Zoology - Theses
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ItemImpacts of habitat fragmentation on dispersal of native mammals( 2011)Habitat fragmentation is widely regarded as a major threat to biodiversity worldwide. In addition to the reduction of the total area of habitat available for wildlife, clearing of native vegetation creates disconnected patches of habitat immersed in a matrix, which does not provide resources for most species of wildlife. The remaining habitat patches often accommodate small populations of native animals, which suffer increased susceptibility to extinction. Movement of individuals between habitat patches, however, can interconnect such populations so that they effectively behave as larger and more stable metapopulations. Hence the traversability of any matrix for wildlife is an important attribute of fragmented landscapes and impacts on their ability to support viable populations. Further, the capability of different species to cross various matrix types varies. ‘Landscape connectivity’ describes the functional isolation of remnant areas of habitat and considers not only the distance between them but also the species-specific ability to successfully move through the matrix. In this thesis I examine the dispersal behaviour and population genetic structure of two species of marsupials in a fragmented landscape. I describe the movements of bobucks (Trichosurus cunninghami) and agile antechinus (Antechinus agilis) in the Strathbogie Ranges in north-eastern Victoria. Being forest-dependent, both species occur solely in remnants of native forest. Both also have a life-history pattern that is characterised by the dispersal of subadult males. However, they differ greatly in body size. I used GPS (for bobucks) and VHF (for antechinus) tracking to monitor individuals within their home ranges in the pre- and post-dispersal stages of their life cycle as well as during dispersal. I also used genetic parentage assignment tests to identify movements of antechinus. While the literature reports that the vast majority of subadult males of both species disperse, in my study a high proportion remained philopatric. Exploratory movements, beyond the boundaries of their home ranges, were recorded for adult and subadult bobucks; however, subadult males that later dispersed rarely exhibited this behaviour. In the months before the dispersal season, subadult bobucks that would subsequently remain philopatric appeared to occupy larger home ranges than dispersers. One possible explanation for this observation is that resource availability drives dispersal in this species and only those individuals that cannot secure access to sufficient resources in their natal home ranges disperse. In order to examine the response of dispersers to different landscape elements I monitored individuals during dispersal. Some bobucks and antechinus dispersed long distances. The very detailed dispersal pathways of bobucks, that I recorded using GPS tracking, revealed a clear pattern with all individuals dispersing along wooded landscape elements when these were available. Individuals were able to cross considerable distances across pasture using paddock trees and linear forest remnants as stepping stones and dispersal corridors. While some antechinus are likely to have crossed small gaps in linear forest remnant habitat, they appeared to be more restricted to forest during dispersal than bobucks. Genetic parentage analysis revealed that some male antechinus moved very considerable distances during the mating season, a behaviour that could significantly increase gene flow in this species. I applied genotypic analyses to infer gene flow in this system and used a landscape genetic approach to determine the connectivity of habitat in the Strathbogie Ranges for both bobucks and antechinus. I also investigated the influence of wooded landscape elements on gene flow in these two species. These analyses indicated that the distribution of forest remnants and individual trees play no role in determining gene flow in bobucks or antechinus within the Strathbogie Ranges. Surprisingly higher spatial genetic structure, which could indicate restricted dispersal, was detected in the larger, more mobile bobucks compared to antechinus. This seems to contradict my findings based on direct observations of dispersers in both species, namely the importance of paddock trees and linear forest remnants as dispersal conduits for bobucks and stronger restriction to forest of dispersing antechinus in comparison to bobucks. In fact, however, this discrepancy emphasises the difference between measurements of dispersal via direct observations versus genotypic inferences of gene flow. While observations of individual dispersers can provide detailed information on the behaviour of animals when they are traversing fragmented landscapes, data on their survival and breeding success post-dispersal is virtually impossible to obtain for many species. In contrast, molecular genetic analyses yield information on the reproductive outcome of dispersal across several generations, but usually cannot inform about dispersal pathways. The two approaches that I used are complementary, and combining them can help to avoid inaccurate conclusions about landscape connectivity for metapopulations, that are based on either technique alone. In my study, the landscape genetic model applied may have underestimated the true value of paddock trees as conduits for bobuck dispersal. Also the small number of antechinus that I was able to monitor during dispersal probably led to an underestimation of this species’ ability to leave its preferred forest habitat and traverse the matrix. Overall, the availability of linear forest remnants, scattered paddock trees and small patches of forest in the Strathbogie Ranges appeared to create high functional connectivity for populations of both species. My research emphasises the importance of maintaining and/or restoring structurally complex landscape elements, so that these can act as dispersal conduits for different species with differing mobility. This approach is an important tool to mitigate the effects of habitat fragmentation on native wildlife.
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ItemPatterns in the distribution and abundance of reef fishes in South Eastern Australia( 2011)This research investigated patterns in the distribution and abundance of nearshore fishes of south-eastern Australia. I used two methods to survey fishes, underwater visual census (UVC) and baited remote underwater video (BRUV). A comparison between these methods revealed that BRUV recorded higher relative abundance of mobile predators, while UVC observed higher relative abundance of herbivores, territorial species, and small site-attached species. These results suggest that studies surveying diversity would do best to employ multiple methods. In cases where funds are limited, UVC may provide a more complete estimate of diversity than BRUV as UVC recorded higher diversity, species richness and more individuals. Combining measures of abundance with habitat data, I investigated fish-habitat associations, specifically exploring how altering spatial grain influenced the strength of correlations between fish and habitat. Species of different sizes responded to habitat measured over different scales, with large-bodied species only displaying strong correlations with habitat when it was measured over large scales. These results suggest that research quantifying fish-habitat associations needs to take spatial grain into account. In addition, many species may respond to changes in habitat at scales larger than are typically investigated. Understanding not only how species interact with their environment but also the scale at which these associations occur is essential for management and conservation. I investigated biogeographic patterns in the distribution of fishes in Victoria using abundance measured by BRUV and UVC. The BRUV data displayed a cline in change across the state in which dissimilarities between locations were linearly related to distance. In contrast, data collected using UVC indicated the presence of a large faunal break in the vicinity of Ninety Mile Beach, and a second break between Cape Conran and Cape Howe, suggesting that contemporary habitat discontinuity, flow and/or temperature may be important factors structuring communities in this region. At a still larger scale, I explored relationships at upper and lower bounds between body size, geographic range size and abundance using data collected from Australia and New Zealand. At maxima, the relationship between body size and abundance was negative but steeper than expected, possibly driven by diver-averse behaviour of large species. At minima, body size and geographic range size were positively related, implying that body size determines the minimum area that a species must occupy. In contrast, at the upper bound this relationship was negative for non-perciform fishes, a K-selected group whose geographic range size could be constrained by their limited dispersal capacity. Distribution-abundance relationships deviated from predictions, with a negative relationship at the upper bound for Perciformes, which could be driven by the high dispersal potential of widespread species that results in diffuse low-density populations. From these results, I concluded, first, that fishes appear to differ from terrestrial taxa, which may be attributed in part to large-bodied fishes’ limited capacity for dispersal. Second, the approach of applying regressions to maxima and minima uncovered relationships that would have been obscured had they been investigated at the mean, highlighting the importance of exploring limits in macroecological relationships.
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ItemResource partitioning among five sympatric mammalian herbivores on Yanakie Isthmus, south-eastern Australia( 2010)This thesis combines multiple approaches to improve our understanding of large herbivore ecology and organisation in a contemporary assemblage made up of species with independent evolutionary histories on Yanakie Isthmus, Wilsons Promontory National Park, Victoria, Australia. In particular, this thesis compares niche parameters among populations of five sympatric native and introduced herbivore species by simultaneously assessing overlap in resource use along two dimensions (spatial and trophic) at multiple scales, thereby providing insight into resource partitioning and competition within this herbivore assemblage. Faecal pellet counts demonstrated that inter-specific overlap in herbivore habitat use on Yanakie Isthmus was low, suggesting that spatial partitioning of habitat resources had occurred. However, resource partitioning appeared to be independent of coevolutionary history. Low overlap in habitat use implies low competition, and the lack of clear shifts in habitat use from preferred to suboptimal habitats suggested that inter-specific competition was not strong enough to cause competitive exclusion. However, low overlap in habitat use between the European rabbit Oryctolagus cuniculus and other species, and preferential use by rabbits (and avoidance by other species) of the habitat that appeared to have the highest carrying capacity, suggested that rabbits excluded other grazing herbivores from preferred habitat. High overlap in habitat use was apparent between some species, particularly grazers, indicating some potential for competition if resources are limiting. In particular, the eastern grey kangaroo Macropus giganteus had a narrow niche, occurred at low densities and had low population metabolism relative to other species, consistent with competitive suppression. In contrast, the common wombat Vombatus ursinus appears to be the strongest competitor in this assemblage, being numerically dominant, utilising the greatest proportion of resources, and displaying a relatively broad habitat niche. Habitat modification by fire, including changes in vegetation composition and structure, altered fine-scale partitioning of habitat resources by sympatric herbivore species, and changed the composition of the herbivore community. Faecal pellet counts demonstrated a decrease in herbivore densities, particularly grazers, following the burn, probably associated with reduced ground layer vegetation cover. In contrast, there was an increase in the density of browsers, particularly the swamp wallaby Wallabia bicolor, probably associated with increased shrub cover. Fine-scale partitioning of habitat resources was evident through inter-specific differences in abundance, population metabolism and use of fine-scale vegetation strata prior to and after the burn. Inter-specific differences in feeding strategies and thus resource requirements appear to facilitate coexistence within this assemblage. Further, overlap in fine-scale habitat use appears to be greater between native and introduced species than between native species, suggesting that species with independent evolutionary histories have inherently less resource partitioning than species with long coevolutionary histories. Microhistological diet analysis showed that the diets of the relatively large-bodied introduced hog deer Axis porcinus and relatively small native swamp wallaby consisted mainly of dicots. The diet of the small, introduced European rabbit contained similar proportions of monocots and dicots. The diets of the native eastern grey kangaroo, intermediate in size, and large native common wombats, consisted mainly of monocots, but kangaroos also consumed moderate amounts of dicots. Overlap in food use by the five species was high, particularly between native and introduced species, but also between some native species. Despite a high potential for food resource competition, it appears that coexistence of herbivores on Yanakie Isthmus is facilitated by ecological separation. However, patterns of ecological separation, niche breadth and diet overlap in this guild did not conform well to body-size related predictions: the species with the narrowest and the broadest diet niches were intermediate in size, and the largest species consumed a greater proportion of dicots than did several smaller species. Interactions between intrinsic and extrinsic constraints on diet choice are likely to influence the diet of herbivores on Yanakie Isthmus. This study provides important preliminary insights into herbivore community niche dynamics on Yanakie Isthmus. High diet overlap and overlap in habitat use at some scales between some species, coupled with resource limitation is likely to result in inter-specific competition, particularly given indications of resource limitation through diet niche adjustments, broad niches and high diet overlap between the native and introduced herbivores in this community. Experimental manipulation is required to obtain a mechanistic understanding of species interactions and conclusively demonstrate competition.