Predicting targets and costs for feral-cat reduction on large islands using stochastic population models
AbstractFeral cats are one of the most destructive invasive predators worldwide. Due to the high risk of feral predators pushing native species to extinction in Australia, density-reduction or eradication campaigns can improve the persistence probability and recovery of native fauna. Kangaroo Island — Australia’s third-largest island — was recently nominated as a complete cat-eradication site by the federal government. Because many population density-reduction campaigns are costly and not effective in the long term, mathematical models predicting optimal culling regimes can guide management plans, especially if they include estimates of costs under different policy decisions. We constructed a stochastic population model with cost estimates to test the relative effectiveness and cost-efficiency of two main culling scenarios for Kangaroo Island to 2030: (1) constant proportional annual cull (one-phase), and (2) high initial culling followed by a constant proportional maintenance cull (two-phase). We also examined the effectiveness of a trap-neuter-release scenario to compare with the culling outcomes. We found that an average culling proportion of ≥ 0.3 would reduce the population to ≤ 0.1 of the founding population, while a two-phase cull where an initial cull of ≥ 0.6 was followed by a maintenance cull of ≥ 0.45 would reduce the final population to 0.01 of its initial size by 2030. Costs estimates varied widely depending on capture techniques used, but a combination of Felixer™ cat-eradication units, conventional traps, and hunting was the most cost-effective in the two-phase culling scenario when hunting is used to make up culling shortfalls (minimum costs estimated at AU$46.5 million–AU$51.6 million with an initial cull of ≥ 0.6, maintenance cull of ≥ 0.45). Trap-neuter-release was an inefficient approach compared to the culling scenarios. Our model results provided direction for the efficient eradication of feral cats on Kangaroo Island and can be transferred to feral-cat management elsewhere.