Modelling the inter-relationships between habitat patchiness, dispersal capability and metapopulation persistence of the endangered species, Leadbeater's possum, in south-eastern Australia

Abstract

A computer simulation model was used to derive estimates of the probability of extinction of populations of the endangered species, Leadbeater's Possum (Gymnobelideus leadbeuteri), inhabiting ensembles of habitat patches within two wood production forest blocks in central Victoria, south-eastern Australia. Data on the habitat patches were extracted from forest inventory information that had been captured in the database of a Geographic Information System (GIs). Our analyses focussed on a range of issues associated with the size, number and spatial configuration of patches of potentially suitable habitat that occur within the Ada and Steavenson Forest Blocks. The sensitivity of extinction risks in these two areas to variations in the movement capability of G. leadbeateri was also examined. Our analyses highlighted major differences in the likelihood of persistence of populations of G. leadbeuteri between the Ada and Steavenson Forest Blocks. These were attributed to differences in the spatial distribution and size of remnant old growth habitat patches as well as the impacts of wildfires. In addition, simulation modelling revealed a different relative contribution of various individual patches, and ensembles of patches, to metapopulation persistence in the two study areas. In those scenarios for the Ada Forest Block in which the impacts of wildfires were not modelled, our analyses indicated that a few relatively large, linked patches were crucial for the persistence of the species and their loss elevated estimates of the probability of extinction to almost 100%. A different outcome was recorded from simulations of the Steavenson Forest Block which, in comparison with the Ada Forest Block, is characterized by larger and more numerous areas of well connected patches of old growth forest and where we included the impacts of wildfires in the analysis. In this case, metapopulation persistence was not reliant on any single patch, or small set of patches, of old growth forest. We found that in some circumstances the probability that a patch is occupied whilst the metapopulation is extant may be a good measure of its value for metapopulation viability. Another important outcome from our analyses was that estimates of extinction probability were influenced both by the size and the spatial arrangement of habitat patches. This result emphasizes the importance for modelling metapopulation dynamics of accurate spatial information on habitat patchiness, such as the data used in this study which were derived from a GIs. The values for the predicted probability of extinction were significantly influenced by a range of complex interacting factors including: (I) the occurrence and extent of wildfires, (2) the addition of logging exclusion areas such as forest on steep and rocky terrain to create a larger and more complex patch structure, (3) estimates of the quality of the habitat within the logging exclusion areas, and (4) the movement capability of G. leadbeuteri. Very high values for the probability of extinction of populations of G. leadbeateri were recorded from many of the simulations of the Ada and Steavenson Forest Blocks. This finding is the result of the limited areas of suitable old growth forest habitat for the species in the two areas that were targeted for analysis. Hence, there appears to be insufficient old growth forest in either of the two forest blocks to be confident that they will support populations of G. leadbeateri in the long-term, particularly if a wildfire were to occur in the next 150 years. The results of sensitivity analyses indicated that estimates of the probability of extinction of G. leadbeuteri varied considerably in response to differences in the values for movement capability modelled. This highlighted the need for data on the dispersal behaviour of the species.