Michael Mahony

The role of introduced mosquitofish (Gambusia holbrooki) in excluding the native green and golden bell frog (Litoria aurea) from original habitats in south-eastern Australia

Abstract. The introduction of fish has decimated many amphibian populations through increased predation, primarily on their larvae. Some amphibian species now occupy marginal habitats as a response to the presence of introduced fish predators. Such habitats may include ephemeral water bodies where fish do not usually occur, although breeding in these suboptimal environments may incur some cost to a species if its larvae are not adapted to develop under these conditions. We investigated this scenario of amphibian decline using the endangered green and golden bell frog (Litoria aurea) and the introduced mosquitofish (Gambusia holbrooki) in a factorial experiment to determine the responses of tadpoles to declining water levels and the introduced predator. Tadpoles metamorphosed asynchronously but did not accelerate development in declining water or when housed with mosquitofish. Mass at metamorphosis was 30% less in declining water. Mass increased with time to metamorphosis in constant water-level treatments, but decreased in declining water. Tadpoles did not respond to mosquitofish and were therefore assumed to be naive to this predator. These results suggest that ephemeral habitats may be suboptimal for breeding, and tadpoles appear better suited to develop in permanent water bodies free of introduced fish. Intra-clutch variability in larval development (i.e. bet-hedging) may allow L. aurea to cope with unpredictable pond duration, whereby even permanent water bodies may occasionally dry out. The responses observed in L. aurea suggest that introduced fish may have reduced the suitability of permanent water bodies as breeding sites for other pond-breeding amphibian species. The use of less favourable ephemeral habitats as breeding sites may be responsible for some of the declines reported in amphibians since the 1970s.

Management of freshwater wetlands for the endangered green and golden bell frog (Litoria aurea): roles of habitat determinants and space

The range of the green and golden bell frog (Litoria aurea) has undergone a widespread yet unexplained contraction in south-eastern Australia since the mid 1970s, and the species is now listed as endangered in the New South Wales Threatened Species Conservation Act 1995 and as vulnerable in the federal Environmental Protection and Biodiversity Conservation Act 2000. Although the cause of the range contraction is unknown, remedial action is necessary to ensure the long-term viability of remaining populations. This can include habitat creation around existing populations and reintroduction projects. However, the habitat requirements of this species are poorly understood. We examined the habitat at 43 waterbodies on Kooragang Island, Australia, with the aim of producing a set of variables which could differentiate between those occupied by L. aurea and those where the species was absent. L. aurea was found in 21 waterbodies and breeding was confirmed in four. Aspects of waterbody physiognomy and water chemistry could not explain L. aurea distribution. The plant species Juncus kraussii, Schoenoplectus litoralis and Sporobolus virginicus were significant predictors of the presence of L. aurea, as was the diversity of vegetation on the banks of waterbodies. The presence of the eastern mosquitofish (Gambusia holbrooki) did not influence waterbody occupancy and tadpoles were found co-existing with this introduced fish. The distribution of waterbodies occupied by L. aurea was aggregated. A waterbody was more likely to be occupied if neighbouring waterbodies within 50 m were also occupied. Habitat creation projects for this species are most likely to succeed if wetland habitat is created in proximity to an existing population.

The decline of the Green and Golden Bell Frog Litoria aurea viewed in the context of declines and disappearances of other Australian frogs

The Green and Golden Bell Frog Litoria aurea has undergone a reduction in range and significant population declines during the past 15 years. In this paper the decline is viewed from the perspective of declines and disappearances of other species of frog along the east coast of Australia. In contrast to L. aurea most declines affected species occurring at high altitude with relatively restricted ranges. These species typically bred in streams, adults were closely associated with stream habitats, and anthropogenic impacts such as habitat destruction, pollution or introduced predators have not been identified. The decline of the Green and Golden Bell Frog accords in part with this pattern, high altitude populations have disappeared and low altitude populations have declined. However, the habitat of adults and breeding sites are typically associated with ponds and swamps not streams. Because the causal agent responsible for declines has not been identified, an experimental approach is considered to be necess...

Overview of the conservation status of Australian frogs

A review of the current conservation status of Australian amphibians was recently completed as part of a World Conservation Union (IUCN) sponsored Global Amphibian Assessment (GAA). Fifty of 216 amphibian species (23%) in Australia are now recognized as threatened or extinct in accord with IUCN Red List Categories and Criteria. Here we report on the categories and criteria under which individual species qualified for listing and provide a summary of supporting information pertaining to population and distribution declines. Major threatening processes contributing to listing of species are also reviewed.

The distribution and host range of the pandemic disease chytridiomycosis in Australia, spanning surveys from 1956–2007

Chytridiomycosis is the worst disease to affect vertebrate biodiversity on record. In Australia, it is thought to have caused the extinction of four frog species, and it threatens the survival of at least 10 more. We report the current distribution and host range of this invasive disease in Australia, which is essential knowledge for conservation management. We envisage that the data be used in a global and national context for predictive modeling, meta-analyses, and risk assessment. Our continent-wide data set comprises 821 sites in Australia and includes 10 183 records from >80 contributors spanning collection dates from 1956 to 2007. Sick and dead frogs from the field and apparently healthy frogs from museum collections were tested opportunistically for the presence of Batrachochytrium dendrobatidis, the fungal pathogen causing chytridiomycosis, and apparently healthy frogs and tadpoles found during surveys were tested purposively. The diagnostic tests used were histology of skin samples and quantitative PCR of skin swabs. Chytridiomycosis was found in all Australian states and the Australian Capital Territory, but not in the Northern Territory. Currently it appears to be confined to the relatively cool and wet areas of Australia, such as along the Great Dividing Range and adjacent coastal areas in the eastern mainland states of Queensland, New South Wales, and Victoria, eastern and central Tasmania, southern South Australia, and southwestern Western Australia. Batrachochytrium dendrobatidis may have been introduced into Australia via the port of Brisbane around 1978 and spread northward and southward. It did not appear to arrive in Western Australia until 1985. The earliest records from South Australia and Tasmania are from 1995 and 2004, respectively, although archival studies from these states are lacking. We also report negative findings showing that the disease does not currently occur in some areas that appear to be environmentally suitable, including Cape York Peninsula in Queensland and most of the World Heritage Area in western Tasmania. Infection with B. dendrobatidis has been recorded from 63 frog species in Australia to date, all belonging to the Hylidae, Limnodynastidae, and Myobatrachidae, with the exception of one individual of a species from the Microhylidae and the introduced cane toad of the family Bufonidae.

Detecting extinction risk from climate change by IUCN red list criteria

Anthropogenic climate change is a key threat to global biodiversity. To inform strategic actions aimed at conserving biodiversity as climate changes, conservation planners need early warning of the risks faced by different species. The IUCN Red List criteria for threatened species are widely acknowledged as useful risk assessment tools for informing conservation under constraints imposed by limited data. However, doubts have been expressed about the ability of the criteria to detect risks imposed by potentially slow-acting threats such as climate change, particularly because criteria addressing rates of population decline are assessed over time scales as short as 10 years. We used spatially explicit stochastic population models and dynamic species distribution models projected to future climates to determine how long before extinction a species would become eligible for listing as threatened based on the IUCN Red List criteria. We focused on a short-lived frog species (Assa darlingtoni) chosen specifically to represent potential weaknesses in the criteria to allow detailed consideration of the analytical issues and to develop an approach for wider application. The criteria were more sensitive to climate change than previously anticipated; lead times between initial listing in a threatened category and predicted extinction varied from 40 to 80 years, depending on data availability. We attributed this sensitivity primarily to the ensemble properties of the criteria that assess contrasting symptoms of extinction risk. Nevertheless, we recommend the robustness of the criteria warrants further investigation across species with contrasting life histories and patterns of decline. The adequacy of these lead times for early warning depends on practicalities of environmental policy and management, bureaucratic or political inertia, and the anticipated species response times to management actions.

Life history of an endangered amphibian challenges the declining species paradigm

Declining species of vertebrates, including amphibians, have a life history that is characterised by low fecundity, long time to maturity, limited capacity to disperse and habitat specialisation. However, by studying aspects of the life history of an endangered amphibian in south-eastern Australia we show that a paradox may exist for some declining amphibians. We used standard mark–recapture methods over two breeding seasons (2000–01) to study a population of the green and golden bell frog (Litoria aurea) on Kooragang Island, New South Wales, with the aim of determining its age structure, and the growth and survival rates of individuals. Two age classes for males and three for females were derived using von Bertalanffy growth models. Around a quarter of the male and female marked population was a first-year cohort. The male and female age structures appeared to be stable over the two breeding seasons. However, there were fewer female L. aurea in the 24-month age class in 2001, which may indicate that recruitment declined in the study area. A short time to maturity (~3 months) was recorded for males. The maximum-likelihood estimate of survival for males in the 2000 breeding season was relatively high (0.76), although the capture probability was low (0.19). The biology of L. aurea conforms to an ‘r’-strategist, which is characteristic of a colonising or ‘weed-like’ species and not of one that has become extinct over much of its former range.

Genetic evidence for species boundaries in frogs of the Litoria citropa species-group (Anura:Hylidae)

The Litoria citropa species-group comprises several small to medium-sized tree-frog species found from mid-eastern Queensland to eastern Victoria in a variety of habitats along streams associated with the Great Dividing Range. The smaller members of the Litoria citropa species-group, Litoria phyllochroa and L. pearsoniana, have a confused taxonomic history with the taxonomic status of several populations, some regarded as endangered, still in doubt. Multi-locus allozyme electrophoretic profiles and nucleotide sequences of a portion of the mitochondrial 16S ribosomal RNA gene were used to examine the evolutionary relationships of populations that are a geographically comprehensive and morphologically representative sample of the species-group. These data demonstrate the presence of a minimum of three species: L. nudidigitus, L. phyllochroa and a third species whose taxonomic name is yet to be resolved. This third taxon encompasses a wide range of allozyme and mitochondrial nucleotide diversity and can be divided into at least four evolutionarily significant units (ESUs) that replace each other in a linear sequence from north of the Hunter Valley in New South Wales to the Kroombit Tops in central Queensland. A possible zone of hybridisation between the southernmost pair of these ESUs was identified in northern New South Wales. The fourth ESU, a northern outlier of the range of the species-group, is confined to Kroombit Tops, central Queensland. While its phylogenetic relationship with the other three ESUs was not resolved precisely by the present analysis, it nevertheless comprises a distinct and very divergent mitochondrial lineage of considerable antiquity. Resolution of the status of a further name applied to the species-group, L. piperata, awaits a morphological analysis that includes the relevant type material.

Sodium Chloride Inhibits the Growth and Infective Capacity of the Amphibian Chytrid Fungus and Increases Host Survival Rates

The amphibian chytrid fungus Batrachochytrium dendrobatidis is a recently emerged pathogen that causes the infectious disease chytridiomycosis and has been implicated as a contributing factor in the global amphibian decline. Since its discovery, research has been focused on developing various methods of mitigating the impact of chytridiomycosis on amphibian hosts but little attention has been given to the role of antifungal agents that could be added to the hosts environment. Sodium chloride is a known antifungal agent used routinely in the aquaculture industry and this study investigates its potential for use as a disease management tool in amphibian conservation. The effect of 0–5 ppt NaCl on the growth, motility and survival of the chytrid fungus when grown in culture media and its effect on the growth, infection load and survivorship of infected Perons tree frogs (Litoria peronii) in captivity, was investigated. The results reveal that these concentrations do not negatively affect the survival of the host or the pathogen. However, concentrations greater than 3 ppt significantly reduced the growth and motility of the chytrid fungus compared to 0 ppt. Concentrations of 1–4 ppt NaCl were also associated with significantly lower host infection loads while infected hosts exposed to 3 and 4 ppt NaCl were found to have significantly higher survival rates. These results support the potential for NaCl to be used as an environmentally distributed antifungal agent for the prevention of chytridiomycosis in susceptible amphibian hosts. However, further research is required to identify any negative effects of salt exposure on both target and non-target organisms prior to implementation.

Movement Patterns of Adult Green and Golden Bell Frogs Litoria aurea and the Implications for Conservation Management

Abstract Conservation of pond-breeding frogs requires information on movement patterns within populations. The Green and Golden Bell Frog (Litoria aurea) is endangered in New South Wales, Australia; yet little is known about its movement biology. To inform conservation planning and habitat restoration, we conducted a mark-recapture study to describe the movement patterns of adult L. aurea among permanent and ephemeral waterbodies, on Kooragang Island, New South Wales, Australia. Twenty-nine percent and 18% of 551 males and 228 females marked, respectively, were recaptured on ≥ 1 occasions over two breeding seasons (2000 and 2001). Most recaptures were in the same permanent waterbody as the original capture (53% males, 65% females). Twenty-seven (24%) male and seven (50%) female movements were from a permanent waterbody to the nearest permanent waterbody (<50 m), respectively. Male L. aurea, usually large individuals, moved relatively long distances (>200 m) to ephemeral waterbodies from permanent waterbodies after prolonged heavy rain. Of the total number of male and female movements recorded, 44 (39%) and two (14%) were to or from ephemeral waterbodies, respectively. Our results demonstrated high site fidelity of L. aurea to individual waterbodies and groups of neighboring waterbodies, especially permanent ones. Our results showed that L. aurea tended to breed in more permanent waterbodies but reproduced opportunistically in ephemeral waterbodies where recruitment was less successful. Habitat restoration projects for L. aurea should construct permanent and ephemeral waterbodies and provide adequate buffer zones that protect the terrestrial habitat.