In 1894, a pregnant house cat escaped from a lighthouse on Stephens Island, New Zealand. She had her kittens in the wild, where they went feral. Within 13 months, a native bird species known as the Stephen Islands wren was nearly extinct.
It’s a story often cited as an extreme – and by some accounts exaggerated – example of the damage that invasive mammals can do to delicate island ecosystems.
But the plot is hardly unusual. On islands where native species evolved with no natural predators, intruders like rodents, feral cats and goats can quickly outcompete or even eat the locals.
Islands are home to 15% of the world’s terrestrial species, but they represent 61% of recorded extinctions, experts say. Invasive species usually were a factor.
Now a new study is making the case for a tried-and-true method of staving off this island extinction “crisis”: Get rid of the invasive mammals.
“We spend billions of dollars a year on conservation … but you can help a lot of the world’s biodiversity by removing” these invasive animals, said study leader Holly Jones, a conservation biologist at Northern Illinois University. “In terms of gain per dollar spent … it’s a pretty darn good return on investment.”
Jones led a team of 29 scientists who reviewed hundreds of mammal eradication projects on 181 islands. In 251 such efforts over several decades, 236 native species got a boost when the uninvited guests were removed from the habitat. Only seven species suffered after the invasive species were removed, according to results published this week in the Proceedings of the National Academy of Sciences.
A Scripps’ murrelet chick, formerly called Xantu’s murrelet. The tiny seabird’s population, which nests in the cliff caverns of Anacapa Island, rebounded after invasive rats were removed from California’s Channel Islands.(Ricardo DeAratanha / Los Angeles Times)
An adult Scripps’ murrelet. The bird is no longer a candidate for Endangered Species Act protection after the invasive rats that ate its eggs and chicks were eradicted from Anacapa Island.(Ricardo DeAratanha / Los Angeles Times)
The island night lizard, a nocturnal reptile found on Santa Barbara, San Nicolas and San Clemente islands, was threatened by cat predation and habitat loss from goats and sheep introduced to the island. After the invasive mammals were removed, the International Union for Conservation of Nature removed the lizard from its "Red List.”(Don Bartletti / Los Angeles Times)
An island fox on San Nicolas Island. During a three–year program to remove feral cats from the island, Navy officials and environmentalists had to be careful not to catch the fox by mistake.(Arkasha Stevenson / Los Angeles Times)
A feral pig on Santa Cruz Island. The pigs caused concerns about damage to ancient Chumash burial sites.(Stephen Osman / Los Angeles Times)
A young bald eagle to be reintroduced to Santa Cruz Island. When DDT nearly wiped out the island’s bald eagle population, golden eagles arrived and ate invasive pigs and native island foxes.(Spencer Weiner / Los Angeles Times)
A baby New Zealand storm-petrel. The species, once thought to be extinct, was rediscovered in 2014 on Little Barrier Island after feral cats and invasive rats were eradicated.
The tufted puffin was able to return to Hawadax Island, Alaska, also known as Rat Island, after efforts to eradicate Norway rats.(Bob Callahan)
Seychelles magpie robins were moved to a lower extinction risk category after being reintroduced to five islands that had been cleared of invasive mammals.
“This paper is a story of hope,” said co-author Nick Holmes, director of science for Island Conservation, a nonprofit organization that uses this approach to try to prevent extinctions. “Here’s an intervention that we can see an impact within a lifetime. It’s a reason to celebrate.”
The study looked back on eradication projects since the 1970s and ‘80s, when some of the first techniques were developed for removing invasive mammals. The study also included a 1925 effort to remove the feral descendants of the Stephens Island lighthouse keeper’s cat, which came too late for the wren but aided the recovery of the fairy prion bird and a nocturnal reptile called the tuatara.
FOR THE RECORD
An earlier version of this post said visitors to Anacapa Island, Calif., must unpack their luggage in rodent-proof rooms. New Zealand uses rodent-proof rooms, and both use tracking tunnels and bait stations around landing zones.
The analysis includes the eight countries with the most eradications – New Zealand, Australia, Ecuador, Seychelles, the United States, the United Kingdom, France and Mexico. Cases in these countries represent 82% of all invasive mammal eradications around the world.
Of the 236 species that benefited, 62 started out as “endangered” or “vulnerable” and 20 others were categorized as “near threatened” under the International Union for Conservation of Nature’s “Red List.” Four of those 62 improved enough to qualify for down-listing on the organization’s endangered list.
Among them was the island fox of California’s Channel Islands, which had been listed as critically endangered. Feral cats were competing with island foxes for food.
On San Nicolas Island, scientists, the U.S. Navy and Island Conservation painstakingly trapped and relocated the islands’ cats, and by 2012, the island was seemingly cat-free. In addition, the National Park Service and the Nature Conservancy removed feral pigs from Santa Cruz Island. The foxes bounced back, and officials are now considering removing the island fox from the federal endangered species list.
On nearby Anacapa Island, the Scripps’ murrelet – a tiny, rare sea bird – was a candidate for listing on the federal Endangered Species Act. Black rats, which got there by stowing away on ships, were eating the birds’ eggs. Rat removal began in the early 2000s, and the murrelet’s numbers rebounded almost immediately.
Ten years later, another bird considered long gone reappeared on the island: the endangered ashy storm-petrel.
There were a few cases where efforts backfired, including times when native birds of prey experienced temporary population declines due to eating poisoned rats.
There were also examples where native birds had come to rely on the invasive rabbits and baby goats for their food. Once the intruders were gone, some of the bird populations became permanently smaller, Jones said.
On the Channel Islands and those around New Zealand, for example, officials take strict biosecurity measures to prevent invasive species – especially rats – from hitching rides onto the islands. In New Zealand, visitors must unpack all their belongings in rodent-proof rooms and on some islands, including Anacapa, pass by rodent tracking tunnels and rat traps when they land.
Holmes said the results should encourage similar projects on other affected islands.
“There’s thousands of islands that we know have threatened species and continue to have invasive mammals,” he said. “If we want to prevent these species from going extinct, we have to do these interventions in pretty quick clip.”
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