Too Rich A Diet

Forest and Bird, Issue 312, 1 May 2004, Page 38

Too Rich A Diet

HAYLEY MEEHAN

explains why a good

year for plants can be bad for birds.

he prolific seeding of native plants can be a disaster for native birds. While the birds may have an easier time, so do the animals which prey on them. The phenomenon is known among scientists as ‘masting’ — the setting of much larger amounts of seed than normal. The consequence is that native birds find food more easily; ‘masting’ even triggers more breeding in some cases. Unfortunately, ‘masting’ also helps mice and rats, and the animals which prey on them. When stoats and other forest pests run out of this abundance of food, they turn on the newly replenished bird populations and can ravage them. Already ‘masting’ has cost a large part of the population of orange-fronted parakeets, when the Department of Conservation failed to recognise that an explosion of the rat population was possible in the last highcountry valleys the birds inhabit. The Department of Conservation has also blamed two consecutive ‘masting’ events for the loss of kiwi in Okarito Forest in South Westland. In 2002 there was a rimu mast and the following year a kahikatea mast. Both years had high numbers of rats and stoats which led to high levels of predation of the kiwi chicks. Mast seeding is the technical term for the production of large seed crops by plant populations over wide areas. The huge increase in seed abundance can improve the

plants’ reproductive success but it can also drive big changes in the populations of seedconsumers. This in turn can significantly impact on other parts of an ecosystem. Although it is a worldwide phenomenon, heavy seeding appears to be especially common in the New Zealand flora. The reason for this remains a mystery. In New Zealand, the ‘masting’ of beech is probably the most studied. In the extensive beech forests of the South and North Islands, ‘masting’ usually occurs once every two to five years. It is rare for there to be two beech masts in a row. In beech forests kaka and kakariki usually nest only in mast years and ‘masting’ has been found to boost breeding success of other birds such as yellowhead/mohua. The unfortunate downside to a good beech-seeding season is the increase in rodent populations, which in turn results in prolific breeding by stoats. Many native birds, including kaka and mohua, now have greatly reduced distribution and populations due largely to predation by rats and stoats. But ‘masting’ is not confined to our native beech species. It is also known to occur in hinau, pigeonwood, tawa, some podocarps, cabbage trees, flaxes, and tussocks. For beech and the majority of these species, the trigger

is believed to be particularly high temperatures the summer before seed fall. This cue is linked with the "La Nina’ phase of the Southern Oscillation. As they respond to the same trigger, most species mast in the same year. However, because it takes two years from floral development to seed production, rimu ‘masting’ is usually a year later. Many species, like hinau, are patchily distributed throughout the forest so the total of extra food available in a mast year is generally a lot less than in a ‘masting’ beech forest. On southern islands such as Stewart and Codfish/Whenua Hou, however, there are stands in which rimu forms a continuous canopy. In these forests the huge production of rimu fruit is crucial to the breeding success of kakapo. The population of kakapo was boosted by 24 young in the last mast season (2002), but last year kakapo did not breed at all. In the winter of 2002, rimu fruit production in Southland’s Waitutu forest attracted flocks of kaka, but this was followed by an irruption in mouse populations. A rapid build-up in stoat numbers followed — bad news for the survival of hole-nesting kaka. This was the first time this pattern had been documented in response to rimu ‘masting. The ups and downs of mast years and their impact on rodent populations have been observed only once in non-beech forests in the North Island. In the mixed podocarphardwood forest of the Orongorongo Valley east of Wellington, in the summer of 197172, some 711 kilograms of fruit and seed — chiefly hinau, and pigeonwood — fell per hectare. This heavy crop of fruit was observed to boost the numbers of ship rats and to also improve the survival rate of young possums still in the pouch. It may not just be the increased seed production which can lead to the increase in

rodent and therefore stoat populations. Often in mast years invertebrate numbers also increase. The invertebrates may be feeding directly on the flowers and seed, or they may be responding to the same climate signals as the ‘masting’ plants. For example, successful breeding in the previous warm summer may lead to good survival rates into the start of the following spring. This could set the scene for high invertebrate numbers coinciding with the seed mast. High numbers of invertebrates in spring may explain the rodent increases prior to the fall of the bumper seed crop. In the North Island the increased seeding of podocarps, and hardwoods such as hinau and pigeonwood, appears to increase the number of breeding attempts by fruit-eating birds such as New Zealand pigeon, kokako and kaka. Sadly, in these mixed forests, the normal number of rodents and stoats is generally much higher than is usual in beech forests. This means that while more chicks may be produced in a mast year, this can be offset by the death of adult birds, particularly females, in non-mast years. ‘Masting’ is an energy-intensive process, Plants reallocate significant resources away from vegetative growth, such as leaves and twigs, into the production of flowers and seed. If climate change leads to warmer summers, we may be in for more regular ‘masting’ events. Not only could this increase the stress on the plants, it could also increase the prevalence of high rodent and stoat populations. Unfortunately, for much of our already hard-pressed native fauna, this would be a bleak outlook. — HAYLEY MEEHAN was till recently a conservation officer with Forest and Bird in Wellington. She now works for the Environmental Risk Management Authority.