Foxes and feral cats are wildly unpopular among conservationists. The two animals are infamous for killing off the Australia's native species, and they've been the targets of numerous government-backed eradication campaigns. But new research suggests that on 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 like bilbies and bandicoots. All of this has given large, non-native predators like cats and foxes a bad name. "We hate them," biologist Emily Hanna of the Australian National University in Canberra said last month at the International Congress for Conservation Biology in Baltimore.

A feral cat roams among baby penguins on Macquarie Island. Credit:Geoff Copson/ Tasmanian NPWS

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 like the bandicoot. To date, few studies have looked at which type of predator is actually most likely to drive native animals extinct.

To determine which island invaders were doing the most damage, Hanna and her research adviser Marcel Cardillo created and analysed 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 dingoes, which some scientists believe help control invasive predators. The researchers also included other factors that might affect extinction risk, such as the size of the island and distance from the mainland. (Ecologists have found that island populations close to continents are more easily replenished, while more distant populations more easily go extinct.) Hanna then analysed these data to find which factors most often correlated with native mammal extinctions.