Landcare Research - Manaaki Whenua

Landcare-Research -Manaaki Whenua

Eradicating invasive species on big inhabited islands

Black browed albatross chicks on the Kerguelen Islands. Image - Hélène De méringo.

Black browed albatross chicks on the Kerguelen Islands. Image - Hélène De méringo.

One legacy of the New Zealand physicist Sir Paul Callaghan was his vision of a pest-free New Zealand. John Parkes has been investigating whether such a vision will divert pest managers from practical solutions and waste a lot of money, or whether it is in fact feasible. All pest eradication projects face this question and there are two ways of answering it – by looking at precedents (who has done it before under similar circumstances) and by analysing the general obligatory rules and particular constraints affecting projects and determining whether they can be met or overcome.


Most of the 31 species of exotic mammals but only a few of the exotic birds, invertebrates and weeds that occur on the main islands of New Zealand have been individually eradicated from some smaller islands (Table). For mammals, the question is whether these successes could be scaled up to deal with one or more species across the larger islands of New Zealand – D’Urville (16,782 ha), Great Barrier (28,510 ha), Chatham (90,650 ha), Stewart (173,500 ha), and the North (11.3 million ha) and South Islands (15 million ha).

Some of the constraints

Scale is a problem for the eradication of some species. What is unclear is whether factors (e.g. topographic complexity, habitat and natural foods, and the size of the target population that might allow for individuals with odd behavioural or physiological characters to avoid being killed) that correlate with scale increase the risk of failure. Scale also means very big islands have to be treated in some sort of ‘rolling front’ strategy with increased risks of reinvasion of cleared areas and increased costs to detect and deal with immigrants. This risk of backfill probably determines the time frame over which any large-island eradication must take place, as the cost cannot be spread over decades without a huge increase in the risk of failure. Thus scale, and things that correlate with it, makes eradication difficult but not intrinsically impossible.

Tools such as aerial baiting with an anticoagulant can put all rodents at risk in one event, but no method exists that reliably kills 100% of populations of other pest species in a single application. For possums, cats, rabbits and stoats, some animals survive each control event and have to be killed, usually by applying other methods, until the last event kills the last animal.


Table. Large islands in New Zealand and overseas from which the ‘target’ pest species have been eradicated.

Species Largest island in New Zealand Area (ha) Largest island elsewhere in the world Area (ha)
House mouse Rangitoto/Motutapu 3820 Macquarie (Australia)3 12 785
Ship rat Rangitoto/Motutapu 3820 Macquarie (Australia)3 12 785
Norway rat Campbell 11 200 Saint Paul (Kerguelens, France) 800
Kiore Raoul 2938 Vahanga (Tuamotus, France) 382
Possum Rangitoto/Motutapu 3820    
Stoat Resolution1 20 860    
Cat Rangitoto/Motutapu 3820 Marion (South Africa) 29 000
Goat Great Barrier 28 510 Isabela (Galapagos, Ecuador)2 500 000
Pig Kapiti 1970 Santiago (Galapagos, Ecuador) 58 465
Rabbit Rangitoto/Motutapu 3820 Macquarie (Australia)3 12 785
Hedgehog Rangitoto/Motutapu 3820    

1 Eradication pending but stoats are likely to reinvade even if they are eradicated.
2 To be confirmed.
3 Eradication pending.

Costs make many operations impractical. The Campbell Island rat eradication cost $220 per hectare so a rough estimate to aerially bait all of New Zealand to eradicate rodents is $6 billion, assuming a perfect kill. What is not known is the cost to remove other pest species that require some sequence of control. The cost to eradicate the suite of mammal pests from Rangitoto/ Motutapu islands was about $3.5 million or $914 per hectare, and that did not include the earlier costs of removing possums and wallabies. Judging by this case, the cost to remove a similar suite of pests from New Zealand would be at least $24.6 billion! This is a minimum figure as managers of Rangitoto/Motutapu islands did not have to spend much money to mitigate non-target effects, manage people, or deal with reinvaders in the short term.

The presence of people poses problems for most eradication projects. There are all sorts of smart tools these days to target sustained control of pests but none (by themselves) are remotely likely to kill 100% of a population, especially of rodents. For that, toxins are essential and aerial baiting will be required in most areas. What an urban human population would think of aerial baiting in their vicinity or how such a technique could be imposed on, for example, organic farmers hardly bears thinking about.

What about just some species?

If just one pest could be eradicated from large islands, which one would be picked? If possums were chosen, a plague of rats and stoats would likely follow; if rabbits were chosen, an increase in hares is likely; if ship rats, a plague of mice is likely. The Animal Health Board would be likely to pick possums, DOC might pick rats, and Otago sheep farmers might nominate rabbits.

The positive solutions

John believes a pest-free New Zealand would be wonderful but in this case ‘perfect’ is the enemy of the ‘good’. The ‘good’ is practical and includes (a) doing rigorous feasibility plans on one or more small islands with human inhabitants, (b) choosing the best places to protect the most valued assets (how much of this can be done depends on the national budget, and national optimisation depends on trade-offs between individual, regional and national priorities (who pays?), (c) improving efficiency by developing new tools and better ways to intervene with current tools, and better ways to monitor effects, (d) investing in research to find out why some individuals of some species always survive our best efforts, and (e) ensuring that the capacity to deliver action is accepted not just as a job for government but is sustained by landowners and the community.

John Parkes

Clarification Note

This article credited Sir Paul Callaghan with the ‘Pest-Free New Zealand’ vision. It has been pointed out that others (e.g. the Predator-Free New Zealand lobby group set up by Les Kelly) have been promoting this idea for some years. It is of course the grander successor to the failed ‘last rabbit’ or ‘last deer’ campaigns of the 20th century. Sir Paul’s vision was slightly different; he focused on the ‘Zealandia’ or ‘halo’ model. This model, with its focus on protecting biodiversity in core areas with export of benefits from the core and a gradually enlarging core, arose out of the mainland island strategies (fenced or not) developed by DOC and private trusts. In a management sense it is the mirror of earlier ‘onion’ models, first discussed by Parkes and Nugent in 1995, where the core and layers of management around it were focused on the pests with the flow of biodiversity benefits an outcome of the layers of pest management.

Parkes JP, Nugent G 1995. Integrating control of mammal pests to protect conservation values in New Zealand. Landcare Research Contract Report LC9495/104.