A SIMULATION STUDY OF THE IMPACTS OF POPULATION SUBDIVISION ON THE MOUNTAIN BRUSHTAIL POSSUM TRICHOSURUS-CANINUS OGILBY (PHALANGERIDAE, MARSUPIALIA) IN SOUTH-EASTERN AUSTRALIA .1. DEMOGRAPHIC STABILITY AND POPULATION PERSISTENCE

Forest fragmentation and the associated problem of population subdivis ion is a major factor threatening the longterm persistence of many for est-dependent taxa. VORTEX, a computer program for population viabilit y analysis (PVA), was used to simulate the impacts of fragmentation an d subdivision on the demography of populations of the mountain brushta il possum Trichosurus caninus Ogilby, which is a forest-dependent spec ies of arboreal marsupial that inhabits wet sclerophyll forests and ra inforests in eastern Australia. In this study, hypothetical population s of 100, 200 and 400 animals were partitioned into one to 10 subpopul ations that were, in turn, linked by varying rates of inter-patch migr ation. These levels of population subdivision may be typical in extens ive areas of wood production montane ash forest in central Victoria wh ere there are limited and generally very small patches of old growth f orest (< 10 ha) that Will become increasingly important refugia for ma ny species of hollow-dependent fauna such as T. caninus. The results o f the analysis demonstrated that the impacts of population subdivision on population demography were usually negative. In almost all of the scenarios, a single population was more stable than an ensemble of sub populations of comparable initial size, irrespective of the rate of in ter-patch migration. There were marked differences in the behaviour of subpopulations initially comprised of 10 and 20 T. caninus and those comprised of approximately 40 or move animals. Small subpopulations we re predicted to exhibit high inherent levels of population instability , probably as a result of factors such as demographic stochasticity an d inbreeding depression, and were characterised by high rates of extin ction, small or negative values for subpopulation and metapopulation g rowth, and large fluctuations in population size. The effects of the d ispersal or migration of animals between subpopulations on their persi stence were complex and related to the final size of the populations t hat resulted from the process of subdivision. Amongst ensembles of sub populations which initially comprised 40 or move animals, increasing t he rate of dispersal had a range of positive effects on subpopulation and metapopulation dynamics. These included: higher rates of and small er fluctuations in, subpopulation and metapopulation growth, and lower probabilities of subpopulation and metapopulation extinction. Many of these trends were reversed when the metapopulation structure was comp rised initially of small subpopulations of 10-20 T. caninus. In such c ases, accelerated rates of migration were predicted to have a detrimen tal impact on most measures of population demography including the rat e of extinction. The results of the study demonstrated the potential v alues of PVA, both in examining the role of dispersal in metapopulatio n dynamics and studying changes in population demography and persisten ce associated with population subdivision.