Benefits and feasibility of local elimination of possum populations

Introduction of the brushtail possum (Trichosurus vulpecula) to New Zealand has resulted in serious ecological and economic impacts and considerable control efforts. Recovery of possum populations after control occurs through immigration from adjacent areas and breeding of survivors and immigrants. If complete local elimination can be achieved, the recovery of populations will depend solely on immigration and therefore should be substantially slowed (particularly in very large areas). To compare the cost-effectiveness of four control strategies over the long term (60 years), we constructed a deterministic bioeconomic model based on 23 variables describing population characteristics, sizes of the sink (i.e. area controlled) and source (of reinfestation) areas, and costs. Sensitivity analysis showed that the most influential variables related primarily to cost and effectiveness of control, whereas factors describing immigration after control had relatively little influence. When the most influential variables were varied, the model predicted that local elimination of possums followed by ‘perimeter’ control is likely to be a more cost-effective control strategy under most scenarios than the current ‘knockdown-then-maintenance-control’ approach. Possum-control technology and its application have improved greatly in the last three decades such that it now appears that local elimination is, technically, a realistic goal, and is possibly already being achieved occasionally. Constraining factors include unreliable monitoring/detection at ultralow densities, inappropriate selection and use of control options, lack of incentive under the present contracting system, initial cost, contracting capacity, and the future regulatory status of poisons. However, these difficulties can be overcome, facilitating the adoption of long-term local elimination strategies that are better suited to managing possum populations in perpetuity.