Sitting on the Fence: Are Predator-Proof Fences a Solution to New Zealand’s Biodiversity Challenges?

Only about a third of New Zealanders are concerned about their country’s species, while most of the public believes that native animals and plants are in a good state.

In a way, it does make sense to think that New Zealand’s biodiversity is doing well. The country has a rich conservation history, a dedicated Department of Conservation and a constant increase in numbers of conservation NGOs and community projects.

But, well… to be honest, our biodiversity state isn’t doing THAT well. Or rather, not so good at all.

New Zealand is actually rated as the worst country in the world by the proportion of threatened species: about 2,800 NZ native species are either threatened or endangered.

The kākāpō (Strigops habroptilus): even though its population size is increasing, the species is still critically endangered (Photo by Dianne Mason)

A study conducted in 2010 suggested that the best way to conserve endemic species effectively in New Zealand is to place them in a natural ecosystem without introduced predators. Introduction of mammalian predators into the country is one of the main drivers of biodiversity loss. One approach to the current biodiversity crisis is fencing natural areas, as building a predator-proof fence is considered as the most financially beneficial method to achieve conservation goals.

The purpose of predator-proof fences is to create a safe sanctuary for native species. A typical fence is about two metres above the ground and one metre below it, has a very small stainless steel wire mesh and a rolled hood on the top to prevent climbing.  In total, 18 predator-proof fence projects are protecting an area of about 8,118 hectares – nearly twice the size of Kahurangi National Park – with a total fence length of approximately 135 kilometres (click here for a detailed list). One of the best-known fenced projects in New Zealand is Zealandia (Karori Sanctuary), a 225 hectares ecosanctuary in Wellington.

A predator-proof fence
A predator-proof fence: a 3D visualisation on the left, a cross section on the right (Photo by the Department of Conservation: Bell 2014, page 3)

A paper published in 2011 by scientists Scofield, Cullen and Wang from Canterbury Museum and Lincoln University questions the ability of predator-proof fences to answer New Zealand’s biodiversity problems.

If the main goal of fenced sanctuaries is to protect native species, will a small (yet fenced) habitat be able to support healthy populations for… well… forever? Moreover, a self-sustained population needs to survive without human intervention, but wouldn’t a fence be considered as an intervention? Fences must be maintained annually to insure their predator-proof state, which is certainly a kind of interference. Also, no studies have yet proven that fences increase the survival of native birds, their breeding success or that they prevent predator invasions.

While fences are predator-resistant, they are not necessarily predator-proof. Fenced sanctuaries are always prone to invasions and they do experience incursions of predators every couple of years, if not yearly. For example, in 2015, a female stoat lived inside the fenced Orokonui Ecosanctuary near Dunedin for several months, regardless of the staffs’ intense efforts to catch it.

Orokonui Ecosanctuary
Orokonui Ecosanctuary (Photo by Kathrin and Stefan Marks)

Off-shore islands are often a better solution to ensure the survival of native species. They lack the high expenses of building and maintaining a fence, reduce the possibility for predator return and have long-term conservation benefits. Several New Zealand species have been rescued from extinction by introducing them to predator-free islands, for example the Black Robin (Petroica traversi).

Furthermore, while fences might be considered more cost-efficient than conventional pest control by some studies, such calculations often exclude depreciation and maintenance costs, which can accumulate to hundreds of thousands of dollars per year.

Many sanctuaries attempt to create “self-sustaining ecosystems representative of the pre-human state that existed in New Zealand approximately 1,000 years ago” (as stated on Zealandia’s website), However, the main priority of conservationists should be preserving species, rather than conserving a pre-human habitat.

Lake Opouahi Reserve
Lake Opouahi Reserve’s predator-proof fence in Hawke’s Bay (Photo by Dafna Gilad)

Although predator-proof fences can definitely assist the conservation of native species, the rapid increase of fenced projects seems to be unwarranted when observing the limited conservation benefits they offer.

However, while examining the conservation and economic side of the fences, a key role is the social aspect.
Another group of scientists argued that fenced sanctuaries promote conservation education, especially for urban population and school pupils. For instance, the number of visitors who visited Zealandia last year was 125,849 – a record for the ecosanctuary. Visitors can observe conservation “success” and enjoy seeing iconic species that no longer inhabit the mainland, unlike conservation projects on off-shore islands with limited public access (for instance, Codfish Island).

Sealers Bay in Codfish Island/Whenua Hou, a nature reserve North-West of Stewart Island with many native species – no public access is allowed (Photo by Dafna Gilad)

Islands also do not provide the necessary ecosystem diversity as the mainland and not many of them have the appropriate qualities to become sanctuaries. Moreover, they can still be invaded by invasive mammals – just as fenced areas – if they are located close enough to the mainland.

One of the current goals of Zero Invasive Predators (ZIP), a non-profit organisation, is to create predator-free peninsulas. A fence, however, is not an option, as it would be too expensive to build and maintain, especially in residential areas or over roads and bridges. Apparently, fences will not necessarily promote a predator-free New Zealand by 2050.

Nevertheless, the success of mainland islands should not be ignored: a saddleback (Philesturnus rufusater), an endangered bird, bred successfully in the wild after being declared extinct for 100 years from the mainland. It was introduced to Zealandia in 2002 and has been nesting outside it in Polhill Reserve since 2014.

A saddleback/tieke in Zealandia (Photo by Jelly Knife)

So, are predator-proof fences the answer to New Zealand’s biodiversity? No, but then again – yes. Fences alone cannot save New Zealand’s biodiversity; however, they do create mainland islands that conserve species and encourage people to appreciate and value conservation efforts.

For more information: Scofield, R.P.; Cullen, R.; Wang, M. (2011). Are Predator-Proof Fences the Answer to New Zealand’s Terrestrial Faunal Biodiversity Crisis? Forum Article, New Zealand Journal of Ecology, Volume 35(3): 312-317.

The author Dafna Gilad is a postgraduate student in the Master of International Nature Conservation taught at Lincoln University and University of Göttingen. She wrote this article as part of her assessment for ECOL 608 Research Methods in Ecology.

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