Spatial model finds broad, frequent aerial culling best limits invasive fallow deer in Tasmanian park

A validated spatially explicit population model projected that thermally assisted aerial culling every year or every two years across a broad zone offered the best balance for limiting invasive fallow deer in Tasmania’s Walls of Jerusalem National Park, reducing helicopter effort and the total number of animals culled. These were simulation results for 2023–2060, not direct observations of management outcomes. The study examined the 518-km² national park and surrounding landscapes at the expanding edge of Tasmania’s fallow deer population.

Non-native fallow deer were introduced in the 1830s, and since 1985 the population had increased about 40-fold. An October 2024 aerial survey estimated 71,655 deer in the island’s traditional core range, with an uncertainty of ±19.6%. Researchers adapted a Tasmania-wide model previously validated against observed changes in deer occupancy and abundance from 1985 to 2019.

The model tracked female deer across four age classes on a 5-by-5-km habitat grid and ran 1,000 simulations for each scenario. Six active management scenarios combined culling every year, every two years or every five years with either a broad 1,056-km² zone or a concentrated 425-km² zone. A separate scenario tested annual culling across an expanded 2,013-km² area.

Management effort was estimated with a catch-per-unit-effort model informed by helicopter flight paths and kill locations from the May 2023 cull. Under the baseline, which included the 2023 cull but no subsequent intervention, the model projected that the female population across the full study area would increase from 2,390 in 2023 to 9,548 in 2060. It also projected 314 deer in the park by 2060, with a 95% confidence interval of 307–322, occupying 52.01% of the park.

The model estimated 5,005 deer-years in the park as a comparative index of deer presence and potential impact, not a direct measure of ecological damage. Annual culling across the broad zone was the most effective active scenario. The model projected 90 deer occupying 5.24% of the park in 2060 and 3,237 deer-years.

By comparison, five-yearly culling in the concentrated zone projected 201 deer occupying 35.64% and 4,125 deer-years. Broad-zone culling stabilised deer numbers by about 2040, whereas restricted culling was followed by increasing numbers late in the projection. Broader culling required more helicopter time but reduced invasion pressure.

Five-yearly broad culling had the greatest mean effort at 298.56 hours, with a 95% confidence interval of 189.47–884.96 hours. Annual broad culling projected 954 deer culled, compared with 1,152 under five-yearly broad culling. Annual and two-yearly broad culling tied for the lowest modelled cost-benefit ratio, at 0.15 helicopter hours per deer-year avoided relative to baseline.

The expanded-zone scenario kept the park free of deer in 65.5% of simulations and had a mean estimated cost of 1,151.36 helicopter hours, but it did not guarantee exclusion and produced the worst cost-benefit ratio. Its best-case assumption of complete removal within culled cells and the large uncertainty in flight-time estimates limit interpretation. The analysis was further limited by uncertain initial abundance, no direct Tasmanian estimates of fallow deer dispersal and a catch-per-unit-effort model fitted to one operational dataset.

Rankings were generally stable when initial abundance was varied, although five-yearly culling performed substantially worse when the assumed starting population was 1,050 females.

Subject of Research: Simulation of aerial culling strategies for controlling invasive fallow deer in Tasmania

Article Title: Using a spatially explicit population model to evaluate management scenarios for an invasive deer population

Article References: Botterill-James, T., Buck, R., Cunningham, C. X., Driessen, M. M., Johnson, C. N., Pople, A., & Forsyth, D. M. (2026). Using a spatially explicit population model to evaluate management scenarios for an invasive deer population. NeoBiota, 106, 191-213. https://doi.org/10.3897/neobiota.106.173542

Image Credits: AI Generated

DOI: 10.3897/neobiota.106.173542

Keywords: fallow deer, Tasmania, invasive species, aerial culling, spatially explicit model, simulation, Walls of Jerusalem National Park, population control, conservation, wildlife management, spatially, explicit