Above picture:  Percentage change in total vegetation cover at each photograph location of the repeat photography analysis across Marion Island between 1965 and 2020.  Graduated symbols indicate percentage change from decreasing to increasing total vegetation cover.  Yellow indicates the sites with a decrease in cover and red indicates sites with an increase in plant cover (from the publication by Stephni van der Merwe and colleagues)

Stephni van der Merwe (Department of Biological Sciences, University of Cape Town, South Africa) and colleagues have published an open access article in the Journal of Vegetation Science on vegetation changes on Marion Island. The analysis was done by comparing photographs taken at matched localities in 1965 and 2020.  The study is stated to provide the first island-wide demonstration that vegetation cover has increased, with invasive plants contributing to the overall change.  Whereas there is no evidence that the introduced House Mice consume alien plants on Marion, their disturbance of natural vegetation may provide the opportunity for invasive plants to spread.

The paper’s abstract follows:

“Questions
At high latitudes, anthropogenic climate change and invasive species threaten biodiversity, often with interacting effects.  Climate change not only impacts native plant species directly by driving distribution and abundance of species, but indirectly through the influence on community dynamics and habitat suitability to invasive species.  A key obstacle to quantifying vegetation change in the sub-Antarctic is the scarcity of cloud-free satellite imagery in a region with near-permanent cloud cover and lack of long-term plot data.  In this paper, we aim to address the following questions: how has vegetation in the sub-Antarctic changed between 1965 and 2020?  What are the roles of climate change and invasive species in driving these changes?

Location
The study was conducted on Marion Island in the sub-Antarctica.

Methods
We quantified vegetation change by analysing repeat ground photography between 1965 and 2020, accompanied by an analysis of climate trends and invasive plant species’ cover changes over the same period.

Results
Total vegetation cover was significantly higher in 2020 than in 1965 in all habitats other than in the coastal saltspray habitat, indicating an increase in overall biomass on the island. The more responsive ‘generalist’ plant species have expanded across the island, whilst the more ‘specialised’ plant species have not significantly changed in cover, with the exception of the mire graminoids, which have declined.  Marion Island has thus undergone significant vegetation change, showing a greening trend across most habitats in the last five decades.  This 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, especially in areas influenced by animal disturbance and nutrient input.

Conclusions
In congruence with research from Northern-hemisphere tundra and other islands in the sub-Antarctic, these results provide substantive empirical evidence for the interacting effects of climate change and invasive species on sub-Antarctic tundra vegetation, as has long been predicted.”

Matched photograph examples showing the most prominent vegetation change within each habitat on Marion Island. Historical photographs from 1965 are on the left and repeat photographs captured in 2019/2020 are on the right (from the publication)

 

In a popular account of the scientific publication, the senior author is quoted as saying “A successful result – eradicating the mice – should give the native species more resilience to deal with the warmer and drier conditions they are facing”.  She goes on to say “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.  The most destructive of the invaders on Marion Island is the house mouse.”

Reference:

van der Merwe, S., Greve, M., Hoffman, M.T., Skowno, A.L., Pallett, N., Terauds. A.,  Chown, S.L. & Cramer, M.D. 2024.  Repeat photography reveals long-term climate change impacts on sub-Antarctic tundra vegetation.  Journal of Vegetation Science 35(6) e70002.

John Cooper, News Correspondent, Mouse-Free Marion Project, 21 January 2025

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A Southern Giant Petrel broods its chick on Marion Island. Watercolour by Flávia F. Barrato of Artists and Biologists Unite for Nature (ABUN) for the Conservation Campaign for Marion Island Special Project. After a photograph by Michelle Risi

The Mouse-Free Marion Project is a registered non-profit company (No. 2020/922433/08) in South Africa, established to eradicate the invasive albatross-killing mice on Marion Island in the Southern Ocean.  The project was initiated by BirdLife South Africa and the South African Department of Forestry, Fisheries and the Environment.  Upon successful completion, the project will restore the critical breeding habitat of over two million seabirds, many globally threatened, and improve the island’s resilience to a warming climate.  For more information or to support the project please visit mousefreemarion.org.