RAT-FREE ISLANDS

Forest and Bird, Issue 279, 1 February 1996, Page 18

RAT-FREE ISLANDS

IAN McFADDEN,

How rodents are. being removed from our offshore sanctuaries For many native species in New Zealand, offshore islands offer the last refuge against the predations of introduced animals, in particular rats. Yet while the conservation problems for mainiand survivors worsen, the number and size of available islands is.severely limited.

One of the key players,

looks at how bigger

isiands are being lidperated trom rodents.

EW ZEALAND was the last major land mass settled by humans: a temperate paradise, brimming with unusual life forms, and free of the ravages of mammalian predators. The arrival of Polynesians about 1,000 years ago, accompanied by foreign animals including the kiore or Pacific rat, began a massive alteration of the New Zealand environment and the beginning of the rapid loss of some of those strange plants and animals. The first of a whole new suite of predators and competitors arrived with Europeans some 800 years later. When Cook sailed out of Fiordland, he left behind the Norway rat which would have quickly adapted to the larder of food in that untouched utopia. During the next 100 years mice and ship rats, cats, mustelids, pigs, ungulates, hedgehogs, rabbits, possums and others joined the scramble for niches in the New Zealand environment — a process of invasion still going on today. These alien animals have had a devastating effect on both native flora and fauna. The most recent

deliberate introduction, in 1985, was chinchilla, a small South American rodent, which will sooner or later establish in the wild if previous events are any guide. Along with this wave of animals came the destruction of huge tracts of forest on a scale quite unimaginable. Within less than 1,000 years — a blink in evolutionary time — the flora and fauna of this country — most of it found nowhere else in the world — had to adjust to these modifications. Unable to do this and having nowhere to go, many species simply vanished forever. Today many of the survivors exist only on offshore islands — refuges that had escaped some of the alien introductions. Islands in a true pristine state do not exist around New Zealand any longer, but there are examples such as some of the more innaccessible islands in the Three Kings group, or the Poor Knights, which give a suggestion of what this corner of the universe must have looked like. The examples are few but the good news is that the trend of invasion is being pushed back and islands once occupied by introduced mammals

are being given a second chance. The methodology of pest removal developed on these islands is now also being applied to isolated pockets of forest on mainland New Zealand. The short-term solution for those endemic plants and animals that simply cannot coexist with introduced fauna, is marooning on offshore islands. With careful management it may be possible eventually to reintroduce some back onto the mainland, but in the meantime, more islands are required as holding areas. Until about 15 years ago there were very few islands entirely free of animal pests. Although larger, more easily located mammals such as possums and deer, were being eradicated from islands around New Zealand as early as 1910, there was an assumption among scientists and managers that rodents simply could not be removed. The stumbling block with rodent eradication was more of a mindset than a lack of available technology. Looking back, the equipment was always there; it just required a different attitude to put it all together.

F ALL NEW ZEALAND’S offshore and outlying islands bigger than five hectares, there are 151 with at least one species of alien animal, and 88 without. There is uncertainty about the status of another 91. Most of those 330 islands are the responsibility of the Department of Conservation, which aims to either maintain them in their present state or to restore them to what is perceived as their former condition. Despite having been associated with humans for thousands of years and numerous attempts at eradication, rats and mice are still with us. Their ability to adapt is well known as they can respond quickly to changes in food source or living conditions. This is partly due to their high reproductive rate and partly their general physiology. In New Zealand, the four species known to have become established are ship

rat, Norway rat, kiore and house mouse. Ship rats are able climbers, occur throughout New Zealand and are the rats most commonly seen by humans. Because of their ability to climb so well, and widespread distribution throughout the country, ship rats are probably the most dangerous pest of the four. Norway rats are usually associated with water, drains and sewers, and have only limited climbing ability. The kiore (see box above) has been ousted from most of the mainland by the other rats and now occurs only in Fiordland, and on certain offshore islands. It is a good climber. Mice are found throughout the country and build up to huge numbers in the South Island beech forests every five to seven years, when beech trees seed profusely. This has serious implications for birds because stoats cash in on this abun-

dance of mice, are able to breed more successfully and so the resulting high numbers of stoats the following year play havoc with vulnerable species like yellowhead. The behaviour of a pest species determines the methods used to eradicate it. Rodents are vulnerable to anti-coagu-lant poisons. The most effective are brodifacoum (under the commercial label Talon), bromodialone (Ridrat), and flocoumafen (Storm). All are derived from the first generation anti-coagulant, warfarin, which caused rats to die from internal bleeding after one feeding. The rapidity of the effects, however, resulted in cautious rats, which only had a small taste of bait on their first day of exposure, developing the symptoms of poisoning but then recovering. Those rats then became bait shy and were no longer susceptible to that bait.

Research showed, in fact, that small amounts of warfarin deliberately fed to rats meant they developed an ability to change the clotting characteristics of their blood so they could survive on a warfarin bait diet. This is little different from prescribing small doses of anti-coagulants to humans to reduce the risk of a stroke from a blood clot. The big difference with the second generation toxins is that they have a delayed action. Rats can feed on them for three or four days before developing the symptoms of poisoning, by which time they have normally consumed many times a lethal dose so are almost guaranteed to die. It was about 1970 that these toxins became commercially available in New Zealand and since then they have been the mainstay of all rodent control. In 1982 I was assigned the task of developing methods suitable for eradicating rodents on islands up to 50 hectares. I began on Rurima Rocks in the Bay of Plenty by putting to use some results of basic bait trials I had carried out on Lady Alice Island with kiore. These had provided a list of flavours, baits and dispensing methods that kiore accepted. It took some time to eradicate kiore from this group of islets. Initial attempts used 1080 but failed because the rats refused to eat toxic baits. However, once a change to anti-coagulant was made the rats were soon eradicated. Once the department had been convinced that a formula had been found, the next step was to move up in size. I had already selected 18-hectare Korapuki Island in the Mercurys, after flying over it en route to Red Mercury to release little spotted kiwi. Permission was granted after some persuasive talking and in 1986 rats were eradicated from that island. The standard method of using bait stations was labour intensive, however, and for very large islands logistically difficult. On Mana Island, for example — at 230 hectares the largest island cleared of rodents to date — about 5,400 bait stations were required to remove mice in 1989. Y ABOUT 1987 it was obvious that the use of anti-coagulant toxins was producing the desired results, and any doubts about their usefulness were gone. At that time two paths of development were being pursued. One continued with fine-tuning the use of bait stations, leading to the successful operation against Norway rats on Hawea and Breaksea Islands off Fiordland in 1988. The other path began with bait stations of a differ-

ent design, but progressed to broadcasting bait by hand on small islands, then onto aerial applications. Sights turned to larger islands and more efficient methods of applying bait, including the use of helicopters with underslung buckets. However, applying tonnes of bait to the environment might appear to be against all conservation ethics. Unlike ground application where the placement and shape of bait stations gives considerable control over which animals can take the bait, aerial spread provides all

opportunistic animals with free access to it. The species likely to be at risk are easy to determine, but quantifying the risk is not so simple. There are two basic issues. Non-target species might eat the bait and die. This depends on how much bait is eaten and the species’ tolerance to the toxin; some birds, for example, are remarkably tolerant to Talon. The other very complex issue is the extent to which the toxin enters the environment or food chain. Theoretically a

bird could die from eating insects that have fed on bait. But, as with the aerial drop of 1080 against possums, the decision has to be based on net benefit. Some basic facts: brodifacoum is not water soluble, but once it enters the subsoil it is broken down by soil micro-organ-isms. This process may take several weeks during which time bait could be available to non-target species. The toxin once fixed into animal tissue may persist for several months. The mode of action also varies according to what animal has eaten the bait. Invertebrates are not affected as they do not have blood and simply pass the toxin through their system in four or five days. Reptiles, on the other hand, while not affected by the anticoagulant properties of the bait, die from overheating as the bait interferes with their ability to thermo-regu-late. Although lizards are not attracted to the actual bait, they can in some cases prey on insects that have just eaten bait. The numbers of most ground-dwell-ing invertebrates and native birds or lizards on islands with rodents is relatively very low. Intensive searches have to be made to find any animals of size, and whole groups are absent. This means that the chance of a saddleback, for example, preying on the weta that has just eaten a bait is remote. In addition, most large ground-dwelling insects are nocturnal, and as saddleback are day-time feeders they do not have easy access to this food source anyway. Even if they did, the number of weta that would have to be consumed to provide a lethal dose far exceeds the relative abundance of weta on rat-infested islands. Most deaths of native birds and lizards that have occurred to date have been from the direct consumption of bait. The application of aerial methods for use against rodents was developed slowly, and began with the manual trial broadcast of bait on Double Island in the Mercury group in 1989. Over a long weekend a group of volunteers traversed the island and saturated it with bait. All rats were killed and the next step was to try the same principle on some of the Mokohinau group, using a helicopter. This was a crude method with a fire fighting monsoon bucket and bait applied at the high rate of 30 kilos a hectare. The rationale for the overdosing was the absence of non-target species on these highly modified islands. Again the operation worked. Success brings attention, and a sponsorship deal was established with ICI Cropcare to attempt eradication of kiore from three islands in the Mercury group, starting with Stanley Island in 1991.

Intensive ground searches for dead birds after the drop revealed four saddleback, one morepork and a few introduced passerines. This level of mortality is well below the threshold of concern. The birds probably died from eating bait directly, apart from the morepork which would have eaten a rat full of bait. Subsequent monitoring of the island's saddlebacks showed that, despite a very dry summer which usually results in increased juvenile mortality, there were higher than normal numbers of young birds alive. Survival of the banded adult population was equal to that in a normal year. The conclusion had to be that saddleback fared better after the removal of kiore than before, and that any deaths due to the poison were more than compensated for by increased productivity due to the absence of the rats. Since those early days of "suck it and see", a lot of resources have been applied to ensuring environmental safety. The persistence of toxins in soil, effects on invertebrates, freshwater animals and, in particular, native birds have all been studied. To date, we have been unable to detect the presence of the toxins in the soil using the most sensitive methods of analysis available. Certainly there have been deaths of non-target species but the level of these has always been well within the limits of natural mortality, and more than compensated for by the productivity achieved after the removal of the rodents. Although we know there could be a toxin persistence issue, every possible step is being taken to ensure those species we are attempting to protect do not suffer. F COURSE the problem of introduced predators is not just a story about rats. Of all the pest predators in New Zealand, stoats are by far the most difficult to control. Unlike rodents they seem to need live prey, so developing a bait is very difficult. In particular, female stoats need a stimulus derived from catching live animals to trigger breeding. Also it is very difficult to detect stoats as they are essentially solitary, secretive and very wary. Fortunately, apart from islands within about a kilometre of the mainland (stoats are good swimmers), they are not a problem on offshore islands. If by some chance, however, they were to get onto a place like Little Barrier we are ill-equipped to do much about it. The recent report of a stoat on Stewart Island highlights this dilemma. There is some stoat-control work being carried out at present with poisoned

eggs which, while labour intensive, is producing encouraging results. The reality, sadly, is that because the problem of stoats is so widespread any control method is only going to be localised and for specific threatened species. It is too soon yet to predict exactly where this will all lead, but, if past progress is any indication, by the turn of the century a long hard look will be being taken at islands like Campbell (11,000 hectares) which is infested with Norway rats. Already DoC is putting in place a strategy that will answer those questions relevant to eradication on Little Barrier, Raoul, Codfish, and Mayor Islands. We may be a long way off actually ordering bait for those operations, but if the majority of New Zealand’s endemic species are to make it into the next century with a realistic chance of unaided survival then ambitious projects like these need to be started now. International interest in New Zealand’s achievements in island pest eradication is growing. Many overseas wildlife agencies also want to create pest-free environments for endangered species or embark on more ambitious restoration programmes, and New Zealand is seen as a world leader in island restoration. Requests for advice and assistance have come from as far away as Canada, the US, Saudi Arabia, France, South Africa and Australia. This is encouraging, not just because it gives New Zealand recognition for its pest eradication achievements, but it gives us a way of contributing in a practical way to conservation around the world. Methodology has changed somewhat — from putting out bait stations in the early ’80s and recording the position using a hand held compass, to plotting weigh points into the computer as an island’s perimeter is flown. Ten years ago a global positioning system (GPS) to provide accurate flight paths was not even a consideration. Now it is a regular tool of the business. Logistically some of our big islands will be a bit of a nightmare. But with good planning those problems should be as easy to overcome as the original scepticism apparent when work first began all those years ago. @

IAN McFADDEN 1s DoC's national technical coordinator for island pest eradications.