Many native New Zealand plants, including beech trees and tussocks have occasional bumper years of flowering and seeds - known as mast seeding.
Rises in average temperature could be contributing to an increase in the frequency of mast seeding years.
Latest research indicates that mast seeding is triggered by the change in average summer temperature between the two previous years, not just the temperature in the previous summer.
The incidence of mast years has been increasing since 2000 and this has been linked with the rise in average New Zealand temperatures.
Mast years have a significant impact on New Zealand's ecosystems. Before the arrival of mice and rats, the large amount of seed in a mast year was eaten by birds and insects. Now the seed is eaten by mice and rats – and populations of these fast-breeding introduced pests explode thanks to the abundance of food. Numbers of introduced stoats also increase as the stoats feed on the high numbers of rats and mice. Once the seeds are gone, the huge numbers of rats, mice and stoats turn to eating our native birds and other native wildlife.
When mast years were infrequent, populations of native species could recover between the mast years. Now there is little chance for them to recover. This has taken a devastating toll on our native species:
In pre-European times mohua (yellowhead) were found throughout South Island forests but their numbers have been greatly reduced, largely due to predation by introduced pests. Their decline has accelerated in the last decade as predator numbers have increased due to frequent mast years.
From 1999-2001 some populations of mohua were hit hard after a beech mast. Mohua in Mt Stokes in Marlborough became extinct. In the Eglinton Valley in Fiordland the numbers collapsed, almost to extinction.
Already mohua have been wiped out in many areas, and if we do not do more to protect them they will only remain on offshore pest-free islands and become extinct on the mainland.
Mohua are not the only species threatened by increasing mast years. Whio (blue ducks), native bats, kiwi and kakariki are also severely affected.These species are only just hanging on in areas of intensive pest control, particularly thanks to the use of aerial 1080. The Department of Conservation's Battle for our Birds will protect important populations of mohua and other native birds following the 2013-14 mast year.