Marion, a sub-Antarctic island, turns green — but at a cost

When veteran sub-Antarctic scientists Brian Huntley and Marthán Bester snapped photos of Marion Island decades ago, they could not have any idea how valuable those images would one day prove to science. Today, a new generation of South African scientists has taken repeat photos of the landscapes in those 1965 and 1980 images, ending years of scientific speculation.

Marion Island, South Africa’s species-rich sub-Antarctic territory some 1,700km southeast of Cape Town, and about halfway to Antarctica, is in the firing line of an army of advancing plants.

According to photos published in a new study led by the University of Cape Town (UCT) and the South African National Biodiversity Institute (Sanbi), the culprit is human-caused climate change. And the green assailant is unmistakable. The framing between the historic and 2019/2020 photos is unerringly similar, and yet each shows a brownish, lonely, windswept habitat transforming into something like the Swiss Alps in summer.

Marion Island, the study notes, has weathered “significant vegetation change, showing a greening trend across most habitats in the last five decades”.

Historic and contemporary photos documenting Marion Island’s greening phases. (Photo: https://doi.org/10.1111/jvs.70002)

That greening “has been accompanied by aridification, an increase in mean air temperature, changes in wind direction and wind speed, and an increase in invasive mouse populations. The three most widespread invasive plant species have also expanded their ranges … ”

An island getting greener and drier at the same time? How is that possible? The island became hotter between 1965 and 2020, while total annual rainfall has fallen sharply.

Dr Stephni van der Merwe of UCT/Sanbi, an author on the study, notes that “lower rainfall actually provides more suitable conditions for many plants of the island” — such as ferns with an aversion to waterlogged soils now expanding into wet mires like bogs.

“The cover of the cushion plant Azorella selago, which is the most widespread plant on the island, decreased in all habitats,” says Pretoria University’s Prof Michelle Greve. “The decrease of Azorella selago is particularly concerning because it is a keystone species on Marion Island — a species that supports the existence of many other species.”

Thanks to its “cushion growth form”, Greve points out, it “forms a haven for many invertebrates and sometimes also other plant species that grow within the cushion”.

Marion’s deep-jade peaks are the spongy, swamp-sodden tips of a volcanic undersea mountain that rears 5,000m from the ocean floor.

Here, millions of penguins and seals squawk and belch a pelagic cacophony heard only in the southern parts of the Earth.

A brown skua hovers over thousands of penguins at Marion Island. (Photo: Tiara Walters)

In spring, the world’s largest breeding population of wandering albatrosses gather on the island to court each other, clapping their bills and fanning out their amorous wings. However, this once-isolated Southern Ocean refuge is now also under threat from suspected deadly bird flu, adding yet another stressor upon existing stressors.



Marion Island’s wandering albatrosses woo each other during the mating season of March 2011. (Photo: Tiara Walters)

History’s single largest mouse-eradication effort 

One of those stressors comes in the form of predatory invasive house mice.

“There is no evidence that mice consume any part of the alien plants on Marion Island,” the study says. “However, the disturbance of natural vegetation caused by mice may provide the opportunity for invasive species to spread.”

It is tough to appreciate just how much destruction these little killer critters may have sown since sealers probably introduced them in the 19th century by accident.

The hotter it gets, the more the house mice breed and eat. Longer foraging periods require more energy. This is probably why the mice have switched prey in recent years, says Van der Merwe, “by starting to eat helpless seabird chicks who evolved without predators and thus have no defence against them”.

But “Mouse-Free Marion”, a project by Birdlife South Africa and the government, is now hoping to zap them. They need R600-million ($33-million) to pull off what is billed as history’s single largest mouse-eradication effort by slinging bait all over the 30,000-hectare island: this intervention, says Birdlife, is a best-practice, effective approach inspired by similar operations elsewhere.

The project’s ongoing fundraising campaign includes a pelagic cruise to Marion and neighbouring Prince Edward in January. A successful result — eradicating the mice — “should give the native species more resilience to deal with the warmer and drier conditions they are facing”, Van der Merwe observes.



WATCH | Seabird expert Peter Harrison talks about the January 2025 ‘Flock to Marion Again’ fundraising cruise. 

To arrive at these seminal results, Van der Merwe and colleagues from the University of Pretoria, the Australian Antarctic Division and Monash University’s Securing Antarctica’s Environmental Future (Saef) also analysed climate and invasive species trends since the 1960s.

“It was extremely challenging to find the exact same locations of photographs taken in 1965 without GPS coordinates or field notes and only being able to head out on days where visibility was good enough to capture good quality photographs,” Van der Merwe recalls.

“Doing any fieldwork on Marion is difficult because of constant rain, regular gale force winds and unforgiving terrain. This makes walking across the island, which has no infrastructure, an interesting obstacle course,” she points out. Another challenge is avoiding sinking into mires or falling on sharp volcanic or loose rock.

Van der Merwe’s co-authors, including Marion Island expert Prof Steven Chown from Australia’s Monash University, hunted down many of the sites of the original photographs.

Groundbreaking data: why scientists believe there’s hope

The scientist quips that she “also gathered the experienced Marionites” at the research base one night in April 2019. “Those who could identify where the historical photos were taken won Lindt Easter eggs.”

Van der Merwe and UCT colleague Nita Pallett subsequently “spent weeks zooming in and out of photographs to make sure everything is identified correctly”.

There have been some high-profile findings on Antarctic greening recently. How does this study offer a new perspective?

According to Van der Merwe, the research “provides fine-scale, ground-based evidence of vegetation greening in the sub-Antarctic, a region previously less studied compared to the Arctic or mainland Antarctica.

“While greening trends have been well-documented in the Arctic for decades, and more recently in parts of Antarctica, our study bridges the gap by demonstrating similar patterns in the sub-Antarctic, further underscoring the widespread and interconnected impacts of climate change on cold ecosystems.”

Beautiful Marion is under serious threat. What, if anything, gives Van der Merwe and colleagues hope that it might be possible to reverse this damage?

“The profound ecological differences between Marion Island and neighbouring Prince Edward Island, to which several of the invasive species found on Marion have not been introduced, gives us as scientists hope that Marion may recover partly if the more problematic invasives are eradicated,” she says. “The most destructive of the invaders on Marion Island is the house mouse, but plans are afoot to eradicate the species in the near future.” DM

• The open-access study is published in the December 2024 edition of the Journal of Vegetation Science.  


• Birdlife South Africa’s “Flock to Marion Again” fundraising cruise leaves Durban on 24 January and returns on 31 January. Book here.