Quantifying Short-Term Foraging Movements in a Marsupial Pest to Improve Targeted Lethal Control and Disease Surveillance

Abstract

In New Zealand, the introduced marsupial brushtail possum (Trichosurus vulpecula) is a pest species subject to control measures, primarily to limit its ability to transmit bovine tuberculosis (TB) to livestock and for conservation protection. To better define parameters for targeted possum control and TB surveillance, we here applied a novel approach to analyzing GPS data obtained from 44 possums fitted with radio-tracking collars, producing estimates of the animals’ short-term nocturnal foraging patterns based on 1-, 3- or 5-nights’ contiguous data. Studies were conducted within two semi-arid montane regions of New Zealand’s South Island High Country: these regions support low-density possum populations (<2 possums/ha) in which the animals’ home ranges are on average larger than in high-density populations in forested habitat. Possum foraging range width (FRW) estimates increased with increasing monitoring periods, from 150-200m based on a single night’s movement data to 300-400m based on 5 nights’ data. The largest average FRW estimates were recorded in winter and spring, and the smallest in summer. The results suggest that traps or poison-bait stations (for lethal control) or monitoring devices (for TB surveillance), set for > 3 consecutive nights at 150m interval spacings, would likely place >95% of the possums in this type of habitat at risk of encountering these devices, year-round. Modelling control efficacy against operational expenditure, based on these estimations, identified the relative cost-effectiveness of various strategies that could be applied to a typical aerial poisoning operation, to reduce the ongoing TB vectorial risk that possums pose in the High Country regions. These habitat-specific findings are likely to be more relevant than the conventional pest control and monitoring methodologies developed for possums in their more typical forested habitat.

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

“…Possums in southern South Island dryland habitats occur at relatively low densities, and range over large areas, compared with possums in other habitat types in New Zealand, such as native forest and farmland . Larger possum home ranges in non‐forested grassland habitats, compared with favoured forest environments, have been previously documented, but the ranges we recorded, particularly at Poolburn, were substantially larger again. We recorded home ranges of up to 100–200 ha for some possums.…”

“…Home range expansion may reflect reduced competition for resources, given that dominant possums often prevent subordinates from accessing limited resources, or it may be socially driven by reduced interference competition (aggressive interactions) or the need to maintain contacts with more sparsely distributed conspecifics (Nugent G, private communication). Home range expansion is likely to benefit maintenance control because possums will have greater probability of encountering control devices …”

Field Trial of an Aerially-Distributed Tuberculosis Vaccine in a Low-Density Wildlife Population of Brushtail Possums (Trichosurus vulpecula)

“…bovis infection. Previous cross-sectional surveys in the vicinity had recorded TB prevalences of 1–3% in possums since the mid-2000s [28,32]. A detailed wildlife disease survey conducted in the study area 4 years prior to the present study recorded a mean prevalence of gross TB lesions among resident possums of 2.7% (n = 450 [28]).…”

“…That is unlikely because our deployment rate (= 300 BCG baits/ km 2 ) was substantially higher than maximum deployment rates of up to 150 baits/km 2 used in aerial rabies vaccination campaigns [56–60], while modelling probabilities of possum encounters with poison baits or traps in this type of habitat [32] leaves little doubt that possums would have encountered the BCG baits. Second, the supply of baits could have been depleted before all possums were able to encounter them.…”

“…Captured possums were euthanized and necropsied for TB following the procedure in the supplementary material (S1 File). At the same time, a sample of feral pigs (which are highly sensitive ‘sentinel host’ indicators of TB presence in possums [37]) was obtained from the study area by shooting, and these were also necropsied for TB diagnosis [32,33]. …”