By PHILIPPA JONES and ANNE BESTON
The breeding rate of tuatara on the Marotiri Islands near Whangarei has delighted and surprised scientists and conservation staff.
A survey last month found 43 per cent of the tuatara were juveniles, up from 2 per cent in previous surveys.
"The number of juveniles hatching has
just gone through the roof ... They're hatching all over the place," said Richard Parrish, Department of Conservation officer for the Whangarei area.
The breeding success is being attributed to a programme to eradicate the native kiore rat and the Norwegian (or ship) rat.
The islands, known as the "chickens" of the Hen and Chickens group, have been rodent-free since 1999 after a two-year poisoning operation.
Mr Parrish said it was not known how many tuatara were living on the Marotiri Islands, but a rough estimate put the population at 12 to 25 a hectare.
The number of tuatara on Stephens Island in Marlborough, which has one of the highest populations, is estimated at 1000 to 2000 a hectare.
"We've got a long way to go before we get to that level, but the rat eradication has created a niche in the ecosystem for tuatara and they're obviously making the most of it," Mr Parrish said.
Tuatara are sometimes known as New Zealand's living fossil because they appeared about the same time dinosaurs were evolving.
They are fully protected and confined to 30 offshore islands in 12 island groups, from the Poor Knights in the north to Stephens Island and the Brothers Islands in Cook Strait.
Their name means "spiny back" and the total population is estimated at about 100,000.
Their slow breeding rate means they were slow to recover from deforestation and introduced predators during early European settlement.
Tuatara lay about a dozen leathery-shelled eggs between October and December. These are then abandoned, becoming vulnerable to native birds and other tuatara.
They live to between 60 and 100 years and have a variable body temperature which allows them to survive in cold climates. They feed on wetas, worms, lizards and small seabirds.
There are two species. The more common is Sphenodon punctatus, or the Northern tuatara. The other is the Brothers Island tuatara, or Sphenodon guntheri.
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