Except for two small species of bats, there are no native mammals here to prey on or compete with introduced animal pests.
Few plants have evolved the prickly, bitter, or noxious leaves that deter grazing.
As a result, countless possums now treat New Zealand's lush forests as an all-you-can-eat buffet, denuding trees, killing forest canopies, and altering the makeup of forest plants.
The pests' destructive potential is enhanced by their tree-climbing habits.
Possums will "have a go at any adult bird, nestling, [or] egg" they find, said Greg Napp, a DOC community relations manager near Abel Tasman National Park on the northwest tip of the South Island (see map).
He adds that the predators have also learned to hunt the area's endangered giant land snail, a meat-eating species found nowhere else in the world.
Not surprisingly, many New Zealanders haven't taken kindly to their unwelcome visitors.
Some Kiwis resort to possum road-rage.
"We squash them on the roads a lot," Field, the Christchurch weaver, said, "but we're not making any headway."
Others have launched cottage industries that utilize possum fur and skins.
Grant Fitz-William and Jocelyn Rae sell one of the more wry products on the marketpossum-leather lampshadesfrom their South Island possum tannery and café, the Naked Possum.
But Christophers, the DOC official, says possum numbers can only be tamed through large-scale pest management.
A raft of national and regional government agencies engage in possum control.
The largest by far is the national Animal Health Board, which polices some 20 million acres (8 million hectares), or one-third of the country, to protect diary and beef cattle and farmed deer from bovine tuberculosis.
Other agencies include DOC, which annually targets about 2.5 million acres (1 million hectares) of its total 20-million-acre (8-million-hectare) portfolio for possum control.
Extermination methods include trap lines, ground-based poisoned bait stations, and aerial drops of sodium fluoroacetate, also known as 1080.
The biodegradable poison is mixed into dyed food pellets and spread by GPS-guided helicopters to blanket large swaths of backcountry.
"Even though we get a small by-catch in species we want, we also kill a lot of possums," DOC's Napp said, noting that net the effect for native birds is positive once possums are removed.
Some deer hunters and environmentalist object to aerial 1080 drops. But Forest and Bird, New Zealand's largest conservation organization, has endorsed the technique.
Gerry McSweeney, past president of Forest and Bird, says trapping is too labor intensive and expensive to keep pace with possums.
"We simply can't save enough animals fast enoughnative animalsto prevent their extinction," he said.
"Which is why we've had to look at more widespread and cost-effective and ultimately ecologically effective systems. And probably top of that list is the use of sodium fluoroacetate, or 1080."
Here to Stay?
No one knows just how many brush-tailed possums call New Zealand home. One study put the tally at 70 million. But experts say the projection was flawed.
"In a native forest, it doesn't matter how many [possums] there are," Christophers, the DOC official, said.
"If they're in the wrong place, then there's too many."
It appears unlikely that New Zealand can rid itself of its unwelcome dinner guests anytime soonif ever.
"They're having a bloody good time over here," Christophers said.
"In my humble opinion, if we could give them back to the Aussiescome and get them."
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