Eradicating an invasive mammal requires local elimination and reduced reinvasion from an urban source population

Invasive mammal eradications are increasingly attempted across large, complex landscapes. Sequentially controlled management zones can be at risk of reinvasion from adjacent uncontrolled areas, and managers must weigh the relative benefits of ensuring complete elimination from a zone or minimizing reinvasion risk. This is complicated in urban areas, where habitat heterogeneity and a lack of baseline ecological knowledge increase uncertainty. We applied a spatial agent-based model to predict the reinvasion of a well-studied species, the brushtail possum (Trichosurus vulpecula), across an urban area onto a peninsula that is the site of an elimination campaign in Aotearoa New Zealand. We represented fine-scale urban habitat heterogeneity in a land cover layer and tested management scenarios that varied four factors: the density of possums remaining following an elimination attempt, the maintenance trap density on the peninsula, and effort expended toward preventing reinvasion by means of a high-density trap buffer at the peninsula isthmus or control of the source population adjacent to the peninsula. We found that achieving complete elimination on the peninsula was crucial to avoid rapid repopulation. The urban isthmus was predicted to act as a landscape barrier and restrict immigration onto the peninsula, but reliance on this barrier alone would fail to prevent repopulation. In combination, complete elimination, buffer zone, and source population control could reduce the probability of possum repopulation to near zero. Our findings support urban landscape barriers as one tool for sequential invasive mammal elimination but reaffirm that novel methods to expose residual individuals to control will be necessary to secure elimination in management zones. Work to characterize the urban ecology of many invasive mammals is still needed.

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Targeted Mop up and Robust Response Tools Can Achieve and Maintain Possum Freedom on the Mainland

Unfenced sites on mainland New Zealand have long been considered impossible to defend from reinvasion by possums, and are thus unsuitable for eradication. In July 2019, we began eliminating possums from 11,642 ha (including approximately 8700 ha of suitable possum habitat) in South Westland, using alpine rivers and high alpine ranges to minimise reinvasion. Two aerial 1080 (sodium fluoroacetate) applications, each with two pre-feeds, were used. Here, we detail the effort to mop up existing possums and subsequent invaders in the 13 months following the aerial operation. Possums were detected and caught using a motion-activated camera network, traps equipped with automated reporting and a possum search dog. The last probable survivor was eliminated on 29 June 2020, 11 months after the initial removal operation. Subsequently, possums entered the site at a rate of 4 per year. These were detected and removed using the same methods. The initial elimination cost NZD 163.75/ha and ongoing detection and response NZD 15.70/ha annually. We compare costs with possum eradications on islands and ongoing suppression on the mainland.

Spatial ecology of the quokka (Setonix brachyurus) in the southern forests of Western Australia: implications for the maintenance, or restoration, of functional metapopulations

We used radio-telemetry to investigate the home-range size and movement patterns of the quokka (Setonix brachyurus) in the southern forests of Western Australia to assess the ability of animals to move between increasingly segregated habitat patches and to identify implications for metapopulation function. We found that quokkas in this region have a much larger home range (71 ± 5.8 ha) and move larger distances (up to 10 km per night) than previously reported for this species in other regions. Temporal and sex variations in home-range size, overlap and movement patterns provided insights into the social structure, reproductive strategies and resource availability for the species in this part of its range. Quokkas moved up to 14 km between habitat patches, where these patches were connected by dense riparian vegetation. While riparian vegetation was used exclusively for movement between habitat patches, quokkas spent only 40% of their time in this ecotype. The current management paradigm of protecting linear riparian vegetation as habitat for quokkas is important for maintaining habitat connectivity, but is unlikely to meet broader habitat and spatial requirements. Management of preferred habitat as well as riparian corridors is necessary for the maintenance of a functional metapopulation.

Estimating disease survey intensity and wildlife population size from the density of survey devices: Leg-hold traps and the brushtail possum

Wildlife disease surveillance requires accurate information on the proportion of managed populations sampled or their population density, parameters that are typically expensive to measure. However, these parameters can be estimated using spatially explicit modelling of capture probabilities, based on the distribution and deployment times of capture devices, given accurate information on the relationships between these variables. This approach is used in New Zealand's surveillance programme aimed at confirming areas free of bovine tuberculosis (bTB¹) in brushtail possums (Trichosurus vulpecula). However, there is uncertainty about the accuracy of the underpinning parameters characterizing possum trappability (g), given the distance between where a trap is placed and the possum home range centre. Sampling intensity (SI: the percentage of the population sampled during a population survey) and sigma (σ; 95% home range radius/2.45) were measured, using leg-hold traps deployed under a set protocol to standardize survey effort, at four sites containing previously radio- and GPS-collared individuals. Those data were used to derive an estimate of the nightly probability of capture of possums in a trap set at their home range centre (g₀). Those estimates were compared to the standard assumptions currently used as defaults in the day-to-day approach used by bTB managers. Home-range size (and therefore σ) varied widely between sites (range 3.6-49.4 ha), probably largely in response to differences in possum density. Field measured SI also varied widely between sites, and was closely positively correlated with home range size (R² = 0.967; P =  0.017); wide-ranging possums were more trappable than sedentary ones. We found that g₀ was inversely related to σ, but the magnitude of increases in g₀ with declining σ appeared to be insufficient to compensate for the fewer places at which each possum could be trapped when those home ranges were small. SI was, therefore, not constant across sites where a standard survey effort was applied. The assumed relationship between g₀ and σ in the current spatial model may, therefore, need reassessment. The management implication of these result is that the sampling effort required to attain a target sampling intensity is dependant on the target animal density, and for bTB management of possums in New Zealand, is under-estimated by the current default parameters in a model of freedom-from-disease for higher density possum populations.

Brushtail possum (Trichosurus vulpecula) social interactions and their implications for bovine tuberculosis epidemiology

The brushtail possum is the main reservoir of bovine tuberculosis in New Zealand. Disease prevalence is generally higher in males than in females. This has conventionally been assumed due to greater infection rates of males, but recent work has raised the hypothesis that it may instead be driven by survival differences. With bovine tuberculosis transmission among possums most likely occurring between individuals in close proximity, here we analyse social networks built on data from wild possums collared with contact loggers inhabiting a native New Zealand forest, to investigate whether there is mechanistic support for higher male infection rates. Our results revealed that adult female possums were generally just as connected with adult male possums as other adult males are, with male–female connection patterns not being significantly different. This result suggest that the new ‘survivorship’ hypothesis for the sex bias is more likely than the conventional ‘infection rate’ hypothesis.

Movements and habitat preferences of pests help to improve population control: the case of common brushtail possums in a New Zealand dryland ecosystem

BACKGROUND

Introduced brushtail possums are controlled in New Zealand to mitigate their spread of bovine tuberculosis in livestock. Given the low rainfall and extreme variation in seasonal temperatures in dryland areas of the South Island, the habitats of possums in these areas differ in many respects from those in the rest of New Zealand. We investigated the movements and habitat preferences of possums at two dryland sites to identify where they aggregate following population control by using GPS collars and cards chewed by possums. At one site, possum numbers were reduced from high levels by 65%, and at the other site, possums had already been reduced to low levels for some time beforehand but were further reduced to maintain them at low levels. This resulted in different possum densities.

RESULTS

Possum home ranges were about 3 times smaller at the higher-density site, but average ranges expanded by 27% following initial control. Home ranges were already large at the lower-density site but did not expand further after maintenance control. No preference for habitat types was apparent at the higher-density site, but at the lower-density site possums selected rock and shrubby habitats and avoided open grassy areas.

CONCLUSIONS

Home range sizes and habitat preferences were density dependent: the lower the density, the larger was the home range; and habitat preferences were highly variable between individuals, but less so for possums at low density. Preference for shrubs and rocks is likely to benefit population control if population control devices are focused on these habitat types. © 2016 Society of Chemical Industry.

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

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.