Department of Conservation efforts to wipe out cats on Little Barrier Island to protect the Cook's petrel backfired as rats became an even more deadly threat.
Efforts to rid the island of feral cats in 1980 failed to reverse the decline in numbers of the native Cook's petrel, says Matt Rayner, a PhD student at the University of Auckland's School of Biological Sciences.
While 32 per cent of breeding burrows fledged a chick when both cats and rats were preying on the petrels, only 9 per cent produced chicks after cats were removed from their breeding areas, he said in research published yesterday in the latest issue of the Proceedings of the National Academy of Sciences.
"The cats were suppressing the rat numbers, so the removal of the cats allowed rat numbers to increase, and as rat numbers increased ... they [hunted] petrels more heavily," said Mr Rayner.
Cook's petrels used to be found everywhere in New Zealand, but they disappeared from the mainland in the 19th century through human impact on their habitat.
The size of pigeons, the burrow-dwelling Cook's petrels on 2817ha Little Barrier Island faced local extinction by the late 1990s.
But in 2004, DoC mounted a massive eradication campaign against rats on the island, including the so-called Polynesian rat, the kiore. The $700,000 eradication was planned years earlier, but plans were aborted in October 2003 because some Maori regarded kiore as taonga (treasure).
Other researchers said at the time that DoC had come under strong political pressure over kiore eradication on Little Barrier and had "bypassed" it for 10 years while it tried to work through the issues with local iwi Ngati Wai, he said.
DoC wiped out the rats with 55 tonnes of brodifacoum anti-coagulant rat bait, except for a couple of hundred Ngati Wai took to the mainland.
Little Barrier was the petrel's world stronghold, with a population of 50,000 breeding pairs. The only other population was on Whenua Hou, near Stewart Island, which hosted several hundreds of the birds, and was declared rat free in 1991.
On Little Barrier, more than 90 per cent of Cook's petrel chicks were being predated by the rats.
"They are poorly adapted to predation by mammals but can live up to 30 years," Mr Rayner said. "They live a long time but breed very slowly ... one egg every couple of years."
After 2004, the breeding rates of petrels shot up to 59 per cent, though Mr Rayner noted that at lower altitudes on the island petrel breeding rates were little changed even after the rats were removed. Researchers attributed this to the rats having had a wider range of food sources on the lower slopes.
It was important for conservationists to fully understand ecological systems before carrying out eradication exercises.
"You had better be sure that you understand what is going on in the community ecology between introduced predators and native prey.
"In a lot of cases, these introduced species have been introduced for quite some time, so they have been here for 50 to 150 years ... The result could be quite unpredictable."