Using probabilistic models to investigate the disappearance of a widespread frog-species complex in high-altitude regions of south-eastern Australia: Disappearances of bell frogs at high altitudes

Amphibian populations have suffered declines and disappearances around the world. It is now recognized that many of the disappearances were the result of infection by the amphibian chytrid fungus Batrachochytrium dendrobatidis (Bd). Most global declines were first observed in the 1980s. Here, we used information from museum collections, natural-history databases and the literature to quantitatively assess the disappearance of the bell frog species complex (Litoria aurea, Litoria castanea and Litoria raniformis) in high-altitude Tableland regions of the Great Dividing Range, south-eastern Australia. Based on the sighting record, we estimated that Li. castanea disappeared on the Northern Tablelands in 1979, then from the Southern and Central Tablelands in 1984. Estimated dates of disappearance of Li. aurea and Li. raniformis from the Southern and Central Tablelands were 1981 and 1982, respectively. The Northern Tablelands populations of Li. castanea may have been one of the first high-altitude populations of frogs to disappear from Australasia and perhaps one of the first globally. We documented targeted searches of suitable habitat within the historic range of the three species in these Tableland regions since 1985, noting that only one bell frog population was discovered (Li. aurea, Southern Tablelands, 2000). Our Solow equation estimate of the probability of extant populations of bell frogs persisting up to the year 2000 was < 0.001, so the rediscovery of Li. aurea was mathematically exceptional. Although the cause of these declines remains unknown, the latitudinal (i.e. north to south) and rapid pattern of disappearance of Li. castanea, and the rapid contraction in the range of Li. aurea and Li. raniformis from high-altitude regions, strongly suggests a pathogen such as Bd was responsible. We recommend further surveys for bell frogs on the Tablelands, as the rediscovery of Li. aurea provides faint hope of finding other extant populations.