Age truncation due to disease shrinks metapopulation viability for amphibians

Metapopulations often exist in a fragile balance between local extinctions and (re)colonisations, in which case emerging threats that alter species vital rates may drastically increase metapopulation extinction risk. We combined empirical data with metapopulation simulations to examine how demographic shifts associated with amphibian chytrid fungus (Batrachochytrium dendrobatidis, Bd) have altered metapopulation viability for threatened amphibians in Australia. Comparing the ages of museum specimens collected before Bd emerged in Australia with individuals from geographically matched remnant populations revealed significant truncation of age structures post-Bd, with a halving of annual adult survival probabilities. Spatially realistic metapopulation modelling demonstrated that reduced adult survival led to major reductions in the parameter space over which persistence was possible for the focal species, with contractions to landscapes with higher landscape connectivity, lower environmental stochasticity and considerably higher recruitment rates. Metapopulation persistence post-Bd required greater landscape connectivity than pre-Bd. This arises from a landscape-level analogue of compensatory recruitment at the population level, in which higher (re)colonisation rates can offset more frequent local extinctions, enabling persistence of amphibians susceptible to Bd. Interactions between recruitment rate, environmental stochasticity and landscape connectivity were also more important for metapopulation persistence post-Bd. Higher recruitment was required to mitigate the impacts of environmental stochasticity, and higher landscape connectivity was required to mitigate the impacts of environmental stochasticity and poor recruitment. Increased reliance on these interdependencies shrunk the parameter space over which metapopulations could persist post-Bd. Our study demonstrates that emerging threats that alter species vital rates can drastically reduce the capacity of certain environments to support metapopulations. For our focal species, reductions in adult survival rates due to Bd produced major reductions in the conditions under which persistence was possible, providing a mechanistic insight into the processes underpinning observed range and niche contractions of amphibians impacted by this pathogen. More broadly, our study illustrates how environmentally mediated host resilience can enable persistence following the emergence of novel pathogens. This pathway to persistence is worthy of greater attention on both conceptual and applied grounds.

Age structure of amphibian populations with endemic chytridiomycosis, across climatic regions with markedly different infection risk

Threatening processes, such as disease, can drive major changes in population demographics of the host. Chytridiomycosis, caused by the fungal pathogen Batrachochytrium dendrobatidis (Bd), has led to the decline of at least 500 amphibian species across the globe and has been shown to truncate host age structure by lowering adult survival rates. This results in heavy reliance on annual recruitment and the inability to recover in the event of periodic recruitment failure. We used skeletochronology to determine the age structure, growth, and survival rates of populations of an endangered amphibian, Litoria raniformis, with endemic chytridiomycosis, across two climatically disparate regions in south-eastern Australia: semi-arid and temperate. Contrary to predictions, populations in the semi-arid region (in which chytrid prevalence is substantially lower due to high temperatures) displayed a more truncated age structure than populations in the temperate study regions. Maximum recorded age was only two years in the semi-arid region compared with up to four years in the temperate region. Wetland hydroperiod and average seasonal air temperature were correlated with age, and males had a slightly higher survival rate than females (0.31 for males and 0.27 for females). Despite the previously documented differences in chytrid prevalence between the two climatic regions, water availability and wetland hydroperiods appear the over-riding determinants of the age structure and survival rates of L. raniformis. Targeted management which ensures water availability and improves survival of 1-year-old frogs into their second and third breeding season would reduce the impact of stochastic events on L. raniformis, and this may be true for numerous frog species susceptible to chytridiomycosis.

Factors influencing persistence of a threatened amphibian in restored wetlands despite severe population decline during climate change driven weather extremes

Biodiversity is in global decline during the Anthropocene. Declines have been caused by multiple factors, such as habitat removal, invasive species, and disease, which are often targets for conservation management. However, conservation interventions are under threat from climate change induced weather extremes. Weather extremes are becoming more frequent and devastating and an example of this was the 2019/2020 Australian drought and mega-fires. We provide a case study the impacts of these extreme weather events had on a population of the threatened frog Litoria aurea that occurs in a constructed habitat which was designed to reduce the impact of introduced fish and chytrid-induced disease. We aimed to determine what factors influenced persistence so that the design of wetlands can be further optimised to future-proof threatened amphibians. We achieved this with 4 years (2016-2020) of intensive capture-recapture surveys during austral spring and summer across nine wetlands (n = 94 repeat surveys). As hypothesized, drought caused a sharp reduction in population size, but persistence was achieved. The most parsimonious predictor of survival was an interaction between maximum air temperature and rainfall, indicating that weather extremes likely caused the decline. Survival was positively correlated with wetland vegetation coverage, positing this is an important feature to target to enhance resilience in wetland restoration programs. Additionally, the benefits obtained from measures to reduce chytrid prevalence were not compromised during drought, as there was a positive correlation between salinity and survival. We emphasize that many species may not be able to persist under worse extreme weather scenarios. Despite the potential for habitat augmentation to buffer effects of extreme weather, global action on climate change is needed to reduce extinction risk.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s10531-022-02387-9.

Rapid population increase of the threatened Australian amphibian Litoria aurea in response to wetlands constructed as a refuge from chytrid-induced disease and introduced fish

Amphibians have declined due to multiple impacts including invasive fish and the disease chytridiomycosis caused by the chytrid fungus Batrachochytrium dendrobatidis (Bd). Wetland restoration can be used to increase amphibian populations. However the design of created wetlands must account for threats such as Bd and introduced fish. There have been no attempts on a landscape level to manage these threats with habitat design. Here we monitored the green and golden bell frog (Litoria aurea) in 2.6 ha of constructed wetlands designed to enhance breeding and increase survival through passive mitigation of Bd and exotic fish. We compared the fecundity, adult population sizes, introduced fish occupancy, Bd prevalence and survival rates of frogs in created wetlands (CW) to three control sites to determine if and why the habitat design was successful. Monitoring involved weekly capture-recapture during the austral spring and summer for three L. aurea breeding seasons. We hypothesised that (1) if the CWs were successful in passively limiting fish colonisation, a larger number of breeding events would be detected compared to control sites which are known to be widely colonised by introduced fish. (2) If the wetlands were successful in passively mitigating Bd, then we would observe an equal or greater survival rate and equal to or lower Bd prevalence compared to control wetlands. We observed a 3.3-fold increase in adult population size in CW from season 1 to 2, and the population increased further in season 3.We found strong support for hypothesis (1) and weak support for (2). Based on these results, we conclude that this design was beneficial shortly after their formation primarily due to fish exclusion, but further study is required to determine if these benefits extend long-term. Future amphibian restoration studies are needed to improve the design of wetlands to enhance suppression of Bd.

Multi-species occupancy modeling provides novel insights into amphibian metacommunity structure and wetland restoration

A fundamental goal of community ecology is to understand species-habitat relationships and how they shape metacommunity structure. Recent advances in occupancy modeling enable habitat relationships to be assessed for both common and rare species within metacommunities using multi-species occupancy models (MSOM). These models account for imperfect species detection and offer considerable advantages over other analytical tools commonly used for community analyses under the elements of metacommunity structure (EMS) framework. Here, we demonstrate that MSOM can be used to infer habitat relationships and test metacommunity theory, using amphibians. Repeated frog surveys were undertaken at 55 wetland sites in southeastern Australia. We detected 11 frog species from three families (Limnodynastidae, Myobatrachidae, and Pelodryadidae). The rarest species was detected at only one site whereas the most common species was detected at 42 sites (naive occupancy rate 0.02-0.76). Two models were assessed representing two competing hypotheses; the best-supported model included the covariates distance to the nearest site (connectivity), wetland area, presence of the non-native eastern mosquitofish (Gambusia holbrooki), proportion cover of emergent vegetation, an interaction term between Gambusia and emergent vegetation cover, and the proportion canopy cover over a site. Hydroperiod played no detectable role in metacommunity structure. We found species-habitat relationships that fit with current metacommunity theory: occupancy increased with wetland area and connectivity. There was a strong negative relationship between occupancy and the presence of predatory Gambusia, and a positive interaction between Gambusia and emergent vegetation. The presence of canopy cover strongly increased occupancy for several tree frog species, highlighting the importance of terrestrial habitat for amphibian community structure. We demonstrated how responses by amphibians to environmental covariates at the species level can be linked to occupancy patterns at the metacommunity scale. Our results have clear management implications: wetland restoration projects for amphibians and likely other taxa should maximize wetland area and connectivity, establish partial canopy cover, and eradicate Gambusia or provide aquatic vegetation to mitigate the impact of this non-native fish. We strongly advocate the use of MSOM to elucidate the habitat drivers behind animal occupancy patterns and to derive unbiased occupancy estimates for monitoring programs.

Population stability in an unmanaged population of the green and golden bell frog in northern New South Wales, Australia

Population monitoring is required to guide conservation programs. We conducted a capture–mark–recapture study of a population of the vulnerable green and golden bell frog (Litoria aurea) at the northern end of its range. Frogs were captured and marked over three breeding seasons (2015/16, 2016/17, 2017/18) in a large coastal lagoon. We aimed to: (1) produce annual estimates of population size to describe population trajectory, and (2) investigate monthly variation in abundance, capture probability, and temporary emigration to understand how these factors change at a finer temporal scale. Frog abundance varied across the three annual breeding seasons: 60–280 adult males, 120–190 adult females, and 90–420 subadults. We infer that the population is stable because adult abundance estimates were higher after 2015/16. Because our study sampled only half the available breeding habitat, the overall population may number 350–850 adults. Our modelling revealed >40 males but <20 females were detected in the sample area in our monthly samples. Estimates of temporary emigration were high (males: 0.54; females: 0.79), suggesting behaviour that made frogs unavailable for capture between months. Our results suggest that monitoring at greater than annual intervals should be adequate to monitor the future trend of this population.

Can a common snake provide conservation insights?

The small-eyed snake (Cryptophis nigrescens) is a common non-threatened species in eastern Australia. It coexists with the threatened broad-headed snake (Hoplocephalus bungaroides), a species adversely affected by habitat disturbance and subject to poaching. The small-eyed snake is a habitat generalist and not subject to poaching. It may prey on other snakes, including the broad-headed snake, and, like the broad-headed snake, may shelter under thermally favourable loose rocks during the cooler months of the year. This may lead to interactions between these species due to the limited availability of such rocks, and possibly exacerbate other threats to the broad-headed snake, such as poaching and the loss of thermally favourable rocks. I conducted repeat surveys for snakes at 64 rock outcrops in Royal National Park over a 16-year period. I predicted that site use by the small-eyed snake would not be influenced by a disturbance variable previously documented to influence site use by the broad-headed snake. Observations were consistent with this prediction, confirming the unique vulnerability of the broad-headed snake. I used my long-term data to analyse the co-occurrence of the two species. The broad-headed snake was detected as frequently at sites with and without the small-eyed snake, suggesting that these species occupy outcrops independently of each other. Therefore, interactions with the small-eyed snake will not reduce the effectiveness of habitat restoration for the broad-headed snake in Royal National Park.

Population monitoring of an urban gliding mammal in eastern Australia

Long-term monitoring is an important element of species conservation. This study describes changes in the size of a squirrel glider (Petaurus norfolcensis) population over a 10-year period. The population occupied a 45-ha forest remnant within the urban area of Brisbane. Gliders were tagged from 25 nights of trapping during 2006–08 and from 16 nights of trapping in 2015. Population modelling was used to estimate adult population size. This suggested the adult population comprised 30–40 individuals at the beginning and end of the 10-year period. It reached a peak of 70 individuals in mid-2007. These data suggest that the study area contains a small population that is prone to interannual variation but there was no evidence of it being in decline. Survival estimates during 2006–08 were equivalent to those estimated for a larger population in Victoria. Population monitoring should be continued to determine how resilient this population is to population decline and to investigate factors that may cause decline. This study provides an example of an approach that could be used to monitor threatened populations of the squirrel glider.

Informing compensatory habitat creation with experimental trials: a 3-year study of a threatened amphibian

The creation or restoration of habitat to mitigate biodiversity loss is a common conservation strategy. Evidence-based research via an extensively monitored trial study should be undertaken prior to large-scale implementation to predict success and identify potential limiting factors. We constructed an experimental trial habitat for the threatened green and golden bell frog Litoria aurea, in Australia, to inform a broader programme of compensatory habitat creation. Individuals were released into the trial plot and a nearby natural wetland for comparison to determine if the created habitat would support their growth, survival and persistence. Half of the trial waterbodies were enclosed within an exclusion fence to separate the effects of habitat suitability from ecological processes. We found the habitat provided L. aurea with sufficient resources to grow, survive and persist for 3 years. However, no breeding occurred, and further investigations need to focus on understanding the drivers of reproduction. Although a disease outbreak occurred during the study, persistence continued for the next 2 years. This was attributed to the large number of individuals released, a strategy we recommend for future mitigation strategies to account for low survival and high turnover rates. Dispersal probably affected abundance in the unfenced areas, and landscape-level initiatives are suggested for this species. This study demonstrates that experimental trials are valuable, as they can inform future habitat management by identifying limitations that could hinder success prior to the implementation of large-scale initiatives.

Population ecology of a cryptic arboreal snake (Hoplocephalus bitorquatus)

Biologists have traditionally been reluctant to study arboreal snakes due to low rates of capture. Overlooking such taxa can mislead interpretations of population trends for data-deficient species. We used regularly spaced transect searches and standard capture–mark–recapture techniques to describe population structure, growth rates, survival and capture probability in a population of the pale-headed snake (Hoplocephalus bitorquatus) in southern Queensland. We obtained data from 194 captures of 113 individual snakes between 2009 and 2015. Using the Cormack–Jolly–Seber method, we estimated apparent annual survival in subadult snakes at 0.23 ± 0.01 (s.e.) and 0.81 ± 0.08 for adults. Capture probability was estimated at 0.16 ± 0.14 per session in subadult snakes and 0.33 ± 0.06 for adults. Within the red gum forests of our study site, we estimate pale-headed snake density at ~13 ± 7 ha–1. Using von Bertalanffy growth modelling, we predict that snakes reach sexual maturity after about four years and may live for up to 20. Our results suggest that the species is a ‘k’ strategist, characterised by slow maturation and low fecundity. These traits suggest that populations will recover slowly following decline, exacerbating the risk of local extinction.

No evidence of protracted population decline across 17 years in an unmanaged population of the green and golden bell frog in north-eastern New South Wales

Describing the population trends of threatened species over time is central to their management and conservation. The green and golden bell frog (Litoria aurea) is a formerly common species of south-eastern Australia that has declined to ~40 populations in New South Wales, and experienced a substantial contraction of its geographic range. We aimed to determine whether an unmanaged population at the northern end of its range had declined across a 17-year period. We estimated population size at the beginning and end of this period, using several population models to fully characterise this population. Different modelling approaches gave different population estimates. Based on a similar number of survey occasions the adult male segment of the population was estimated using the Popan model at 112.0 (±13.5, s.e.; 95% CI: 85.5–138.8) in 1998/99 and 95.2 (±17.6; 60.8–129.7) in 2015/16. With the inclusion of maturing subadults following the practice of earlier studies, the population was estimated at 163.6 (±25.9; 112.8–214.5) males in 2015/16. These estimates represent an index of a larger population because the largest wetland was subsampled. Our data provide no evidence of a declining population. Our study highlights the need to understand the implications of using different population models and two age-classes to estimate population parameters.

Visual lures increase camera-trap detection of the southern cassowary (Casuarius casuarius johnsonii)

Context Monitoring is a key component in managing wildlife populations and is critical for revealing long-term population trends of endangered species. Cryptic or highly mobile species that occur in low densities and in remote terrain require the development of specific monitoring methods. The southern cassowary is an Australian endangered species that poses many challenges for conducting population surveys. Aims The aims of the present study were to determine the effectiveness of camera traps in detecting cassowaries at a site, to determine whether visual lures increased detection rates, and to explore the potential of camera traps in population surveys. Methods Coloured lures (mimicking large blue and red fruit) were placed in front of a set of camera traps and compared with controls (no lures) at 29 survey sites on the Daintree coast, northern Queensland, Australia. Key results Camera traps with lures (1) detected more birds, (2) had a shorter detection latency, (3) had a marginally greater number of captures, (4) experienced a longer capture duration, (5) were more likely to have the cassowary stop in front of the camera, and (6) achieved a 95% probability of detecting cassowaries in 12 trap days, compared with 28 trap days without lures. Conclusions An increase in the number of cassowaries detected, the reduction in camera latency times and the ability to identify the birds enables a more efficient approach to estimating population sizes over existing methods. This is the first published study to use visual lures to conduct camera trapping of birds. Implications The use of camera traps with lures is a practical and cost-efficient technique for the rapid detection of cassowaries at a site and lends itself to studies of population structure, size and trends.

Differences in microhabitat selection patterns between a remnant and constructed landscape following management intervention

Context Achieving successful conservation outcomes in habitat creation and reintroductions requires an understanding of how species use their habitat and respond to these interventions. However, few initiatives directly compare microhabitat selection between remnant and managed habitats to measure effectiveness and evaluate outcomes. Probability of detection is also rarely included in studies on microhabitat use, which may lead to erroneous conclusions if detectability varies between variables. Methods In this study, we used the endangered green and golden bell frog (Litoria aurea) to compare differences in microhabitat-use patterns in both a remnant and a constructed habitat. A detectability study was also conducted to determine detection probabilities among microhabitats. Key results Aquatic vegetation was used more than expected in both the remnant and constructed habitats, and rock piles were utilised less than expected in the constructed habitat, despite their recommendation in most habitat templates. We found that detection probabilities altered the outcomes of abundance estimates for nearly all the measured microhabitat variables. Conclusions Future management for this species should focus on providing high proportions of aquatic vegetation. Furthermore, although rock piles have been utilised greatly in past L. aurea habitat creation, placing large rocks on a managed site is expensive and time consuming. Future management initiatives may need to focus on providing smaller proportion of rocks, which would be a more appropriate use of resources. Implications With conservation management projects increasing over the next few decades, understanding habitat use before implementing strategies should be a priority as it will provide important insights and inform decision-making for optimum habitat creation and restoration. Furthermore, accounting for detectability in microhabitat use studies is essential to avoid wrong conclusions that may negatively affect the success of ecological management strategies.

We Made Your Bed, Why Won't You Lie in It? Food Availability and Disease May Affect Reproductive Output of Reintroduced Frogs

Mitigation to offset the impacts of land development is becoming increasingly common, with reintroductions and created habitat programs used as key actions. However, numerous reviews cite high rates of poor success from these programs, and a need for improved monitoring and scientific testing to evaluate outcomes and improve management actions. We conducted extensive monitoring of a released population of endangered green and golden bell frogs, Litoria aurea, within a created habitat, as well as complementary surveys of a surrounding wild population. We then compared differences between the created habitat and natural ponds where extant frogs either bred or didn’t breed in order to determine factors that contributed to the breeding failure within the created habitat. We evaluated differences of L. aurea abundance, abundance of other fauna, vegetation, water quality, habitat structure, invasive fish, and disease between the three pond types (created habitat, breeding ponds, non-breeding ponds). We discovered that vegetation and invertebrate diversity were low within the created habitat, potentially reducing energy and nutritional resources required for breeding. Also, a greater proportion of frogs in the created habitat were carrying the chytrid fungal pathogen, Batrachochytrium dendrobatidis, compared to the wild populations. In addition to causing the potentially fatal disease, chytridiomycosis, this pathogen has been shown to reduce reproductive functioning in male L. aurea, and subsequently may have reduced reproductive activities in the created habitat. Conspecific attraction, pond hydrology, and aquatic vegetation may also have had some influence on breeding behaviours, whilst the presence of the invasive mosquitofish, Gambusia holbrooki, and heterospecific tadpoles were unlikely to have deterred L. aurea from breeding within the created habitat. Through the use of scientific testing and monitoring, this study is able to make recommendations for future amphibian breed and release programs, and suggests planting a diversity of plant species to attract invertebrates, creating some permanent ponds, connecting habitat with existing populations, trialling artificial mating calls, and following recommendations to reduce the prevalence of disease within the population.

Low disease-causing threshold in a frog species susceptible to chytridiomycosis

A simple diagnosis of the presence or absence of an infection is an uninformative metric when individuals differ considerably in their tolerance to different infection loads or resistance to rates of disease progression. Models that incorporate the relationship between the progression of the infection with the potential alternate outcomes provide a far more powerful predictive tool than diagnosis alone. The global decline of amphibians has been amplified by Batrachochytrium dendrobatidis, a pathogen that can cause the fatal disease chytridiomycosis. We measured the infection load and observed signs of disease in Litoria aurea Receiver operating characteristic curves were used to quantify the dissimilarity between the infection loads of L. aurea that showed signs associated with chytridiomycosis and those that did not. Litoria aurea had a 78% probability of developing chytridiomycosis past a threshold of 68 zoospore equivalents (ZE) per swab and chytridiomycosis occurred within a variable range of 0.5-490 ZE. Studies should incorporate a species-specific threshold as a predictor of chytridiomycosis, rather than a binary diagnosis. Measures of susceptibility to chytridiomycosis must account not only for the ability of B. dendrobatidis to increase its abundance on the skin of amphibians but also to determine how each species tolerates these infection loads.

The role of non-declining amphibian species as alternative hosts for Batrachochytrium dendrobatidis in an amphibian community

Context Pathogens with reservoir hosts have been responsible for most disease-induced wildlife extinctions because the decline of susceptible hosts does not cause the decline of the pathogen. The existence of reservoirs for Batrachochytrium dendrobatidis limits population recovery and conservation actions for threatened amphibians. As such, the effect of reservoirs on disease risk within host community assemblages needs to be considered, but rarely is. Aims In this study we aimed to determine if amphibian species co-occurring with the green and golden bell frog Litoria aurea, a declining species susceptible to B. dendrobatidis, act as alternate hosts. Methods We quantified B. dendrobatidis infection levels, sub-lethal effects on body condition and terminal signs of disease in amphibian communities on Kooragang Island and Sydney Olympic Park in New South Wales, Australia, where two of the largest remaining L. aurea populations persist. Key results We found L. aurea carried infections at a similar prevalence (6–38%) to alternate species. Infection loads ranged widely (0.01–11 107.3 zoospore equivalents) and L. aurea differed from only one alternate host species (higher median load in Litoria fallax) at one site. There were no terminal or sub-lethal signs of disease in any species co-occurring with L. aurea. Conclusion Our results suggest that co-occurring species are acting as alternate hosts to L. aurea and whether their presence dilutes or amplifies B. dendrobatidis in the community is a priority for future research. Implications For L. aurea and many other susceptible species, confirming the existence of reservoir hosts and understanding their role in community disease dynamics will be important for optimising the outcomes of threat mitigation and habitat creation initiatives for their long-term conservation.

Population age structure of the spotted tree frog (Litoria spenceri): insights into population declines

Context. Despite increased scientific attention on amphibian conservation in recent years, knowledge of population demography of amphibians remains scarce, hampering evaluation of population declines and development of appropriate management responses. Aims. The aims of this research were to examine variation in population demography of the spotted tree frog (Litoria spenceri), a critically endangered species in Australia, and to evaluate the role of various factors potentially responsible for population declines such as introduced trout, chytridiomycosis and habitat changes. Methods. Skeletochronology combined with mark–recapture sampling were undertaken in two different river systems, Bogong Creek and Taponga River, to determine population age structure. Age-specific survival estimates were derived from each population and were then used to examine variance in age-specific mortality. Key results. Relative population density per 200 m of stream was 67.7 adults and 131.3 juveniles at Bogong Creek and 10.7 adults and 33.8 juveniles at Taponga River. Ages were determined for 578 frogs across the two populations. Age-specific survival was lowest in the first year of life compared to all other age classes, and highest in sexually mature adults. Differences in age-specific survival were similar between the populations, with the exception of first-year survivorship, which averaged 1.9% at Bogong Creek and 0.4% at Taponga River. This difference was large enough to explain most of the marked difference in population density between the two streams. Key conclusions. The difference in first-year age-specific survival is consistent with trout predation as the most parsimonious explanation for the large differences in population density between the populations, and lends further weight to the role of introduced trout in the decline of this species. Implications. This study has contributed to informing management actions for conservation of this species, and demonstrates that population age structure data may provide valuable insights into demographic variability within and between populations and species. This may have important implications for interpretation of population declines, and conservation and management responses.

Longevity and larval development among southern bell frogs (Litoria raniformis) in the Coleambally Irrigation Area - implications for conservation of an endangered frog

Context. With the flow of many of the world’s rivers regulated such that water can be diverted for agriculture and human consumption, basic ecological information on the current status of key biota in significant floodplain wetlands and their response following inundation is needed. The maintenance of natural habitat to ensure amphibian survival is gaining increasing recognition, given the ongoing decline of anuran populations. Information on longevity, time required to emerge from the water and to reach sexual maturity, all provide important information about the required timing, frequency and duration of environmental water allocations to ensure successful recruitment among populations of southern bell frogs (Litoria raniformis Keferstein, 1867). Aims. The aims of this research were to establish the longevity of southern bell frogs in the Coleambally Irrigation Area (CIA) in the Riverina region of New South Wales, Australia, and to evaluate the capacity for southern bell frog tadpoles to survive and successfully metamorphose following an extended overwintering period. Methods. Skeletochronology studies were carried out using toe-clips taken from adult and juvenile frogs captured in irrigation channels and rice fields over two rice-growing seasons. For the metamorphosis assay, southern bell frog tadpoles were held back in their development by low temperatures and low food allocation for 290 days, before temperatures and food allocation were increased adequately to allow metamorphosis to occur. Key results. The study indicated that skeletochronological examination of toe-bones was a useful technique for establishing the age structure of southern bell frogs in this region. The oldest animals in the population were found to be 4–5 years old, although the majority of frogs were typically 2–3 years old. Also, the metamorphosis assay indicated that successful metamorphosis was the exception rather than the rule if tadpole development was held back by low food ration and low temperatures. Conclusions. If southern bell frogs reach sexual maturity only after 2 years, and the oldest animals observed in the field are 4 or 5 years old, then there is a very narrow window of opportunity – two to three seasons – for each individual to successfully breed. Implications. The implications for environmental flow management are that habitats for key species identified for protection such as the endangered southern bell frog will need water every 1–2 years to enable each cohort to breed and maintain the wild populations. The extent of the environmental flows needs to be adequate to ensure that water persists long enough for critical biological events such as anuran metamorphosis to occur during the spring and summer months.

Visible Implant Alphanumeric tags as an alternative to toe-clipping for marking amphibians - a case study

The removal of toes in unique combinations (‘toe-clipping’) has historically been the most popular technique for individually marking amphibians. However, recent studies have questioned the utility and ethics of this technique, and provided impetus for research on the efficacy of alternative methods. In this study, the efficacy of Visible Implant Alphanumeric (VIA) tags for marking juvenile growling grass frogs (Litoria raniformis) was assessed with reference to the fundamental assumptions of the mark–recapture approach. Three questions were investigated: (1) What is the probability of tag loss? (2) What is the probability of tag misidentification, and does it vary between frogs or observers, or with observer experience? and (3) To what extent does tagging influence survival or growth? Laboratory and field trials demonstrated that the probability of tag loss for juvenile L. raniformis was ~0.07 (95% CI: 0.03–0.13) when tags where implanted on the dorsolateral region of the thorax, eight times lower than that estimated for tags implanted on the thigh (mean 0.59; 95% CI: 0.29–0.86). Tags were rarely misidentified by naïve observers (mean probability of tag misidentification <0.01), but tag read accuracy varied amongst observers. In the laboratory, tagging did not adversely influence survival or growth of L. raniformis during the 12-week study period. Given that modification of the techniques used here may eliminate tag loss, and that variation in tag read accuracy between observers may be easily avoided, it is concluded that VIA tags represent a promising alternative to toe-clipping for marking L. raniformis and other amphibians.