On the heavily glacierized island of South Georgia, a British overseas territory in the southern Atlantic, rats have been completely eradicated. The island, once traversed in 1916 by polar explorer Ernest Shackleton during an ill-fated Antarctic expedition, was declared rodent-free on 8 May 2018 by the South Georgia Heritage Trust (SGHT), a Scottish-based organization which helps conserve indigenous species and the historical heritage of the island.
The rat eradication plan, which began in 2008, was developed and carried out by the SGHT’s Habitat Restoration Project and covered 108,723 hectares (1087 km2), making it the largest successful rodent eradication project ever, over eight times bigger than any previously completed undertaking.
Why rid the island of all of its rodents?
The story begins in the 18th century when rats were first introduced to South Georgia by ships hunting seals and whales in the nearby waters. The human introduction of rats proved disastrous for the island’s native bird species, specifically the South Georgia pipit and South Georgia pintail, which are found nowhere else in the world. The species evolved in an environment that lacked natural predators. The rats preyed upon the birds’ eggs and chicks along the island’s coastal areas, driving the species nearly to extinction. These coastal areas, devoid of trees, were the only viable habitat for the birds, which made their eggs and chicks in ground nests easy targets for the rats.
Glaciers also played an important role in the story of this project. Glaciers cover over 50 percent of the island due to its sub-Antarctic location and high elevation. Through a genetic analysis of the island’s rat population, the project found that the rats did not cross the glaciers, which served as natural barriers, preventing the rats from infesting rat-free areas, according to Dickie Hall, one of the directors of the project who spoke with GlacierHub about its success. With help from the glaciers, the SGHT could bait and poison the island’s rats in stages over the course of several years, safe in the knowledge that rodents from a neighboring part of the island would not be able to re-invade, according to Hall.
However, the glaciers on South Georgia like most around the world are retreating due to climate change. When the glaciers retreated from the island’s coast, they provided a “local benefit to some species by increasing habitat areas,” Hall said. The retreating glaciers left wide flat beaches that became breeding grounds for penguins and seals, for example, which attracted the rats due to new food sources like eggs and carrion. The beaches also acted as bridges, allowing rats to infiltrate rat-free areas of the island once protected by the glaciers, Hall continued.
The retreating glaciers also meant SGHT had to move fast to respond to the South Georgia rat population while the ice still separated South Georgia “into islands of habitat,” added Hall. Overall, he said, it was the presence of the glaciers dividing the rat populations that made the baiting project feasible.
The SGHT project’s field operations employed helicopters to drop poisoned bait across the island’s ice-free areas. Over the course of the project’s three phases, over 300 metric tons of bait were dropped.
The first of these operations began back in 2011 as a pilot phase that successfully eradicated the target area’s rats. Two additional field operations followed the pilot in 2013/14 and 2015/16 to eradicate the rest of the island’s rats. While early indicators for rat eradication appeared favorable, the SGHT waited two years before conducting a final surveying expedition and declaring South Georgia rat-free.
The expedition, dubbed “Team Rat,” searched for any remaining rats on the island over the course of six months. The team utilized chewsticks, tracking tunnels (rectangular boxes used to trap animals), and a team of skilled rodent detectors that included three dogs and their human handlers.
Throughout the six-month search for any remaining rats, the dogs walked close to 2,500 km and climbed an astounding distance equivalent to hiking Mt. Everest close to thirteen times. At the conclusion of the arduous survey by Team Rat, not one single rat was found left on the island. With the eradication of the rats complete, bird populations, most notably the South Georgia Pipit, have begun to recover across the island.
Alternatives besides complete rat eradication were also considered by SGHT, according to Hall. One potential option was to reduce and subsequently maintain the island’s rat population at a low level to limit their impact on bird populations and the environment. However, this option was ultimately not pursued. One of the reasons for this was because of the option’s high labor intensity. In addition, the island’s rapidly retreating glaciers made the island more interconnected than ever before, according to Hall, meaning control would become progressively more difficult as the glaciers continued to retreat and the rats continued to increase in other areas of the island.
For eradication to have the best chance for success, the SGHT had to act now before South Georgia’s remaining glaciers vanished. In the future, it will also be crucial to prevent a re-invasion of rats. Fortunately, South Georgia is one of the most remote places on Earth, so humans and ships are the sole way for invasive rats to be reintroduced. Even so, to prevent unintended reintroductions, Hall indicates that the government of South Georgia and South Sandwich Islands has already tested the use of rodent detector dogs and implemented stringent biosecurity policies regulating imports on to the island.
Overall, preventing a bird species from becoming extinct might not come to mind when one thinks of retreating glaciers. Nonetheless, the project serves as a reminder of our interconnected world and of the vulnerability of birds and all living things.