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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ItemThe importance of body size: scaling of physiological traits in insects( 2015)Biological phenomena occur across wide scales in space, time, and organisational complexity. Molecules, which are small, quickly transforming units, exhibit new emergent properties when they are arranged into ecosystems. These properties of ecosystems, such as species diversity, distribution, standing biomass, or rates of nutrient turnover involve large spatial and temporal scales, as well as many underlying processes that make their study inherently complex. Integration across disciplines and across levels of biological organisation is one of the grand challenges in biology. Towards this end, novel methods are required so that cross-disciplinary phenomena can be quantified using a common metric. Energy and mass are two universal currencies that are able to cut through the hierarchy of biology, which must be both conserved irrespective to the scale of inquiry. Dynamic Energy Budget (DEB) theory builds upon the laws of energy and mass conservation by identifying other universal constraints on the metabolic organisation of diverse species. While DEB theory is commonly perceived to be relevant at fine biological resolutions, particularly the individual level, it has received little recognition from population, community, and ecosystem biologists despite its application to many supra-individual topics. In this thesis, I bring principles of DEB theory to bear against several current problems in biology that each span multiple organisational levels. As pointed out by renowned mathematical ecologist, Richard Levins, different models may take a different emphasis on precision, generality, or realism and do so antagonistically (at the expense of the other qualities). In thesis I take an emphasis on generality by developing simple, parameter-sparse DEB-based models that are able to yield predictive synthesis on cross-disciplinary issues, demonstrating the parsimony of DEB approaches. This departs from previous DEB studies on macro-ecological patterns, which take more of an emphasis on precision. I also focus on the taxonomical group of the insects – a group which is comparatively understudied in the DEB literature. The first of these problems surrounds a theoretical underpinning to the famous pattern of metabolic scaling. Metabolic scaling is the observation that as organisms increase in size, the energy turnover in a fixed unit of biomass decreases. This pattern has great biological importance and now forms the basis of the emerging field of metabolic ecology. Much of the current interest and controversy in metabolic scaling relates to recent ideas about the role of supply networks in constraining energy supply to cells. I show that an alternative explanation for physicochemical constraints on individual metabolism, as formalised by DEB theory, can contribute to the theoretical underpinning of metabolic ecology, while increasing coherence in the topic of metabolic scaling. In particular, I emphasise how DEB theory considers constraints on the storage and use of assimilated nutrients, and illustrate how this explains the frequently observed quarter-power scaling of many biological rates without relying on optimisation arguments or implying cellular nutrient supply limitation. Because the DEB theory mechanism for metabolic scaling is based on the universal process of acquiring and using pools of stored metabolites, it applies to all organisms irrespective of the nature of metabolic transport to the cells, but without necessarily excluding insights from transport-based models. Design constraints imposed by increasing size cause metabolic rate in animals of different species to increase more slowly than mass. However, mechanistic explanations for interspecific metabolic scaling do not apply for ontogenetic size changes within a species implying different mechanisms for these scaling phenomena. Next, I show that the DEB theory approach of compartmentalizing biomass into reserve and structural components provides a unified framework for understanding both ontogenetic and interspecific metabolic scaling. I formulate the theory for the insects and show that it can account for ontogenetic metabolic scaling during the embryonic and larval phases, as well as the U-shaped respiration curve during pupation. After correcting for the predicted ontogenetic scaling effects, which I show to follow universal curves, the scaling of respiration between species is approximated by a ¾ power law, supporting past empirical studies on insect metabolic scaling and my theoretical predictions. The ability to explain ontogenetic and interspecific metabolic scaling effects under one consistent framework suggests that the partitioning of biomass into reserve and structure is a necessary foundation to a general metabolic theory. The uptake of resources from the environment is a basic feature of all life. Consumption rate has been found to scale with body size with an exponent close to unity across diverse organisms. However, like metabolic rate, past analyses have ignored the important distinction between ontogenetic and interspecific size comparisons. I present a mechanistic model, based on DEB theory, for the body mass scaling of consumption, which separates interspecific size effects from ontogenetic size effects. The model predicts uptake to scale with surface-area (mass2/3) during ontogenetic growth but more quickly (between mass3/4 and mass1) for interspecific comparisons. Available data for 41 insect species on consumption and assimilation during ontogeny provides strong empirical support for the theoretical predictions. In particular, consumption rate scaled interspecifically with an exponent close to unity (0.89) but during ontogenetic growth scaled more slowly with an exponent of 0.70. Assimilation rate (consumption minus defecation) through ontogeny scaled more slowly than consumption due to a decrease in assimilation efficiency as insects grow. Again, these results highlight how body size imposes different constraints on metabolism depending on whether the size comparison is ontogenetic or inter-specific. Finally, I use the principles of DEB theory to explore the universality of growth patterns in insects. Insects are typified by their small size, large numbers, impressive reproductive output, and rapid growth. However, insect growth is not simply rapid; rather, insects follow a qualitatively distinct trajectory to many other animals. I present a mechanistic growth model for insects and show that the up-regulation of assimilation during the growth phase can explain the near-exponential growth trajectory of insects. The presented model is tested against growth data on 50 insects, and compared against other mechanistic growth models. Unlike other mechanistic models, the presented growth model predicts energy reserves per biomass to increase with age, which implies a higher production efficiency and energy density of biomass in later instars. These predictions are tested against data compiled from the literature whereby it is confirmed that insects increase their production efficiency (by 24 percentage points) and energy density (by 4 J/mg) between hatching and the attainment of full size. The model suggests that insects achieve greater production efficiencies and enhanced growth rates by up-regulating assimilation and increasing energy reserves per biomass, which are less costly to maintain than structural biomass. My findings illustrate how the explanatory and predictive power of mechanistic growth models comes from their grounding in underlying biological processes. These applications of DEB theory highlight novel insights on some well-studied, but unresolved issues in biology. More importantly, the theoretical basis of these insights demonstrates the value of a quantitative framework for metabolic organisation to the study of macro-physiological patterns, and how simplified DEB models can contribute to the emerging field of metabolic ecology. While the grand challenge of unification across scales still remains, the results of this thesis hold much promise for metabolic theory as a platform for synthesis in biology.
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ItemResource use in a community of large herbivores in south-eastern Australia( 2014)Many theories seek to explain herbivore community ecology. While much work has focused on African ungulates, I examined some of these theories in the complex macropodid community of Victoria Valley, Grampians National Park, Victoria, Australia. I investigated the influences of body size, diet type, ecological specialisation, and co-evolutionary history on resource use at multiple scales. Furthermore, I took advantage of macropodid sexual dimorphism to simultaneously explore differential resource use between and within species. In Chapter 2, I explored the relationships between body size, diet type, co-evolutionary history and habitat use of eastern and western grey kangaroos, red-necked and swamp wallabies, red deer and European rabbits in Victoria Valley, Grampians National Park, Australia. I used unbaited camera traps to estimate occupancy by these species in seven habitat types. None of these theories explained habitat use in this community. Red deer used a more narrow range of habitats than expected, which may be due to low habitat suitability for this species. In Chapter 3, I examined ecological specialisation in habitat selection during foraging and resting periods in western grey kangaroos, red-necked and swamp wallabies. I used radiotracking to quantify habitat selection. Western grey kangaroos were specialists in this community during the foraging period. Niche data for the two wallaby species were equivocal, and I was unable to determine the degree of specialisation of either species. Within species, I found no evidence of sex-based specialisation (sexual segregation). However, western grey kangaroos and red-necked wallabies specialised by activity period, that is they used different habitats for foraging and resting. In Chapter 4, I sought evidence of the effects of body size on microhabitat use during foraging and resting periods both between and within species in western grey kangaroos, and red-necked and swamp wallabies. I used radio-tracking to quantify habitat use, and characterised the locations used by recording the cover of plant functional groups and species. Foraging western grey kangaroos and swamp wallabies behaved as predicted; red-necked wallabies used more open, poorer-quality habitats than expected. Only western grey kangaroos showed a sex effect on habitat use: the relatively smaller females foraged in higher-quality patches. Habitats used during resting generally offered greater concealment cover than those used during foraging, but body size did not influence the density of vegetation used In Chapter 5, I assessed the effects of body size on diet quality in eastern and western grey kangaroos, red-necked and swamp wallabies. I quantified diet composition through microhistological analysis of faecal pellets, and then measured the quality (nitrogen and fibre) of the plant genera that contributed up to 85% of each species’ diet. The percentage of fibre (cellulose and hemicellulose) explained differences in diets; nitrogen concentration (total nitrogen x availability of nitrogen) did not. The two, relatively larger, kangaroo species consumed poorer quality diets (higher levels of fibre: cellulose and hemicellulose) than the smaller wallaby species. Male and female red-necked wallabies responded differently to fibre, but western grey kangaroos showed no sex-based diet quality differences. The findings reported in this thesis contribute to our understanding of community ecology in large herbivores more generally, and macropodid habitat and diet use specifically. Overall, I found the most support for the influence of body size on community structure at my site. Resource use both between and within species at the habitat, microhabitat and diet scales broadly matched predictions from body size theory. The fit to the predictions was strongest during the foraging period, suggesting that foraging resources are the key to structuring our herbivore community.