Applying the seven rules for eradication to pest-free New Zealand
Image - Andrea Byrom.
A key challenge for pest-free or predator-free NZ (PFNZ) will be to decide whether mammal pests can be eradicated or, if not, whether biodiversity goals can be achieved by suppression. As progress is made towards a zero or low predator/herbivore environment, managers of large areas of New Zealand or some of our largest inhabited islands may well tackle eradication of selected pests, or local elimination with management of reinvasion.
Achieving eradication from very large islands such as Stewart, Chatham or Great Barrier or defendable areas of the mainland such as Banks Peninsula will require systematic planning. Fortunately, there is a framework of seven ‘rules’ that we can apply, summarised here by Andrea Byrom and John Parkes. Rules 1–3 are regarded as critical, i.e. unless met, eradication cannot proceed. Rules 4–7 are desirable, i.e. eradication can still proceed even if they are not strictly met.
Rule 1: All animals must be put at risk
Usually, this rule is applied to one species at a time, and there are numerous examples of single-species eradications worldwide. Some species can be removed with a single method applied once (e.g. aerial baiting of rodents); others require a series of control events, often changing methods to get the last pest (e.g. ground and helicopter hunting of ungulates). Multi-species eradications have been achieved at small scale in New Zealand (e.g. in fenced sanctuaries such as Maungatautari).
Rule 2: Pest species must be killed at rates faster than their rate of increase
Intuitively, this rule makes sense for pests that require a series of control events. New Zealand has made huge strides in very large scale suppression of possums and for species such as goats, where control methods are deployed sequentially. Attempting eradication on the scale of PFNZ will challenge this rule for species with fast rates of increase like mice.
Rule 3: The risk of recolonisation must be zero
It’s easy to envisage this rule working for islands with water barriers that prevent or slow the movement of animals (e.g. Tiritiri Matangi). However, when considering large areas of the mainland such as Banks and Otago peninsulas (suggested as first steps for mainland eradication), preventing recolonisation will be challenging.
Rule 4: Social and economic conditions must be conducive to meeting the critical rules
A range of stakeholders have an interest in the outcome of any eradication programme; communities always have a range of values and aspirations. However, when eradication is the aim, only some control methods can meet the critical rules – anticoagulant aerial baiting will need to be used to eradicate rodents on big islands despite people being present. Others oppose the killing of animals on ethical grounds, and some value some pests as a resource. The hard fact is that as managers work towards PFNZ they will be confronting the public with techniques that will cause angst and objections.
Rule 5: Where the benefits of management can be achieved without eradication, discounted future benefits should favour the one-offcosts of eradication
In other words, if the benefits can be achieved more cheaply by suppressing pest animals in perpetuity, then it’s a better option. This is a simple calculation when the benefits have monetary value, but it is more complex when the value of biodiversity or other non-monetary values are considered.
Rule 6: Animals surviving the campaign should be detectable and dealt with before numbers can increase
Detection of pests is a growing field in the management of invasive species internationally and is often regarded as easy, but it is not! Detection is both a technical problem (which devices are most suitable?) and a statistical problem, because managers must put a probability on their belief that no pests are present when they cannot find any. Scaling up to large areas of the New Zealand mainland will pose an even greater challenge, especially for cryptic species like mice and stoats.
Rule 7: There must be no net adverse effects
The method chosen to eradicate a pest must not affect valued species (unless the latter can be replaced) or permanently damage the environment. Less clear are the problems of removing some pest species while leaving others. New Zealand’s mammal pests interact, so removing possums alone may lead to an increase in rat (and stoat) numbers, such that threats to plants are reduced but threats to birds may increase.
These ‘seven rules’ provide an excellent framework that can help us work towards PFNZ. However, they will need to be applied to multiple species simultaneously; pest species that cannot be removed in one hit have rapid rates of increase at low densities; recolonisation will be an ongoing challenge; some methods of control will generate public debate; even if eradication is a more logical option than sustained control, the one-offcosts may still be too high; managers need to get smarter in detecting re-emerging pests; and removal of an individual pest species may have adverse ecological consequences. Nevertheless, the rules will keep managers grounded in reality as they begin to think about local elimination of multiple pests at very large scales on mainland New Zealand.
This article was funded by the Ministry of Business, Innovation and Employment (contract C09X0909).