Estimating realistic costs for strategic management planning of invasive species eradications on islands

Environmental managers regularly face decisions about how to counteract threats. These decisions require an understanding of both the conservation benefits and economic costs of candidate actions. However, transparent frameworks for how to accurately calculate costs for management are rare. We worked with island managers in Australia to develop eradication protocols for six invasive species- four mammals and two weeds. We used the protocols to create an accounting framework for invasive species eradications to produce realistic cost estimates for eradications across multiple locations. We also used our models to test common cost assumptions: (1) that costs scale linearly with area, (2) that terrain does not influence costs, and (3) that eradication costs stay constant through time. By explicating testing assumptions, we found that costs largely scaled linearly with area, that terrain influences costs, and that costs decline as populations decline in response to ongoing management. Estimated mammal eradication costs were driven in large part by the area of an island and the cost of transport. However, when area alone was used as a proxy for costs, the calculated costs deviated from our modelled costs by 40-56%. Weed eradication cost estimates were driven by the size and density of an infestation as well as the terrain of the island, with the effect of terrain becoming more pronounced as area to be treated increased. We provide a method to calculate realistic costs across several sites, which can be used to guide strategic management decision-making, including prioritisation, and on-ground management actions.