Exploring the influence of disturbance history and forest type on an Arboreal Marsupial, the Common Brushtail Possum (Trichosurus Vulpecula), using a Multi-disciplinary Approach

The level of impact of forest disturbance on mammal communities depends on a species’ degree of forest-dependence as well as the intensity and scale (temporal and spatial) of the disturbance. Where land-use practices such as logging, agriculture, and urbanisation alter the habitat characteristics of a forest, a wide range of effects may lead to a reduction in species diversity or the decline of a population. Arboreal marsupials are the faunal group considered to be most at risk from habitat disturbance in Australian forests. Despite this, there have been relatively few studies examining the influence of forest disturbance on arboreal marsupials, particularly in Tasmania. The overall aim of this thesis is to determine whether habitat disturbance (as a result of harvest and associated activities (and wildlife in one site)) and forest type influence the physiology and population biology of an arboreal marsupial, the common brushtail possum (Trichosurus vulpecula), in Tasmanian forests. A multi-dimensional approach integrating physiological, genetic, and ecological studies was employed to address this aim at both the individual and population levels. In particular, this thesis examines potential impacts of habitat disturbance and forest type on physiological parameters of well-being, reproductive parameters, and population parameters (demographic and genetic) in the brushtail possum. Trapping was conducted in spring/summer and autumn/winter during 2007–2008 at six dry Eucalyptus forest sites (three regenerating after harvest and three in relatively undisturbed forest) in southeast Tasmania and four wet Eucalyptus forest sites (two regenerating after harvest and two in relatively undisturbed forest) in northeast Tasmania. Disturbed sites were 4–11 years post harvest. All sites were embedded within a matrix of mature or older-aged regenerating forest. Vegetation and structural attributes of the study site and the land immediately surrounding it were assessed. At each site, data were collected on the characteristics of the faunal community (community composition and biodiversity, species abundance, body mass, sex, and breeding success), which included the brushtail possum. Blood samples, tissue biopsies, and late lactation milk samples were collected from brushtail possums to assess physiological well-being, population genetics, and reproductive parameters. Plasma cortisol concentration was assessed via radioimmunoassay as a proxy for adrenal status, white blood cell differential was performed on blood smears as a proxy for generalised immune response, and haematocrit was measured as a proxy for body condition. DNA was extracted from tissue for genetic analyses using microsatellites to explore mating system, genetic diversity, and implications of gene flow between brushtail possum populations. Brushtail possum milk was analysed for lipid, solid, carbohydrate, and protein concentrations. Analysis showed that, in general, habitat structural complexity within sites and age structure of the forest in the surrounding landscape did not vary significantly, indicating that broad resource availability (food and refuge) was equivalent across sites. However, basal area, the number of hollow-bearing trees, and the percentage of mature forest, young forest, and overstorey cover varied with disturbance, while understorey cover and numbers of both tree hollows and hollow-bearing trees varied by forest type. Faunal distribution was dictated by forest type. While most species showed little influence of habitat disturbance, brushtail possums exhibited lower abundance, decreased breeding frequency, and a male-biased adult sex ratio at the disturbed sites. Comparison of physiological parameters across sites suggest that the brushtail possum is physiologically resilient, with no clear influence of habitat disturbance or forest type on adrenal status, generalised immune response, or body condition. There was a subtle difference in fat content of brushtail possum milk, driven primarily by milk composition at one site; this probably reflects site level differences in maternal diet. However, there were no differences in breeding success, distribution of births, and timing of developmental features, survival, or body condition of young across sites. Genetic analysis suggests that there are two populations of brushtail possums influenced by geographic distance, with near-random mating and moderate genetic diversity, across eastern Tasmania. There were no effects of disturbance, as examined in this study, on genetic diversity or mating system. However, disturbance resulted in an altered landscape with decreased female-specific resources (e.g., tree hollows which are necessary for reproduction). Results suggest that the male-biased adult sex ratio observed in brushtail possum populations living in disturbed sites was due to a lower abundance of adult females at these sites, rather than maternal sex allocation, retention of subadult males (e.g., lack of dispersal), or increased immigration of adult males. The outcomes of this study demonstrate the value of using a multi-dimensional approach that integrates physiological, genetic, and ecological investigations of the potential influences of habitat disturbance and forest type on animal species. Such an approach reduces uncertainty about the relationship between disturbance and the response and subsequent recovery of fauna, which is important for the development of effective forest management strategies. Brushtail possums are resilient and able to cope with habitat disturbance at the individual level. However, subtle population-level responses may have implications for population growth and long-term viability of brushtail possums in areas subject to intensive and extensive forest harvesting. These results also illustrate the importance of retaining mature habitat elements in the landscape to allow forest-dependent fauna to recolonise harvested areas and persist into the long term