AFTER NEARLY 200 years of infestation, Campbell Island/Motu Ihupuku may finally be free of rats. A Department of Conservation team spent a month on the island in the winter of 2001 spreading 120 tonnes of poison bait in an attempt to rid the island of the rodents, which have decimated much endemic wildlife.

Almost 700 km south of Bluff, and with an area of 11,300 ha, Campbell Island was the greatest challenge yet for the pest eradication techniques that have been developed, largely in New Zealand, over the past 20 years. Sea cliffs rising to over 300 m—high-rise havens for the agile rodents—and the island’s consistently adverse weather added to the challenges of size and isolation.

Even 10 years ago, ridding so large an island of rats was an impossible dream. But following a number of successful smaller-scale operations—notably on Breaksea Island (26 ha, 1987), Kapiti Island (2200 ha, 1996) and Codfish Island/Whenua Hou (1800 ha, 1998)—and with the help of technological advances such as GPS-equipped helicopters, and growing political support, such dreams are now becoming reality.

Two further island nature reserves due for eradication operations are Raoul Island, in the Kermadecs (3000 ha, to be cleared in July 2002) and Little Barrier/Hauturu Island (1800 ha, scheduled for 2003).

Norway rats arrived on Campbell Island soon after its discovery in 1810 by Captain Hassellburg, of the sealing ship Perseverance. Cats followed, probably introduced by farmers in the early 1900s. Together, these intruders wiped out all the native land birds from the main island, such as the Campbell Island pipit, snipe and teal, along with smaller sea birds, including the storm petrel, diving petrel and various prions. They also reduced the colonies of larger sea birds, such as the grey petrel, white-chinned petrel and sooty shearwater, to a tiny fraction of their former abundance.

Parakeets and rails, too, are thought to have fallen victim to the mammalian onslaught, while a range of invertebrates, including ground weta, giant slugs and several kinds of large weevil, also fared badly. Rats have even been seen diving for mussels.

Cats, never especially numerous, had died out naturally by the 1990s. Derek Brown, a member of the Campbell Island eradication team and a specialist in island management, thinks the cats might have disappeared as a result of the recovery of the island’s vegetation following the removal of sheep.

“A cat can easily run across a grazed paddock. But with no sheep to crop them, the island’s tussock and megaherbs grew taller and thicker. This would have made hunting more difficult, and cats would also have had to push though the vegetation, which, given the island’s climate, gets very wet. So the cats would have been wet and cold, which would certainly have inhibited their breeding, and may thus have led to their demise.”

Rats, on the other hand, continued to prosper, and by the time the cats had gone the rodent population was estimated to be fluctuating seasonally between 50,000 and 200,000. The whole island was infested, from the sea-bound cliffs, to the open, swampy ridges, to the shore, where easy foraging along the tide line made for the highest concentrations.

Given the island’s size and remoteness, the usual application of two drops of poison bait totalling 12 kg/ha was considered neither financially nor logistically feasible. Instead, a single drop of 3 kg/ha was planned, but with adjacent swaths overlapping by 50 per cent to eliminate the risk of gaps in the bait-spread, meaning the whole island would receive a total drop of 6 kg/ha.