CAPE TOWN, South Africa — Mice that were accidentally brought to a remote island near Antarctica 200 years ago are reproducing uncontrollably due to climate change. They are consuming seabirds and causing significant harm in a unique nature reserve with diverse wildlife.
Now, conservationists are preparing for a large-scale extermination using helicopters and hundreds of tons of rodent poison, which must be distributed across every part of Marion Island’s 115 square miles for maximum effectiveness.
If a single pregnant mouse survives, their high breeding capacity means all efforts may end up being futile.
The Mouse-Free Marion project — a massive pest control initiative — is considered crucial for the ecology of the uninhabited South African territory and the broader Southern Ocean. If successful, it would mark the largest eradication of its kind.
The island is home to globally important populations of nearly 30 bird species and provides a rare undisturbed habitat for wandering albatrosses with their 10-foot wingspan, among others.
The island remained undisturbed until house mice stowed away on seal hunter ships in the early 1800s, introducing the island’s first mammal predators.
The past few decades have been the most significant period for the damage caused by the mice. Dr. Anton Wolfaardt, the Mouse-Free Marion project manager, stated that their numbers have significantly increased primarily due to rising temperatures from climate change, which has transformed the cold, windswept island into a warmer, drier, and more inhabitable environment.
“They are probably one of the most successful animals in the world. They’ve got to all sorts of places,” Wolfaardt said. But now on Marion Island, “their breeding season has been extended, and this has resulted in a massive increase in the densities of mice.”
Mice are highly prolific, able to reproduce from about 60 days old, with females giving birth to four or five litters a year, each containing seven or eight babies.
Rough estimates suggest there are over a million mice on Marion Island. They are feeding on invertebrates and, increasingly, on seabirds — consuming both chicks in their nests and adult birds.
A single mouse can consume a bird several times its size. Conservationists captured a photo of one perched on the bloodied head of a wandering albatross chick.
The phenomenon of mice preying on seabirds has been observed on only a few of the world’s islands.
The scale and frequency of mice preying on seabirds on Marion has risen dramatically, according to Wolfaardt, following the initial reports in 2003. He mentioned that the birds have not developed defense mechanisms against these unfamiliar predators and often remain stationary while being nibbled by mice. Sometimes, multiple mice gather around a bird.
Conservationists predict that if no action is taken, 19 seabird species will disappear from the island within 50 to 100 years.
“This incredibly important island as a haven for seabirds has a very uncertain future because of the impacts of mice,” Wolfaardt said.
The eradication project represents a solitary opportunity for success, with no margin for error. Growing mouse and rat populations have posed challenges for other islands. South Georgia, in the southern Atlantic, was declared rodent-free in 2018 after an eradication effort, but it was a multi-year project; the one on Marion could be the largest single intervention.
Wolfaardt mentioned that they plan to use four to six helicopters to spread around 550 tons of rodenticide bait across the island. Pilots will follow specific flight paths, and Wolfaardt's team will be able to track the drop using GPS mapping.
The bait has been made in a way that it won't harm the soil or the island's water sources. It should not impact the seabirds, who feed out at sea, and won't have negative effects on the environment, according to Wolfaardt. Some individual animals will be affected, but those species will recover.
He mentioned that in these situations, there is no perfect solution. Nothing can just get rid of mice without affecting anything else.
BirdLife South Africa and the national Department of Forestry, Fisheries and the Environment are partnering on the eradication project. Marion Island has been designated as a special nature reserve under the highest level of environmental protection. It has a weather and research station, but is otherwise uninhabited and dedicated to conservation.
The department stated that the elimination of mice is crucial for preserving the unique biodiversity of the island.
Wolfaardt mentioned that the extensive planning required means a probable start date in 2027. The project also needs to raise around $25 million, some of which has been funded by the South African government, and obtain final regulatory approvals from authorities.
Scientists have previously attempted to manage Marion's mouse population.
In the 1940s, the mice were already causing problems for researchers, so they introduced five domestic cats. By the 1970s, there were about 2,000 feral cats on the island, which were killing around half a million seabirds per year. The cats were eradicated by introducing a feline flu virus and hunting down any survivors.
Islands are crucial for conservation efforts but are delicate. The Island Conservation organization describes them as “extinction epicenters”, and 75% of all species that have gone extinct lived on islands. About 95% of those were bird species.
Wolfaardt explained that this is truly an ecological restoration project. It’s a rare conservation opportunity to permanently address a conservation threat.
