The legacy of deliberate and accidental introductions of invasive alien species to Australia has had a hefty economic toll, yet quantifying the magnitude of the costs associated with direct loss and damage, as well as for management interventions, remains elusive. This is because the reliability of cost estimates and under-sampling have not been determined. We provide the first detailed analysis of the reported costs associated with invasive species to the Australian economy since the 1960s, based on the recently published InvaCost database and supplementary information, for a total of 2078 unique cost entries. Since the 1960s, Australia has spent or incurred losses totalling at least US$298.58 billion (2017 value) or AU$389.59 billion (2017 average exchange rate) from invasive species. However, this is an underestimate given that costs rise as the number of estimates increases following a power law. There was an average 1.8–6.3-fold increase in the total costs per decade since the 1970s to the present, producing estimated costs of US$6.09–57.91 billion year-1 (all costs combined) or US$225.31 million–6.84 billion year-1 (observed, highly reliable costs only). Costs arising from plant species were the highest among kingdoms (US$151.68 billion), although most of the costs were not attributable to single species. Of the identified weedy species, the costliest were annual ryegrass (Lolium rigidum), parthenium (Parthenium hysterophorus) and ragwort (Senecio jacobaea). The four costliest classes were mammals (US$48.63 billion), insects (US$11.95 billion), eudicots (US$4.10 billion) and monocots (US$1.92 billion). The three costliest species were all animals – cats (Felis catus), rabbits (Oryctolagus cuniculus) and red imported fire ants (Solenopsis invicta). Each State/Territory had a different suite of major costs by species, but with most (3–62%) costs derived from one to three species per political unit. Most (61%) of the reported costs applied to multiple environments and 73% of the total pertained to direct damage or loss compared to management costs only, with both of these findings reflecting the availability of data. Rising incursions of invasive species will continue to have substantial costs for the Australian economy, but with better investment, standardised assessments and reporting and coordinated interventions (including eradications), some of these costs could be substantially reduced.

Invasive alien deer (known in Australia as ‘feral deer’; hereafter, ‘alien deer’) are some of Australia’s worst emerging pest species. Recently, the Government of South Australia launched a four-year program to reduce the populations of alien fallow deer (Dama dama). The program will focus on coordinating landscape-scale aerial culls and seeks to deliver the most efficient and humane approach to aerial culling. We sourced data from a recent program trialling a new approach to aerial culling that incorporated advanced thermal technology and a second shooter with a shotgun to target fallow deer. We reviewed available video and audio records of 104 deer culled in the program to assess efficiency and welfare outcomes. We collected information on the number of shotgun and rifle rounds fired per animal, time between first shot with a shotgun and apparent death, and pursuit time. We completed field dissections of 20 individuals targeted in the program to assess the lethality of wounds inflicted with shotgun pellets. We also compared program costs and efficiency against published and unpublished data from ten other aerial-culling programs for alien deer in South Australia since 2009. A total of 383 shotgun rounds and 10 rifle rounds were used on 104 fallow deer in the focal program. We documented strong improvements to animal welfare for alien deer targeted with shotguns. The mean (± standard error) time between first shot and apparent death with a shotgun was 11.1 ± 0.7 seconds; mean pursuit time between detection and apparent death was 49.5 ± 3.4 seconds. Pursuit time increased with subsequent deer controlled within a group; the maximum pursuit time for any individual was 159.0 seconds. All autopsied animals had received lethal wounds from shotgun pellets, with 100% receiving lung-penetrating damage and 70% also receiving heart-penetrating damage. While a program that uses a shotgun and rifle combined with a second shooter and thermographer can cost more to mobilise, the outcomes measured in cost deer-1 made it the most cost-effective approach of any program we assessed. Control options that deliver improved animal welfare outcomes and increase efficiency are desirable for managing expanding populations of alien deer in South Australia and elsewhere.

Helicopter-based shooting is an effective management tool for large vertebrate pest animals. However, animals in low-density populations and/or dense habitat can be difficult to locate visually. Thermal-imaging technology can increase detections in these conditions. We used thermal-imaging equipment with a specific helicopter crew configuration to assist in aerial culling for feral pigs (Sus scrofa) and fallow deer (Dama dama) in South Australia in 2021. Seventy-two percent of pigs and 53% of deer were first detected in dense canopy/tall forest habitat. Median time from the first impact shot to incapacitation was < 12 s. The culling rate (animals hour−1) doubled compared to visual shoots over the same populations and the wounding rate was zero resulting in a incapacitation efficiency of 100%. The crew configuration gave the shooter a wide field of view and the thermal operator behind the shooter provided essential support to find new and escaping animals, and to confirm species identification and successful removal. The crew configuration allowed for successful target acquisition and tracking, with reduced target escape. The approach can increase the efficiency of aerial culling, has the potential to increase the success of programs where eradication is a viable option, and can improve animal welfare outcomes by reducing wounding rates and the escape of target animals.

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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.