High up in the sub-alpine bogs of the Snowy Mountains lives one of Australia’s most iconic and rarest creatures, the Southern Corroboree Frog (Pseudophryne corroboree). This small, vividly-coloured species is restricted to Kosciuszko National Park, in the south-east of New South Wales. Found only at altitudes above 1300 metres, this species historically had a range of 400km2.
The Southern Corroboree Frog breeds throughout summer. Males call from moist terrestrial nest chambers, typically around the edge of pools in sphagnum bogs. The large eggs, with a diameter of 9mm when hydrated, wait for autumn rains or early snow melt before the tadpoles hatch and enter the pools. Adapted to life in cooler climates, this species may take three to five years to mature and can live for at least nine years.
Its striking yellow and black longitudinal markings make it one of our most easily recognised frogs, but also indicate the lethal alkaloids within its skin. These protect them against predation. But even with no known predators and a distribution almost entirely within a pristine wilderness area, the Southern Corroboree Frog is still one of Australia’s most threatened vertebrates.
The Southern Corroboree Frog is listed on the IUCN Red List as Critically Endangered. It is also listed as Critically Endangered under the Commonwealth EPBC Act, and Endangered under the NSW Threatened Species Conservation Act.
The species has declined steadily since the mid-1980s, and there may now be as few as 50 adult Southern Corroboree Frogs left in the wild.
The most recent surveys have detected only 15 calling males across all historic and reintroduction sites. No breeding was recorded in 2013. With its ongoing decline and the small population remaining it’s likely that without human intervention the species will become extinct in the very near future.
The primary cause of decline of the Southern Corroboree Frog is chytridiomycosis, the disease associated with amphibian chytrid fungus (Batrachochytrium dendrobatidis). This fungus has caused the disappearance of many species around the globe, especially in Central America and eastern Australia.
Studies show that the pathogen was not present in the Corroboree Frog population before the declines in the 1980s. The arrival of the fungus in disease-free populations has shown the devastating impact on this species.
The spread and persistence of chytrid fungus in the population is facilitated by a species living alongside the Corroboree Frog, the Common Eastern Froglet (Crinia signfera). This species appears to sustain high infection levels, but doesn’t develop the disease. As a result, it acts as a reservoir host, sustaining the disease in the ecosystem and allowing transmission to other species.
An additional threat to the Southern Corroboree Frog is climate change. Reduced precipitation and warmer temperatures are likely to eventually affect breeding pools and vegetation around them. Droughts already result in egg and tadpole deaths, and as the frequency of droughts increases with climate change, the capacity for the Southern Corroboree Frog to recovery greatly reduces.
The goal of the Corroboree Frog conservation strategy, co-ordinated by the NSW Office of Environment and Heritage, is to ensure the frogs survive in the wild, but it is clear that without human intervention the species will become extinct. To make sure that doesn’t happen a captive population has been established for research and for re-establishing wild populations.
These populations are based at Taronga Zoo, the Amphibian Research Centre, Melbourne Zoo and Healesville Sanctuary. All the populations are held in climate-controlled, quarantine facilities that can mimic cold winter temperatures to simulate natural climate conditions.
Over recent years, captive breeding has been very successful. Earlier this year, Taronga and Melbourne Zoos alone produced over 1400 live, fertile eggs.
Back in the wild the focus is on translocating captive-bred eggs into Kosciuszko National Park. The eggs are released into a series of artificial pools designed to be fungus and drought proof. Survival of the eggs so far has been relatively high. But, it takes four to five years for the frogs to mature, so whether the eggs will survive to adulthood and breed isn’t known yet.
In 2013, the program further expanded with the construction of a large disease-free enclosure in Kosciuszko. The enclosure was designed with frog-proof fences to prevent frogs from climbing in or out of the enclosure. This colony will produce offspring for other recovery efforts at relatively low cost. To date, 120 frogs from the Amphibian Research Centre and 120 eggs from Taronga Zoo have been released within it.
James Cook University are researching the immunity of frogs to chytrid fungus. The work has been investigating whether there are differences in resistance between individuals and between sites. Genetic differences may pave the way for selectively breeding for disease resistance.
The Southern Corroboree Frog is a species at the very brink of extinction. Due to a dedicated recovery team it still exists, and a large insurance colony has been established to ensure its persistence.
There are many research efforts worldwide investigating potential strategies for threatened amphibians to survive chytrid fungus. Although there is no clear answer yet, our knowledge of the disease has expanded enormously in recent years. There is hope that future developments might allow critically endangered species such as the Southern Corroboree Frog to once again survive in the wild.
This article was co-authored by David Hunter, threatened species officer for the New South Wales Office of Environment and Heritage.
The Conversation is running a series on Australian endangered species. See it here