Plan to eradicate island rats that breed out of control and eat birds

Rats were accidentally introduced to a remote island near Antarctica 200 years ago. Due to climate change, their reproduction was out of control. They fed on seabirds, causing major harm to a special nature reserve with “unique biodiversity”.

Now, conservationists are planning a mass extermination using helicopters and hundreds of tons of rodent poison, which would need to be dropped on every part of Marion Island’s 297 square kilometers (115 square miles) to ensure success.

Even if a pregnant mouse survives, their ability to reproduce means it may all be in vain.

The Marion Rat-Free Project – large-scale pest control – is considered vital to the ecology of the uninhabited South African territory and the wider Southern Ocean. If successful, it would be the largest eradication operation of its kind.

The island is home to nearly 30 globally important bird species and a rare undisturbed habitat for the 10-foot wingspan wandering albatrosses and many other bird species.

It was an undisturbed habitat at least until the early 1800s, when smuggled house rats arrived on sealing ships, introducing the island’s first mammalian predators.

Dr Anton Wolfat, project manager for Rat-Free Marion, said the past few decades have been the worst time for damage caused by rats. Their numbers have increased significantly, he said, largely due to rising temperatures due to climate change, which has transformed a cold, windy island into a warmer, drier and more livable place to call home.

“They’re probably one of the most successful animals in the world. They’ve been to all kinds of places,” Wolfart said. But now on Marion Island, “their breeding season has been extended, which has resulted in a significant increase in rat density.”

Rats need no encouragement. They can breed from about 60 days old, and females can have four to five litters per year, with each litter containing seven to eight children.

A rough estimate is that there are over 1 million rats on Marion Island. They feed on invertebrates and, increasingly, seabirds – both nestlings and adults in the nest.

A mouse can eat a bird several times its size.

Conservationists took a photo of a bird perched on the blood-stained head of a wandering albatross chick.

There are only a few islands in the world where rats eat seabirds.

Wolfart said the scale and frequency of seabird predation by rats on Marion Island has increased alarmingly since it was first reported in 2003. He said the birds haven’t developed defense mechanisms to protect themselves from these unfamiliar predators and often sit there while rats nibble on them. Sometimes multiple mice will swarm on a single bird.

Conservationists estimate that 19 species of seabirds will disappear from the islands within 50 to 100 years if nothing is done, he said.

“This extremely important island’s future as a seabird haven is extremely fragile due to the impact of rats,” Wolfart said.

The eradication program was a successful endeavor with no room for error. The booming rat population has caused problems on other islands. South Georgia, an island in the southern Atlantic Ocean, was declared rodent-free in 2018 after eradicating rodents, but that was a multi-year project; the intervention in Marion may have been the largest yet.

Wolfart said four to six helicopters may be used to drop up to 550 tons of rodenticide bait on the island. Pilots will be given accurate flight paths, and Wolfart’s team will be able to use GPS maps to track the crash.

The bait is designed not to affect the soil or the island’s water source. Wolfart said it would not harm seabirds that forage offshore and would not have a negative impact on the environment. Some animals will be affected at the individual level, but these species will recover.

“There’s no perfect solution to this kind of thing,” he said. “Nothing kills rats or anything else.”

The eradication project is a partnership between BirdLife SA and the national Department of Forestry, Fisheries and Environment, which has designated Marion Island as a Special Nature Reserve with the highest level of environmental protection. It has a meteorological and research station but is uninhabited and dedicated to environmental protection.

“Eradicating rats is vital if the island’s unique biodiversity is to be protected,” the department said.

Wolfaardt said the amount of planning required meant a start in 2027 was likely. The project still needs to raise approximately US$25 million (partly funded by the South African government) and obtain final regulatory approval from the authorities.

Scientists have tried to control Marion’s rats in the past.

In the 1940s, they had become such a pest to researchers that five species of domestic cats were introduced. By the 1970s, there were approximately 2,000 feral cats on the island, killing 500,000 seabirds each year. These cats were wiped out by introducing the cat flu virus and hunting down the survivors.

Islands are vital to conservation efforts, but they are also vulnerable. Island Conservation calls islands “extinction centers” with 75 percent of extinct species living on them. About 95% of them are birds.

“This is truly an ecological restoration project,” Wolfart said. “This is one of the rare conservation opportunities to address conservation threats once and for all.”