Each spring, millions of seabirds congregate on remote Campbell Island in the Southern Ocean to take part in the bustling, confused and hazardous business of reproduction. Waiting for them are one of humankind's most destructive hangers-on—rats, by the tens of thousands. Of all our species' travelling companions, these small rodents, which have held the island for almost 200 years, are among the most dangerous.
Adult seabirds can keep rats at hay most of the time, but eggs and chicks are vulnerable to predation. In one sense, the birds are fortunate: they are not dependent on the island for food. The Southern Ocean provides them with rich and extensive feeding grounds. Land birds have had a more difficult time of it. Many of the insects they rely on for food are also taken by the marauding rats. Ground-nesting birds, in particular, have fared poorly, and rats have ousted many of the island's original inhabitants and modified the centuries-old ecology by eating plants and their seeds.
The Department of Conservation has long wanted to eradicate Norway rats from Campbell Island, but the expense of the operation was always prohibitive. New funding, announced with the New Zealand Biodiversity Strategy in March 2000, has now made the huge project possible. The government has committed an extra $187 million to biodiversity protection between 2000 and 2005. Eradicating rats from Campbell Island will be the largest of the projects made possible by this injection of money, with $2.6 million budgeted over four years.
As well as being the largest of the new projects, Campbell Island rat eradication will also be the largest island-restoration project to have been attempted anywhere in the world, and follows the success of similar operations on Kapiti Island, near Wellington, and Whenua Hou (Codfish Island) off the north-west coast of Stewart Island.
The Campbell Island rat campaign will pose major difficulties for project leader Peter McClelland and his team. The location is extremely remote-700 kilometres south of mainland New Zealand—and the climate is harsh, with winds gusting to 240 kilometres an hour. Although DoC has perfected eradication techniques on smaller islands, Campbell Island, at 11,300 ha, is around four times the size of Kapiti, the largest New Zealand island to be cleared of rats so far.
In addition to the problems of working on a huge, remote and rugged island in the often bitterly cold climate of the Southern Ocean, the department will also be dealing with the toughest and largest of the three rat species in New Zealand. In the absence of predators, Norway rats on Campbell Island have developed the highest rat population density known anywhere in the world. Even in winter, when rat numbers are at their lowest, there are estimated to be 50,000 on the island.
Ensuring that the team gets enough poison bait on the ground at the right time to mount an effective strike against the rats will take careful planning. Only one poison-bait drop will be made, using three helicopters to spread 120 tonnes of bait. A fourth helicopter will carry bait up to a high central loading point from the shoreline.
Just getting helicopters to Campbell will be difficult. To reach the island, they will have to refuel twice: at the southern end of Stewart Island and again on Enderby, in the Auckland Islands. The poison drop is scheduled for late winter 2001, before the thousands of seabirds make their annual pilgrimage to the island for nesting and before female rats go underground to have their young.
At this time of year the days are short in southern latitudes, and there will be few when the relentless winds are weak enough to allow the helicopters to fly. Spreading bait over the entire island at the required sowing rate of 6 kg per hectare will take close to four days of reasonable weather. If weather forces a break in the spreading, it will be necessary to make a substantial overlap in the spread area.
Trials have been carried out already to determine the optimal bait for Campbell's rats. Brodifacoum will be used in a 6 g cereal bait. The exact formulation was chosen for its palatability and its persistence in Campbell's weather. "On Codfish we used a smaller bait which broke down rapidly, because we were concerned about non-target species," says McClelland. "Here we don't have any of those concerns, and persistence is what's important. Of course, the rats have to like the stuff. We tested one even more weatherproof formulation, but the rats wouldn't touch it We only get one crack at this job, and we'll either succeed or we won't. If there are any rats left, we've failed. A long lasting bait will help our chances." So will the habits of the Norway rat. It caches food underground, and if one rat dies, another will feed from its cache.
Assuming the clearance is successful, there are plans to return some threatened birds to their former home. Among them will be the Campbell Island teal, a flightless relative of the endangered New Zealand brown teal. A small population of these ducks has been bred in captivity at the Mt Bruce Wildlife Centre, and a breeding colony has been established on Whenua Hou in anticipation of the Campbell Island eradication project. There are also plans to re-establish a population of Campbell Island snipe, a diminutive wading bird which was found on a small rock stack off the coast of Campbell Island in 1997 (see New Zealand Geographic, Issue 37).
Eventually, it is hoped that Campbell Island will take its place among the growing number of island sanctuaries where some of the world's rarest bird, insect and plant species can live without harassment from mammalian predators.