Summary and Conclusions
This scoping analysis has drawn from a wide range of predator extermination experiences to understand the broad scale parameters involved in managing rats, possums, stoats, and ferrets at zero density on Banks Peninsula. Inevitable reinvasions mean it is not possible to eradicate each of these species once and then for Banks Peninsula to remain predator free without further action. Ongoing border protection and interior monitoring would be essential for sustained control at zero density. Existing predator eradication technology includes distribution of toxins on the ground and from the air, and trapping. Research is seeking to broaden this suite of tools, but for now that is what is available, and what we have evaluated here. Extermination requires exposure of all target animals to the control method, so home ranges determine how closely control points must be spaced. This varies, by species, as does the type of control apparatus required. We used that information to identify three main predator removal scenarios, for which we derived order of magnitude costs and time required for removal. Our primary purpose was to evaluate methods that did not entail aerial poisoning, but we evaluated aerial brodifacoum, as well as aerial sodium fluoroacetate for comparative purposes. We do not believe these aerial poisoning methods are suitable for Banks Peninsula. Aerial brodifacoum has significant adverse environmental consequences and the present value of costs for Scenario Four exceeds that for Scenario One and is similar to Scenario Two. Aerial sodium fluoroacetate has some potential cost advantages, but on its own is least likely to obtain zero density and is not suitable for Peninsula-wide application.
Regardless of the method of predator removal used, ongoing biosecurity measures (and ongoing costs) will be required to maintain predators at zero densities. Without ongoing biosecurity measures predator numbers on Banks Peninsula would likely return to similar levels to now.
Non-aerial predator removal is an extremely labour intensive and expensive business, so application at the Banks Peninsula scale is unprecedented. Our estimates are that predator removal operation costs (i.e. exclusive of planning, consent, administration, and ongoing predator management costs) would be in the range of $88m to $134m. However, ongoing costs are also significant - our estimate is in the order of $1.65m per annum without buffer, quarantine, or other biosecurity measures. In comparison, the national strategy to eliminate predators from the whole of New Zealand by 2050 has a budget of $84m ($28m from central government on a one-for-two top-up basis for others’ contributions) and the Department of Conservation’s 2016 Battle for our Birds budget was $20.7m for one-off predator reduction operations on 800,000 ha of Conservation land.
Even with a significant workforce of 100 full-time equivalent field staff, attaining zero predator densities across Banks Peninsula would take a considerable number of years. Acknowledging that is important to ensure that sufficient long-term funding is available on commencement to take the project to completion – or risk its complete failure in the long run. It is also important to understand the quantum of labour required, and for how long, where volunteer labour is a significant contributor. Loss of volunteers runs exactly the risks as loss of funding – previous investments can be undermined totally.
Options to reduce costs and/or labour requirements are to change the programme target to either controlling fewer species, to maintaining low (but not zero) predator densities, improving community engagement, including provision of voluntary labour, or some combination of these. In the future, new technology may provide the answers, but for now is unavailable.
Removing rats and/or ferrets could also have significant implications. Rats have the smallest home ranges of the species assessed and so the required grid pattern is much denser than for other species, with rats requiring control features at the rate of 4 ha-1, whereas the next most dense control is for
possums at 1 ha-1 (ferrets are 0.1 ha-1). Removing rats from the programme would reduce both
removal and monitoring costs. For example, under Scenario One removal costs associated with the OSKA traps would decrease from $84.5m to $21.1m because the required grid density is only one fourth that required for rats. Time to removal would fall by a commensurate amount. Of course, the rats would remain. It was beyond our remit to estimate the suppression effect that would have on protected species recovery, or the associated reduction in benefits.
Savings from removing ferrets from the programme are far less than from removing rats. Looking again at Scenario One, DOC250 traps would no longer be required, saving $3.4m in removal costs. Again, there is likely to be an adverse effect on protected species, but we have not investigated that. The aerial sodium fluoroacetate example (Scenario Five) is an example of the implications of changing the programme target to control at low densities, which would result in an order of magnitude reduction in present value of costs, despite the requirement of ongoing aerial toxin applications. Community residents can affect project viability in two main ways, as a source of volunteer labour and by their decisions about predator control activities undertaken by them or others on the land they manage. The draft community survey in Appendix 3 sheds light on these matters.
There are significant logistical, legal, and financial challenges to achieving a predator free Banks Peninsula. These potential barriers are sensitive to the species targeted and the methods used. We recommend further investigation of the effects of a restricted programme, particularly the exclusion of rats, on conservation outcomes and community support to assess whether a more affordable scheme that could be completed in a much shorter time has merit relative to the proposal we assessed (controlling all five of the target species at zero density).
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