Could museum specimens from 1973 help us solve frog disappearances that occurred decades ago?
The amphibian chytrid fungus (Batrachochytrium dendrobatidis) is thought to be responsible for declines and extinctions of frog species around the world. We suspect its involvement in many frog declines in Australia, but for most species, we lack evidence. We investigated the cause of mysterious frog disappearances that occurred from the early 1970s on the New England Tablelands of New South Wales. We found no evidence of the potentially fatal fungus prior to disappearances, but it’s now widespread in the area. Once rare species that seem to tolerate the disease are now common, while species that are more susceptible are largely missing. Our study provides the first evidence that the amphibian chytrid fungus was responsible for dramatic frog declines on the New England Tablelands and that it remains a significant threat to some frogs in the region.
Disease is among the greatest threats to biodiversity, after habitat loss and over exploitation. Nine percent of all species are threatened by invasive diseases and species according to the IUCN Red List of Threatened Species. If you look more specifically at frogs, that number more than doubles. This jump is largely due to chytridiomycosis, a catastrophic wildlife disease caused by the amphibian chytrid fungus (Batrachochytrium dendrobatidis; Bd).
Bd emerged as a global pandemic around the 1970’s and caused severe, simultaneous and, at the time, mysterious amphibian declines in Central and South America and eastern Australia. Since then, the pathogen is thought to have contributed to the extinction of almost 100 species, and population declines in about 500. No other wildlife disease has even come close to this level of destruction.
Despite Australian frogs being among the hardest-hit, Bd’s prevalence across the continent is poorly known aside from a few early-detected hotspots in the Snowy Mountains, southeast Queensland and the Wet Tropics. However, similarly mysterious and sudden declines have occurred outside of those Bd hotspots, most notably in the New England Tablelands, a plateau of around 1,000 metres elevation in north-eastern New South Wales.
The area is home to two ‘missing’ frog species, the Peppered Tree Frog (Litoria piperata) and Spotted Bell Frog (Litoria castanea), both of which are feared extinct. The Booroolong Frog (Litoria booroolongensis) was believed to be locally extinct until we rediscovered a small population in late 2017. While common in the rest of its range, the Tusked Frog (Adelotus brevis) is now endangered on the New England Tablelands.
Frog species losses in the New England Tablelands occurred around the same time as those in the ‘Bd hotspots’, were similarly sudden, and lacked any obvious explanation. Was this disease actually responsible for the disappearance of the Peppered Tree Frog and other locally missing species? To find out, we compared Bd prevalence on the Tablelands before and after the disappearances of the Peppered Tree Frog and the Booroolong Frog.
To figure out whether Bd was now present (and at what prevalence) on the Tablelands we took skin swabs of Stony Creek Frogs (Litoria wilcoxii), now by far the most common species where Booroolong and Peppered Tree Frogs once dominated but can now no longer be found. We then analysed those samples back at the Australian Museum, testing for the presence of Bd DNA.
But how could we determine if Bd was infecting frogs before they disappeared over forty years ago? Fortunately for us Bd has a helpful quirk - much of its DNA is protected in hard capsules, making it detectable even in old museum specimens. We can therefore swab these frog specimens in the same way as live ones, allowing us to test Booroolong Frogs and Peppered Tree Frogs collected in 1973, just prior to their disappearance.
All the museum specimens collected before population declines tested negative, but about a third of the live Stony Creek Frogs we tested were positive for Bd. So, it appears that sometime after 1973, Bd arrived on the Tablelands and frog populations of many species crashed. Species that were once common, but appear to be very susceptible to Bd, vanished. However, species such as the Stony Creek Frog, which appears to have been relatively rare in the 1970s, is now thriving on the Tablelands.
Our study provides the first evidence that disease is likely to have driven the rapid frog population declines and disappearances on the New England Tablelands. It seems likely that Bd is responsible for the loss of the Peppered Tree Frog and Spotted Bell Frog, and local disappearance of the Booroolong Frog and Tusked Frog. The threat is still very much present, but our rediscovery of the Booroolong Frog is a glimmer of hope for the Tablelands – some missing species may still be hanging on and can perhaps reclaim their old haunts yet.
Timothy Cutajar, Research Assistant, Herpetology, Australian Museum Research Institute.
Christopher Portway, Research Assistant, Herpetology, Australian Museum Research Institute.
Dr Jodi Rowley, Curator, Amphibian & Reptile Conservation Biology, Australian Museum Research Institute & UNSW Sydney.
Portway, C.D., Cutajar, T.P., King, A. & Rowley, J.J.L. (2020) First evidence of the amphibian chytrid fungus likely driving dramatic frog community changes on the New England Tablelands of Eastern Australia. Herpetological Review. 51 (2), 247-251.
Our work on the New England Tablelands has only been possible through the support of Glen Innes National Parks and Wildlife Service team and the amazing generosity of local landholders who allowed us to survey their properties. We also thank David Coote and David Hunter from the New South Wales Department of Department of Planning, Industry and Environment and Rebecca Webb from James Cook University for Bd detection advice. This project has been assisted by the New South Wales Government through its Environmental Trust.