Abstract
Invasive species are often favoured in fragmented, highly-modified, human-dominated landscapes such as urban areas. Because successful invasive urban adapters can occupy habitat that is quite different from that in their original range, effective management programmes for invasive species in urban areas require an understanding of distribution, habitat and resource requirements at a local scale that is tailored to the fine-scale heterogeneity typical of urban landscapes. The common brushtail possum (Trichosurus vulpecula) is one of New Zealand’s most destructive invasive pest species. As brushtail possums traditionally occupy forest habitat, control in New Zealand has focussed on rural and forest habitats, and forest fragments in cities. However, as successful urban adapters, possums may be occupying a wider range of habitats. Here we use site occupancy methods to determine the distribution of brushtail possums across five distinguishable urban habitat types during summer, which is when possums have the greatest impacts on breeding birds. We collected data on possum presence/absence and habitat characteristics, including possible sources of supplementary food (fruit trees, vegetable gardens, compost heaps), and the availability of forest fragments from 150 survey locations. Predictive distribution models constructed using the programme PRESENCE revealed that while occupancy rates were highest in forest fragments, possums were still present across a large proportion of residential habitat with occupancy decreasing as housing density increased and green cover decreased. The presence of supplementary food sources was important in predicting possum occupancy, which may reflect the high nutritional value of these food types. Additionally, occupancy decreased as the proportion of forest fragment decreased, indicating the importance of forest fragments in determining possum distribution. Control operations to protect native birds from possum predation in cities should include well-vegetated residential areas; these modified habitats not only support possums but provide a source for reinvasion of fragments.
Highlights
Within and across environments the spatial distribution of animal species reflects the availability and distribution of speciesspecific resources and the ability of species to reach and exploit them [1,2,3,4,5,6,7]
Of the 30 survey locations for each habitat type, possums were detected at most forest locations (28), just under half of the amenity (13) and Residential 1 (Res 1) (12) locations dropping to 6 locations in Residential 2 (Res 2) and 0 in Residential 3 (Res 3) locations
Habitat occupancy of the common brushtail possum in the urban Dunedin landscape during summer reflected the heterogeneous distribution of resources and patch connectivity, emphasising the influence that landscape structure and resulting resource availability have on the distribution of this species [57]
Summary
Within and across environments the spatial distribution of animal species reflects the availability and distribution of speciesspecific resources and the ability of species to reach and exploit them [1,2,3,4,5,6,7]. Landscape structure determines habitat availability and connectivity between resources, influencing species distributions and occupancy of habitat patches [8,9]. Landscape structure is especially significant in fragmented habitats where important resources and habitat patches become heterogeneously distributed across the environment [7,10,11,12]. Fragmentation of landscapes results in an overall loss of natural habitat, both spatially and structurally, and species which are sensitive to urbanisation (‘urban avoiders’) [5] may be limited to small patches of natural habitat isolated within a matrix of modified habitats [9,13]. The impacts of habitat fragmentation on animal distributions are significant in urban landscapes, which are becoming increasingly prevalent as the world’s human population continues to grow [5,17,18]. The ecosystem has been transformed into hybrid systems consisting of fragments of original habitat that are often small and embedded within a matrix of highly-modified and human-dominated habitat [14]
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