This reservoir host amplifies the impact of chytrid in corroboree frog populations. Although the prognosis for both corroboree species is alarming, understanding the mechanism controlling chytrid dynamics is facilitating the development of conservation strategies.

For other species such as the endangered alpine tree frog, the situation appears a little better. The alpine tree frog also experienced major declines but remnant populations now appear relatively stable despite the continued presence of chytrid. Studying these populations, we have found that their persistence is facilitated by high juvenile frog recruitment.

During the breeding season, the prevalence of chytrid can exceed 90% in adults, resulting in very low survival. Crucially, though, the pathogen is rare in tadpoles and juvenile frogs. This allows the next generation to disperse into woodland habitat free of the disease. Chytrid is aquatic and individuals have a low risk of becoming infected in terrestrial environments. However, when they return to wetlands to breed they become infected. Luckily, adults are able to breed before succumbing to the disease.

In the foothills of the Snowy Mountains, the whistling tree frog, a close relative of the alpine tree frog, is bouncing back. Surveys in the 1970s found that whistling tree frogs were everywhere on the NSW southern tablelands, but populations crashed in the 1980s.

When we commenced our surveys in 2011, we found that the species was present in many areas where it had been absent two decades earlier. Ongoing surveys in 2012 and 2013 demonstrated that the species is re-expanding into habitat occupied decades ago. While more work remains to be done on the mechanism facilitating recovery, we found that sites that retained frogs during the cycle of population decline and recovery had high quality habitat.

Without conservation interventions, the number of Australian species driven to extinction by chytridiomycosis will almost certainly rise. While preserving habitat is important, it is not enough on its own; direct management is needed. We are now developing management actions that focus on reducing chytrid fungus in the environment or on frogs, and increasing the capacity of populations to persist despite the high mortality associated with disease.

The chytrid threat has been extremely daunting over the past 15 years. However, our research indicates we are now at a turning point, with the potential to make real progress in the management of this terrible disease.

Ben Scheele is an associate of the ARC Centre of Excellence for Environmental Decisions. He is currently based at James Cook University.