Improving possum ground control in the southern South Island
Brushtail possums are the main wildlife reservoir of bovine tuberculosis (TB) in New Zealand, and are primarily responsible for the transmission of TB from wildlife to livestock. Intensive control of infected possum populations undertaken under the New Zealand National Bovine Tuberculosis Pest Management Strategy has led to substantial reductions in the number of infected cattle and deer herds. However, such possum control costs about $80 million a year. In addition, possum grazing reduces farm production by about $35 million a year. Improving the efficacy of possum controls is therefore a national priority.
Nineteen percent of New Zealand’s land area is dryland habitat, consisting mostly of grassland and shrubland. Such lands include some of the most threatened native ecosystems and species in New Zealand, and are poorly represented in reserves. Possums do not use these landscapes randomly but instead concentrate their activities in particular habitats such as shrublands and rocky outcrops. This being so, habitats such as open grassland could be omitted from ground-based possum control operations, with little loss of control efficiency. In addition, possums may vary their movements and use of habitats throughout the year depending on the availability of favoured foods and their breeding behaviour. Understanding this seasonality will help managers decide on the timing and location of possum control, and increase control efficiency. Further, habitat use by possums in uncontrolled, high-density populations may differ from that of possums at low density following control. This can affect the way managers target control for initial versus maintenance (follow-up) operations. For example, after control, surviving possums may concentrate their home ranges in the very best habitat, or they may aggregate depending on the availability of surviving conspecifics. On the other hand, post-control possum distribution may simply reflect the level of difficulty control teams face when accessing different terrains.
This project enables spatially-targeted control of possums to be applied to southern South Island (SSI) habitats by providing critical information on habitat types preferred by possums and its seasonality of use.
Carlos Rouco and Grant Norbury monitored an uncontrolled possum population living at a range of elevations in a diverse array of rock outcrop, grass and shrub habitats between Alexandra and Cromwell. The site was representative of typical dryland habitat across the SSI. The team set out 260 chewcards spaced at 100-m intervals over 900 ha for 4 days. The habitat type at each card location was derived from a habitat map (Fig. 1A) based on aerial photographs using ArcGIS 10 software (ESRI, Redlands, California, USA). Cards were deployed during summer, autumn, winter and spring for a year, before the possum population was controlled to 30% of its pre-control density (i.e. c. 70% decline). Chewcards were then redeployed for eight seasons (24 months) and the data analysed using Jacobs' (1974) index to quantify possums’ preference for each habitat type.
The chewcard data showed that possums in the SSI prefer dense shrub (75–100% coverage), followed by less dense shrub (50–75% coverage) and rock outcrops (Fig. 2). Improved pastures were avoided. Before control, preference for shrubs and rock outcrops was most evident in winter when there was unusually high snow cover. After control, preference for all shrublands and rock outcrops increased in all seasons, presumably because lowering the density of possums reduced competition for preferred habitats. This greater preference for shrub and rock habitats should benefit maintenance control operations provided contractors target these habitats (Fig.1B).
Possum control takes place over 3.5 million ha of Vector Risk Area (infected wildlife habitat) in the SSI, therefore more targeted control in preferred habitat will have substantial cost savings.
This work is part of a larger project funded by the former Animal Health Board entitled Improved efficiency and effectiveness of ground-based possum control and monitoring in the southern South Island (R-10737).
Carlos Rouco, Grant Norbury
Fig. 1 A. Map of the study site showing different habitat types. Red dots represent chewcard locations. B. Map showing the habitats most preferred by possums (based on chewcard records) and likely to yield the most possums in maintenance possum-control operations.
Fig. 2 Habitat selection by possums, indicated by chewcards after one night before and after control at the study site. Positive values indicate habitat preference and negative values indicate habitat avoidance relative to availability. There was high snow cover during the winter before control (blue diamonds, pre-control).
