Stochastic population models to identify optimal and cost-effective harvest strategies for feral pig eradication

Abstract

Eradicating feral pigs from island ecosystems can assist in restoring damaged biodiversity values and protect commercial industries such as agriculture. Although many feral pig eradications have been attempted, management decisions are often led by practitioner experience rather than empirical evidence. Few interventions have been guided by population models to identify harvest rates necessary to achieve eradication within a specified time frame, nor have they applied data on control effort and cost to evaluate the relative cost-effectiveness of proposed control strategies. We used effort and cost data from a feral pig-control program on Kangaroo Island, South Australia over 17 months to derive functional-response relationships between control effort (hours pig-1) and pig abundance for four control methods: (i) ground-based shooting, (ii) trapping with remote triggers, (iii) poison baiting, and (iv) thermal-assisted aerial culling. We developed a stochastic Leslie matrix with compensatory density feedback on survival and fertility to project population trajectories from an initial population (N0) of 250 female pigs with an estimated island-wide carrying capacity (K) of 2500 over 3 and 10 years for populations subjected to an annual harvest of 35% to 95%. We built functional-response models to calculate annual effort and cost for six cull scenarios across all harvest rates. We derived total cost and effort over 3- and 10-year projections from the sum of annual cost and effort within the projection intervals. Pig populations were reduced to < 10%N0based on harvest rates > 70% and 50% for culls of 3- and 10-year duration, respectively. In all scenarios except ′trapping only′, the total cost to reduce population to ≤ 10% ofN0decreased with increasing harvest proportion, with lower total costs incurred over 3 years compared to 10 years. The simulations suggest that the most cost-effective approach for most scenarios is to maximise annual harvest and complete eradication effort over the shortest periods.

Aerial culling invasive alien deer with shotguns improves efficiency and welfare outcomes

“…Aerial culling can be an effective, rapid, and humane means for removing large numbers of alien deer (Husheer and Robertson 2005;Bengsen et al 2022;Pulsford et al 2023), alien pigs (Cox et al 2022;Hamnett et al 2023), and other pest species in vast, remote, and inaccessible landscapes. In 2020, 2021, and 2022, South Australia's aerial culling programs have removed approximately 3,000 alien deer per year (BDO Econ-Search 2022).…”

Thermal aerial culling for the control of vertebrate pest populations

“…Decades of intense feral animal control has eradicated both deer and goats [23]. Eradication is now focused on feral pigs [17,43] and cats (Felis catus) [44]. After the 2019-2020 fires, the pig population was estimated at < 450 individuals [43].…”

“…Landholders also engaged contract, volunteer, and/or recreational shooters to assist with deer management and a commercial harvester also worked across most of the seven holdings regularly between 2018-2020. A visual aerial cull was done over the area 10 days prior (13)(14)(15)(16)(17) to the thermal aerial culling program. The visual shoot crew flew 89.9 hours, using 2 helicopters over 5 days, and culled 603 deer (6.8 deer hour -1 ) across 128,103 ha, with the aim of removing as many deer as quickly as possible.…”

“…However, effectiveness depends on visual detection of the target species, which can be difficult in tall forests and/or dense vegetation. The success of aerial culling can also be reduced when the target population size is small [17] (e.g., an emerging population), or when populations have already been reduced substantially by previous intervention. In artificially reduced populations, survivors of previous aerial culling often flee to the safety of vegetation cover, reducing the efficacy of subsequent control [18,19].…”

“…We describe the efficacy of the method for two vertebrate pest species: a low-density (< 0.2 animals km -2 ) population of feral pigs, and a highdensity (> 6 animals km -2 ) population of feral fallow deer. The pig population had been reduced by 90% during the 2019-2020 bushfires [17,43] and this presented a unique opportunity to attempt to eradicate the low-density population while vegetation recovery enabled high detectability. We measured several aspects of the approach to test the following hypotheses: (i ) the number of shots taken and shots impacting an animal do not vary relative to vegetation-density category (open, sparse, dense) or species (pig, deer), (ii ) four temporal welfare indicators (time from start of pursuit to first bullet impact [chase time], time from first bullet impact to incapacitation, time from first shot to incapacitation, time from pursuit to incapacitation) do not vary among vegetation-density categories and species, and (iii ) no wounded animals escaped.…”