Summer of discontent
The humble beech seed is an unlikely agent of chaos, but over the past 12 months it has been responsible for a chain of small disasters in New Zealand’s forests and countryside.
For many people, the enduring ecological memories of the start of the third millennium will be of plagues of stoats and rodents, threatened bird populations and the long hours put in to protect the most vulnerable species.
At Te Urewera National Park, kiwi researchers will remember the summer of 1999/2000 for the forced evacuation of kiwi hatchlings and eggs out of their study area in order to save them from almost certain stoat predation.
In Fiordland’s Eglinton Valley, a whopping 220 stoats were trapped as part of a research programme, compared to 35 the year before.
For Department of Conservation workers on Mt Stokes, in the Marlborough Sounds, the millennial summer will be remembered as the time when 15 years of hard work to save a tiny remnant population of mohua, or yellowhead, was almost ruined by a rat invasion.
And at the Rotoiti Nature Recovery Project in Nelson Lakes National Park, exceptionally high mouse numbers over winter had conservation staff throwing up their hands in despair.
That the humble beech seed is ultimately responsible for all this carnage and chaos reflects a feature of New Zealand flora called “masting.”
Many New Zealand plants—from beech and rimu through to cabbage
trees and tussocks_ do not flower
regularly, and usually flower heavily only once every few years. These seasons of heavy flowering are commonly known as mast years.
Landcare Research has found that masting in beech species, in particular, is linked to warm summer temperatures in the preceding summer, and usually occurs every four to five years.
The result of hot, dry conditions in the South Island during the El Nino summer of 1997-8 was a heavy masting in the summer of 1998-9. It was a summer when beech forests took on a reddish hue from the massed male flowers, when clouds of pollen filled the air, and eventually, in autumn, when a confetti of beech seeds began to fall. At Mt Thomas, in Canterbury, Landcare recorded densities of up to 15,000 seeds per square metre—the heaviest seeding there since records began in 1965. In Te Urewera’s mixed beech/podocarp forests, locals claimed it was the biggest mast year in living memory.
The carpet of rich, nutritious seed that results from a mast year provide a major injection of food into ecosystems. Everything benefits, from forest-litter invertebrates through to insectivorous birds such as robins and tomtits, seed-eating birds such as parakeets and kaka, and lizards.
The influx of food typically translates into good breeding seasons. Native parakeets, for example, breed furiously in the winter following a mast year. Mice and rats also benefit from the increased seed and invertebrate supply, and can often reach plague numbers by winter. Stoats, weasels and ferrets then benefit in turn, as they gorge on the high numbers of rodents and birds. In the complicated web of predator-prey relationships, even trout do well from a mast year. Mice swimming across rivers provide a unique treat for trout, and a nutritional boost far above that which comes from the normal diet of small invertebrates. One Canterbury record is of a trout being caught with 18 mice in its stomach.
The high number of stoats, in particular, causes native birds to suffer, typically leading to population crashes in the late summer following a mast year. Most seriously affected are hole-nesting birds such as kaka, mohua and parakeets, and ground-nesting birds such as kiwi. But even the less vulnerable tui, kereru and robins are affected.
Landcare kiwi researcher John McLennan says 1999/2000 has been an “incredible” stoat plague year at Te Urewera. After losing seven out of nine of the first clutch of kiwi chicks to stoat predation, the joint Landcare, DoC and iwi research team realised they were likely to lose all the chicks from the second clutch as stoat numbers increased further.
“We admitted defeat. We took the chicks out of their nests as either hatchlings or as eggs about to hatch and raised them in captivity. It is the first time I have ever had to do that,” says McLennan.
At Mt Stokes and in the Eglinton Valley, DoC staff working to protect mohua populations kept stoats under control through intensive management, but in both areas they struck an unexpected threat: rat predation. At Mt Stokes, the population crashed from 90 to fewer than 30 birds over winter, while in the Eglinton Valley 10 out of 28 pairs of mohua suffered predation. Rats were confirmed as the villains in six of the 10 cases.
On Mt Stokes, the loss was particularly frustrating. Since the rediscovery of mohua on the mountain in 1985, their numbers had painstakingly been built up from six to 90. But because ship-rat numbers built up over winter and rats moved for the first time into the mohua breeding zone, the result was a crash of the fragile population back to a critical level. Plans to transfer the birds to a predator-free island sanctuary over the summer were abruptly put on hold.
If these were the problems suffered in intensively-managed areas, elsewhere in the country’s beech forests stoats and rats must surely have run amok. Here the ecological damage from the beech mast will never be quantified, but, as any conservation worker will gloomily acknowledge, the impacts are more than likely to have been bad.
And there may be worse to come, for beech trees flowered once again in many parts of the South Island and eastern North Island over summer. Early indications are that seed is setting for a second year in a row. A double mast threatens an even more violent roller-coaster ride over the coming 12 months.
If, for example, the number of stoats being caught in managed areas this summer represents a threefold increase on the back of last year’s beech mast, then a further mast this year could see a similar increase next summer. Where there was one stoat in 1999, there could be nine in 2001.
Such a possibility is likely, says DoC predator research scientist Elaine Murphy. “But a second year of abundant beech seeding is very unusual, and at this stage we don’t know what will happen.”
Much will depend on how heavy the seeding turns out to be. The numbers of rats and mice surviving from the first beech mast and the severity of the 2000 winter will also be key factors.
There is already one bright spot on the horizon. This summer’s miserable South Island weather means that the cycle of hot summers has been broken, and there is unlikely to be a third heavy beech flowering this spring.