An important prerequisite for the conservation management of forest-dependent mammals is a sound understanding of variations in species-habitat relationships within different parts of their geographic range. This is particularly important at the edge of the range where information on ecology and physiology is often limited. Furthermore, understanding species-habitat relationships, and their spatial and temporal variability, is essential for tree-dependent species because specialization on forest or woodland resources can restrict the movements and dispersal capacity of such species. The overall aim of this thesis was to investigate the ecological and physiological responses of the koala (Phascolarctos cinereus) to various environmental variables at the semi-arid margins of its geographic range in Queensland. Species at their range edge are predicted to be more vulnerable to climate change and climate extremes because they may already be close to their physiological limits. This research advances our understanding of the movement ecology, resource selection and physiological stress of an arboreal marsupial in a highly dynamic, semi-arid environment such as semi-arid Australia. Results will underpin the development of effective and scientifically informed management strategies for western koala populations.
The research was conducted across three different biogeographic regions within the semi-arid landscapes of southwest Queensland, Australia. Koalas in this area are at the western limits of their geographic range and form a trailing edge population. Three facets of koala ecology and physiology were examined: home range size and movement patterns, physiological stress levels, and dietary composition. Surveys were conducted at sites along transects collecting fresh faecal pellets to enable measurement of cortisol metabolites and assessment of diet. Twenty-one koalas were GPS-collared within a subset of these sites to assess movement patterns and resource selection. Vegetation surveys were carried out at each site to provide information on the availability of tree species, and data were obtained on climate and other environmental variables. Home ranges, fractal dimensions (tortuosity of movement paths) and daily distances travelled were calculated, and structural equation modelling and multiple regression were used to quantify the potential influence of environmental variables on koala movement behaviour. ANOVA was used to compare the average time spent in riparian habitat for each bioregion. Methods for measuring concentrations of cortisol metabolites in faecal pellets of captive koalas following adrenocorticotropic hormone (ACTH) treatment were validated prior to the assessment of faecal cortisol in wild koala populations. Mixed effects modelling and regression analysis were used to relate the physiological stress of wild koalas to environmental variables. Diet composition was calculated from faecal cuticle analysis, and regression was used to assess variations in diet between drought and post-flood conditions.
In this research I demonstrated that koalas living close to the arid periphery of their distribution required larger home ranges and showed higher habitat selectivity (riparian habitats) than those living closer to the centre of their distribution. In semi-arid regions, home range sizes were in general smaller and movement patterns more tortuous with higher annual rainfall and more freestanding water. However, in the short-term (< 2 months) higher rainfall increased male home range size and resulted in larger, more linear distances travelled. This is likely to reflect breeding and dispersal behaviours during good conditions. Faecal cortisol metabolites (FCM) concentrations were also related to rainfall, with higher FCM levels associated with lower rainfall levels. This is consistent with physiological stress when moisture levels are low, including during drought conditions. In particular, rainfall from the previous two months had the strongest influence on FCM levels. The delay of the new growth after a rainfall causes the consequential time lag of about two months between the rainfall and the maximum reduction in FCM levels in koalas. Low minimum temperatures also increased FCM concentrations, and these effects were greater during drought conditions. Diet changed between drought and post-flood conditions, with diets during drought being mainly composed of species that are likely to have higher leaf moisture content.
The dependencies of movement patterns, physiological stress and diet with moisture availability (including drought, rainfall, leaf moisture and freestanding water) are likely to affect population dynamics, which highlights conservation concerns for the continued survival of inland koala populations with decreased rainfall and increased variability from climate change. To ensure the continued survival of these populations in extreme weather, the most important actions for koala conservation within these semi-arid landscapes are the maintenance of the quality and quantity of riparian habitats, expanding the availability and accessibility of freestanding water, such as around farm dams, and providing specific trees as food resources.
This study advances our understanding of the movement ecology, resource selection and physiological stress of koalas in a highly dynamic, semi-arid environment in Australia. The results demonstrate the importance of integrating physiological assessments into ecological studies to identify stressors that have the potential to compromise the long-term survival of threatened species. The differences in movement patterns and resource use within the different koala populations, in response to rainfall and water availability, shows that we cannot rely upon behavioural traits of animals located towards the core of their geographic range to make assumptions about the movements and resource selection at the semi-arid edge of their range. Therefore, for conservation actions to be effective, it is imperative to distinguish differences between edge and core populations, particularly for threatened species such as the koala.