Islands Rescued From Invasive Rats

Huge gains have been made over the last twenty years in stopping invasive rats and mice from destroying island biodiversity. So far, black rats (Rattus rattus), Norway rats (R. norvegicus), Polynesian rats (R. exulans) and house mice (Mus musculus) have colonized 80 percent of the world's major islands.

For centuries the rodents have stowed away on boats to settle on distant islands. The rodents' propensity to quickly reproduce and consume a varied diet has often wreaked ecological havoc in places where they've landed. In doing so, the non-native rats and mice have ravaged at least 170 types of plants and animals and caused 50 species to go extinct.

A recent review of hundreds of attempts to rid islands of exotic rodents finds that new eradication techniques developed in New Zealand have turned the tide on rat and mice invasions. Documented initiatives to eliminate rats and mice from islands have had a 90% success rate.

The destructive rodents have been eradicated from 284 islands worldwide, totalling 47,628 hectares. While most the islands are small, under 100 hectares, with the 1990s advent of using helicopters to distribute rodenticide, larger and more rugged islands are also being freed of rats. The biggest success is 11,300 hectare Campbell Island in New Zealand, which became free of Norway rats in 2002.

New Zealand and Australia account for nearly half of the treated islands. Many other countries have also successfully removed rodents including from three islands in Canada's Queen Charlotte Archipelago, four small Hawaiian islands, four of the Galapagos Islands, islands of the United Kingdom's Bristol Channel, France's Sept-Isles and many islands of New Caledonia, Seychelles and Falkland. Mexico recently protected over 200 seabird colonies by removing black rats from several islands in the Gulf of California.

A rodent removal program is considered successful if there's no sign of the animals for two years afterwards. However some successes need to be repeated, as rats and mice continue to arrive and re-establish, particularly on islands near mainland populations.

Despite these encouraging results, there are still some limitations to ridding islands of exotic rodents. Attempts to remove house mice have particularly been less effective, with a 19% failure rate.

Avoiding harming native animals is another challenge. Wild birds and reptiles are sometimes held captive while rodenticide is in place to prevent their poisoning. Where these animals have ingested toxins, their populations fortunately rebounded rapidly after rats were gone. Still, only two islands that are home to a unique species of mammal have had invasive rodents completely removed.

Despite the benefits for native species, not everyone wants to see islands rid of rats. An animal rights group tried, unsuccessfully, to legally block eradicating black rats from Anacapa Island of California's Channel Islands.


Gregg Howald, C. Josh Donlan, Juan Pablo Galván, James C. Russell, John Parkes, Araceli Samaniego, Yiwei Wang, Dick Veitch, Piero Genovesi, Michel Pascal, Alan Saunders, Bernie Tershy. 2007. Invasive Rodent Eradication on Islands. Conservation Biology. 21(5): 1258-1268.

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