Weed invasion trajectories: How they inform CBAs

Abstract

Weeds proposed for management under a Regional Pest Management Plan are typically species that are of limited current distribution (and hence limited impact) in the region, but are expected to cause a significant impact if allowed to spread to realise their potential distributions. The trajectory of the infested area over time can be modelled by a logistic equation with three parameters: current area occupied (e.g. number of hectares invaded), maximum area that could be occupied (ha) and the rate at which the maximum area would be approached in the absence of the proposed management. This invasion trajectory plays a pivotal role in the cost-benefit analysis (CBA) required by the Biosecurity Act (BA) for any species being proposed as the target of a regional management programme because it defines how the benefits of the proposed regional weed management programme, i.e. the losses prevented by the programme, accrue over time. In the CBA, these future benefits are discounted to a present value with those nearer to the present day discounted less than are those oc curring further out in time. As a result, the greater the rate of spread expected in the absence of the proposed management programme, the more likely it is that the CBA will give a positive net present value (NPV) and hence provide the economic justification for the programme. The other two invasion trajectory parameters, current area occupied and maximum area that could be occupied, also affect the NPV. The economist conducting the CBA for a proposed regional weed management programme needs robust estimates of all three of the invasion trajectory parameters. Such estimates will be possible through bio-data sharing applications such as the web-based weed mapping application being developed in AgResearch under the Beating Weeds programme.