20-year DoC project in South Island valley shows value of pest control.

A 20-year effort at the Department of Conservation's longest-studied area for pest control has led to native bird numbers doubling - an encouraging sign for New Zealand's bold predator-free 2050 mission.

A programme in South Westland's Landsborough valley is DoC's longest study charting the response of birds to pest control, giving conservation scientists insights into what approaches work in beating back predators.

Predator control began in the valley in 1998 after the impact of predators on birdlife was observed.

Since then, DoC has carried out valley-wide trapping and six aerial 1080 operations timed with increasing rodent levels. The most recent two, in 2014 and 2016, covered the entire valley.

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The monitoring programme involved a team of bird experts doing 175 five-minute bird counts each spring at fixed points in the valley, providing an index of relative bird numbers.

All native birds showed increasing or stable population trends apart from two species— tautou (silvereye or waxeye) and the migratory long-tailed cuckoo (koekoeā) — which had declined.

Numbers of mohua (yellowhead), tuī (parson bird), korimako (bellbird), pīpipi (brown creeper), tītitipounamu (rifleman), riroriro (grey warbler) and kākāriki (yellow-crowned parakeet) have all steadily increased during the past 20 years in response to a sustained programme to suppress rats, stoats and possums.

Other species — kākā (bush parrot), pīwakawaka (fantail), ngirungiru (tomtit) and kereru (wood pigeon) — have stayed stable.

Kākāriki (yellow-crowned parakeet) numbers in the South Westland's Landsborough valley steadily increased over the last 20 years. Photo / Nir Ketraru
Kākāriki (yellow-crowned parakeet) numbers in the South Westland's Landsborough valley steadily increased over the last 20 years. Photo / Nir Ketraru

"Mohua is one of the most threatened birds there and has increased 24-fold over the time of the study — going from 14 to 338 birds in the monitoring area," Conservation Eugenie Sage said.

"These results highlight that where we control pests over whole valleys and forests, we can turn around the fortunes of our native birds and help address our biodiversity crisis where 82 per cent of our birds are threatened or at risk of extinction."

Sage said new funding for DoC of $181.6 million over four years would allow more of the work to happen.

A specific allocation of $81.3m dedicated to predator control would result in 1.85m ha — and an area larger than Northland and Auckland combined — where predators were kept low on an ongoing basis.

"The investment reflects New Zealand's longstanding commitment to the international Convention on Biological Diversity, and our responsibility to protect the species and ecosystems found nowhere else in the world."

Mohua numbers increased 24-fold over the time of the study-going from 14 to 338 birds in the monitoring area. Photo / DoC, James Rearden
Mohua numbers increased 24-fold over the time of the study-going from 14 to 338 birds in the monitoring area. Photo / DoC, James Rearden

The results follow other new data from a Landcare Research-led meta-analysis of 23 sanctuaries, three "mainland islands" and other published studies, showing pest control was working for some native species better than others.

Birds that benefited most were typically deeply endemic species, such as kiwi, North Island kōkako and hīhī. There were weaker benefits among more recently endemic species, like tui and tomtit.

Worst off were "recent natives" - including grey warbler, silvereye and fantail - whose populations often show little effect or even decline in response to control.

Generally, pest control tended to bring greater benefits within fenced sanctuaries where mammal pests had been wiped out, compared to unfenced mainland islands where some pests were being kept at low levels.

New Zealand has a goal of ridding possums, rats and stoats from the country by 2050.

The plan has four goals with a 2025 deadline: to sweep pests from another million hectares of land, to develop a scientific breakthrough that could exterminate at least one mammalian predator, to demonstrate areas of more than 20,000ha could be predator-free without the use of fences, and to complete removal of all introduced predators from offshore island nature reserves.