Investigating brushtail possum (Trichosurus vulpecula) home-range size determinants in a New Zealand native forest

Abstract

Context The Australian brushtail possums (Trichosurus vulpecula) introduction to New Zealand has exacted a heavy toll on native biodiversity and presented the country with its greatest wildlife reservoir host for bovine tuberculosis (TB). Management efforts to control both possums and TB have been ongoing for decades, and the biology of possums has been studied extensively in Australia and New Zealand over the past 50 years; however, we still do not have a clear understanding of its home-range dynamics. Aims To investigate determinants of home range size by using a uniquely large dataset in the Orongorongo Valley, a highly monitored research area in New Zealand and compare our findings with those of other studies. Methods Possum density was estimated, for subpopulations on four 13-ha cage-trap grids, by the spatially explicit capture–mark–recapture analysis of trapping data from 10 consecutive months. Home ranges were estimated from trap locations using a 100% minimum convex polygon (MCP) method for 348 individuals and analysed with respect to grid, age and sex. Key results Mean (standard error) possum density, estimated as 4.87 (0.19), 6.92 (0.29), 4.08 (0.21) and 4.20 (0.19) ha–1 for the four grids, was significantly negatively correlated with mean MCP home-range size. Grid, age, and the interaction of age and sex were significantly related to home-range size. Older possums had larger home ranges than did younger possums. When ‘juvenile cohort’ and ‘adult cohort’ data were analysed separately, to investigate the significant interaction, males in the ‘adult cohort’ had significantly larger home ranges than did females, with the grid effect still being apparent, whereas neither sex nor grid effects were significant for the ‘juvenile cohort’. Conclusions Our findings indicate that, in addition to density, age and sex are likely to be consistent determinants of possum home-range size, but their influences may be masked in some studies by the complexity of wild-population dynamics. Implications Our findings have strong implications regarding both disease transmission among possums and possum management. The fact that adult males occupy larger home ranges and the understanding that possum home range increases as population density decreases are an indication that males may be the primary drivers of disease transmission in possum populations. The understanding that possum home range increases as population density decreases could be a direct reflection of the ability of TB to persist in the wild that counteracts current management procedures. If individuals, and particularly males, infected with TB can withstand control measures, their ensuing home-range expansion will result in possible bacteria spread in both the expanded area of habitation and new individuals becoming subjected to infection (both immigrant possums and other control survivors). Therefore, managers should consider potential approaches for luring possum males in control operations.

Spatial utilization predicts animal social contact networks are not scale-free

“…We have used connectance , a network characteristic, to estimate the size of the individual's home-range (i.e. 0.4–2.1 ha), which is realistic compared with the observed average home-ranges for possums at our study area of 1.3 ha and 1.8 ha for females and males respectively [ 16 ]. We have generated a suite of random networks and compared their characteristics to empirical data.…”

“…This work was part of a wider study [ 36 ] on how TB persists across the year, and thus was undertaken primarily outside the mating season. It is also known that there are differences in home-range area between males and females, juveniles and adults [ 16 ], and between populations inhabiting different habitats [ 13 , 15 , 16 ]. In particular juveniles have smaller home-ranges which are generally within the maternal home-range.…”

“…The site, made up of mixed native broadleaf-conifer forest, is a long-term study site for the management of TB in possums [ 22 ], and supports moderately high possum densities (approx. 7 ha −1 ) [ 16 ]. Each of four study sites (A, B, C and D) consisted of a ca 13 ha trapping grid, made up of 100 traps at 40 m spacing.…”

“…Possums in New Zealand occur in densities ranging from 0.4 to 12 per hectare, depending on the habitat type [ 13 ], with reasonably well-defined home-ranges of a size negatively correlated with density [ 14 ]. For example, home-range area is commonly between 0.5 and 2 hectares in high density populations in native forest, but can reach up to 100 ha in generally low-density populations in more open pasture areas [ 15 ], with variation also driven by other factors including sex and age [ 16 ]. Wild possums are traditionally considered largely solitary animals [ 17 ]; however, with no social groupings, they often have widely overlapping home-ranges (both between and within sexes), in which interactions among individuals occur [ 18 ].…”

Measuring Home‐Range Changes Following Density Reduction of Australian Brushtail Possum

“…The Australian brushtail possum ( Trichosurus vulpecula ) in New Zealand is an invasive species that is classified as an agricultural and conservation pest because they transmit Mycobacterium bovis (bTB) to livestock and eat pasture, crops, native plants, and native animals (Butcher , Coleman and Caley , Nugent et al , Payton , Sadleir ). Possum home‐range characteristics are likely density‐dependent, with larger home ranges generally observed in lower‐density populations (Whyte et al , Efford et al , Richardson et al , Rouco et al ). Some previous live‐capture trapping and very high frequency (VHF) radio‐tracking studies indicate that home‐range sizes (Clinchy et al , Morgan et al ) and den‐site use (Whyte , Whyte et al ) of possums increase following density reduction.…”