Zoology - Theses
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ItemTriple jeopardy in the tropics: assessing extinction risk in Australia's freshwater biodiversity hotspot( 2017)Freshwaters are the most degraded and imperiled ecosystem globally. Despite this high vulnerability, conservation efforts in freshwaters often lag behind those in terrestrial and marine ecosystems. In Australia this is particularly evident; despite high levels of river degradation, few freshwater fishes have had their conservation status assessed and only 14% of fishes are listed. Most listed species are restricted to southern Australia where rivers are particularly degraded. Northern Australia’s rivers are very diverse with many highly range restricted fishes. Yet almost no species are listed, despite potential vulnerability and an increasing number of threats across the north. Nowhere is this more evident than the Kimberley region in the north-west, where 49% of species are restricted to three or fewer rivers, and 10% are restricted to an area of <20 km2. Very little is known about the ecology of the region’s endemic fishes, so their vulnerability cannot be assessed. In my thesis I assess extinction risk in the freshwater fishes of the Kimberley using the triple jeopardy framework, that is whether they have small geographic ranges, low abundances and/or narrow ecological niches. Specifically I aim to (1) determine the relationships between range size, body size and abundance in all Australian freshwater fishes and (2) whether these relationships can be used to identify species at risk of extinction. I then determine whether (3) small ranged Kimberley endemics have narrow habitat, dietary or thermal niches compared to closely related widespread species and (4) synthesize these results to identify the fishes most at risk of extinction in the Kimberley. First, I test for a relationship between geographic range size and body size in all Australian freshwater fishes. I then investigate how this relationship varies with conservation status. I identify currently unlisted freshwater fishes that share traits with listed species and map their distribution, along with freshwater fish research effort, across Australia. I found a positive relationship between range size and body size. For a given body size, conservation listed species have a range less than one tenth the size of unlisted species. Based on this relationship, I identified 55 additional species that may be vulnerable to extinction. Most of these species are restricted to northern Australia where freshwater fishes are poorly known due to low research effort. Second, I test for abundance-geographic range size and abundance-body size relationships in Australian freshwater fishes and investigate how these relationships vary with conservation status. I identify and map currently unlisted freshwater fishes that are numerically rare, and combined with the results outlined above, map species with a double jeopardy risk of extinction. I found a negative body size-abundance relationship and no correlation between range size and abundance. Although relative abundance was a poor predictor of current conservation listing, I identified 59 consistently rare species. Twenty of these species (34%) currently suffer a double jeopardy risk of extinction and all were restricted to northern Australia. Third, using closely related widespread and endemic congeneric pairings of Kimberley freshwater fishes, I investigate whether endemic species have narrow dietary niches at any stage during their development. Using qualitative measures of habitat and presence/absence data, I also assess habitat specialization. Most range-restricted species have narrower ecological niches making them more vulnerable to extinction. Fourth I test the thermal performance of two pairs of congeneric species that are sympatric in the Drysdale River, with one widely distributed species and one range restricted species in each pair. In the Syncomistes pair, resting metabolic rate (RMR) was similar between species at low temperature but at higher temperatures the RMR of the widespread species was lower due to the onset of anaerobiosis. The range-restricted Syncomistes also has a higher critical thermal limit (CTL). In the Melanotaenia pair, the results were the opposite, with the widespread species having a higher CTL and RMR. The thermal performance of each species was related to their distribution within the catchment rather than their geographic range size, with the thermally sensitive species dominating the cooler, perennial downstream reaches, and the hardier species being more abundant in the hotter, more ephemeral upper catchment. Finally, I use the above information to assess the triple jeopardy extinction risk in the fishes of the Kimberley. Seventy-nine per cent of Kimberley endemic fishes are vulnerable on one or more axis, and two species had a triple jeopardy risk of extinction. The majority of vulnerable species are found in the remote rivers of the north-western Kimberley, but the most imperiled species (Hypseleotris kimberleyensis) is restricted to the heavily degraded Fitzroy River. My thesis shows that, despite fundamentally different environments, life histories and dispersal capacity, Australian freshwater fishes exhibit range size, body size and abundance relationships largely similar to terrestrial fauna. By identifying northern Australia as a hotspot of unrecognized vulnerable species, I provide an important context for guiding targeted research and informing future conservation management of Australia’s freshwater fishes. Combined with their small ranges and/or low abundance, the narrower niches of most Kimberley endemic species makes the region’s fishes particularly extinction prone. By identifying which endemic species are most vulnerable, my study provides specific information for targeting conservation efforts in the region. As the Kimberley and northern Australia more broadly are earmarked for major development, substantial effort is needed to effectively manage fish populations, design and manage developments with the environment as a major stakeholder and preserve remote rivers with high endemism and extinction risk. However, as northern Australia’s rivers are in good condition, with planning and research there is an excellent opportunity for proactive, properly informed freshwater conservation across the region.
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ItemCompetition and coexistence: the ecology of sympatric common and mountain brushtail possums (Trichosurus spp.)( 2014)Closely related species often possess similar ecology due to their shared phylogenetic history, and consequently they usually require similar resources. As a result, potential for competition between closely related species is high. Without the rapid renewal of shared resources, coexistence between such species can usually only occur in the presence of resource partitioning. I studied the ecology of sympatric common and mountain brushtail possums (Trichosurus vulpecula and T. cunninghami) within a network of linear forest remnants in Boho South, north-east Victoria, with the aim of uncovering the processes facilitating coexistence between these congeneric species. Both common and mountain brushtails are primarily folivorous, arboreal/semi-arboreal marsupials, and occur sympatrically through some parts of their distributions. I examined three-dimensional resource partitioning by the foraging guild of arboreal marsupial folivores to which the two brushtail species belong. At Boho South, this guild included the koala, the greater glider and the common ringtail possum, in addition to the two brushtail species. I found that significant resource partitioning occurred in both the horizontal and vertical plane, with each species adopting a particular foraging niche with regard to tree species use and the available vegetation strata. Vertical stratification of such a diverse marsupial guild has not previously been demonstrated in temperate forest systems, and this result highlights the role of habitat heterogeneity in maintaining diverse faunal assemblages. I radio-tracked pairs of adult female common and mountain brushtails occurring in close sympatry in the linear forest remnants. Female mountain brushtails maintained home range areas more than twice the size of those of common brushtails, and the home ranges of mountain brushtails overlapped those of sympatric common brushtails more significantly than the reverse. Although the degree of total (95 %) home range overlap was high, the core (50 %) home ranges of study pairs overlapped significantly less. Patterns of den use between the two species were similar, with common and mountain brushtails using a mean of 6.0 ± 1.1 and 5.7 ± 1.4 dens per individual, respectively, over a mean of 15.5 months monitored per individual. The majority of den fixes were in tree hollows (75.7 % and 89.5 % of common and mountain brushtail den fixes, respectively). Vertical vegetation strata were used differently by each species, with common brushtail using the canopy and midstorey significantly more, and the understorey and ground layer significantly less, than mountain brushtails. The maintenance of relatively discrete core home range areas combined with fine-scale differences in use of tree species and vegetation strata appeared to facilitate the high degree of home range overlap between adult females. I conducted an extensive analysis of the diet of each species, examining faecal samples for both plant and fungal content. Dietary resources were strongly partitioned between the two brushtail species: common brushtails foraged primarily on Eucalyptus, while mountain brushtails foraged primarily on silver wattle. Common brushtails also consumed significantly more mistletoe foliage. Both species used fungal and floral food resources, however, these were also strongly partitioned between the two species: common brushtails supplemented their diet more often with flowers, while mountain brushtails consumed a greater amount and diversity of epigeal and sequesterate fungal taxa. Given the tripartite relationship occurring between sequesterate ectomycorrhizal fungi, the forest tree and shrub taxa with which the fungi form symbiotic relationships, and the mycophagous mammals that disperse fungal spores in their faeces, the consumption of sequesterate fungi by mountain brushtails is likely to play an important role in ecosystem health at Boho South, particularly given the local extinction of specialist fungivores, the potoroids, in this area. I propose that the strong partitioning of foliage between the two brushtail species, combined with an abundance of shelter resources in the form of suitable tree hollows, facilitates the stable coexistence of common and mountain brushtails at Boho South. By significantly reducing competition for food resources, dietary partitioning appears to allow each species to maintain home ranges that overlap strongly with those of their congeners. As the foraging niches of each brushtail species are consistent with other research on their digestive tolerances, I suggest that the divergent diets of common and mountain brushtails are a result of ecological character displacement having evolved in the two species to minimise competition for food resources. Current fine-scale partitioning of habitat components and supplementary food sources by common and mountain brushtails at Boho South are a possible further means by which these two species avoid competition for resources.