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Green and Golden Bell Frog

The Green and Golden Bell Frog (Litoria aurea), also named the Green Bell Frog, Green and Golden Swamp Frog and Green Frog, is a ground-dwelling tree frog native to eastern Australia. It can reach up to 11 centimetres (4.3 in) in length, making it one of Australia's largest frogs.

Many populations, particularly in the Sydney region, inhabit areas of frequent disturbance, such as golf courses, disused industrial land, brick pits and landfill areas. Though once one of the most common frogs in south-east Australia, the Green and Golden Bell Frog has endured major declines in population, leading to its current classification as globally vulnerable. Its numbers have continued to decline and are threatened by habitat loss and degradation, pollution, introduced species, and parasites and pathogens, including the chytrid (Batrachochytrium dendrobatidis).

Taxonomy

The common name, "Green and Golden Bell Frog", was first adopted by Harold Cogger in his 1975 book Reptiles & Amphibians of Australia. Before this, its common names were "Golden Frog" and "Golden Treefrog". The Green and Golden Bell Frog has many physical and behavioural characteristics representative of ranids; hence its original classification as Rana. It has a pointy snout, long legs, and almost complete toe webbing; the tympanum is large and distinct; and the overall body shape is similar to many Rana species. Like many frogs in the Rana genus, Green and Golden Bell Frogs are mostly aquatic, and only travel over land during periods of rainfall. It was removed from the genus because of anatomical differences with the family Ranidae. The bone and cartilage structural formations of the Green and Golden Bell Frog are closest to those of species in the family Hylidae; it was therefore reclassified.

The Green and Golden Bell Frog was first described as Rana aurea by Lesson in 1827. It has changed classification 20 times; it was first named Litoria aurea in 1844 by Günther, and changed another nine times before being named again as Litoria aurea. The specific epithet aurea derived from the Latin aureus for 'golden'. The species is now classified within the Litoria aurea complex, a closely related group of frogs in the Litoria genus. This complex is scattered throughout Australia: three species occur in south-east Australia, one in northern Australia, and two in Southwest Australia. The complex consists of the Green and Golden Bell Frog (Litoria aurea), Growling Grass Frog (Litoria raniformis), Tablelands Bell Frog (Litoria castanea), Dahl's Aquatic Frog (Litoria dahlii), Spotted-thighed Frog (Litoria cyclorhyncha) and the Motorbike Frog (Litoria moorei). The ranges of L. raniformis and L. castanea overlap with the Green and Golden Bell Frog; this as well as physical similarities may make it difficult to distinguish between the species. The Tablelands Bell Frog has not been seen since 1980 and may now be extinct, although the large yellow spots present on its thighs help distinguish it from the Green and Golden Bell Frog. The Growling Grass Frog, which is very similar to the Green and Golden Bell Frog, can only be readily distinguished by raised bumps on the dorsal surface.

Distribution

The Green and Golden Bell Frog is native to south-eastern Australia. Before its decline in population, its distribution ranged from Brunswick Heads, in northern New South Wales, to East Gippsland, in Victoria, and west to Bathurst, Tumut and the Australian Capital Territory.

The Bell Frog's current distribution now ranges from Byron Bay, in northern New South Wales, to East Gippsland, in Victoria; populations mostly occurring along the coast. In New South Wales, it has declined severely in range and abundance since the 1960s, although no similar declines have been reported in Victoria. In New South Wales, it has disappeared from highland areas above 250 metres (820 ft), except for a population in Captains Flat. A study of populations along coastal New South Wales indicated that many populations were very small, usually of fewer than 20 adults. But there are six known populations of more than 300 frogs: two in the Sydney metropolitan area, two in the Shoalhaven and two in the New South Wales mid-north coast. It is estimated that the Green and Golden Bell Frog has disappeared from at least 90% of its former range.

The Green and Golden Bell Frog survives in some areas of Sydney, such as the Brickpit at Sydney Olympic Park (the proposed site for the tennis courts for the 2000 Sydney Olympics). When the Green and Golden Bell Frog was found there, the tennis courts were built elsewhere, and the population has since been monitored. The Green and Golden Bell Frog has become an unofficial mascot for the Homebush Bay area.

The Green and Golden Bell Frog occurs on two islands off the east coast of Australia: Broughton Island off Port Stephens and Bowen Island at Jervis Bay. It has been introduced to New Zealand, where it is now common, and to the Pacific Islands of New Caledonia and New Hebrides.

Description

The Green and Golden Bell Frog is a large, stout frog; adults range from 4.5 centimetres to 11 centimetres (1¾ to 4¼ in) in length; typical specimens measure 6 to 8 centimetres (2½–3¼ in). The Green and Golden Bell Frog is therefore one of the largest Australian frogs. Mature male Green and Golden Bell Frogs are generally smaller than mature females, and the colour on their dorsal surface differs greatly to females. It may be almost completely green, of shades from dark pea-green to bright emerald; green with metallic, brassy, dull copper-brown, or gold markings; or almost completely bronze. During the cooler months (May-August), when Green and Golden Bell frogs are inactive, colouration may darken almost to black.

A creamy-white or pale yellow stripe, bordered above with gold and below with black, extends from behind the eye, across the tympanum to the groin. This stripe rises to form a dorso-lateral fold towards the groin. Another stripe of the same colour begins below the eye and continues to the shoulder. The abdomen is cream or white, and has a coarsely granular texture. The legs are green, bronze, or a combination of both, and the inside thigh and groin are blue-green. Mature males develop a yellowish colouration to the vocal sac on the throat. The pupil is restricted to a horizontal slit and the iris is golden-brown with a black streak that runs from the corners of the pupil to the horizontal limits of the eye. The tympanum is distinct and ovular in shape, and the species has enlarged toe discs to aid in climbing. As this species is often found in water, the fingers are free from webbing while the toes are almost completely webbed. When in breeding condition, males develop nuptial pads on their thumbs, which are used to grip females during mating.

Ecology and behaviour

As a member of the tree frog family, the Green and Golden Bell Frog spends much time basking in the sun on rocks and reeds. Unlike most frog species it is often active during the day. When handled, this species secretes a slimy acrid mucus which consists of 17 aurein peptides. Thirteen of these show broad-spectrum antibiotic and anti-cancer activity, which is useful in fighting off harmful microorganisms.

The Green and Golden Bell Frog is generally associated with coastal swamps, woodland, and forest; but populations have also been found at former industrial sites (for instance, the Brickpit). The requirements of its habitat have been difficult to determine, for it has been found in a wide range of water bodies except fast-flowing streams. It is most typically found in short-lived freshwater ponds that are still, shallow, unshaded and unpolluted; and it tends to avoid waters that contain predatory fish, whether native or introduced. The frog prefers water bodies that support emergent vegetation such as reeds and bullrushes for basking, and winter habitats consist of available shelters around the breeding site. Grassy habitats are usually close at hand to provide suitable terrestrial feeding grounds.

The Green and Golden Bell Frog's reproduction depends on salinity and water temperature. Salinity affects tadpoles' development and metamorphosis, and breeding generally only occurs in ponds that measure 20 °C (68 °F) or above. The Green and Golden Bell Frog tadpoles can tolerate salinity levels of six parts per thousand (ppt) without any apparent effects, while salinity of 8 ppt or higher decreases growth rates and increases mortality rates. On the other hand, salinity levels of at least 1–2 ppt can be beneficial to the Green and Golden Bell Frog because this kills pathogens such as the chytrid fungus.

The voracious adults have a very broad diet, including insects and other frogs, even of the same species. The tadpoles feed on detritus, algae and bacteria. Natural predators include wading birds and snakes, and are eaten by tortoises, eels, other fish, and a range of invertebrate predators.

Reproduction

The Green and Golden Bell Frog breeds in the warmer months from October to March. During the breeding season, males call while floating in the water, with a deep growl that has been described as a four-part "craw-awk, crawk, crok crok" — likened to the sound of a motorbike changing gears. An average of 5,000 eggs are deposited amongst aquatic vegetation in a gelatinous mass; however, a clump of 11,682 has been recorded. Initially, the mass floats, but sinks up to 12 hours after laying, or when disturbed. Two to three days later, the tadpoles hatch out and will complete metamorphosis between 2 and 11 months, depending on the temperature of the water and available food. Breeding occurs in a significantly higher proportion of sites where no predatory fish are present, and water bodies are ephemeral rather than permanent. Populations in Victoria, however, have been recorded as breeding in permanent ponds as readily as they do in ephemeral ponds.

The tadpoles of the Green and Golden Bell Frog are large, reaching 80 millimetres (3.15 in) in length, but size varies greatly; smaller tadpoles are more common. The body is usually as wide across as it is deep. The fin has a yellow tinge and is considerably arched. The musculature is moderate and tapers to a fine point as does the fin. The body wall is translucent yellow with darker areas over the abdomen. Just before its limbs form, the tadpole begins to develop the greenish colouration of the adults. Metamorphs resemble the adults and average about 2.6 centimetres (1 in) in length.

Conservation status

They like to get jiggy then... they have "it" The numbers of Green and Golden Bell Frogs are estimated to have declined by more than 30% in the past ten years. It is listed as globally and nationally vulnerable, and as endangered under the New South Wales Threatened Species Conservation Act, 1995. There are many factors thought to be responsible for the dramatic decline of this species: these include habitat fragmentation, the introduction of predatory fish and alteration of drainage regimes. Population declines are closely related to the introduction of the Eastern Mosquitofish (Gambusia holbrooki), a species native to North America that was introduced in order to control mosquito larvae. Laboratory studies have demonstrated that the tadpoles of the Green and Golden Bell Frog are extremely susceptible to predation by this fish. Other factors thought to affect this species include predation by introduced mammals such as cats and foxes, changes to water quality at breeding sites, herbicide use, and loss of habitat through the destruction of wetlands. But, because of this species ability to survive in degraded, polluted environments, habitat destruction and pollution are unlikely to be the main causes of population declines. The amphibian chytrid fungus appears to have led to at least some, and perhaps most, of the major declines recorded through the 1970s and 1980s.

The Green and Golden Bell Frog has been the subject of much research and monitoring, which is important to improving its conservation. This research focuses on the development of management measures to keep the introduced mosquitofish under control. Other strategies being devised will allow for the development and improvement of suitable habitat, to increase the reproductive success of the species. Parallel to these measures, community awareness programmes have also been proposed.

In 1998 a captive breeding program was set up by the herpetofauna staff at Taronga Zoo in Sydney, sponsored by the ASX Frog Focus. The purpose of the program was to help preserve declining populations of Green and Golden Bell Frogs in the Sydney region. It involved the captive breeding of wild frogs and releasing large numbers of tadpoles back into the wild, habitat restoration and post-release monitoring. The program was initially titled Frog Focus Botany as Botany was the original focus site. Thousands of tadpoles were released into a site in Sir Joseph Banks Reserve and post-release monitoring was done by the local community. It was also the first time that school students had been involved with endangered species monitoring. The program has since branched off into several other areas. Between 1998 and 2004, tadpoles were released into specially designed ponds and dams on Long Reef Golf Course at Collaroy in northern Sydney, with little success. Although Green and Golden Bell Frogs had previously been located in the area, the population had since been lost. Adult Bell Frogs are occasionally found on the golf course; however, a permanent breeding population is yet to be established.

Notes

References

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