Pest-proof fences 'of limited value'

The annual cost of maintaining fenced forest was calculated at $3365 per hectare. Photo / Christine Cornege

The benefits of New Zealand's $24 million worth of predator-proof fences has been challenged by research which argues that they create "small, expensive zoos".

Two New Zealand studies question the conservation value of the country's 109km of protective fencing, and compare their success to island sanctuaries.

The New Zealand Journal of Ecology study of 18 predator-proof fences found that the fencing model was very expensive, required intensive and ongoing monitoring and might not be backed by strong evidence.

Author and Canterbury Museum vertebrate zoology curator Paul Scofield wrote: "The creation of these sanctuaries enclosed by predator-proof fences often creates small expensive zoos surrounded by degraded habitat that will never be able to sustain the animal and plant species contained within the fence."

The annual cost of maintaining fenced forest was calculated at $3365 per hectare, compared to $29 per hectare for maintenance of the Little Barrier island sanctuary.

Despite this cost, there was no local research which indicated that fenced mainland areas improved native bird numbers, increased their survival, or definitely excluded predators such as stoats and rats.

A study led by University of Auckland ecologist Bruce Burns was more positive.

It indicated that fencing played an important role in New Zealand's conservation strategy, in practice and as an advocacy tool.

But his article also said protective fences were only stop-gap measures: "The ultimate [and currently unrealistic] measure of conservation success will be when the threat is removed outside the fence, and the fence itself can be taken down."

Dr Burns also spelled out the problems faced by regular breaches and constant maintenance.

Once a fence had been breached, there was a 99 per cent likelihood it would be reinvaded within 24 hours - usually by mice or ship rats.

Both studies noted that island sanctuaries, on the other hand, were far cheaper to maintain, had less risk of reinvasion and a long-lasting conservation benefit. New Zealand species such as black robin and kakapo have been saved from extinction by their placement on isolated islands.

Predator-proof fences were most valuable in cases of intermediate intervention for at-risk species, such as conservation of endangered skinks in Central Otago.

Dr Scofield said the rapid growth of predator-proof fences could be because of their visible, tangible qualities which secured community grants.

"The rate of growth in predator-proof-fence building is out of proportion to its benefits ... time should be allowed for existing fences to show they can make contributions to species management, and have their worth evaluated scientifically."

He believed that recreating an exact copy of a prehuman ecosystem was impossible and an "illusory goal".

"We need more thinking 'outside the box' to ensure cost effective management occurs," Dr Scofield wrote.

HELPING HAND

Predator-proof fences in NZ:

* 109km of fencing, protecting 7133ha of forest.
* $24 million construction cost, plus annual maintenance, plus $800,000 annual depreciation cost.

Annual costs:

* Predator-free islands - $29 a hectare
* Pest-controlled areas of mainland - $11-$96 a hectare
* Predator-proof fences sites - $3365 a hectare

Source: New Zealand Journal of Ecology