There are approximately 4000 frog species in the world. Australia and New Guinea have a high diversity of frogs. Currently 208 Australian species are formally recognised although some await description.
The Action Plan for Australian Frogs classifies 20 of Australia's 208 described species as Endangered and 7 as Vulnerable. Eight of the endangered species may have disappeared altogether. An additional 14 species are described as ‘insufficiently known' but are of conservation concern. Thus 13% of known species are considered threatened with extinction and there are concerns for an additional 7%.
In the past 200 years the Australian landscape has been altered by urbanisation, vegetation clearing, and water diversion and pollution on a large scale. Dramatic declines in some Australian frog species have been reported since the 1980s, with concern increasing more recently following the disappearance of a number of species from apparently natural ecosystems.
For most species, there is no clearly identifiable cause of decline, although several factors are implicated. Some declines are associated with loss, degradation and fragmentation of habitats, land use practices, changes to hydrology, pollution and predation by introduced fish, but others are as yet inexplicable. Disease is being investigated as a possible cause for some declines in north Queensland, and this may have relevance elsewhere.
So far no factors of a global nature have been shown to be related to frog declines, but these are being investigated around the world. Possible factors include air pollution, climate change influencing precipitation patterns and increased ultra-violet radiation resulting from depletion of the ozone layer. This last has implications for basking frogs in particular.
The first evidence that Australian frog populations were in serious decline was provided by the disappearance of two species throughout their known range. The Mt Glorious Torrent Frog Taudactylus diurnus, only described in 1966, was last sighted in 1979 (Czechura and Ingram 1990). The Gastric Brooding Frog or Platypus Frog, Rheobatrachus silus, which was unknown until 1973, was last sighted in 1981 (Richards et al, 1993). No possible causal agents were identified at the time (Tyler and Davies 1985), and there has been no subsequent clarification of possible causes for these disappearances.
Although there were isolated suggestions in the early 1980s that there had been declines in other species such as the Green-thighed Frog Litoria brevipalmata and the Yellow-spotted Tree Frog L. castanea on the New England Tablelands, no verification was made. It was not until 1989, at the First World Congress of Herpetology in Canterbury, England, that there was any perception of a significant pattern of declines on a global basis, and any recognition of the need to take special measures to ensure the survival of amphibians.
There is an urgent need to raise the level of commitment to the conservation of frogs and frog habitats by conservation agencies, and also, increase community awareness and involvement in frog conservation.
Fauna surveys and frog habitat description, monitoring and protection need greater attention. Increased investigation into little known species, including those of the semi-arid zone, such as Cyclorana and Neobatrachus species, is needed to adequately assess their status.
An enormous array of human activities impinge upon the viability of frogs. Those summarised in The Action Plan for Australian Frogs include:
- Insecticide use in agricultural and horticultural areas, particularly aerial spraying.
- Land reclamation by drainage in wetland areas, resulting in loss of breeding sites.
- The conversion of temporary ponds to dams for stock use resulting in the destruction of peripheral sheltering sites.
- Introduction of the Mosquito Fish Gambusia holbrooki which preys on frog eggs and tadpoles.
Other factors being considered as potentially implicated in frog declines include: global changes to air and water quality, increased exposure to ultra-violet radiation caused by depletion of the ozone layer, habitat modification, impacts of introduced species such as trout, pollution, hormonally active pesticide residues, pathogens and disease, acidification and climate change, including changes to precipitation patterns and climatic extremes.
While some local losses of frogs are attributed to specific causes, such as drainage of wetlands, there are many disappearances that cannot be explained, and examples such as Rheobatrachus vitellinus, in Eungella National Park of Queensland, defy any reasonable explanation. This species existed in creeks that were difficult to access and which were not subjected to any human pressure. In the case of R. silus and Taudactylus diurnus in the Conondale and Blackall Ranges, a portion of their habitat was less secure, and siltation resulting from an inadequate streamside buffer zone during adjacent forestry operations would have eliminated a portion of habitat. Nevertheless, this activity was not responsible for their total disappearance there, nor from the isolated Kondalilla National Park, or the headwaters of Kilcoy Creek.
As most frog species are susceptible to dehydration, it is possible that the recent decline of some populations may be attributable to drought. In the case of the stream-dependent species Rheobatrachus silus, drought would seem the most likely option to explain its demise. However there is no evidence that the disappearance of R. silus was linked to any extreme reduction in rainfall or available surface water.
Experience elsewhere in Australia and overseas rarely supports the concept that drought has been instrumental in major declines. A contrary view, however, has been put by Pechmann and Wilbur (1994), and Osborne (1996) considers it may be implicated in the decline of Pseudophryne corroboree.
Direct human activities have been a significant causal factor in the decline of species, but there is no evidence that these activities have accelerated over the past 15 or so years in which declines have become so noticeable. It remains possible that the declines are the summation of long-term effects of human activities, or that there is an unknown synergism involving several factors. The existence of frogs in urban areas has been a feature of suburban life, but many suburban declines have also been noted like Pseudophryne australis.
Some chemicals such as herbicides which are widely used drain into waterways. The active constituents of herbicides currently in common use are exposed to reasonably rigorous toxicological scrutiny during their developmental stages, and their approval for release in Australia hinges upon an assessment of their action upon non-target organisms such as frogs. However, many herbicides, such as glyphosate, are mixed with chemical dispersants which are not subject to the same scrutiny. Glyphosate is now marketed in over 100 countries.
An Australian study confirmed that glyphosate-based herbicides are toxic to frogs, especially tadpoles, possibly because of the effect of the dispersant on tadpole gills (Bidwell and Gorrie 1995). The authors recommended that a list of low impact dispersants suitable for use in aquatic environments be developed. The National Registration Authority has responded to this report, and from July 1997 a total of 74 herbicide preparations were banned from use near water.
There have been reports in several south-eastern areas of increased numbers of foxes. Frogs that bask, such as Litoria raniformis, L. aurea and L. castanea could be particularly vulnerable to such predators.
The Action Plan summarises the following information in seeking patterns to frog declines detected during the last 15 years:
- Twenty-four of the twenty-seven threatened species are confined to eastern Australia.
- Of the seven riparian species in the Wet Tropics which have disappeared at upper altitudes, three remain at lower altitudes. The other four species have disappeared from all sites.
- All fossorial species in eastern Australia are considered secure.
- In south-eastern Australia every species known to bask is locally or totally in decline, although the same is not true for south-western species.
- Eighteen of the twenty seven species are continuously associated with static or running water.