Rodent Eradication From Islands — The Conservation Potential

Forest and Bird, Volume 19, Issue 1, 1 February 1988, Page 32

Rodent Eradication From Islands — The Conservation Potential

by

Dr David Towns,

scientist with the

Science and Research Directorate of the Department of Conservation.

W ith the introduction of an array of rodent, feline and mustelid predators to New Zealand over the last 150 years, the fauna of the main islands has taken a battering. So great is the extent of this destruction, Jared Diamond (1984) was recently moved to comment that New Zealand no longer has an avifauna, just the wreckage of one. Predator-free offshore islands provide the only means of preserving some unique remnants of this fauna. They remain a last refuge not just for birds, but also many lizards, insects and even some plant species. The continuation of these outposts has not always been regarded as assured. As recently as 10 years ago, the slow creep of rodents through these remaining predatorfree islands seemed inevitable. This view was reinforced by the irruption of ship rats on Big South Cape Island in 1964, followed by the extinction of two endemic forms of birdlife, and the invasion of Whenuakura Island by Norway rats in the early 1980s, causing the destruction of the tuatara population. In a workshop on the impacts of rodents on nature reserves in 1978, both Dr lan Atkinson and the late Dr Kazimierz Wodzicki concluded that eradicating rodents from islands, however small, was an unrealistic goal. Because of this previously well justified view, predator-free islands, especially those free of rodents, took on a dual role. In addition to their value as the least modified of New Zealand's ecosystems, they have acquired the guise of living arks — havens to which the inhabitants of other islands could

be moved once their original homes had been invaded by rodents. Recently our whole approach to this problem has undergone a revolution. New forms of safer, effective rat poisons, and some innovative methods of dispensing them, have meant that we can now change from constantly battling to save a diminishing resource to a phase of pre-emptive strikes. Choosing islands for their potential conservation value and removal of rats is feasible, and the first attempts are already under way. While DSIR teams from Ecology Division have been developing their rodent eradication techniques on islands in the Marlborough Sounds and Fiordland, a second group, now in the Department of Conservation, has been developing methods for removal of Norway rats and kiore from islands in the Hauraki Gulf and Bay of Plenty. Korapuki Island The most recent of these has been on Korapuki Island, the first project designed specifically with endangered species conservation as the ultimate goal. Reaching this point has taken several years and covered four different groups of islands as techniques have been developed and refined. The sequence really began in the late 1970s when Department of Conservation (DoC) science technician, lan McFadden, became involved with Dr Phil Moors’ programme for eradicating Norway rats from the Noises Islands in the inner Hauraki Gulf. When the need for bait dispensing silos became apparent, Ian tried a number of designs with kiore on Lady Alice Island, in the Hen and Chickens Group, prior to their use on the Noises. Further work on Lady Alice enabled Ian to test various forms of baits and attractants (McFadden 1984). As work concluded on the Noises, the focus shifted to Rurima Island, a small (6 ha) island in-

habited by kiore, off Whale Island, in the Bay of Plenty. This is where the final phase of design was required — the selection of the most suitable rodenticide. After about 6 months of trials, all rat sign ceased on Rurima and no kiore have been seen for over three years. The way was now open to design a rat extermination programme around the conservation needs of rare species. The one chosen was Whitaker's skink, a large (20 cm) lizard confined to two tiny offshore islands (the largest just 10 ha), and a small and highly vulnerable area at Pukerua Bay near Wellington. In 1986 the Hauraki Gulf Maritime Park Board accepted a proposal

by myself and lan McFadden to eradicate kiore and rabbits from 18 ha Korapuki Island as a prelude to establishing a new population of Whitaker's skink, most of which would be transferred from Middle Island about 2 km away. The eradication campaign began in early November 1986, when Ian McFadden and Derek Brown (the latter now DoC, Nelson/ Marlborough Region) put in long days setting up bait silos, and filling them with unpoisoned grain as bait. After a rather tense wait of four days when little happened, it became obvious that the kiore were starting to find the grain very much to their taste. It was then time to switch to poisoned grain,

which the kiore clearly found no less attractive. When Ian, Derek, DoC technician Murray Douglas and I left Korapuki in midNovember, none of us suspected that our next trip to the island would be to New Zealand's newest rodent-free habitat. Korapuki has now been checked five times, but despite a range of baits, lures and traps, not one rat has been seen, nor evidence found of their presence. All going well, by early 1988 Whitaker's skinks may have their first opportunity to expand, rather than decrease, in number for 150 years or more. The success of this and other eradication programmes lessens the pressure on rare species, and also provides an opportunity for a complete change in the way we look at those islands which are already free of rodents. They are no longer a diminishing resource, nor has their value as biological time capsules of ancient New Zealand decreased. If anything they have become even more biologically important. Because rodent extermination on islands up to 40 or 50 ha might be possible (or even larger, if Bruce Thomas and Rowley Taylor, of Ecology Division, DSIR, are successful), we have to be extremely careful how island conservation should proceed. It now becomes difficult to justify shifting any species to small islands which are naturally free of predators. Fortunately most such islands remain untouched, so their values as living laboratories demonstrating the effects of over 10,000 years of isolation are relatively intact. We now have the responsibility of ensuring that those same values are passed to other generations Note of Caution The queue of organisms proposed for ro-dent-free islands seems to grow without check, and includes such diverse groups as large flightless insects, lizards and threatened landsnails. Island rehabilitation can be designed around the requirements of each

of these. In addition, new advances in population genetics can now help to define the right combination of individuals within each species which should establish new island populations. The time is now right for a note of caution. Our successes could, without careful thought, lead to some islands becoming little more than open zoos housing odd combinations of species which have never previously occurred in the region, or which have never been naturally associated with each other. Our challenges are now in defining how island liberations should be approached so that subsequent generations will thank us for our forward thinking as well as our abilities as rodent eradicators. Who would have thought a few years ago that soon we would have the luxury of contemplating challenges like these? Acknowledgements My thanks to the Hauraki Gulf Maritime Park Board for providing challenges to meet, and to Drs Phil Moors and Richard Sadleir of the Department of Conservation for their comments on a draft of this article.

References Atkinson I.A.E. (1978) Evidence of the effects of rodents on the vertebrate life of New Zealand islands. pp 7-30. In The Ecology and Control of Rodents in New Zealand Nature Reserves. Eds P R Dingwall, I.A.E. Atkinson, C. Hay. New Zealand Department of Lands and Survey Information Series 4. Diamond, J. (1984) Distributions of New Zealand birds on real and virtual islands. New Zealand Journal of Ecology 7: 37-55. McFadden, I. (1984) Composition and presentation of baits and their acceptance by kiore (Rattus exulans). New Zealand Wildlife Service Technical Report No 7 (unpublished). Moors, PJ. (1985) Eradication campaigns against Rattus norvegicus on the Noises Islands, New Zealand, using brodifacoum and 1080. pp 143-155 In Conservation of Island Birds Ed PJ. Moors. ICBP Technical Publication 3. Wodzicki, K. (1978) A review of existing control measures. pp 195-205. In The Ecology and Control of Rodents in New Zealand Nature Reserves. Eds P.R. Dingwell, I.A.E. Atkinson, C. Hay. New Zealand Department of Lands and Survey Information Series 4.