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New Zealanders have been using fences to exclude unwanted animals for years. For the most part, keeping domestic stock out of waterways. But a predator exclusion fence is much more complicated. The fence needs to account for the climbing ability of possums, the jumping ability of cats and stoats, and the burrowing ability of rats. The mesh also needs to be small enough to prevent mice from squeezing through. A typical fence is over 1.8 m tall, has a woven mesh skirt below the ground to prevent burrowing and uses a mesh with tight wire weave. A hood generally sits over the top to prevent jumping and climbing.
In 1994 experiments began at Zealandia (originally the Karori Wildlife Sanctuary) in Wellington to develop fencing to exclude all pest mammals. It also had to stand up in the Wellington wind! Following extensive testing, the final 8.6 km fence was installed in 1999. At completion, the expected lifespan of the fence was 25 years. Luckily, it looks like these fences can last significantly longer. To aid in the conservation movement Zealandia has made their fence design and maintenance programme available through a creative commons licence (here).
Keeping a fence in good repair is ongoing work. Maintenance work is required to check in on rust, wear and any foliage touching the fence. At Zealandia, they do a yearly full fence audit. This includes digging up parts of the skirt to check for rust.
Ring fences form a complete enclosure around the edges of a sanctuary. This reduces the risk of reinvasion but increases the initial costs.
- After Zealandia’s fence was completed in 1999, the eradication programme began. A combination of bait and trapping resulted in rodents, mustelids, hedgehogs, possums, cats, rabbits, hares, deer, pigs, goats and cattle all being eradicated within a year. The fence has made a phenomenal difference. In the last 20 plus years bird life has boomed at Zealandia with kākā and flocks of tūī regularly seen in Wellington suburbs. It is not only birds that have benefited from Zealandia; tuatara, Maud Islands frogs and kākahi (freshwater mussels) also call it home.
- Our biggest fenced sanctuary is Maungatautari in Waikato. A 47 km fence circles 3,400 ha of broadleaf podocarp forest with two smaller fenced enclosures that have been used as trail sites. The sanctuary has led to increases in many species inside the fence, including the endangered Hochstetter’s frog. The surrounding areas have benefited too, with an explosion in the number of tūī.
- Bushy Park Tarapuruhi, outside of Whanganui, is a sanctuary built around an old homestead. Since the 1960s, the 89 hectare forest has been managed by Forest and Bird, and in 2005 this area was fenced. The birdlife at Bushy Park is predominantly native species, further enriched by reintroductions of North Island robins in 2005 (the year the fence was erected), tīeke (North Island saddleback) in 2006 and hihi (Stitchbird) in 2013.
- In Taranaki, a concerned community set out to improve the state of the Rotokare Scenic Reserve. A trust was formed in 2004 and through massive community effort it was fenced soon after with an 8.2 km perimeter fence. Twelve pest species were removed from the area, and unusually they managed to remove mice and keep them out too.
- Orokonui is a 307 hectare fenced sanctuary of coastal Otago forest, just a short drive from Dunedin. It includes a precious population of Otago skinks and a population of kākā that has gone from six in 2008 to close to 50 in 2021.
- The Brook Waimarama Sanctuary in Nelson became fully fenced in 2016. This fence protects 691 hectares of mainly pristine old-growth forest. In eight years of trapping before the fence was completed, about 30,000 pests were removed, including rats, stoats, possums, mice, feral cats and hedgehogs. In 2021 they celebrated the release of 40 tīeke – a milestone as their first translocation and ending 120 years of tīeke being absent from the Nelson region.
Tawharanui is one hour drive to the north of Auckland at the edge of the council boundary. The fence crosses the base of the peninsula with a 2.5 km fence enclosing 588 hectares. Unfortunately, the fence is “leaky” as it doesn’t meet the sea – this has led to issues including repeated stoat invasions. Bellbirds returned after the predators were removed, establishing a population on the Auckland mainland for the first time since 1890. A host of other birds have been translocated, including kiwi, pateke (Brown teal), kākāriki, takahe and tīeke.
Kaipupu Point, in the Marlborough Sounds, is another peninsula sanctuary. A 600 metre predator proof fence was installed in 2008.
Wharariki Sanctuary is a new peninsula sanctuary with a fence completed in 2020. The fence protects 2.5 hectares for nesting seabirds, rare coastal plants and giant snails and geckos. Work continues to remove predators from the area and it is now part of the broader Onetahua Restoration project.
The benefits and the challenges
The major challenge for fenced sanctuaries are the initial cost of the fence and the ongoing need for maintenance. The higher the fence to area protected ratio, the more it makes sense to use a fence.
Fenced sanctuaries have created the first predator free areas on mainland New Zealand since the arrival of invasive species. Our most sensitive native species, like tīeke and hihi who need near-zero predators present for nesting success, now breed on the mainland. Native bird species benefit the most from fenced sanctuaries, with a boom in their numbers and a decline in introduced bird species like blackbirds and sparrows.
The removal of all predator species in one go prevents the boom of one species; for example, when stoats are removed, rat numbers increase. There is an ongoing struggle to remove mice from fenced sanctuaries. Very few have managed to eradicate them, and while they are small mice, they can significantly impact invertebrate recovery.
Fenced peninsulas and large ring fences are more cost effective than other forms of predator control. While maintenance of the fence remains a requirement, the fences are so far aging well.
Despite the challenges, fenced sanctuaries remain one of the best tools in the predator free NZ toolbox. John Innes, a senior researcher at Manaaki Whenua, proposes fenced sanctuaries form cores which larger predator control programmes radiate around. Predator Free Wellington and Predator Free Dunedin are examples of this.
One big driver is that our predator-free islands are getting close to capacity for some species. It is a good problem to have, but we need to make decisions about where species like kākāpō can move next. Two options being explored are Maungatautari and fencing the Wainuiomata mainland island. This will require another innovation; kākāpō are great climbers, so the current fences in use will need revamping to keep them inside.
The use of exclusion fences are also becoming more refined, targeting just one species to better understand their response to predator control. The Manahuna Aoraki project, in the Mackenzie basin, has used a predator proof fence to protect 6000 m sq of habitat for the endangered robust grasshopper. While built to protect the grasshopper, it has also become a safe haven for the nationally endangered Tekapo ground wētā.