Letteratura scientifica selezionata sul tema "Litoria aurea"

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.

Phylogenetic relationships among hylid frogs of the Australian region were studied by micro-complement fixation comparisons of serum albumin. Although our data support current species-group arrangements, we do not find good agreement between our phylogenetic hypotheses and those derived from morphological and karyological studies. Immunological analyses provide data which allow the construction of a phylogeny for the Australian radiation of the speciose genus Litoria, and suggest dividing the species of Litoria examined into five major species-assemblages, each of which is probably monophyletic. The sister- group relationship between the Litoria aurea group and Cyclorana is confirmed, and the diphyletic origin of the terrestrial hylids is supported. The radiation of Australian hylids is monophyletic with respect to the outgroup taxon (Hyla) used in this study, and the origin of diversification within the genus correlates well with estimates of the final separation of Australia from Antarctica-South America. Preliminary data suggest that the endemic New Guinean taxa (Nyctimystes and the montane Litoria) are closely related to the Australian 'freycineti' assemblage within Litoria. Albumin from Litoria infrafrenata cross-reacted poorly with all available Australian antisera, suggesting that this species may have originated independently of the rest of the Australian hylids. Our data support the classification of Australian tree frogs as hylids, rather than as leptodactyloid offshoots.

In south-eastern Australia, the pelodryadid Litoria aurea Group (sensu Tyler & Davies 1978) comprises three species: Litoria aurea (Lesson, 1829), Litoria raniformis (Keferstein, 1867), and Litoria castanea (Steindachner, 1867). All three species have been subject to declines over recent decades and taxonomic uncertainty persists among populations on the tablelands in New South Wales. We address the systematics of the Group by analysing mitochondrial and nuclear DNA sequences to assess divergence in the Litoria raniformis from across its current range in New South Wales (NSW), Victoria, South Australia (SA) and Tasmania. We also included samples of Litoria castanea from a recently rediscovered population in the southern tablelands of NSW. Our phylogenetic and population genetic analyses show that Litoria raniformis comprises northern and southern lineages, showing deep mitochondrial DNA sequence divergence (7% net average sequence divergence) and can be diagnosed by fixed allelic differences at more than 4,000 SNP loci. Samples of the northern lineage were collected from the Murray-Darling Basin while those of the southern lineage were collected from south-eastern South Australia, southern and south-eastern Victoria and Tasmania. Analysis of the morphology and bioacoustics did not unequivocally delineate the two lineages. The presence of a hybrid backcross individual in western Victoria at the northern margin of the southern lineage, leads us to assign sub-species status to the two lineages, L. r. raniformis for the northern lineage and L. r. major for the southern lineage. Our data do not unequivocally resolve the taxonomic status of L. castanea which will require molecular genetic analyses of museum vouchers from those parts of the range where L. castanea and L. raniformis are no longer extant. Our data also suggest that human mediated movement of frogs may have occurred over the past 50 years. Our genotyping of vouchers collected in the 1970s from the Mount Lofty Ranges in South Australia detected mitochondrial haplotypes of both sub-species and SNP analysis showed that a single Tasmanian specimen was a backcross with L. r. raniformis ancestry. Movement of L. r. raniformis into Tasmania and both sub-species into the Mount Lofty Ranges are not likely due to passive movements of animals through agricultural commerce, but due to the attractiveness of the species as pets and subsequent escapes or releases, potentially of the larval life stage.

AbstractWe investigated the effects of various salinities on larval growth, metamorphosis and survival in the green and golden bell frog (Litoria aurea). This large hylid was formerly common on the east coast of Australia, but now occurs in only a few scattered localities where rising salt is one of a number of potential threats. Tadpoles tolerated salinity up to 4% seawater (sw; 1.58‰) without apparent effect, but salinity of 5.5% sw (1.87‰) and above significantly decreased growth rates and increased mortality. Salinity of 10% sw (4.41‰) caused mortality of all individuals within 72 days, 20% sw within four days, and 25% sw within five hours. Salinities from 10-15% sw (4.41-6.73‰) were tolerated for short periods (up to 3 weeks) before significant weight loss and mortality resulted, indicating a detrimental cumulative effect with high salinity. Metamorphosis occurred frequently below 5.5% sw but did not occur above 5.5% sw. Although L. aurea tadpoles can tolerate moderate salinities, increased salinisation caused by substantial land clearing, urban run-off and accumulation of salt in some waterbodies may contribute to the reduction of populations.

The green and golden bell frog (Litoria aurea) is an Australian hylid, which was once common with a relatively continuous distribution. Historically, this distribution extended from northern New South Wales (NSW), as far as Ballina, to East Gippsland in Victoria; with inland populations as far west as Bathurst and Tumut. Today the species is reported to have disappeared from 80% of its former range and remaining populations are mostly fragmented and typically restricted to the coastline, extending from Yuraygir National Park (northern NSW) to East Gippsland. In this thesis, I report a comprehensive study designed to identify the phylogeographic and conservation genetic parameters of L. aurea. In doing so, I also investigate evolutionary relationships within the ???bell frog??? species group. In this study, microsatellite and mitochondrial DNA (mtDNA) markers are employed. The development of species-specific microsatellite markers and the collection of samples was a substantial component of the study. These markers and samples should prove useful for future studies of L. aurea and perhaps more generally the ???bell frogs???. Initially, a large-scale assessment of genetic structure and diversity in L. aurea using microsatellite markers was undertaken. Twenty-one locations were sampled from throughout the species range covering 1000 kilometres of the east coast of Australia. Levels of allelic diversity and heterozygosity were high (uncorrected mean alleles/locus and HE: 4.8-8.8 and 0.43-0.8 respectively) compared to other amphibian species and significant differences among sampled sites were recorded. Despite recent population declines, no sites displayed a genetic signature indicative of a population bottleneck. Significant genetic structuring (overall FST = 0.172) was detected throughout the species range, but was relatively low compared to previous amphibian studies that used microsatellites. In addition, some areas sampled within continuous habitat showed evidence of weak genetic structuring (data subset FST = 0.034). Next, relationships among extant bell frogs (Litoria aurea species-group) were investigated, using mitochondrial ND4 nucleotide sequence data. Analyses supported a clade comprised of the temperate members of the species-group, L. aurea, L. cyclorhyncha, L. moorei, and L. raniformis but failed to support the inclusion of the tropical bell frog L. dahlii in this group. Relationships among the four members of the bell frog clade correlated with geographical distribution: the south-western Australian bell frogs (L. cyclorhyncha, L. moorei) and the south-eastern Australian bell frogs (L. aurea, L. raniformis) were reciprocally monophyletic. Results also indicated that divergence of these two lineages occurred during the late Miocene, which was consistent with results of previous studies and with more general assertions that much of the major differentiation and radiation of the Australian biota predated the Quaternary. Following this, intraspecific phylogeography of L. aurea using two mitochondrial genes COI and ND4 was investigated. I examined extant populations from throughout the species??? range, sequencing 263 individuals from twenty-six locations. Recent evolutionary history, as well as the current population structure of L. aurea, was inferred from the resulting pattern of genetic variation amongst haplotypes, in conjunction with demographic and population analyses. Results indicated that there were no phylogeographic divisions within L. aurea, despite a general consensus that amphibians are highly structured. However, I did still detect significant structure amongst extant populations (FST = 0.385). Overall, patterns of haplotype relatedness, high haplotypic diversity (mean h = 0.547) relative to low nucleotide diversity (mean Pi= 0.003), and mismatch distribution analysis supported a Pleistocene expansion hypothesis with continued restricted dispersal and gene flow. Taken together, the results of this thesis indicate that L. aurea is a species with relatively weak population and phylogeographic structure compared to other amphibians. The data provide no support for the existence of distinct evolutionary lineages within L. aurea, implying that there are no historically isolated populations that should be viewed as separate evolutionarily significant units. Nevertheless, remaining populations are still significantly structured but not all populations are genetically distinct. Low phylogeographic structure, coupled with evidence for recent gene flow among many sites would permit ???well managed??? intervention to mediate gene flow amongst currently isolated populations, and I provide some guidelines for the implementation of such conservation strategies. However, there is no evidence to suggest that supplementation through artificial immigration is at this time necessary given current levels of genetic variation within populations. In the short-term, conservation management in L. aurea should focus on the protection of local populations and habitat to promote population connectivity to ensure processes that maintain adaptive diversity and evolutionary potential are conserved. Preservation of the species??? natural range and the maintenance of dense networks of suitable habitat, in conjunction with maximising local carrying capacity and reproductive output, as well as minimising known threats, are key to securing the long-term persistence of the green and golden bell frog.

Research Doctorate - Doctor of Philosophy
An understanding of a species’ population ecology can be used to determine the impacts of threatening processes and to estimate the probability of local extinction. For this thesis I estimated the major demographic rates of a population of the green and golden bell frog through the use of mark recapture studies at Sydney Olympic Park, Australia. Through the analysis of historic and contemporaneous data we found that the mitigation of habitat destruction with habitat offset resulted in no net loss and an approximate 2-fold increase in population size. However, this required the construction of 19 times the amount of pond area than was destroyed. Females were detected less frequently during surveys suggesting they may have a smaller population than males. Analysis that included capture probability indicated that this difference was because females were more difficult to detect and the sex ratio was near parity. Mark recapture also estimated low survival, high recruitment and rapid growth of post-metamorphic frogs. Additionally, survival and recruitment were found to be highly variable which resulted in variable population sizes. Growth rate also varied on a spatial scale which may suggest a difference in productivity in different areas. This demographic information was used to construct a model of population viability with sensitivity analyses. The high variability in demographic rates resulted in highly variable predictions of population size, with a 42% chance of extinction over 100 years. Sensitivity analysis predicted that the probability of extinction was most sensitive to changes in female survival with male survival and growth rate also having considerable impact. We then ran a posthoc experiment to validate an assumption that the recruitment rate was not density dependent by increasing the survival of females over the winter. We found that breeding of mature females was not restricted by density dependent factors, which was in concordance with the population viability model. Together, these results have expanded the knowledge of the population ecology of this species which should induce targeted management actions to female survival and improve habitat offset programs through knowledge that such programs require substantial offset ratios. The use of these findings should therefore provide better conservation outcomes for the species as a whole.

Research Doctorate - Doctor of Philosophy (PhD)
The myriad of concurrent threats driving species declines make it difficult for managers to identify causal agents and preserve threatened populations. The decline of the green and golden bell frog (Litoria aurea) in Australia is an example of how multiple threatening agents such as the novel pathogen, Batrachochytrium dendrobatidis (Bd), invasive mosquitofish Gambusia holbrooki, and habitat degradation, can act independently or synergistically to rapidly decimate populations and prove difficult to manage. The broad focus of this study was to assess the impact of these threats on L. aurea, with the aim of improving current management techniques. Multi-state mark recapture analysis of three years of monitoring data from an extant and released population of L. aurea revealed that Bd significantly reduced survival probability, even within a habitat where the frogs persist. Analysis of data from the released population suggests the severity of Bd was affected by host age and condition, temperature, density, and food availability, and that both nutrition and Bd likely contributed to a lack of reproduction. Increased salinity may be tool for passively reducing Bd severity, as frogs used waterbodies of 3 parts per thousand (ppt), but care needs to be taken as salinities above 5 ppt elicited avoidance behaviour. Generalised linear models showed a marked separation between G. holbrooki and tadpole occupied ponds with a lack of avoidance by adult frogs. These patterns suggest that adults are naïve to G. holbrooki and the fish likely reduces recruitment through larval predation. A laboratory experiment identified few synergistic effects of Bd and G. holbrooki on tadpoles, although we argue that this may change in the wild. This thesis affirms the negative impact Bd and G. holbrooki have on L. aurea, and suggests why our study population persists in the face of these threats. We highlight factors that can improve in situ conservation projects of this nature, such improvements to habitat that may mitigate Bd, the importance of habitat that promotes a healthy abundance of prey species to supply nutrients for immune function and breeding, the importance of excluding G. holbrooki from protected sites, and the importance of habitat close enough to extant sites or large enough to support dispersal behaviours; all potentially achievable goals that may greatly benefit many threatened amphibians.

Research Doctorate - Doctor of Philosophy (PhD)
Traditional management of species and ecosystems is reliant on the ability to accurately predict the outcomes of management actions. Due to the high level of uncertainty in natural systems, this level of predictability is unlikely to be possible in the vast majority of management situations. Adaptive management is an alternative approach that can be effective regardless of how much is known about the species or system and the way in which it will respond to management, as actions are used as a series of experiments to test hypotheses and build knowledge about the way it functions. I used this approach to investigate the effectiveness of a range of management actions aimed at conserving the threatened green and golden bell frog (Litoria aurea), a species which was once common but has experienced large-scale declines since the 1970s. I found that the removal of the exotic predatory fish Gambusia holbrooki via pond draining was effective in greatly increasing the reproductive success of this species, and the addition of sodium chloride to increase the salinity of ponds was effective in reducing the prevalence of the pathogenic amphibian chytrid fungus, and in turn increasing L. aurea survival. Actions aimed at creating early-successional habitat (the removal of overgrown aquatic vegetation and shade trees) do not appear to be necessary to maintain habitat quality at sites that are dominated by urban or industrial land uses which provide incidental disturbance (though they may still be required to maintain pond function). Population supplementation via the release of captive bred tadpoles had temporary benefits for naive occupancy and abundance, but it is likely that this strategy can only be effective in the long term if the cause of declines can be determined and mitigated. No evidence was found to suggest that poor habitat quality is the cause of observed low occupancy at sites where L. aurea persists. Regardless of whether these actions were effective in meeting conservation goals in the short term, they were all valuable in the long term as they added to our understanding of the habitat requirements of this endangered amphibian and can inform and improve future management strategies.

Research Doctorate - Doctor of Philosophy (PhD)
Successful management of threatened species requires an understanding of how they use their habitat and respond to management intervention. For this thesis, I investigated the habitat features driving the distribution of the threatened green and golden bell frog (Litoria aurea) toward industrial landscapes, and constructed an experimental trial habitat to compare habitat use patterns and determine whether the constructed habitat would support its growth, survival, and persistence. A detectability study was also conducted to determine detection probabilities among various habitat types. The results revealed the most important landscape feature was the number of permanent waterbodies within a kilometre distance which determined L. aurea occupancy, colonization, and breeding. The most utilized habitat for all demographic groups was aquatic vegetation while the least utilized was open water. Moreover, accounting for detection probabilities altered the outcomes of nearly all habitat variables, and incorporating them is essential to avoid wrong conclusions. Lastly, the constructed habitat provided L. aurea with sufficient resources to grow, survive, and persist for three years. Implications for future conservation management is that a landscape level approach is required to successfully manage this species. Although a relatively small animal, L. aurea disperse large distances to waterbodies. While it has been recognized as a colonizing species, this aspect of its life history has been underestimated and largely ignored. The creation of permanent waterbodies is necessary in areas with little interconnectivity, many ephemeral ponds, or where droughts are common. A greater proportion of aquatic vegetation should also be the focus of future L. aurea management. Furthermore, large release numbers are necessary to combat its high mortality rates and as buffer against predation and disease. The findings of this thesis also demonstrate that experimental trials prior to implementation of large scale initiatives are highly valuable for informing future habitat management decisions.

Research Doctorate - Doctor of Philosophy (PhD)
Over the past 50 years dramatic declines in the world’s amphibian fauna have resulted in the possible extinction of up to 159 species and the emerging infectious disease chytridiomycosis has been implicated as a causal agent. Chytridiomycosis is caused by the newly described amphibian chytrid fungus Batrachochytrium dendrobatidis which infects keratinised cells in the outer epidermal layers of post-metamorphic amphibians that impair the osmoregulatory function of the skin, leading to circulatory collapse and death in susceptible species. The chytrid fungus also infects the keratinised mouthparts of tadpoles but this is rarely reported to be fatal. The existence of reservoir hosts, in the form of tadpoles and less susceptible species allows the pathogen load to remain high in an area, driving susceptible species to extinction. As a result, population declines and extinctions caused by chytridiomycosis are often rapid and occur alongside a suite of non-declining species. The green and golden bell frog Litoria aurea is an endangered Australian anuran that was once widespread throughout eastern NSW and Victoria but underwent a dramatic range contraction in the 1980s. This species currently persists in less than 10% of its former range in a series of highly isolated and disturbed sites along the coastline. Historically, the decline of the green and golden bell frog has been attributed to the effects of habitat loss and predation by the introduced mosquito fish Gambusia holbrooki. However, at the landscape level, the distribution of these threats is inconsistent with the bell frog’s pattern of decline. Green and golden bell frogs are highly tolerant of disturbance and appear to have disappeared from apparently suitable areas only to persist in highly modified sites, often in the presence of mosquito fish and other non-declining frog species. These observations suggest another agent may be operating. The decline pattern of the green and golden bell frog has a number of consistencies with global disease-induced declines but the potential role of chytridiomycosis in the bell frog has not been established. Several small mortality events of infected bell frogs have been observed, indicating susceptibility, but nothing is known about infection and disease dynamics. There has also been some speculation that the persistence of bell frogs in coastal and often contaminated sites may be due to the antifungal properties of water solutes in those environments but this has not been investigated in detail. The overall objective of this thesis was to establish the susceptibility of bell frogs to chytridiomycosis and to model this impact on population dynamics. It also investigated the role of environmental inhibitors of chytrid in allowing bell frog populations to persist in their current range and the potential for these inhibitors to be used as a mitigation tool. This was done by addressing four primary aims. The first primary aim of this thesis was to determine whether bell frogs were susceptible to chytridiomycosis upon exposure to the chytrid fungus and this was investigated in both captive and free-living animals. An infection experiment exposing green and golden bell frogs to a chytrid isolate in captivity monitored infection loads and signs of disease over time (Chapter 2). All exposed bell frogs were found to become infected and infection loads increased significantly over time. Within 100 days all infected bell frogs showed terminal signs of chytridiomycosis. In free-living bell frogs, mark-recapture surveys and radiotracking were used to determine infection status over a 12 month period to estimate the impact of infection on survival probability (Chapter 3). Infection was found to impact survival over the colder non-breeding season with significantly lower survival probabilities found in infected bell frogs (0.1), compared to uninfected (0.56). The second primary aim of this thesis was to determine the impact of infection on population dynamics. This was investigated using the survival and infection transition probabilities from multistate models and comparing population size scenarios when chytrid was present or absent (Chapter 3). The results reveal that an infected population would decline at twice the rate of an uninfected population. The ability of the chytrid fungus to cause population decline and extinction in bell frogs was further supported by the unintentional exposure of a reintroduced population at the Hunter Wetland Centre and the resulting chytridiomycosis epidemic observed (Chapter 4). The third primary aim of this study was to determine whether inhibitors of chytrid survival, growth or transmission occur in current bell frog habitat. Infection levels in the dwarf green tree frog Litoria fallax were compared between sites formerly and currently occupied by bell frogs in the Lower Hunter Region of NSW, where bell frogs have undergone a directional range contraction that echoes the species decline (Chapter 5). Infection loads were found to be higher in sites where bell frogs have disappeared and were correlated with the abundance of fish (positive relationship), the degree to which the water body dried, water temperature and salinity (negative relationships). A series of controlled and replicated experiments were then conducted to test for causation and an inhibitory effect of water temperature and salinity on infection load was confirmed. The fourth primary aim of this thesis was to determine whether inhibitors of the chytrid fungus can be used to mitigate the impact of chytridiomycosis on bell frog populations. The antifungal effect of dissolved sodium chloride in aquatic habitat was investigated in controlled experiments conducted in the laboratory and then in the field. When grown in culture media with 4 and 5 ppt sodium chloride, chytrid growth, zoospore density and zoospore motility were significantly inhibited compared to 0 ppt (Chapter 6). Given that chytrid fungi are generally adapted to freshwater habitats and are have low sensitivities to desiccation, this effect is likely the result of energy investment into osmoregulation rather than growth and motility. To investigate the impact of this on infection outcomes, an infection experiment was conducted and found that hosts housed in water bodies with 3 or 4 ppt sodium chloride had significantly lower infection loads relative to those housed in 0 ppt and had significantly higher survival rates (Chapter 6). These results confirmed the antifungal effect of dissolved sodium chloride at environmentally available concentrations. A field based experimental reintroduction was then used to trial the practicality and effectiveness of elevating dissolved sodium chloride concentrations in ponds as a mitigation tool for the chytrid fungus (Chapter 7). Naturally derived sodium chloride was added to eight water bodies to equal a maximum of either 2 or 4 ppt. An additional four water bodies were left at close to 0 ppt to act as controls. Captively bred green and golden bell frog tadpoles were released into each independent water body and monitored for body size, infection levels and survival. A negative effect of exposure to 4 ppt was found on the body length of tadpoles but a positive effect was found for the outcome of infection, with significantly lower prevalences and higher survival rates post-metamorphosis. Monitoring of non-target amphibians and macro-invertebrates found a negative effect of 4 ppt on the relative abundance of several species. These results support the role of sodium chloride manipulations in natural amphibian habitat as a potential mitigation strategy but cautions that these effects must be weighed against any negative effects on co-occurring species. In the process of addressing the four primary aims of this thesis, a number of additional secondary aims were also investigated that, although not the focus of this thesis, still contributed to our understanding of chytridiomycosis and the green and golden bell frog. The first of these secondary aims was to determine the dynamics that drive infection outcomes for both the individual host and the host population. In determining the susceptibility of bell frogs to chytridiomycosis following exposure to the chytrid fungus, it was found that the terminal stages of chytridiomycosis were determined by infection loads crossing a threshold, representative of the point at which epidermal function is irreversibly impaired (Chapter 2). In host populations, it was the survival rate of infected individuals as well as the transition probabilities between infected and uninfected states, both of which were driven by seasonal variability in temperature, that determined the impact on population size (Chapter 3). The second secondary aim was to determine whether non-declining amphibian species that co-occur with bell frogs are less susceptible to chytridiomycosis and was investigated in a comparative infection experiment (Chapter 2). The comparison of infection outcomes when exposing both green and golden bell frogs and the non-declining co-occurring striped marsh frog Limnodynastes peronii to the chytrid fungus revealed significantly different outcomes. Infection loads in striped marsh frogs did not increase over time and did not result in the mortalities seen in bell frogs. Rather, infection loads remained the same or declined over time and survival rates did not differ from uninfected controls. This suggested that this species has a mechanism of maintaining infection loads below the disease causing threshold. These susceptibilities also correlate with the decline patterns and threat status of these two species. Further evidence that bell frogs are more susceptible to chytridiomycosis than co-occurring species was obtained through observations of a reintroduced bell frog population extinction alongside populations of three other species that did not disappear (Chapter 4). Finally, the third secondary aim was to determine the most effective vital rate to target to manage the negative effect of chytridiomycosis on population persistence. This was done by comparing scenarios of population persistence models when the probability of acquiring infection, the probability of losing infection, the recruitment rate or the survival probabilities of infected and uninfected hosts were improved by a relative measure (Chapter 3). Comparisons revealed that when the degree of ‘improvement’ is kept constants across rates, an increase in the recruitment rate or survival probability of uninfected individuals resulted in the greatest benefit for population persistence. These results suggest that even if methods to control the chytrid fungus were available, counteracting its effects may be the most effective management strategy.

Research Doctorate - Doctor of Philosophy (PhD)
This thesis investigated the potential of water salinity to limit the pathogenicity of Bd (the amphibian chytrid fungus) in a created habitat mosaic. Results demonstrate that determining the outcomes of varying aquatic habitat treatments in a constructed, constrained system is a complex problem, with many interactions. Nevertheless, there was no evidence that any of the habitat components (treatments) incorporated into the experimental design negatively impacted the fitness, or reduced the survival of the model amphibian species, Litoria aurea. The results of this thesis in fact suggest that the most appropriate strategy for optimising the probability of persistence of L. aurea in a landscape in which they have been shown to persist, is to offer a landscape mosaic of waterbodies of varying hydroperiod and salinity. Such a mosaic offers choice for optimal breeding and dispersal throughout the seasonal climatic cycle, while exposing individuals, even intermittently, to elevated salinities that may confer survival across the whole habitat. Taken together, these environmental variables in a complex mosaic may tip the balance of population dynamics from decline and extinction towards persistence. The implications of this approach extend beyond L. aurea to the management of other amphibian species that persist with Bd in complex environments.

Research Doctorate - Doctor of Philosophy (PhD)
The accelerated decline and extinction of thousands of species is termed the global biodiversity crisis. A detailed understanding of the processes driving variation among species that lead to threatened status is essential in order to develop effective management strategies and curb the current rate of species decline. For many threatened species, there exists a paucity of information on their basic ecology and the factors that influence population dynamics. The fundamental ecological factors influencing a species population dynamics are natality, mortality, emigration and immigration; collectively referred to as demographic vital rates. Amphibians have undergone the most severe decline of any vertebrate group in recent decades, with over one third of species at risk of extinction. However, for most threatened amphibian species there is generally a paucity of information on their ecological vital rates is generally accepted that the majority of threatened amphibian species possess life-history traits situated towards the K-selected end of the life-history spectrum, with specialised life histories such as low fecundity and habitat specialisation. Interestingly, there are few threatened species that possess r-selected life-history traits (high fecundity and non- specialised habitat association). The green and golden bell frog (Litoria aurea), is something of an enigma, as it possesses life history characteristics of r-selected species synonymous with those of a weed species, yet has suffered a 90% decline in its distribution and corresponding decline in overall population abundance. Remaining populations are disjunct and generally restricted to coastal areas of NSW and Victoria where they remain at risk of extinction from threatening processes such as habitat loss, invasive predatory fish and disease. Investigating and gaining a better understanding of the ecological aspects of this declining r-strategist species will allow us to address the current gaps in knowledge about L. aurea so that appropriate and improved management actions can be developed and implemented for its conservation. This thesis explores the major vital rates that are currently least understood for Litoria aurea and focusses on one of the last remaining populations occurring at Sydney Olympic Park (SOP), NSW, Australia. It begins by investigating the survival, recruitment and individual growth rates of the population to determine the sources of variation in demographic rates. Generating and analysing a six year mark-recapture dataset revealed a highly variable population size, with the population estimated to have high recruitment and rapid growth of post-metamorphic frogs, but is offset by low survival rates into older age classes. In addition, individual growth rate varied across the spatial scale indicating resource availability as a limiting factor. Using this information, we were able to further explore these sources of population variation in more detail, being resource use, sources of mortality, reproductive success and movements. I investigated the diet of L. aurea and its prey availability among habitat types within the population investigated, which revealed the source of variation in growth rate within the population. Furthermore, I used radio-tracking techniques to determine winter habitat use of L. aurea, which demonstrated different levels of habitat use compared to that used in summer, and differences in winter habitat occupation between sexes. These results highlighted the importance of understanding resource usage of L. aurea when considering habitat creation for the purposes of reintroduction or management of existing populations. Moving on to the sources of mortality in the species, I was able to develop a seasonal profile for the infection and prevalence of the chytrid fungus, Batrachochytrium dendrobatidis (Bd) within the SOP population. This enabled me to assess the relationship environmental temperature has on the pathogen and also identify infection recoveries from individuals during a time when prevalence was highest. I also investigated predation by the invasive black rat (Rattus rattus) as a source of L. aurea mortality in the SOP population by analysing the stomach contents of rats using traditional morphological methods and novel molecular techniques. I was able to show proof of concept of molecular stomach analysis in a wild population, which yielded better detectability of prey items than traditional methods with the identification of one amphibian and one reptile species among stomach samples. These data suggested that predation rates by rats on L. aurea might not be high, but this finding will need to be confirmed by further studies. Finally, I focussed on the reproductive success and movement of L. aurea by exploring the distribution and habitat predictors of ponds used in breeding events and by investigating the significance of juvenile dispersal in relation to biotic and abiotic pond characteristics. Given that breeding pond choice was seasonally dynamic in this study, understanding the drivers of these seasonal shifts may be informative in managing existing populations of the species. In summary the key findings in this thesis with respect to the demographic vital rates of L. aurea relevant to its conservation are: the SOP L. aurea population experiences a high level of demographic turnover with low survival and high recruitment, which has implications for the persistence of this population; growth rates of individual L. aurea reflect the diversity of prey item assemblages available in their environment, which varies depending on the quality of habitat; specific winter micro-habitat was used by L. aurea at higher rates than would be predicted by chance and had specific characteristics selected for by the species that may assist in their survival over winter; wild individual L. aurea recovered from Bd infection during the coolest months of the year when Bd prevalence is highest and risk of infection is greatest, according to a seasonal disease profile developed for the pathogen; physical and genetic L. aurea material were not detected in the stomach contents of a wild population of a potential predator, the introduced black rat, and; breeding ponds varied in occupancy based on L. aurea life stage with adult males calling in larger, well connected ponds and juveniles readily dispersing from breeding ponds. By focussing on the demographic vital rates that drive population dynamics, this study contributes to understanding the fundamental processes of L. aurea ecology. I conclude that studies leading to the understanding of a species’ demographic vital rates are valuable, since previously overlooked components of a species’ ecology may identify mechanisms useful for better informing conservation management practices and potentially reduce further species declines.

Research Doctorate - Doctor of Philosophy (PhD)
Translocations and reintroductions are an increasingly important tool in conservation. The focus is often on the breeding and reintroduction, and less consideration is given to the environment into which animals are released. Many such programs achieve less favourable outcomes, or fail completely, because the impacts of the environmental conditions in the reintroduction landscape are not adequately taken into account or mitigated. There is a need for studies in real-world scenarios to test the paradigm that environmental manipulation could improve fitness and survival in populations, and increase the probability of establishment of viable, self-sustaining populations. Reintroductions of amphibians impacted by chytridiomycosis into environments where the disease is present provide a scenario where this paradigm can be tested. This thesis explores that paradigm, investigating ways in which real-world environments might be able to manipulated to adversely impact the disease organism responsible for chytridiomycosis, while remaining favourable to the amphibian host. It begins by exploring the state of translocation programs around the world, before moving on to study the relationship between environment and disease/host relating specifically to amphibians and the chytrid fungus, Batrachochytrium dendrobatidis (Bd). Using a model species that is both susceptible to Bd and subjected to many conservation translocation programs, many of which have failed in the past, I explored whether it might be possible to increase translocation success of wild populations by manipulating certain environmental stressors. Along the way, I developed and optimised an assay for studying ecoimmunology for amphibians (necessary for the current studies), investigated the outcomes of translocating my study species in the presence of the disease without intervention by environmental manipulation, the role of ontogeny in the susceptibility to the disease, and looked at the effect of temperature and seasonality on wild populations. Finally, I directly tested the core paradigm of mitigating the impact of emerging disease by environmental manipulation, and demonstrated that it is feasible, leading to potentially significant conservation outcomes. I conclude that studies leading to the understanding of mechanisms of disease transmission and dynamics as they play out in realistic environmental scenarios is a conservation strategy worth pursuing, since such investigations may identify management strategies that increase resilience of susceptible species at the landscape level.