The sharp population decline of kakapo is associated with European colonisation of New Zealand, rather than Polynesians, researchers say.

The decrease in population size and genetic diversity for one of New Zealand's most iconic parrot species coincided with European arrival, University of Otago associate professor of zoology Dr Bruce Robertson said.

Previously abundant throughout the country, there are now only 155 living kakapo, and they are intensively managed on offshore island sanctuaries.

Dr Robertson said the study, published in the Journal of Heredity, supported the hypothesis that the introduction of mammals such as stoats may have been the primary cause of the parrot's decline.

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The study compared modern day and historical kakapo genetic variation and used population modelling to estimate the timing and magnitude of the decline since the arrival of Polynesians to New Zealand.

"We found that population decline in kakapo was not associated with Polynesian arrival, but with the more recent European colonisation," said Laura Bergner, who undertook the research for her Masters at Otago's department of zoology.

While Polynesians hunted kakapo for food and skins, introduction of mammalian predators may have been more important in kakapo decline than hunting.

"It is possible that an earlier impact of Polynesians on kakapo demography may have been eclipsed by the more recent impacts of European colonisation, but examining older kakapo samples, such as those from Maori middens, is required to shed more light on any impacts of Polynesian settlement on kakapo," Bergner said.

"Our results lend support to the current strategy of maintaining predator-free offshore islands for kakapo to live on. Although it was previously known that kakapo exhibit low genetic diversity, understanding how quickly this loss occurred can also help manage what genetic diversity remains."

Dr Robertson said estimates of historical population sizes can give managers accurate conservation targets.

"If we succeed in the ambitious plan of eradicating all introduced mammalian predators from New Zealand by 2050, historical population size estimates could give managers a target for the number of wild kakapo," he said.