Unravelling the frog diversity of Gondwanan rainforests

Continuing research in rainforests of northern New South Wales and southern Queensland has recently resulted in the scientific description of two unique frog species.

Australia has 247 recognised species of frogs, with the largest diversity occurring in the eucalypt forest and rainforest of eastern Australia. The Gondwanan rainforests along Australia’s Great Dividing Range are refuges of wet habitat that have direct evolutionary connections with the ancient forests that once covered much of the ancient Gondwanan supercontinent. A great diversity of Australia’s frogs is found in these rainforests with many found nowhere else. Some of these species have even evolved unique breeding strategies to take advantage of the continuously wet environment. Despite longstanding recognition of the importance of these habitats and the frogs within them, their remote nature has slowed research efforts with two species recently identified after long-term efforts by a collaboration of scientists.

Over the last 30 years, Professor Michael Mahony (University of Newcastle), Professor Stephen Donnellan (South Australian Museum), and Mr Harry Hines (Queensland Parks and Wildlife Service) have led a team of scientists, including researchers from the Australian Museum, in an effort to investigate the diversity and ecology of rainforest frogs in northern New South Wales and southern Queensland. The results of this work have contributed to the recent description of two frog species: the Wollumbin Pouched Frog (Assa wollumbin) from Wollumbin (Mount Warning) and Knowles’ Mountain Frog (Philoria knowlesi) from the western part of the Border Ranges.

Pouched frogs are fascinating because they have male parental care! The wet conditions of Gondwanan rainforest allow the male to initially lay their eggs directly onto the rainforest leaves. When the tadpoles hatch, they ‘swim’ up the father into pouches on his side (giving its common name), which support the tadpoles as they develop into metamorphs (baby frogs). Pouched frogs occur in numerous rainforest patches scattered between Dorrigo National Park in the south, to Conondale National Park in the north. Over the course of this research, the team identified these populations represented two distinct species, Assa darlingtoni and Assa wollumbin.

The team speculated that each rainforest isolate may host a genetically distinct pouched frog population because of the large distances between them. Following collection and analyses of frog genetic material from each rainforest isolate, it was surprising that only one location was strongly genetically divergent, and that it was separated by less than 15 km from the more widespread Pouched Frog (A. darlingtoni). In fact, the ancient Tweed volcano hosts both the new Wollumbin Pouched Frog and the well-known Pouched Frog, with the new species occupying the volcanic plug and the Pouched Frog inhabiting the caldera wall. Although surprising, genetic, acoustic, and morphological evidence supported the formal description of the Wollumbin Pouched Frog, which only occurs in a small area of Wollumbin National Park and some surrounding properties.

Mountain frogs (Philoria) also have a unique reproductive strategy. The males avoid open water, and instead build a small subterranean chamber to house their eggs and tadpoles which live where water is trickling through the soil. The eggs are very yolky, and development happens entirely within the nest with no need for the tadpoles to eat until they metamorphose into baby frogs. The discovery of Knowles' Mountain frog in the western Border Ranges involved extensive surveys and investigations of many mountain frog populations between Dorrigo and Brisbane. The species in the western Border Ranges looked very similar to its eastern Border Ranges counterpart but is often more colourful and was found to be genetically isolated. It was named after Ross Knowles to highlight his contributions to environmental conservation and work on the mountain frog group.

Ross Knowles, who worked on the taxonomy of mountain frogs in 2004 and described two new species (Phioria pughi and Philoria richmondensis), found genetic divergence among populations of mountain frogs across the Border Ranges, which allowed our team to speculate that further undescribed species may occur in the area. Starting with a genetic sample collected in the western Border Ranges in 1997, it was initially a challenging task to locate frogs and collect more genetic samples from the remote areas of the western Border Ranges to enable a thorough comparison. As the project continued, advances in technology and methods of genetic analyses allowed comparisons of mitochondrial and nuclear DNA among the various populations. Such techniques proved vital to the identification and taxonomic uncoupling of these frog populations, and an east-west divide across the Border Ranges became apparent.

Owing to the long-term work on these frog groups across the rugged and beautiful Gondwanan rainforests, we now have a greater understanding of the true diversity of endemic species in these relictual habitats. Unfortunately, both recently described species are under threat. Each occur almost exclusively in protected National Parks, but these refuges are now facing increasing threat from climate change. The black summer mega-fires of 2019/2020 burnt significant areas across the distribution of Knowles’ Mountain Frog and burnt close to the habitat of the Wollumbin Pouched Frog, increasing the risk of future fire. We now know that these unique and fascinating frogs exist and protecting them in the face of changes to climate, disease prevalence, and disturbance processes will be an especially challenging task.

Stephen Mahony, Research Associate, Herpetology and Mammalogy, Australian Museum; and, Research Assistant, University of Newcastle.