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Long-tailed Macaque (© Arco Images GmbH / Alamy)

The World's Worst Invasive Mammals

Animals as common as goats, deer, rabbits or mice can have a devastating effect on other wildlife

Brushtail Possum

Brushtail Possum
(© Juniors Bildarchiv / Alamy)
The brushtail possum (Trichosurus vulpecula) lives only in its native Australia and nearby New Zealand, where it was introduced in 1837 to establish a fur trade. The nocturnal marsupial is about the size of a cat and lives mostly in trees. In Australia, dingoes and bush fires keep the population in check. But in New Zealand, an environment that evolved almost completely devoid of land mammals for 65 million years, until the arrival of the Maori around 1250 A.D., the possum invasion is a very different story.

Possums are now ten times more abundant in New Zealand than they’ve ever been in Australia. With an absence of predators, the possums are free to roam and graze on whatever’s palatable. Their feeding on eucalyptus leaves has created a large imbalance in the island forest vegetation, and the possum’s appetite for birds has depleted some species like the threatened kokako bird and the kereru, a native pigeon.

“The trouble is, they’re actually quite nice animals,” says Mick Clout, a conservation ecologist at the University of Auckland in New Zealand. “If you see them in their native Australia where they belong, they’re fantastic. But they don’t really belong here [in New Zealand].”

Of utmost economic concern is that possums are the main wild vector of bovine tuberculosis, which can devastate cattle. Though the animals are still trapped for their pelts, this does not completely control the population and wildlife authorities have been forced to use other, sometimes controversial, methods, such as aerial poisoning.

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