Department of Conservation monitoring has confirmed rat numbers are building to extremely high levels in many parts of Kahurangi National Park posing a major threat to native wildlife.
DOC is planning to carry out aerial 1080 pest control over about 270,000 hectares of the park this spring as part of its Battle for our Birds predator control programme to protect native species from rat and stoat plagues.
Monitoring shows rat levels have climbed in the park’s beech forest between May and August. Rat tracking levels increased from 54% in May to 90% in August in the Fyfe River area, from 31% to 51% in the Cobb valley, from 43% to 78% in the Waingaro River area and from 34% to 63% in the Oparara Basin.
The monitoring also shows rising rat numbers in the park’s Tasman Wilderness Area. In the Spey River area rat tracking increased from 25% in May to 58% in August, and in the Roaring Lion River area from 13% to 50%.
The escalating rat numbers are due to an exceptionally heavy beech seed-fall, known as a mast, providing abundant food. With plentiful rodents to feed on stoats produce more young than usual causing their numbers to explode in summer.
DOC Motueka Conservation Services Manager Mark Townsend said based on scientific research it was estimated that without pest control rat tracking levels would reach 100% in November in parts of Kahurangi National Park.
“We can’t let Kahurangi’s vulnerable native species suffer heavy losses from this growing predator onslaught. We are on track to carry out aerial 1080 pest control pest control in western, northern and eastern parts of the park in coming weeks, when weather conditions allow.
“We want to prevent increased predator attacks on nesting birds during their crucial spring breeding season so more chicks can survive to increase populations.
“We are particularly concerned to safeguard threatened populations of whio, great spotted kiwi, kea, kaka, rock wren, long-tailed bats and Powelliphanta snails.”
Aerial application of cereal baits containing biodegradable 1080 pesticide enables large-scale protection in difficult terrain. It rapidly knocks down rats to near zero levels, and stoat numbers are also substantially reduced through their eating poisoned rodent carcasses.
The Kahurangi aerial1080 pest control will also reduce possum numbers. Possums cause browsing damage to native vegetation and also prey on native birds, their eggs and native snails.
The Kahurangi operation is one of 25 confirmed Battle for our Birds operations using aerially-applied 1080 over about 680,000 hectares of conservation land to knock down rising predator numbers fuelled by the unusually heavy seeding in South Island beech forests. Six operations have been completed.
The pest control is targeted to protect the most at-risk populations of mōhua/yellowhead, kākāriki/parakeet, kiwi, whio/blue duck, kea, kākā, rock wren, giant land snails and native bats at sites across the South Island.
Rats usually don’t breed in winter and their numbers start to die off due to lack of food. However, when a heavy beech mast produces an abundant beech seed food supply, rat breeding occurs in winter. With plentiful food, rats can breed every six weeks and they produce litters of 5 – 8 pups with high pup survival. The pups mature in 12 weeks and in turn start breeding.
Research indicates that with an abundant beech seed food supply rat numbers grow about 1.2% a day – 35% a month – until about November when the seed food supply is running out due to seeds germinating.
Stoats breed only once a year but during a beech mast their breeding is especially productive, with about 10-14 kits produced in spring. As a result, stoats can undergo a five to seven-fold population increase in summer when the young stoats become independent and emerge from dens.
Rat tracking indicates rat densities using lines of tracking tunnels. Bait lures rats inside tunnels and their footprints are tracked using ink pads. The rat tracking figure is the percentage of tunnels showing evidence of rats over one night within a monitoring site.
Mice numbers can also rise in response to a beech mast but the monitoring shows their numbers have remained comparatively low in Kahurangi due to the high numbers of rats which prey on mice and compete with them for food.