Biggest Rat Eradication Project Ever Deemed A Success

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.

Photo of the South Georgia Pipit
The South Georgia Pipit (Source: Ingo Arndt/SGHT).

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.

Photo of South Georgia glacier
A glacier on South Georgia (Source: Oliver Prince/SGHT).

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.

Photo of Helicopter with poisoned bait flying over a glacier
Helicopter with poisoned bait flying over a glacier on South Georgia (Source: Tony Martin/SGHT).

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.  

Photo of rodent detection dogs and pengiuns
Project worker Miriam Ritchie and her rodent detection dogs Ahu and Willl at Macdonald Cove on March 08, 2018 (Source: Oliver Price/SGHT).

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.

Killing Rats to Save Birds as Glaciers Recede

Wandering Albatross chick, South Georgia island, ©Stephen Williams
Wandering Albatross chick, South Georgia island, ©Stephen Williams

The biggest island-based rat-killing operation in history is under way on South Georgia, an island north of Antarctica and east of the Falklands. The island was once one of the richest seabird breeding territories in the world, but bird populations fell into severe decline after rats arrived aboard sealing and whaling ships in the 19th century. Scientists are racing to eradicate the rats before glaciers that currently serve as natural barriers between the rodents and remaining bird populations melt away. They are hoping to save the South Georgia Pipit, a species found nowhere else in the world, from extinction.

South Georgia pipit, ©Brian Gratwicke
South Georgia pipit, ©Brian Gratwicke

During the 18th and 19th centuries, the bays of South Georgia served as whaling harbors. Accompanying the whaling expeditions were non-native species, particularly Norwegian brown rats, which have greatly reduced the populations of some native species on the island. South Georgia’s unique geography has complicated this process, however, and offered some protection against extinction.

The UK overseas territory stretches over 7,000 square kilometers, and boasts 11 mountain peaks over 2,000 meters high. Given the combination of latitude and elevation, half the island is covered in snow and ice year round. A number of the glaciers reach from the high peaks all the way to the ocean. This glacial ice has fragmented the island, acting as a barrier between different sections of bird habitat.

Grytvikken at South Georgia, whaling station. source: Wikimedia Commons
Grytvikken at South Georgia, whaling station. source: Wikimedia Commons

 

In 2010, scientists published a study in the journal Antarctic Science that analyzed the impact of glacial retreat on this habitat separation. The study revealed that the glaciers play an important role in preventing the invasive Norwegian brown rats from reaching the ground nesting sites of native birds. The birds inhabit the coastal sections of the southern end of the island, while rats thrive in the north. Glacial retreat over the last several decades is quickly eliminating that natural barrier, however, allowing rats to move to sections of coastal habitat that were formerly isolated. According to the study, the average rate of retreat has increased more than four-fold since the 1950s. Activists and scientists are concerned that the two sides of the island’s coasts will soon be connected by ice-free land. They estimate that under current conditions, the rats would completely eliminate the endemic pipit from the island, and greatly reduce other birds as well.

Spacial distribution of rats and pipits on South Georgia island. A.J. Cook, S. Poncet, A.P.R. Cooper, D.J. Herbert and D. Christie, “Glacier retreat on South Georgia and implications for the spread of rats” Antarctic Science, Volume 22 | Issue 03 | June 2010, pp 255-263 Figure 6 b. Doi:10.1017/S0954102010000064. Reproduced with permission.
Spacial distribution of rats and pipits on South Georgia island. A.J. Cook, S. Poncet, A.P.R. Cooper, D.J. Herbert and D. Christie, “Glacier retreat on South Georgia and implications for the spread of rats” Antarctic Science, Volume 22 | Issue 03 | June 2010, pp 255-263 Figure 6 b. Doi:10.1017/S0954102010000064. Reproduced with permission.

To prevent this loss, the South Georgia Heritage Trust and a group of biologists from around the world are pursuing an intensive extermination campaign to eliminate rats completely from South Georgia Island. Organizing themselves under the name of Team Rat, in March 2011 they spread 50 tons of Brodifacoum, a rat poison, on the island. This action was the first phase of the eradication process. The initial results were encouraging, with no rats subsequently found in the areas where the poison was spread. According to the South Georgia Heritage Trust website, it seems impossible to prevent all ill effects on other animals on the island from the poison. However, several factors limit the risk of this substance on the native fauna. It is not water soluble, most birds on the island eat marine rather than terrestrial prey, and the rats, once poisoned, tend to retreat into their burrows. The team is carefully monitoring effects on other wildlife each season and has said it will cease work if there is evidence of long-term population-level damage due to the project.

To commemorate the rat eradication campaign, South Georgia issued a set of stamps, pictured here.
To commemorate the rat eradication campaign, South Georgia issued a set of stamps, pictured here.

The fact that the sections of the island are separated by glaciers means that the rats can be eradiated pocket by pocket without risk of previously baited areas becoming repopulated, and the organizers of the effort are confident they will be able to eradicate the rats from South Georgia. The second phase of the project was undertaken in the spring of 2013 when an additional 180 tons of rat poison were dropped on the island, which marked the successful baiting of 70% of all rat-infested habitat.

“After two years of monitoring the work we did back in 2011 . . . we have found no sign of rodents in the areas we cleared then and we are now pretty confident that we were completely successful in eradicating rats from the area,” Howard Pearce, one of the organizers of the project, told The Guardian. The project coordinators expect to complete baiting of the rat habitat by 2015, and expect to declare the island rat-free by 2025. If everything continues according to plan the island will be rat-free before glacial retreat unites the southern and northern shorelines. If this campaign meets these goals, Team Rat will have succeeded in staving off extinction of an endangered species due to glacier retreat.

To see more photos of South Georgia, look here. And here is another story about an invasive species on another sub-Antarctic island with a history of whaling, Signy Island in the South Orkneys.