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

摘要

The level of impact of forest disturbance on mammal communities depends on a species’uddegree of forest-dependence as well as the intensity and scale (temporal and spatial) of theuddisturbance. Where land-use practices such as logging, agriculture, and urbanisation alterudthe habitat characteristics of a forest, a wide range of effects may lead to a reduction inudspecies diversity or the decline of a population. Arboreal marsupials are the faunal groupudconsidered to be most at risk from habitat disturbance in Australian forests. Despite this,udthere have been relatively few studies examining the influence of forest disturbance onudarboreal marsupials, particularly in Tasmania.udThe overall aim of this thesis is to determine whether habitat disturbance (as a result ofudharvest and associated activities (and wildlife in one site)) and forest type influence theudphysiology and population biology of an arboreal marsupial, the common brushtail possumud(Trichosurus vulpecula), in Tasmanian forests. A multi-dimensional approach integratingudphysiological, genetic, and ecological studies was employed to address this aim at both theudindividual and population levels. In particular, this thesis examines potential impacts ofudhabitat disturbance and forest type on physiological parameters of well-being, reproductiveudparameters, and population parameters (demographic and genetic) in the brushtail possum.udTrapping was conducted in spring/summer and autumn/winter during 2007–2008 at six dryudEucalyptus forest sites (three regenerating after harvest and three in relatively undisturbedudforest) in southeast Tasmania and four wet Eucalyptus forest sites (two regenerating afterudharvest and two in relatively undisturbed forest) in northeast Tasmania. Disturbed sites wereud4–11 years post harvest. All sites were embedded within a matrix of mature or older-agedudregenerating forest. Vegetation and structural attributes of the study site and the landudimmediately surrounding it were assessed. At each site, data were collected on theudcharacteristics of the faunal community (community composition and biodiversity, speciesudabundance, body mass, sex, and breeding success), which included the brushtail possum.udBlood samples, tissue biopsies, and late lactation milk samples were collected from brushtailudpossums to assess physiological well-being, population genetics, and reproductive parameters. Plasma cortisol concentration was assessed via radioimmunoassay as a proxyudfor adrenal status, white blood cell differential was performed on blood smears as a proxyudfor generalised immune response, and haematocrit was measured as a proxy for bodyudcondition. DNA was extracted from tissue for genetic analyses using microsatellites toudexplore mating system, genetic diversity, and implications of gene flow between brushtailudpossum populations. Brushtail possum milk was analysed for lipid, solid, carbohydrate, andudprotein concentrations.udAnalysis showed that, in general, habitat structural complexity within sites and age structureudof the forest in the surrounding landscape did not vary significantly, indicating that broadudresource availability (food and refuge) was equivalent across sites. However, basal area, theudnumber of hollow-bearing trees, and the percentage of mature forest, young forest, andudoverstorey cover varied with disturbance, while understorey cover and numbers of both treeudhollows and hollow-bearing trees varied by forest type. Faunal distribution was dictated byudforest type. While most species showed little influence of habitat disturbance, brushtailudpossums exhibited lower abundance, decreased breeding frequency, and a male-biased adultudsex ratio at the disturbed sites.udComparison of physiological parameters across sites suggest that the brushtail possum isudphysiologically resilient, with no clear influence of habitat disturbance or forest type onudadrenal status, generalised immune response, or body condition. There was a subtleuddifference in fat content of brushtail possum milk, driven primarily by milk composition atudone site; this probably reflects site level differences in maternal diet. However, there wereudno differences in breeding success, distribution of births, and timing of developmentaludfeatures, survival, or body condition of young across sites.udGenetic analysis suggests that there are two populations of brushtail possums influenced byudgeographic distance, with near-random mating and moderate genetic diversity, acrossudeastern Tasmania. There were no effects of disturbance, as examined in this study, onudgenetic diversity or mating system. However, disturbance resulted in an altered landscapeudwith decreased female-specific resources (e.g., tree hollows which are necessary forudreproduction). 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 femalesudat these sites, rather than maternal sex allocation, retention of subadult males (e.g., lack ofuddispersal), or increased immigration of adult males.udThe outcomes of this study demonstrate the value of using a multi-dimensional approachudthat integrates physiological, genetic, and ecological investigations of the potentialudinfluences of habitat disturbance and forest type on animal species. Such an approachudreduces uncertainty about the relationship between disturbance and the response andudsubsequent recovery of fauna, which is important for the development of effective forestudmanagement strategies. Brushtail possums are resilient and able to cope with habitatuddisturbance at the individual level. However, subtle population-level responses may haveudimplications for population growth and long-term viability of brushtail possums in areasudsubject to intensive and extensive forest harvesting. These results also illustrate theudimportance of retaining mature habitat elements in the landscape to allow forest-dependentudfauna to recolonise harvested areas and persist into the long term.ud