Recently John Ogden interviewed some key people in the rat eradication business: Professor Mick Clout is Director of the Centre for Biodiversity and Biosecurity, a government funded "Centre of Research Excellence" based at Auckland University. He is also the Chairperson of the Invasive Species Specialist Group of the International Union for the Conservation of Nature (IUCN). Alan Saunders is Director of Cooperative Islands Initiative, based at Auckland University. Dr Charlie Eason is a toxicologist, recently with The Centre for Environmental Toxicology at Landcare Research, now employed by Connovation Ltd.

Between them these people have extensive knowledge of rats, islands, rat eradication and control operations, and the use of toxins. Mick and Alan are both familiar with the Barrier; Alan contributed to the GBICT’s summer lecture series in 2005. Together these three are at the forefront of international experience in pest eradication and we are fortunate to have them involved. This article gives their summarised answers to the questions John put to them.

  Have poisons been widely used in rodent eradications before?

Yes. Over eighty islands have had rodents or other pests eradicated by use of toxins. In many cases poison was distributed by helicopter, but on some small islands ground-based bait-stations were used. Brodifacoum (sold as Talon and Pestoff) is probably the most widely used rodenticide worldwide, and has been used in New Zealand since the early 1990s. It is preferred because it kills rats after one feed. Aerial drops of brodifacoum pellets have been very successful in rat eradication, both here and overseas.

No. The largest island from which rats (R. norvegicus) have been eradicated to date is Campbell Island (11,400 ha.), which is in New Zealand’s sub-Antarctic zone, and is uninhabited. The largest island from which mice have been eliminated is 700 hectares, and for Ship rats (R. rattus) the largest is 88 hectares. GBI would be by far the largest island attempted so far. The real problem however is not the size, but the presence of a large human and domestic animal population.

Yes. Rodents have been eliminated on several inhabited islands, such as Frigate Island (Seychelles) and Viwa Island (Fiji). The residents of Lord Howe Island (New South Wales) are currently considering a proposal to eradicate rats and mice, using a combination of aerial and ground-based approaches. In New Zealand, rats have also been eradicated from Kapiti Island, which is rugged and forested and has a small human population and many visitors. Rodent eradications on inhabited islands are obviously more complicated. Because rodents are likely to live in and under most buildings, the approval and support of all house-owners will be needed for bait to be laid—or traps to be set in and around houses. During an eradication operation toxic baits will be virtually everywhere. All residents will have to support the goals of the operation and accept some limitations on their activities during the period of the operation. Gaining 100% community support is a tall order anywhere, and will probably be the biggest challenge to eradicating rodents from Great Barrier. An eradication campaign is unlikely to succeed without co-operation and goodwill from the vast majority of the public. Without clear public support the agencies involved are unlikely to fund it.

  What sort of difficulties will this impose for individuals?

Clearly poison baits cannot be dropped by Helicopter over inhabited areas. Around (and under) houses there would need to be individually monitored bait stations. Also dogs and stock might have to be moved to other areas temporarily. This is an area where consultation, planning and monitoring are essential. For some individuals, such as farmers with stock, there could be major inconvenience for short periods, but for others the inconvenience would be brief and offset by advantages in employment and subsequently improved living and business conditions.

  What if poison, or dead rats, ended up in watercourses? What is the risk of contamination of drinking water?

Use of poisons is never without risk. You have to weigh-up the short-term bad effects against the long-term good. Because so many areas have now been treated there are clear guidelines for this sort of risk-management. The issue of drinking supplies would have to be managed by careful bait locations. It will depend to some extent on the poison used. Brodifacoum is not readily broken down in soil or water, which is why continued long-term use around houses etc. is potentially a more serious problem than a one-off drop. It’s presence in dead rats can also be a problem if domestic cats or dogs eat them

  New, rodent-specific, poisons are currently being developed by Landcare, and these may change the whole operation in the near future.

See STOP PRESS — page 12

  Why do feral cats need eliminating?

Feral cats are real ‘wild’ cats that live and breed in the bush, feeding on rabbits and rats, as well as native species such as large invertebrates, lizards and native birds. There are several hundred, possibly a thousand, such animals on Great Barrier. If rodents are eliminated the cats might switch diet and start feeding more on birds, such as pateke. Pet cats are not in themselves a problem, but unfortunately people throw out their kittens and let their pets go wild. There’s no point eliminating feral cats if domestic cats are not neutered because the problem will soon come back. Likewise if we eliminate rats it is essential to keep a check for them on boats and aircraft.

  Would trapping be an alternative close to houses?

Trapping might supplement poisons in particularly sensitive areas, but trapping alone has never been successfully used in a large-scale operation to eradicate rats.

Basically the government, but with support from corporate funding and other sources. Department of Conservation would clearly be involved, and ARC. (Both have offered to seriously consider financial support if the community requests it).

  Would there be any financial incentives for individuals?

The direct benefits (like not having to buy rat-poison) might be quite small, but indirectly the economic benefits of a successful eradication campaign would have spin-off in jobs and increased tourism, effecting the whole community. Lord Howe Island might be a good model (see GBIT Environment News No: 5.). The effect on the biodiversity would be huge and rapid—Little Barrier and many other examples prove that.

  But wouldn’t the poison get taken up by some non-target species?

Yes, there would be some deaths. Moreporks and perhaps kingfishers eating wobbly rats would be at risk. Some smaller bush birds would also die from sampling baits. But this is all well studied,  a temporary "knock down" is followed by a huge resurgence in the rat-free environment.

  What about our special species, such as rails and brown teal?

These would have to be carefully considered, just like pet cats, dogs, sheep etc. You’d need to examine the options—like maybe caging some populations over the drop period as an insurance policy. These are the sorts of topics which would be addressed in detail in a technical feasibility study.

That’s correct. I don’t know how dependent people are on the Barrier for wild pork, but perhaps they could manage for a while after the drop while pig-livers were monitored. Once done of course there would be no risk of continuous toxin build up as there is now.

  Any other points?

Mice: Eradicating the rats from Great Barrier—and not the mice, may result in a significant increase in mouse numbers. This may lead to even greater mouse impacts such as food loss and spoilage. With different habits from rats, and smaller ranges, mice are difficult to eradicate using existing baits and approaches. Research is currently focused on improving our ability to confidently eradicate mice. Provided effective techniques were available it would be best to eradicate both the rats and the mice at the same time.

  Biosecurity: Apart from eradicating the rats and mice, another major challenge will be to prevent them from re-colonising Great Barrier. Rodents are commensal animals—they occur virtually everywhere that people live. They are adept at "hitching a ride" in freight, on boats etc. They can also swim and/or raft on flotsam impressive distances. If we are to go to the cost and effort of eradicating the rodents from the Barrier we will also need to put in place and consistently implement quarantine, surveillance and contingency procedures to ensure they do not get back and re-establish.

New toxins: Although Brodifacoum is the poison most commonly and effectively use in eradication operations, new toxins are being developed. Landcare is working on a "rodents only" toxin, which will eliminate the risk to other wildlife, pets etc and may greatly facilitate eradications in inhabited areas in future.

Eradication is fundamentally different from control. Eradication implies getting every last one, and preventing re-invasion. This raises important issues for ‘border security’, but that would certainly create some jobs, both policing and monitoring. With ‘control’ the pests remain, and the costs continue – in this case the environmental and biodiversity costs consequent on long-term use of toxins.

These three experts are enthusiastic to support the project. They, like us, recognise that it is not an easy job, that it is expensive, and that there are risks involved. However, given strong public support we believe it can be achieved. What is needed now is a good economic assessment, followed by a technical feasibility study which will address the above questions, and more, in much greater depth.

‘Turning the Tide: the eradication of invasive species.’ Proceedings of the international conference on eradication of island invasives. 2002. Edited by C.R. Veitch and M.N. Clout. IUCN Occasional Paper No.27.