A study which found predator-proof fences often create "small expensive zoos surrounded by degraded habitat" unable to sustain the animal and plant species within it has been criticised by those who run such enclosures in Otago and elsewhere as simplistic and flawed.
The article "Are predator-proof fences the answer to New Zealand's terrestrial faunal biodiversity crisis?" by Paul Scofield, of Canterbury Museum, and Ross Cullen and Maggie Wang, of Lincoln University, reviewed 14 predator-proof fence projects, finding their goals were frequently not achieved and cost-benefit analysis often did not adequately quantify ongoing costs.
"We believe that the rate of growth in predator-proof fence building is out of proportion to its benefits," they said in the paper being published in the New Zealand Journal of Ecology.
They calculated there was 109km of "predator-proof" fencing in New Zealand, protecting 7133ha of forest, with the overall cost at the end of 2006 exceeding $24 million. In Otago there are two predator-proof fence enclosures - Orokonui Ecosanctuary near Waitati, in Dunedin, and the Department of Conservation's (Doc's) Macraes Flat grand and Otago skink enclosures.
The mean costs per hectare of New Zealand's 18 projects was "one to two orders of magnitude greater" than the annual cost of ongoing predator control using the unfenced mainland island approaches, the study said.
"A perfect re-creation of a pre-human ecosystem is impossible and New Zealand conservation has to accept that the crucial issue of the next few years is to maintain what we now have."
No introductions of species to the enclosures had as yet made any difference to any species' threat status and there were no studies to show whether the fences worked, the study said.
Otago Natural History Trust chairman Neville Peat said the study took a simplistic look at the issue and, while for Orokonui restoration within the fence was a primary objective, in the long run the trust expected gains beyond the fence.
There was predator-control work going on in the buffer zone outside the fence and there was a possibility one day a neighbouring tract of land might offer the opportunity of "mainland island"-type predator control.
Karori Sanctuary Trust founder vice-patron Jim Lynch, of Wellington, said the huge flaw in the study was expecting the same biodiversity outcome from predator-proof fences as achieved in bait-trap operations.
He admitted not all predator-proof enclosures were in sustainable areas or well thought through.
Doc Otago Skink Recovery programme manager Andy Hutcheon said there was an ongoing cost to fences but they provided a strong barrier against predators. The programme also had an extensive trapping programme and both tools had resulted in increases in skink numbers with populations growing by 13% in a year.
