- Title
- What makes an endangered weed? Investigating the ecological aspects of a declining r-strategist amphibian
- Creator
- Garnham, James
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2018
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- 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.
- Subject
- habitat restoration; amphibian chytrid fungus; amphibian demography; threatening processes; molecular DNA; ecological vital rates; Litoria aurea; cytridiomycosis; radio tracking
- Identifier
- http://hdl.handle.net/1959.13/1411173
- Identifier
- uon:36306
- Rights
- Copyright 2018 James Garnham
- Language
- eng
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