Habitat preferences of the common brushtail possum (Trichosurus vulpecula) in an urban environment

Abstract

The common brushtail possum (Trichosurus vulpecula) is one of the most widely distributed and abundant native mammals in Australia. Despite its decline in woodland and forest habitats, its classification as a generalist species – having a broad diet and habitat requirements – has facilitated its success in colonising the urban and semi-urban regions of Australia. However, proliferation into urban environments has increased interactions with humans, particularly when ceiling and roof cavities are used as den sites. Efforts to deter the common brushtail possum from using buildings as dens is complicated by its protected status in Australia. Furthermore, existing management approaches have been subjected to little empirical evaluation, and the constant recurrence of the human-possum conflict suggests the need for alternative strategies. One option that has not received much attention is the manipulation of habitat in the vicinity of buildings to deter the common brushtail possum. In this research, I aim to determine whether the common brushtail possum can be discouraged from using buildings as den sites by manipulating habitats within the home-range of individuals that occupy buildings. Specifically, my research questions are: (1) what are the home-range movements and den use of the common brushtail possum in an urban area; and (2) does the common brushtail possum avoid certain habitat types in urban environments. To investigate the first research question, thirteen common brushtail possums were trapped and tracked on the Acton campus of the Australian National University (ANU). Each individual was radio-tracked for approximately four to eight weeks. The home-range movements and den sites of individuals were determined through day and night-time radio-telemetry. Four individuals were excluded from subsequent analysis because of premature collar-detachment. I found the average (± standard error) home-range (90% Kernel Density Estimate) and core-range (50% Kernel Density Estimate) to be 1.25±0.25ha and 0.33±0.06ha across the nine remaining individuals. The number of den sites ranged from one to three across all individuals, and 15 den sites were identified in total: four located on trees and the remainder within anthropogenic structures. Anthropogenic den sites included ventilation ducts, drains underneath roads, hollow spaces in roofs, ceilings and facades of buildings, water mains, air-exchange units, building ledges and in spaces underneath buildings. The second research question was investigated in three spatial scales: home-range, plot and tree-level. To examine habitat preferences at the home-range scale, aerial imagery of the study area was digitised into habitat classes (i.e., native canopy cover, exotic canopy cover, ground cover, buildings, roads, bare ground). The proportion of habitat classes used within home and core-range were compared relative to availability within the study area across all individuals. I found that all habitats were used proportionally relative to availability by the common brushtail possum with the exception of roads in both the home and core-range. Observations at the plot and tree-level yielded more significant insights regarding the fine-scale habitat preferences of the common brushtail possum. To examine preferences at the plot-scale, 48 plots (50x20m or 0.1ha) were established across four broad vegetation types (e.g., sparse native, sparse exotic, dense native and dense exotic) within the study area. The common brushtail possum was surveyed within plots using camera trapping and spotlighting. Camera traps were deployed for four to seven nights to determine plot-level occupancy, and spotlighting was conducted after dusk for six weeks to assess plot-level abundance. Eighteen potential explanatory variables were recorded in each plot to determine which were associated with occupancy and abundance. Using Generalised Linear Models (GLMs), I found that the common brushtail possum was more likely to occur at plots of a higher total cover of canopy and mid-storey vegetation. The species was more likely to be abundant on plots with: 1) a higher woody cover within 50m of plots, 2) a higher mid-storey vegetation cover, 3) a higher plot species richness; and 4) an exotic tree cover dominance. Observations at the tree-level provided further insights on the habitat preferences by this species. Using Generalised Linear Mixed Models (GLMMs), I found that common brushtail possum was more likely to be abundant on trees of exotic species origin and of a larger diameter at breast height. Selection of tree genera by the common brushtail possum was examined using a Chi-square test of independence. Betula, Cupressus, Populus, Quercus and “other” tree genera were positively associated with occupancy and Casuarina, Eucalyptus, Gleditsia and Ulmus were negatively associated with occupancy by the common brushtail possum in the study area. My results suggest that habitat manipulation is a viable option for discouraging the common brushtail possum from using buildings as den sites. My results for the first research question suggest that: 1) the immediate 0.33ha around a building should be prioritised for habitat manipulation; and 2) remediation of access points should be undertaken and tree guards should be used to deny access to buildings. My results for the second research question suggest that: 1) native flora, particularly from Eucalyptus spp., should be prioritised for use around buildings; 2) a native monoculture (i.e. single species) should be planted around buildings if possible; 3) if monoculture is not feasible, flora diversity around buildings should prioritise the use of native flora from the Myrtaceae family and/or the use of exotic flora with thorns, spines and prickles; 4) exotic tree genera should be avoided in the immediate vicinity around buildings unless proven to be avoided by the common brushtail possum; and 5) a low total canopy and mid-storey cover should be maintained around buildings. This study has contributed to the limited literature regarding the ecology of urban-dwelling common brushtail possums in Australia and is the first to report both home and core-range estimates for both sexes within urban landscapes. This study has extended the recommendations made in previous studies by providing scale and specificity for implementation and experimentation and has also identified knowledge gaps for future investigation. Show more