The island of South Georgia in the southern Atlantic Ocean is free of rats and mice after the world’s largest rodent eradication effort to date. A monitoring survey found no evidence that any rodents remained on the island, the South Georgia Heritage Trust (SGHT) in Dundee, U.K., announced at a press conference in London today.

The charity had dropped about ۳۰۰ metric tons of poisoned bait on the island over the course of several years to kill the pests, which threatened native birds. Conservationists have applauded the results, and say that ongoing vigilance will be necessary to ensure that the work is not undone.

“It’s really exciting to have it official that South Georgia is rodent-free,” says Clare Stringer in Sandy, U.K., head of international species recovery at the Royal Society for the Protection of Birds, who was not involved in the project. “It’s what we were all hoping for.”

Rodents were carried to South Georgia, a British overseas territory with no permanent inhabitants, in the ۱۸th century by sealing and whaling ships. They soon wrought havoc on the island’s bird population, eating eggs and attacking chicks. In response, in ۲۰۱۱ SGHT began an ambitious £۱۰ million elimination project, dropping bait from helicopters in a rodent-infested part of the island. The group followed up with two additional seasons of treatment in ۲۰۱۳ and ۲۰۱۵ that covered the remaining areas inhabited by rodents.

The stepwise strategy was possible, says Mike Richardson, chair of the SGHT Habitat Restoration Project Steering Committee, because the island’s glaciers act as natural barriers between areas of the island home to rats and mice, so a treated region was unlikely to become reinfested between treatments. The project ultimately covered an area of more than ۱۰۰,۰۰۰ hectares across the ۱۷۰-kilometer-long island—several times larger than any previous island elimination efforts.

Baiting finished in ۲۰۱۵, but a declaration of success had to wait for another ۲.۵ years. It is difficult to immediately conclude that rodents have been eradicated because survivors can be hard to find, says SGHT project director Dickie Hall. But a couple years later, failure is easier to spot: The rodents reproduce quickly, and even a few survivors can lead to a population boom.

Hall’s team began a final survey of the island in late ۲۰۱۷, setting up and monitoring devices designed to detect any remaining rodents. The devices included so-called chewsticks, on which hungry rodents leave telltale toothmarks, as well as camera traps and tracking tunnels containing ink that mice and rats walk through, leaving behind footprints. The team was also accompanied by three dogs specially trained to sniff out rodents (and ignore the smell of the thousands of seals and penguins present on the island). After ۶ months of searching—including ۲۴۲۰ kilometers of walking by the dogs—the team found no evidence of any rats or mice.

Richardson says that although they “can’t prove a negative,” all signs point toward a complete elimination of rodents from South Georgia. And although SGHT has yet to conduct a comprehensive survey of the birdlife—it’s next on the agenda—Hall says that since finishing the baiting they have seen an “explosion” in the bird population, including that of the South Georgia pipit and South Georgia pintail, two species found only on the island.

James Russell, a conservation biologist at The University of Auckland in New Zealand, says now that the eradication of rodents from South Georgia is complete, strict biosecurity measures will be vital to ensure that they do not return. “Rats will continue to try and spread around the world and we can be confident that they will come, but … we can stop them,” he says.

Richardson agrees: SGHT has made a series of recommendations to the government of South Georgia and the South Sandwich Isles regarding biosecurity, he says, but it is “the government’s responsibility to effect tight biosecurity measures from now on.” He says it is gratifying to see at least one of these suggestions is already being piloted: Rodent detection dogs are inspecting vessels leaving the Falkland Islands for South Georgia, in an effort to sniff out any stowaways.

The success on South Georgia could also have lessons for other regions looking to eliminate rodents. New Zealand, for example, is aiming to eradicate all major mammalian predators by ۲۰۵۰, but has much larger islands. Russell says the South Georgia project’s “stepwise” approach could make New Zealand’s goal “more technically feasible”: The country could eliminate predators from multiple isolated regions over time, rather than trying to do whole islands at once.

But using the strategy in areas heavily populated by people will raise a host of additional considerations, including people’s acceptance of poisonous baits, Stringer notes. But, “Now that we’ve seen South Georgia succeed with such a huge area,” she says, “well, anything’s possible.”