Alien Possums Gobbling New Zealand Forests, Birds

Sean Markey in Wellington, New Zealand
for National Geographic News
April 25, 2006
With its bushy tail, tall ears, and pink nose, Australia's brush-tailed
possum could be the poster child for cute critters.

But here in New Zealand—where millions of the animals eat native plants, trees, and birds by the bushel—the marsupials are possums non grata.

"They are gobbling through this country as if it was made of ice cream," said Herb Christophers, spokesperson for the New Zealand Department of Conservation (DOC) in Wellington.

The pests were introduced to New Zealand in the 19th century and today spread bovine tuberculosis to livestock and wreak havoc on forests, competing with native birds for food.

Possums can munch half a pound (300 grams) of foliage a day and prey on the eggs and chicks of endangered birds like the kokako, kereru, and kiwi—New Zealand's iconic national bird.

"They're an absolute pest," said Anne Field, a Christchurch weaver who uses yarn spun from merino wool and dead possum fur.

"I don't think anybody has a good word for them."

Evolutionary Isolation

Native to Australia, where they are now protected, brush-tailed possums were first introduced to New Zealand in 1837 to jump-start the fur trade.

The cat-size marsupials gained a permanent foothold here by 1858. Finding abundant food and no natural predators, they have never looked back.

Today possums occupy nearly 99 percent of the country, according to Christophers, and are poised to "take a stranglehold" on the last possum-free scrap of land in New Zealand, the area of Fiordland.

New Zealand's environment is defenseless against invasive animals like possums, stoats, and rats, because it evolved in isolation over the last 65 million years.

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.

Possum Control

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 market—possum-leather lampshades—from 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 enough—native animals—to 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 soon—if ever.

"They're having a bloody good time over here," Christophers said.

"In my humble opinion, if we could give them back to the Aussies—come and get them."

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