Threats to frogs

How threatened are amphibians?

Frogs and amphibians have been making headlines over the last few years because they appear to be declining in numbers. Of the around 6000 species of amphibians worldwide (most are frogs and toads) around a third are threatened and around 160 species are thought to have become extinct in recent times. By comparison, only 12% of bird species and 23% of mammal species are considered threatened. Also concerning is the observation that 43% of all amphibian species are showing a population decline while fewer that 1% of species are increasing.

In Australia, which only has frogs and toads, 50 species or 23% are considered threatened or recently extinct. While still a cause for great concern, this is less than the global average and is much lower than the levels of threat to amphibians in Central America and the Caribbean: more than 80% of amphibians are threatened in the Dominican Republic, Cuba, and Jamaica, and 92% are threatened in Haiti.

Many frogs are likely to be particularly sensitive to environmental changes because they are dependent on a good water habitat for their eggs and tadpoles and also on a suitable terrestrial habitat for adult forms. The aquatic habit of tadpoles and the role of the moist skin in breathing also makes frogs highly susceptible to toxins in the environment.

What are the threats to Australian frogs and toads?

The threats to frogs and toads in Australia can be divided into those that are well-known and have been operating for some time — principally habitat degradation and destruction and depredation from introduced pests; and those that are not well known and appear new.

Habitat degradation and destruction remains the number one threat to frogs and toads in Australia and indeed to amphibians around the world. Habitat changes are associated with the declines in just under half of Australia’s threatened or extinct frogs. Another significant on-going threat is predation on eggs and tadpoles by introduced fish. These threats are seen mostly in the more developed areas of southern and eastern Australia and south-western Australia. Northern Australia, with its much lower level of development and largely intact habitat, has far fewer threatened frog populations (see map below).

A recent potential threat to the frogs of northern Australia is the continued spread of the cane toad, Bufo marinus westward. As of early 2007 it threatens the Western Australian border and the suburbs of Darwin. The toad may impact on local native frog populations by depleting the food resources that native frogs need. But as yet no negative impact has been confirmed.

The main impact of cane toads documented so far has been on frog-eating species, such as yellow-spotted monitor Varanus panoptes and the northern quoll Dasyurus hallucatus, and reduction in predation by these monitors on pig-nosed turtles Carettochelys insculpta. There is currently (mid 2007) a concerted effort to trap cane toads in the north western NT in order to reduce their possible impact on native frogs and other animals (see www.frogwatch.org.au ).

Recent frog declines and the threat from Chytrid fungus

These well-known threats, however, do not seem to explain some recent declines in populations of Australian frogs. Since the 1970s a number of enigmatic and catastrophic declines have been seen in Australian frog populations. Northern Australia has not been immune from such declines.

• four frogs from north Queensland forest stream habitats can no longer be found in the wild: the remarkable northern gastric brooding frog, Rheobatarchus vitellinus; the sharp-snouted day frog, Taudactylus acutirostris; the little waterfall frog, Litoria lorica; and the mountain mistfrog, Litoria nyakalensis.
• The northern tinkerfrog Taudactylus rheophilus from montane forest steams in north Queensland has almost entirely disappeared.
• Four other frogs that live near streams in north Queensland forests have disappeared from most upland areas of their historical ranges: the waterfall frog, Litoria nannotis; the common mistfrog, Litoria rheocola; the Australian lacelid frog, Nyctimystes dayi; the Eungella day frog, Taudactylus eungellensis

These declines, and those that have occurred further south, seem to mostly affect stream-dwelling frogs in upland areas.  The usual factors of habitat modification or predation by pests do not appear to be significant factors.

An emerging culprit is the disease chytridiomycosis, caused by the chytrid fungus batrachochydrium dendrobatidis, which is thought to be involved in the decline of many of these frog populations. However, because this fungus does not seem to cope well with temperatures above 30^oC it does not appear to be a threat in the tropical savannas including Cape York Peninsula. It is however, a significant threat to stream-dwelling frog populations in upland areas of north Queensland’s wet tropics region (see boxed story, The threat of Chytrid Fungus).

What about climate change?

Climate change is expected to bring increased temperatures to most parts of northern Australia over the next century and beyond. These temperature increases could in turn lead to increased drying of some habitats. Of particular concern are frogs that depend on cool, moist high altitude habitats that may shrink or even disappear as it get warmer and drier. The nurseryfrogs (Cophixalus) which mostly live in such high altitude areas of north Queensland are considered threatened in longer term because of climate change.

References

Department of Environment and Heritage (2006) Infection of Amphibians with Chytrid Fungus Resulting in Chytridiomycosis – Background Document for the Threat Abatement Plan, Commonwealth of Australia, Canberra

Hero J.-M., Morrison C., Gillespie G., Roberts J. D., Newell D., Meyer E., McDonald K.R., Lemckert F., Mahony M., Osborne W., Hines H., Richards S., Hoskin C., Clarke J., Doak N. and Shoo L. (2007) Overview of the conservation status of Australian frogs. Pacific Conservation Biology 12, pp 313-320.

Roleants K., Gower D.J., Wilkinson M., Loader S.P., Biju S.D., Guillaume K., Moriau L., and Bossuyt F. (2007) Global patterns of diversification in the history of modern amphibians. Proceedings of the National Academy of Sciences (DOI:10.1073/pnas.0608378104)

Tyler M. J. (1994) Australian Frogs – a natural history. Reed Books, Chatswood NSW