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.
Have rats been eradicated from an island as large as Great Barrier?
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.
Have eradications occurred on other inhabited islands?
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 y. Likewise if we
eliminate rats it is essential to keep a check for them on boats and
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.
You mentioned funding. Who pays for these operations?
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
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
What about pigs? I understand they accumulate brodifacoum in their
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.
Editor’s summarising comment:
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.
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.