Objectives
>Students describe the threats to the frog species in their region
>Students explain how identified threats to frogs indicate larger aquatic habitat concerns
Duration
2 hours
Materials required
>Pencils
>Torches covered in red cellophane
>Thermometer
>Tape recorder or mobile phone with a recording function Frog Field Guide
>Frogwatch Frog Calls CD
>Information about creating frog habitat (hyperlink available on Sustaining River Life cd and web versions)
>“Frogs: The Thin Green Line” video (hyperlink available on Sustaining River Life cd and web versions)
>Frogwatch Field Datasheet (see below)
Green Tree Frog (Litoria caerulea). Photograph: Richard Tate
Background
Beginning in the early 1980s, biologists began to realise that amphibians such as frogs are extremely sensitive to pollution and other environmental stresses. Declines in amphibian numbers and increases in the number of deformed bodies led scientists to investigate the role of habitat loss, increased ultraviolet radiation (due to ozone depletion), and chemical pollution in these important changes.
Frog populations have declined dramatically since the 1950s. Habitat loss is a significant cause of this decline, as are pollutants, climate change, ozone depletion, the introduction of non-indigenous predators/competitors, and emerging infectious diseases including Chytridiomycosis.
Many environmental scientists consider amphibians, including frogs, to be excellent biological indicators of broader ecosystem health because of their intermediate position in food webs, their permeable skins, and their typically biphasic life (aquatic larvae and terrestrial adults). It appears that the species with both aquatic eggs and aquatic larvae are most affected by the decline.
Chytridiomycosis is an infectious disease affecting amphibians worldwide. Originally from Africa, Chytridiomycosis appears to have been brought to Australia with the live animal trade. This highly virulent fungal pathogen of amphibians is capable at the minimum of causing sporadic deaths in some populations, and 100 per cent mortality in other populations. Because Chytridiomycosis requires cool temperatures, frog species in the shrinking alpine regions are most at risk.
A number of studies indicate that the introduction of Gambusia (Mosquito fish) has contributed to the decline of many frog populations. Studies have shown that Gambusia will attack and eat tadpoles.
There is evidence of chemical pollutants causing frog developmental deformities (extra limbs or malformed eyes). Pollutants have varying effects on frogs. Some alter the central nervous system while others cause a disruption in the production and secretion of hormones.
Experimental studies have also shown that exposure to commonly used herbicides such as glyphosate (Roundup) and insecticides greatly increase the mortality of tadpoles. Additional studies have indicated that terrestrial adult stages of amphibians are also susceptible to non-active ingredients in Roundup. The herbicide Atrazine has been shown to cause male tadpoles of African clawed frogs to become hermaphroditic with development of both male and female organs. This has been repeated in many parts of the world.
Habitat loss, degradation and fragmentation have further pressured frogs in the face of climate change, as fewer areas now meet the breeding requirements of frogs.
The construction of dams and weirs across the Murray-Darling Basin has significantly altered the flow regime of the system. Flooding, which once fed off-stream fish-free wetlands, occurs less and less often. Other sites have been impacted by grazing, ploughing or irrigation infrastructure and practices.
Like many other organisms, increasing ultraviolet-B (UVB) radiation, due to stratospheric ozone depletion and other factors, may harm the DNA of amphibians, particularly their eggs. The amount of damage depends upon the life stage, the species type and other environmental parameters.
Climate change has also caused amphibian declines. The general drying trend in eastern Australia associated with climate change bodes poorly for water-dependent species such as frogs. Climate change, coupled with habitat loss and fragmentation, could well be the final stressor that pushes some species to extinction.
Some of the frogs of special concern in the Murray-Darling Basin include:
>Green and Golden Bell Frog
>Yellow-spotted Bell Frog
>Booroolong Frog
>Northern Corroboree Frog
The Green and Golden Bell Frog has been found in association with almost every type of waterbody except fast flowing streams. In NSW, it inhabits many disturbed sites including abandoned mines and quarries. Sites which support breeding populations are waterbodies which are still, shallow, ephemeral, unpolluted, unshaded, with aquatic plants, and free of Gambusia and other predatory fish. They also have terrestrial habitats that consist of grassy areas and vegetation no higher than woodlands, and a range of diurnal shelter sites.
The Yellow-spotted Bell Frog on the southern tablelands of NSW has suffered an extensive decline, with no confirmed records since 1980 where it was formerly known from Namadgi and Kosciusko National Parks and extensive areas of grassland.
The Yellow-spotted Bell Frog occupies habitat which includes permanent ponds, swamps, lagoons, farm dams