Effective pest-proof fences must be windproof and waterproof, but allow water to pass under the fence through pest-proofed culverts or stream crossings. To construct a completely pest-proof fence (that keeps out mice), it must be built with no gap bigger than two-tenths of an inch. Mouse eradication attempts have had mixed success over the years—the Karori Wildlife Sanctuary was actually re-invaded by mice. But fence design is constantly improving and pioneer companies such as Xcluder are now testing fences in Mauritius, Australia and Hawaii.
Not all mainland islands depend on fences. At Lake Rotoiti Nature Recovery Project in Nelson Lakes National Park, 12,000 acres of beech forest will be restored through an extensive predator trapping program. The park will eventually host native species like kaka, kakariki, bush robins, mistletoe, giant snails, lizards, bats and insects.
“Trapping is the key management tool for Lake Rotoiti,” says Brian Paton, DOC biodiversity program manager, adding the program cost is just $136,000 a year. “Fencing 12,000 acres of land would be a logistical and budgetary nightmare just to patrol it, never mind the cost of buying and installing it. We are seeing good increases in native bird life in our restoration area.”
The success of this mainland island is attracting more than 100,000 visitors a year to Lake Rotoiti. The recovery area is right beside a campground so the public can readily experience native flora and fauna.
Elaine Wright, DOC terrestrial sites manager, is non-committal on the future of fenced mainland islands: “Fences are one of the sweetest techniques we can use, but they’re still just one tool in the toolbox, and not appropriate for highly mobile species such as falcons.”
Wright explains that the DOC is directly involved in only six of the country’s 70 mainland islands, including Rotoiti, but supports the other projects as much as resources will allow. “The islands we manage tend to have specific learning objectives and are often more experimental than community-driven islands,” she adds.
“I have no problem with the concept,” says Owen McShane, director of New Zealand’s Centre for Resource Management Studies. “But why do these projects have to be on such a large scale and controlled by DOC? Private landowners know a lot about pest control too. DOC is desperately short of manpower, so why not let the private sector help out?”
McShane suspects a hidden agenda behind the growth in mainland islands. “DOC already controls 40 percent of New Zealand’s landmass. I suspect these mainland islands may be another way of locking people out of areas that could be used for recreational or economically productive purposes.”
McShane’s views seem to be in the minority, since mainland islands are attracting widespread public support through time and money donations. The recent creation of a national group called Sanctuaries of New Zealand reflects the growing number of people who see the benefits. Not the least of these, say supporters, is the fact that mainland islands not only allow the reintroduction and protection of plant and wildlife, they also become education and tourism resources that enhance local employment opportunities.