Modelling the impacts of wildfire on the viability of metapopulations of the endangered Australian species of arboreal marsupial, Leadbeater's Possum

Catastrophic events such as intensive wildfires have a major effect on the dynamics of some wildlife populations. In this investigation, the computer package ALEX (Analysis of the Likelihood of EXtinction), was used to simulate the impacts of wildfires on the persistence of metapopulations of the endangered species Leadbeater's possum (Gymnobelideus leadbeateri) which is restricted to the montane ash forests of the Central Highlands of Victoria. A range of scenarios was examined. First, the response of G. leadbeateri to fires in hypothetical patches of old growth forest of varying size was modelled. Metapopulation dynamics were then modelled in four existing forest management areas: the O'Shannassy Water Catchment and the Steavenson, Ada and Murrindindi Forest Blocks using GIS-derived forest inventory data on complex spatial arrangements of potentially suitable old growth habitat patches. The impacts of different fire frequencies and the proportion of forest area that was burnt in the Steavenson Forest Block and the O'Shannassy Water Catchment were examined. Finally, the combined impacts of both wildfires and post-fire salvage logging operations on the persistence of populations of G. leadbeateri were assessed. Our analyses indicated that, even in the absence of wildfires, populations of G. leadbeateri are very susceptible to extinction within single isolated habitat patches of 20 ha or less. The probability of persistence approached 100% in patches of 250 ha. The incorporation of the effects of wildfire was predicted to have a major negative impact on isolated populations of G. leadbeateri. In these cases, the probability of population extinction remained above 60%, even when a single patch of 1200 ha of old growth forest was modelled. In the absence of wildfires, there was a low probability of extinction of G. leadbeateri in the O'Shannassy Water Catchment where very large patches of old growth forest presently exist. The risk of extinction of the species was significantly higher in the Murrindindi and Ada Forest Blocks where there are lower total areas of, and significantly smaller, suitable habitat patches. Wildfires resulted in an increase in the predicted probability of metapopulation extinction in the four areas that were targeted for study. An investigation of the Steavenson Forest Block and the O'Shannassy Water Catchment revealed that the predicted values for the probability of extinction were sensitive to inter-relationships between the frequency of fires and the proportion of habitat patches that were burnt during a given fire event. The probability of extinction of G. leadbeateri was predicted to be lowest when there were frequent fires that burnt only relatively small areas of a given forest block. Conversely, the results of our analyses suggested that populations of the species are vulnerable to infrequent but intensive conflagrations that burnt a large proportion of the forest. The results of the suite of analyses completed in this study suggest that four management strategies will be important for the long-term conservation of G. leadbeateri. (1) Attempts to suppress wildfires should be maintained as even the largest remaining areas of old growth forest may be susceptible to being burnt by repeated widespread wildfires that could result in localised and/or global extinction of the species. (2) Because the probability of population persistence is greatest in those areas that support more extensive stands of old growth forest, it will be important to grow relatively large patches of existing regrowth forest (over 50 ha) through to ecological maturity. This will be particularly important in some wood production forest blocks where there are only limited areas of old growth forest. (3) Patches that are set aside for the conservation of G. leadbeateri should be spatially separated to minimise the risk that all of the reserved areas in a region are destroyed in a single major fire event. (4) Salvage logging operations should be excluded from stands of old growth forest and reserved areas that are burnt in a wildfire. This is because such activities can have a major negative impact of the development of suitable habitat for G. leadbeateri.