Disappearing frogs

Frogs are disappearing, not only in Australia but worldwide.  The problem is both complex and serious - complex, because we don’t fully understand the causes and serious because it points to   problems in the environment. 

Frogs, like all amphibians, have moist, permeable skins, or skin patches, that allow both inward and outward movement of water and dissolved gases.  This water and gas exchange across a frog’s skin makes it particularly prone to the effects of toxic chemicals such as pesticides and herbicides.  For this reason, frogs are important indicator species.  When frogs start to disappear we know things are seriously wrong in the natural world.

Declines in frog numbers have been reported in North and South America, Europe, and South Africa - all areas with active frog research communities.  However, little to no frog monitoring occurs in south-east Asia and New Guinea, both of which have highly diverse frog faunas.  Whether populations in these areas are stable or also in decline is not known.

Missing Queensland frogs

The Southern Platypusfrog, Rheobatrachus silus, last seen in the wild in 1979.

The alarm bells started ringing for Queensland frogs in the mid 1980s.  At this time, leading frog researchers, Glen Ingram and Greg Czechura, of the Queensland Museum, started to consider the shocking prospect that two South East Queensland frogs had seemingly vanished.  Neither the Southern Platypusfrog, Rheobatrachus silus, nor the Southern Dayfrog, Taudactylus diurnius had been seen in the wild since 1979.  From limited data, we know that the Southern Platypusfrog was not a common frog but the Southern Day Frog was said to be abundant where it occurred.

The loss of the Southern Platypusfrog seems all the worse as its unusual breeding strategy was of immense interest to the scientific community.  This was one of two frog species in which the mother swallows her fertilised eggs which then develop into young frogs within her stomach.  The only other frog to do this, the Northern Platypusfrog, Rheobatrachus vitellinus, was also found in Queensland and this species too has vanished. 

We now consider the Southern Platypusfrog, Rheobatrachus silus, the Northern Platypusfrog, Rheobatrachus vitellinus, the Southern Dayfrog, Taudactylus diurnius, and the Mountain Mistfrog, Litoria nyakalensis to be extinct. 

Other species, if not already gone, may be perilously close to extinction (the Sharp-snouted Dayfrog, Taudactylus acutirostris and the Northern Tinkerfrog, Taudactylus rheophilus).  There was great concern for the Eungella Dayfrog, Taudactylus eungellensis, and the Armoured Mistfrog, Litoria lorica, but these species have both been rediscovered in small numbers following long absences. 

Australian frog disappearances are not unique to Queensland and a number of southern species have also declined.  These include the iconic corroboree frogs, Pseudophryne corroboree and P. pengilleyi that have all but disappeared from the wild.

Causes of declines

A Scarlet-sided Pobblebonk, Limnodynastes terraereginae, with an extra limb - frogs are particularly susceptible to the effects of pollution.

Chytrid (kit-rid) fungus (Batrachochytrium dendrobatidis)

This fungal pathogen is probably the main cause for the Queensland declines.  Most of our missing species are from high altitude rainforests where pollutants are not a major concern.  Chytrid fungus feeds on keratin, attacking the skin on the feet of adult frogs and the mouthparts of tadpoles.  Frogs with chytrid fungus develop thick peeling skin, become lethargic and lose movement in their hind legs.  They eventually die.  Infected tadpoles die shortly after turning into small frogs.

Destruction and alteration of frog habitat

Human impacts on the environment can be detrimental to frogs.  Urbanisation diminishes frog habitat as important wetlands are drained to make way for an expanding human population.  The damming of rivers, cattle damage along drainage lines, introduced fish and pollution all reduce frog numbers.  Frogs are particularly susceptible to the affects of pollution.  Chemical pollutants render habitat unsuitable for frogs and can induce developmental deformities. 

Climate change

Scientists predict that global warming will cause worldwide changes to rainfall and temperature.  In areas where rainfall becomes less regular, the breeding success of frogs will be severely impacted.  A number of poor breeding years in succession may push a species to extinction.  Frogs found in montane rainforest communities are of particular concern as they rely on cool, moist conditions.  In these habitats, changes in temperature and rainfall are predicted to cause an upward migration of both plants and animals.  The net effect being that rainforests, and the animals that live in them, are pushed to the very top of the mountain and have nowhere else to go.  Over time, these habitats are infiltrated and replaced by dry forest communities.

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