The koomal (Trichosurus vulpecula hypoleucus) is the only (of six) subspecies of common brushtail possum with a conservation listing (‘near-threatened’). It is a smaller (1-2 kg), more timid subspecies of T. vulpecula, inhabiting the southwest of Western Australia. The largest forest inhabited by koomal is the jarrah forest, a dry sclerophyll forest set on ancient, nutrient-depleted soils. Over half of the forest is managed for timber harvesting and since 1994, large parts of the forest have been baited for red foxes (Vulpes vulpes). In the context of koomal conservation I investigated whether koomal populations in the northern jarrah forest were limited from top-down and/or bottom-up processes. Specifically, I examined the mechanisms by which food availability and predation influenced koomal abundance, site occupancy, body condition, reproductive output and offspring sex ratios. Koomal may also be limited by den availability if current forest management practices are inadequate at providing sufficient hollow trees for koomal. Thus, I also provided information on koomal home range size, level of territoriality, den tree use and den tree characteristics, which is essential for future evaluations of whether den availability limits koomal. The effects of introduced predators on koomal populations may be exacerbated if koomal fail to recognise them as a threat. Lastly, I explored whether koomal recognised the introduced foxes and feral cats (Felis catus), and the native chuditch (Dasyurus geoffroii) as a threat, and possible antipredator responses koomal used against these predators. Results revealed that several koomal populations in the northern jarrah forest were either locally extinct or present at low densities. Low food availability was associated with decreased koomal site occupancy and body weight and a bias of offspring sex-ratios towards males. Conversely, predators appeared to have no influence on koomal abundance, site occupancy or reproductive output, although increased cat activity reduced koomal body condition. Chuditch were sometimes present at high densities (up to an order of magnitude higher than the introduced predators) while foxes and cats were generally scarce. Thus, chuditch appeared not to be significant predators of koomal in the northern jarrah forest. Foxes and cats have been shown by previous studies to have significant effects on T. vulpecula populations. Thus, their minimum impacts observed in this study are likely the result of foxes and cats being present at densities that were too low to have substantial effects on koomal. Furthermore, koomal may have also effectively minimised their predation threat from the three predators examined. Koomal recognised all three predators as a threat and shifted to dense cover when faced with high predation risk (i.e. multiple predator species present at high densities). The use of cover has been previously shown to be an effective response used by mice (Mus domesticus) to minimise their predation risk from foxes, cats and chuditch. Koomal also remained close to trees while on the ground, probably as a first line of defence against predators. Koomal appeared to maintain exclusive core territories from same sex conspecifics, probably to protect their dens. Males had larger home ranges than females. Den trees used by koomal largely matched trees currently reserved as habitat for hollow-dependent species within logged areas of the forest (i.e. habitat trees), based on the 2004-2013 Forest Management Prescriptions. Den trees used by koomal also had two additional characteristics: they were preferentially of wandoo (Eucalyptus wandoo) species and had some degree of canopy connectivity. If den availability is found to be limiting for koomal, additional trees to be preserved for koomal conservation should incorporate these additional characteristics. Overall, koomal populations appeared to be limited primarily by food availability and to a lesser extent by cats. Effective koomal conservation requires efforts to preserve and enhance ground plant resources and an integrated management of both foxes and cats, which includes monitoring of these introduced predators, and of their effects on native prey. Further research is also required on alternative factors that may limit koomal populations and that were not addressed in this study including the effects of logging practices and the cumulative effects of mining practices in the jarrah forest.