Australia studies effect of cats, foxes on native species

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BALTIMORE — Foxes and feral cats are wildly unpopular among Australian conservationists. The two animals are infamous for killing off the continent's native species, and they've been the targets of numerous government-backed eradication campaigns. But new research suggests that on Australian islands, these predators help control an even more destructive one: the black rat. As a result, eliminating cats and foxes could actually leave native mammals more vulnerable to predation, competition, and ultimately extinction.

Australia is ground zero for the modern biodiversity crisis. The continent has suffered more than a quarter of all recent mammal extinctions, and many other native species survive only as small populations on one or more of the country's thousands of islands. While habitat destruction has caused some extinctions, cats, foxes, and rats introduced around 1800 by British sailors have also played a major role, decimating native animals such as bilbies and bandicoots — both small, ratlike marsupials found only in Australia. All of this has given large, nonnative predators such as cats and foxes a bad name.

"We hate them," biologist Emily Hanna of the Australian National University in Canberra declared at the International Congress for Conservation Biology.

But to plan successful eradication campaigns, scientists must first understand how introduced predators interact with native fauna and with each other. For instance, cats and foxes are infamous for hunting birds and other wildlife, but they can also control rats, which are themselves ferocious killers of and competitors with native animals such as the bandicoot. To date, few studies have looked at which type of predator is actually most likely to drive native animals extinct.

Dangerous invaders

To determine which island invaders were doing the most damage, Hanna and her research adviser Marcel Cardillo created and analyzed what she calls a "ridiculously large" database comprising 934 living and extinct populations of 107 mammal species on 323 Australian islands between the early 1800s and today. For each island, the researchers recorded the presence or absence of various native mammals, and of rats, cats, foxes, and wild dogs known as dingoes, which some scientists believe help control invasive predators. Hanna then analyzed these data to find which factors most often correlated with native mammal extinctions.

The study yielded some surprising results: Native mammals were most likely to die off on islands that had rats, but not cats, foxes, or dingoes. Extinction rates on such islands ranged from 15 to 30 percent, but when cats, foxes, or dingoes were present, the rates plummeted to just over 10 percent — not much higher than on islands without any introduced predators, the scientists reported at the meeting and online this month in the journal Global Ecology and Biogeography.