VANCOUVER—"I have a freezer full of dead rats," says Laurie Wein, project manager at Gwaii Haanas National Park Reserve in western Canada. It's a necessary evil, for the restoration specialist leading Parks Canada's war on rats in the biodiverse archipelago of Haida Gwaii (or the Queen Charlotte Islands). "Invasive species here on Haida Gwaii are the number-one threat to ecosystem functioning."

After nearly three centuries of rat invasion, islands in the Haida Gwaii archipelago in British Columbia—known as the Galápagos of the North—are being restored to their original rat-free state in a bid to save beleaguered populations of nesting seabirds, whose eggs and chicks are eaten by the introduced rodents. "For seabirds that have evolved on these island systems, they haven't developed any defenses, so they're just kind of sitting ducks, so to speak," Wein says.

Already, Wein and her team have removed rats from two islands and there are plans to eradicate them from another two next year. All together, if successful, these rat eradications would be the thin end of a wedge to make other rat-infested islands in the park, which currently total 16, rat-free.

"There is pressure for Canada to take their invasive species issues more seriously," says Wein, who will launch Canada's first aerial rat eradication next fall on Murchison and Faraday islands when helicopters rigged with pellet-spraying buckets on their underbellies will rain rodenticide onto the old-growth forest below.

Late September of this year she and her crew ran a trial study on three smaller islands to establish the minimal amount of bait needed to do the job. For the trials, the crew didn't use real rodenticide but instead used bait pellets laced with pyranine, a biomarker that causes urine and feces to glow neon green under UV light. After spreading a placebo nontoxic bait at rates of 11 to 30 kilograms per hectare, the crew trapped the rats on the island to see whether they had taken the bait. "So you're sort of looking at the personal bits and if they glow nice and green, then the rat was euthanized," says Peter McClelland, program manager of outlying islands for the Department of Conservation (DOC) in New Zealand. "We're now very confident that we know how much bait has to go out so that we can reach 100 percent of those rats," Wein says. "The next step is moving towards the full eradication in the fall of 2013."

Globally, the war on rats ramped up since New Zealand started to perfect the science of clearing their islands of the rodents in the 1970s. To date there have been 466 successful rat eradications on islands worldwide according to Database of Island Invasive Species Eradications, and the numbers are climbing fast. This is important because half of the world's endangered species exist on islands and most of those are at risk of extinction due to introduced mammals that commonly include cats, rats, foxes and raccoons. Although islands make up only 5 percent of the world's landmass, they host 20 percent of the world's unique plant and animal species. The flip side of the same coin is that to date, 80 percent of extinctions recorded have taken place on islands.

Seabird populations suffer especially. Almost half of all seabirds species have declining populations according to BirdLife International, with nearly 100 species threatened by extinction worldwide.

The Haida Gwaii archipelago, of which 16 islands are rat-infested, are no exception. Together the Haida Gwaii islands host more unique subspecies than any equivalent area in Canada, including the morphologically unique Haida Gwaii black bear, the Queen Charlotte goshawk, northern saw-whet owl and Queen Charlotte ermine as well as unique forms of the Stellar's jay and hairy woodpecker. The islands are also home to half of the world's population of ancient murrelets and one fifth of the world's breeding Cassin's auklets. To give you an idea of the devastation the rodents wreak, after the introduction of rats to Langara Island, presumably from fishing boats or log barges in 1946, the number of ancient murrelet breeding pairs plummeted from 200,000 to 20,000. In a bid to boost the bird's numbers, rats were eradicated from the island in 1995 and, sure enough, between 1999 and 2004 the murrelet breeding population is believed to have doubled.

Some of the islands have dealt with rats problems for much longer. "These rats that they're chasing are ship rats that probably go back to the 1700s when the Spanish and English started competing for sea otter pelts," says Gregg Howald, director of the North America Region for Island Conservation, a U.S. conservation group. "So this is turning back the clock hundreds of years." Invasive species, of which rats are forefront, are deemed to be the single largest threat to the ecological integrity of the Gwaii Haanas National Park Reserve by Parks Canada and the Haida First Nation, Wein says.

"Don't underestimate them, they are born survivors," says DOC's McClelland, the world's leading rat eradication expert, who was consulting on the recent trials in Gwaii Haanas. "But if you want the seabirds you can't have rats." With rats gone, the entire island ecosystem and nutrient cycle is expected to be restored because seabirds shift marine nutrients to forest ecosystems via their seafood diet and defecation.

Of course, if ingested, the rodenticide will kill other small mammals like deer mice and native shrews, but wildlife surveys show that the former is absent from these islands and that the latter exists at very low densities, likely due to being outcompeted or predated by the invasive rats anyway. In the worst-case scenario, if these mammals don't bounce back after the eradication, Parks Canada will reintroduce these species to the islands.

As for the birds, "if we see a rebound [of seabirds] within five years we'll be pretty pleased," says Wein, who has her sights set first on removing rats from Kunghit Island, a 12,704-hectare island in the southern part of Gwaii Haanas. "That's sort of the gold standard for us." The current record for the largest rat eradication is held by the Australians, who ran a successful eradication program last winter on Macquarie Island, which is 13,000 hectares.

Parks Canada's upcoming eradications mark the first time rodenticide will be aerially applied in a forested North American ecosystem. "Having that barrier broken through will open up the door to other operations more broadly within the Pacific Northwest," says Island Conservation's Howald.

"It's very addictive, rat eradications, because in so much work we do in conservation it's the same battles year after year or decade after decade," McClelland says. "You do an eradication, you see those birds come back and you really do feel like you've made a difference."