Seasons of decline

In March 2000, we reported that a “beech mast” event—in which large areas of South Island beech forest flowered and set seed—had led to plagues of rodents in spring and a boom in stoat numbers during the 1999/2000 summer, with dire consequences for native wildlife (see GeoNews, Issue 46). The problem has recurred over the past summer, with predictably disastrous results.

Written by      

Grant Stirling

Many native species in the South Island have had a ruinous past six months, after a massive increase in rodent and stoat numbers over spring and summer.

The unlikely trigger has been the flowering of many beech forests for two years in a row. This highly unusual occurrence has caused a compounding effect, with residual pest populations from the first beech mast being boosted further by a mild winter and a continua­tion of the food supply.

The worst fears of conservation workers were realised as mouse, rat and stoat numbers built to unprecedented levels over the past spring and summer.

During summer, Depart­ment of Conservation staff caught 140 stoats in the Rotoiti Nature Recovery Project in Nelson Lakes National Park. The year before, after the first beech mast, the tally was 90. The year before, it had been just 22.

By November last year, staff at Nelson Lakes were recording rats in 100 per cent of tracking tunnels at a nearby site where rats were not being controlled. In early 1999, before the impact of the beech masts had been felt, rats were being recorded in just 12 per cent of the tunnels. According to DoC staff, the number of rats in the area had gone “off the dial.

“In Fiordland’s Eglinton Valley, DoC staff caught 274 stoats this summer, com­pared to 217 in 1999/2000 and just 31 in 1998/1999. From August until Decem­ber 2000, staff caught 867 rats, compared to 361 in 1999 and 35 in 1998.

In towns surrounded by beech forest, such as Murchison, St Arnaud and Springs Junction, locals experienced the rodent plague first-hand. Through­out spring, home-owners laid rat poison and were forced to keep windows and doors closed. Domestic cats had a field day. It was to no avail. Local electricians were called out to repair chewed cables, and rodent nests were found in beds, washing machines and ovens. Casual walks in the bush revealed rodents everywhere, and the Buller Gorge highway was littered with road-killed rats.

DoC’s national predator control officer, Darren Peters, describes the past season as one which will have had significant impacts on a range of native bird, invertebrate and lizard species. “Predator events such as this can produce big declines in surviving populations of wildlife,” he says. “In non-mast years, there will be an ongoing gradual decline in many populations, but events like this represent a major kick in the head.”

One casualty of the mast-fueled rodent boom has been the mohua (yellowhead) on Mt Stokes, in the Marlborough Sounds, where DoC’s 15-year battle to save the region’s last population of the birds has come to an end.

Six pairs of mohua were found on Mt Stokes in 1985, and, by 1998/9, the popula­tion had been painstakingly built up to about 90 birds. On the back of the first beech mast, rat predation saw the population crash to just 27 birds. Four were transferred to a predator-free island, but when staff surveyed the mountain again in spring, they couldn’t find any of the remaining birds. Even though Mt Stokes did not experience the second beech mast, low residual rat numbers appear to have been the cause of the eradication.

In the Eglinton Valley, the first beech mast saw about 30 per cent of the valley’s female mohua killed by rats. This year, although monitoring was not under­taken, staff say higher pest come to an end.

Six pairs of mohua were found on Mt Stokes in 1985, and, by 1998/9, the popula­tion had been painstakingly built up to about 90 birds. On the back of the first beech mast, rat predation saw the population crash to just 27 birds. Four were transferred to a predator-free island, but when staff surveyed the mountain again in spring, they couldn’t find any of the remaining birds. Even though Mt Stokes did not experience the second beech mast, low residual rat numbers appear to have been the cause of the eradication.

In the Eglinton Valley, the first beech mast saw about 30 per cent of the valley’s female mohua killed by rats. This year, although monitoring was not under­taken, staff say higher pest numbers mean even more female mohua and their chicks were probably killed.

Ground-nesting birds are always particularly vulnerable to predator pressure. Kiwi Recovery Programme co-ordinator Hugh Roberston says it is likely the stoat impact on the few great spotted kiwi and Haast tokoeka surviving in beech forests will have been greater than usual this summer.

The known impacts on another ground-nesting bird, the blue duck, illustrate the scale of the problem. Ten blue duck nests were moni­tored in the Te Anau region over spring. No ducklings were hatched, and at least two adult females were killed. The use of video monitors on three nests showed that stoats were the villains.

At DoC’s Hurunui Mainland Island, in north Canterbury, only one rat had been recorded since monitor­ing began in 1995. This summer, rats spread throughout the remote high-country valley for the first time, and mice and stoats were caught in record numbers. Although key species within the mainland island, such as parakeets and great spotted kiwi, will have been protected by DoC’s pest-control programme, co-ordinator Andrew Grant says “major impacts” are likely in adjacent areas that did not have the benefit of pest control.

While conservation workers have held the line in some pockets of countryside over the past two years, the real impact will never be quantified.”We cannot monitor every area,” says Darren Peters. “Populations in isolated and vulnerable areas are sure to have disappeared.”The only witness will be the silent testimony of the species themselves.