Why New Zealand needs more big pest-fenced ecosanctuaries

Tuesday, 28 December 2021

The hills above the Hutt Valley contain some of the oldest native forest in the country, and a long-held dream held by Zealandia founder Jim Lynch could soon see it become a fenced eco-sanctuary, 3310 hectares in size.

OPINION: Hauturu-ō-Toi – better known to most Kiwis as Little Barrier Island – is the treasured whenua of Ngāti Manuhiri, sole preserver of hihi and wētāpunga and the recently rediscovered New Zealand storm petrel, and home by translocation to North Island tīeke, kākāpō and North Island kōkako. And famously free of all of the mammal pests that still ravage mainland ecosystems.

Until the excellent proposal for an ecosanctuary at Wainuiomata, near Wellington, there has been dismally little recognition that we can right now put pest-free sanctuaries of at least 3000 hectares in the best places anywhere on the mainland.

In pest control, people like to say there is no silver bullet, but there is – and it is pest fences.

The sparrow-sized New Zealand storm petrel, thought to have been extinct for more than a century, has been rediscovered on Hauturu-ō-Toi, or Little Barrier Island.

The proof is Sanctuary Mountain-Maungatautari, a Waikato reserve larger than Hauturu and now home to translocated whiteheads, rifleman, kākā, hihi, takahē, robins, the fastest-growing North Island kōkākō population and – in 2022 – kākāpō.

Maungatautari is in fact a cluster of five separate pest-fenced areas, and while eradicating mice from the main 3142ha area has proved impossible, the remaining 100ha of smaller cells are kept mouse-free, allowing sustained restoration of mouse-sensitive species such as tuatara and giant wētā.

A kākā in the Sanctuary Mountain-Maungatautari reserve in Waikato. It is also home to hihi, North Island kōkako and, from 2020, will have kākāpō.

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Rats are among the toughest predators to eradicate at large scale.

Pest fences work. According to Elton Smith, who manages the Orokonui Ecosanctuary near Dunedin, they “keep out most mammals most of the time”. In the past nine years, Maungatautari staff have trapped one hedgehog, 53 ship rats, four norway rats, 17 weasels and three possums inside their reserve.

John Innes at an event in Pureora Forest to mark the North Island kōkako’s recovery from the brink of extinction.

If 53 ship rats sounds like a lot, that is about one rat per kilometre of fence in nine years, a huge tribute to the staff who responded 24/7 to trees crashing through fence sections and other events since 2004, when the pioneering project started.

Pest-fenced ecosanctuaries on the mainland have hosted the removal of more pest mammal species (up to 14) at one site than anywhere else in the world, simply because there were and are more pest mammal species present than on any island. The most that have been removed from any island anywhere in the world is nine, from Rangitoto and Motutapu islands in the Hauraki Gulf, another proud Kiwi conservation record.

In contrast, current Predator Free NZ initiatives target on average just two mammal species at a site, and two-thirds of projects in the 2021 Predator Free 2050 five-year progress report list only one, usually possums, which are the low-hanging fruit of eradication, or stoats (more difficult but wide-ranging, so can they be targeted with comparatively few devices). None focuses in detail on ship rats and mice, the tough ones at large scale.

Predator Free NZ is a magnificent, very long-term, pest-control vision that should not be confused with restoration now. Restoration is much broader than two to four major predators. It requires control of browsing pests such as deer and goats, and of weed plants including wilding pines, as well as planting and numerous other steps to stop destruction of habitat.

My plea for fences is not an argument against Predator Free 2050. I have spent my 40-year career exposing the insidious damage done by the main culprits.

In 1992, using then-pioneering time-lapse video cameras at kōkākō nests at Rotoehu Forest, near Rotorua, we rediscovered that possums eat birds and eggs, and we filmed nightly battles between ship rats and sitting kōkākō females, and since then we have recovered kōkākō populations with intensive, large-scale pest control.

And I supported the significant 2013 Ruapehu meeting that affirmed that a predator-free vision was reasonable. Predator Free 2050 is a fine vision, and its projects seek practical, novel ways to achieve this at scale, something ecosanctuaries are not doing. But they are not even remotely trying to remove the required 14 pest mammal species at a site that pest-fenced ecosanctuaries already do.

It’s time to combine the skills, vision and resources of communities, iwi and agencies, and build more large pest-fenced ecosanctuaries in the best places. It is not just a good way to make small bits of the mainland predator-free; it is the only way. Put most simply, there is no other current tool.

This makes Forest & Bird’s assertion that “… it’s much more cost-effective over a big area to do landscape-scale protection” at best just wishful thinking. If we want to add another mainland site for kākāpō, then for the foreseeable future (20 years) it’s pest fences or nothing.

After decades of successful marooning of taonga species on treasured islands like Hauturu, it is now time – as Auckland Council’s Matt Maitland says – “to take passengers from the lifeboats and put them ashore”.

John Innes is senior researcher, wildlife ecology, at Manaaki Whenua - Landcare Research; a member of DOC’s kākāpō and takahē recovery teams; founding committee member of Sanctuaries of NZ Inc; and chair of the scientific advisory panel at Sanctuary Mountain – Maungatautari.