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A million mice eat live seabirds on a remote island. Environmentalists have a plan | Radio-Canada

The flow3:00 p.m.How to kill a million mice

Warning: this story contains distressing images.

Conservationists are hatching a plan to kill up to a million mice on South Africa’s remote Marion Island, fearing that the invasive rodents could wipe out the seabirds that live there.

“They basically eat them alive,” said Anton Wolfaardt, a conservation scientist in Cape Town and director of the Mouse-Free Marion (MFM) project. “(The birds) sit there while swarms of mice often nibble on them.”

The project aims to drop bait containing rodent poison across the island, with the aim of eradicating every mouse. Wolfaardt said the stakes for not intervening are high.

“(We) predict that the majority of Marion Island seabirds, including the wandering albatross, will become locally extinct within the next 30 to 100 years if the mice are not removed,” he said. The flow Matt Galloway.

WATCH | Aerial views of Marion Island:

Marion is a windswept volcanic island in the Indian Ocean, about 2,000 kilometers southeast of Cape Town. The island is designated a special nature reserve, with Wolfaardt describing it as a “refuge” for albatrosses, petrels, penguins and seals which hunt on sub-Antarctic waves but come ashore to breed and molt.

Mice first arrived in Marion on sealing boats about 200 years ago, but in recent decades their population has ballooned to a million rodents in peak season. Wolfaardt said the increase is due to climate change; Warmer, drier conditions extended the mice’s breeding window.

Mice have learned to attack where the bird’s plumage is thinnest, usually at the head, to reach soft tissues more easily. Photographs taken on Marion show birds with bloody wounds on their heads and necks, sometimes with mice munching on the live bird, in a behavior scientists have called scalping.

Wolfaardt explained that birds on the island have not evolved defense strategies against these relatively new predators and young chicks cannot yet fledge.

“Eventually the birds become so tired that they end up dying, or from some sort of bacterial infection,” he said.

An injured gray-headed albatross on Marion Island. An invasive species of mice on the island is attacking large seabirds, which have not developed a defense against the new predator. (Tom Peschak/Mouseless Marion Project)

Helicopters roam the ground with poison

Project Marion will involve a fleet of helicopters dropping poison along precise, overlapping routes across the island. This approach has worked in eradication programs on smaller islands, but Wolfaardt said Marion’s “size and topographical complexity” present unique challenges.

This complexity means the plan likely won’t be ready for implementation until 2027. Wolfaardt said the team is studying extensive data and developing contingency plans around variables such as weather – but noted that there was no margin for error.

“(We need to) ensure that every inch of the island… is covered in rodenticide bait, to ensure that every mouse… consumes a lethal dose,” he said.

The birds would benefit from a temporary respite if most, but not all, of the mice were eradicated, he noted. But this could be short-lived: Mice can have four or five litters per year, with six to eight babies per litter.

The project is a partnership between BirdLife South Africa, a non-profit conservation organization, and the South African government. The estimated cost is US$26 million, funded through a combination of government support and a fundraising campaign.

WATCH | A mouse attacks a petrel chick (viewer discretion advised):

Wolfaardt said the project’s risk assessment suggests that some birds or animals could inadvertently ingest the poison, perhaps by eating the mouse carcasses. But he stressed that seabirds feed at sea and will be largely uninterested in grain used as bait.

“We are confident that…the net benefit is substantially positive,” he said.

Finding a “sweet spot”

Environmentalist Ted Grosholz believes that the fact that the mice are confined to the island gives the project a good chance of success, and that the potential “catastrophic loss” to seabirds justifies the plan.

But he warned that achieving total eradication could be difficult.

“Trying to eradicate the last individuals — in this case, the last mice on the island — can be very expensive and time-consuming,” said Grosholz, a professor at the University of California, Davis.

Grosholz and Stephanie Green, an associate professor at the University of Alberta, have worked together on how best to manage invasive species, particularly in oceans and waterways where marine life eggs and larvae can spread quickly over long distances.

A house mouse eats an azorella selago plant.
About 200 years ago, mice hitched rides on sealing boats heading to Marion Island. (Stefan Schoombie/Mouse-Free Marion Project)

In March 2021, they published a study on what they called “functional eradication” in the journal Frontiers of ecology and environment.

“Functional eradication is recognizing that in these large interconnected systems, we may not be able to completely eliminate every individual from the population,” Green said.

“But we might be able to identify a sweet spot, a sort of tipping point where we can keep a suppressed invader below a place where it doesn’t have a substantial impact on the ecosystem.”

Green explained that there are certain environments where eradication makes sense, such as an isolated island where the species cannot return quickly or easily. But in other environments, focusing on removal can allow conservation efforts to spread over a wider area.

The hydra effect

Green said keeping a species’ population in check can also guard against the hydra effect, where a species rebounds after an intervention. The phenomenon is named after the many-headed monster from ancient Greek mythology, which grew two new heads for every severed head.

In 2009, Grosholz led a study to see if it was possible to eradicate another invasive species, the European green crab, from a remote California lagoon.

A man on a boat holds a small crab aloft for the camera.
Invasive European green crabs have been wreaking havoc in North American waters for decades. (Submitted by Stephanie Green)

The crabs have invaded North American waters for decades, feasting on clams and mussels and costing the commercial shellfish industry millions of dollars each year. American organizations have experimented with harvesting crabs as fertilizer, food intended for human consumptionor even turn them into whiskey. But authorities have struggled to combat the species, which has no natural predators, breeds prolifically and is spreading farther north into Canadian waters as temperatures warm.

In five years, Grosholz and his team trapped and destroyed about 90 percent of the lagoon’s crabs, reducing their numbers from about 100,000 to just under 10,000.

“Unfortunately, the following year we were again above 200,000, even close to 300,000 crabs,” Grosholz said. The flow.

The team had not taken into account that adult green crabs eat the young – a form of population control when a single female can lay up to 185,000 eggs up to twice a year.

“By removing almost all adult crabs, we allowed the next generation to survive at high levels, leading to a huge increase in the population,” he explained.

A cautionary tale about cats

Back in Cape Town, Wolfaardt discussed a previous eradication program on Marion Island.

Scientists brought five cats to the island in the 1940s to care for the mice at their research station. These unneutered cats quickly multiplied and began preying on seabirds.

A mouse above the head of a live bird, attacking the bird and nibbling exposed tissue from a wound.
A mouse attacking a wandering albatross chick on Marion Island. Mice attack birds where their plumage is thinnest, at the head and neck. (Stefan Schoombie/Mouse-Free Marion Project)

In the 1970s, about 2,000 cats killed about 450,000 seabirds each year.

It was not until the 1990s that the South African government eradicated feral cats from the island.

“Our project is in line with this, aiming to remove the last introduced predator from the island and return it to the refuge status it deserves to be,” Wolfaardt said.

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