Resource selection by the brushtail possum (Trichosurus vulpecula Kerr) in a mixed red - silver beech (Nothofagus fusca - N. menziesii) forest, north Westland, New Zealand : a multi-scale, GIS-based approach
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Resource selection by the brushtail possum (Trichosurus vulpecula) was studied in a beech (Nothofagus) forest in order to elucidate possum ecology in an extensive forest type lacking extensive possum research. The study was conducted in the Upper Grey Valley, north Westland, on a post-glacial terrace dominated by Nothofagus fusca and Nothofagus menziesii and dissected by a gravel road. Five principal research questions were posed for the study of possums within the site. These were:
- What are the movement parameters of the resident possum population?
- What are the patterns of vegetation heterogeneity?
- What constitutes possum diet?
- What are the relationships between possum diet and vegetation heterogeneity?
- What are the relationships between possum movements and vegetation heterogeneity? Twelve possums were radio-collared and radio-tracked over a year. Home ranges calculated with the kernel method averaged 8.1 ± 1.0 ha (mean ± s.e.), which is large when compared to possum home ranges observed in other continuously forested sites. Range lengths were similarly large (456 ± 42 m). Bi-monthly variation was only displayed with significantly smaller home ranges in winter when compared to autumn. Despite the fact that the study site was generally dominated by N fusca and N menziesii, considerable heterogeneity existed with respect to the possum palatable species within the site. Many palatable species such as Aristotelia serrata, Fuchsia excorticata, Muehlenbeckia australis and Trifolium repens occurred predominately along the roadside, whilst Weinmannia racemosa occurred predominately on steep areas within the forest. TWINSPAN classification of the forest identified three vegetation communities. These were open sites, including the road and swamp areas, steep areas, including the hill and escarpments, and terrace areas. Resource selection by possums within the site was investigated with diet preference analysis, habitat selection analysis and modelling of use of space versus resource variables. Diet preference was analysed by comparing the level of consumption of food types with their level of availability within the site at multiple scales. The top five ranked preferred food types, calculated overall, were, in decreasing rank, Fuchsia excorticata foliage, Carpodetus serratus fruit, Rubus cissoides foliage, Muehlenbeckia australis foliage and Pseudopanax colensoi fruit. Habitat selection analysis indicated that although overall selection was not occurring, possums exhibited a degree of preference towards the terrace community. Modelling of resource variables against use of space indicated that although there was latent variability between individual possums, there was a trend for greater use of areas with high abundance of preferred species. Modelling of pooled possum use of space on an annual scale indicated that use was positively correlated with species richness and negatively correlated with N fusca, Quintinia acutifolia, and Coprosma rotundifolia abundance, which are all unpreferred species. Annual home ranges of possums were found to be negatively correlated with forage quality as defined by preferred species. A model of possum resource selection within the site was postulated to the effect that possums choose their home range location randomly, with respect to vegetation heterogeneity, and then modify the size of their home range to suit nutritional needs rather than modifying range location. The ranging behaviour and resource selection of possums observed in this study have implications for the management of possums in similar forest types. Efficiency of control and monitoring operations may be increased by using a spacing regime derived from possum home ranges. Inferences obtained from habitat selection analysis indicate that stratification of control and monitoring operations is not warranted in similar forest types. The development of extensive models with predictive capabilities for the distribution of possums within the environment has the ability to become a useful tool for possum managers. Further research of potential benefit could include investigation into resource quality and availability, the effect of home range size on residual trap catch (RTC) indices, improving diet assessment techniques and assessing possum preference for non-foliar food types.