Cape Town - When the cat’s away, the mice will play – and that old adage is unfortunately also true for Marion Island, one of two islands making up South Africa’s Prince Edward Island sub-Antarctic archipelago deep in the “Roaring Forties” of the Southern Ocean.
The house mouse Mus musculus was accidentally introduced to Marion Island by sealers as long ago as the beginning of the 19th century, and because of its physical adaptability – it’s able to survive temperatures ranging from below freezing to hot desert conditions – this little alien mammal has thrived in the pristine island environment where it has no natural predators.
Mice are known to be omnivorous, feeding on a wide range of plants and animals, and have been shown to have a significant effect on native plants, terrestrial invertebrates, reptiles and birds, particularly in island ecosystems.
In an attempt to deal with the problem of a burgeoning and increasingly invasive mouse population, domestic cats were misguidedly introduced to Marion in 1949, but ended up themselves doing much greater environmental damage by turning feral and preying on the island’s seabird population that totals close to 2.5 million birds. The cats caused huge devastation among the smaller of the 29 bird species that have been recorded here, and by 1977 were taking an estimated 455 000 birds of just one species (burrowing petrels) each year.
A 14-year cat eradication programme, the most successful of its kind anywhere in the world, was completed by 1991, but this again left the mice effectively uncontrolled, and numbers quickly rose.
The need to deal with the mouse problem has become increasingly urgent with the confirmation by South African ornithologists working on Gough Island in 2003/4 that, during winter when other food such as seeds and invertebrates are scarce, mice attacked albatross chicks, eating them alive while they sat helpless on their nests, as well as chicks of other species such as petrels and shearwaters in their burrowed nests. And in recent years similar evidence has emerged on Marion Island.
Earlier, the direct impact of the house mouse on seabird populations at Marion had been considered negligible and restricted to predation of eggs and chicks of very small seabirds, such as storm petrels of which newly hatched chicks may weigh only 10g.
But in 2003 the first wounded Wandering Albatross chick was reported; by 2008 a further 11 had been found; and in 2009 eight Dark-mantled Sooty Albatross chicks from two colonies were recorded with open wounds on the back of their heads and necks, similar to wounds observed on Tristan Albatrosses on Gough Island.
These recent incidents are cause for alarm given the threatened status of the albatross species breeding on Marion Island, coupled with an expected increase in mouse populations linked to global warming, ornithologists say.
In response, the Department of Environmental Affairs’ Prince Edward Islands management committee commissioned a scientific review of the problem by two seabird experts: John Cooper, who has a long association with the islands, and Andrea Angel, who with her husband Ross Wanless had produced the first hard photographic evidence of the gruesome mouse predation on albatross chicks on Gough Island in 2003/4.
Angel and Cooper, who in 2006 had written a review of the impacts of introduced rodents (rats and mice) on Tristan and Gough Islands for the Royal Society for the Protection of Birds, produced A Review of the Impacts of the House Mouse Mus musculus on sub-Antarctic Marion Island, Prince Edward Islands in late 2011.
“The impacts of mice on Marion Island are a striking example of how insidious the effects of the House Mouse can be when it preys upon species that are poorly represented, such as invertebrates and plants, or large charismatic fauna, such as the many seabird species, which together are part of the island’s closely knit ecosystem,” they concluded.
The report’s advice on how to tackle the problem included a rodent eradication expert to accompany this year’s annual relief team to produce a feasibility study for removing the mice.
Angel and Cooper noted that eradicating invasive rodents from islands was now routinely recommended as the best conservation measure, but also pointed to evidence showing that, while many islands had been cleared of mice, they were harder to eradicate than rats. The first reported mouse eradication was on Flatey Island in Iceland in 1971, and there have been more than 50 other attempts worldwide since then. All have involved exposing all individuals of the target species to poison bait.
A mouse eradication effort is being planned for the 6 500-hectare Gough Island, but Marion Island is nearly five times as big at 29 000ha.
Angel and Cooper recommended waiting for final outcomes of several current and planned mouse eradications at other islands, including Gough, before starting at Marion. But they also suggested that an international expert be invited to visit Marion to research and produce a feasibility study.
They submitted their report to the national Department of Environmental Affairs, which manages South Africa’s presence at the Prince Edward Island group and in Antarctica, in December 2011.
Cooper met senior government officials in December last year when there was verbal agreement on getting the feasibility study done, after which he obtained a quote from a New Zealand expert he hopes will join next year’s relief team. Now he’s looking for funding from outside government to pay for this initiative.
“We really want to start moving on this,” he explains. “It’s becoming more and more feasible because elsewhere they’re trying more and more dramatic programmes that we can learn from – for example, the British have just completed phase two of the five-year rat eradication programme on South Georgia, where they have already helicopter-baited a far greater area than Marion Island.”
But there are “really big problems” to be resolved at Marion if the project is to be successful, Cooper warns.
It will have to be done as a one-off complete operation in which the entire island is baited – unlike South Georgia, there are no natural barriers such as glaciers that conveniently “compartmentalise” the island.
Unfortunately, Marion Island’s weather is extremely variable, so although it could be possible to fly bait-dropping helicopters at lower altitudes, they might not be able to cover the high mountainous areas – the highest point is Mascarin Peak at 1 242m. And mice have been seen above the vegetation line at over 1 000m, where they probably survive on small invertebrates that live among mosses and lichens.
Asked to comment, Environmental Affairs spokesperson Zolile Nqayi said the feasibility study was on hold, pending an assessment by the British government of the potential mouse removal implementation plan at Gough Island during the coming relief voyage next month.
“Gough Island is much smaller than Marion and therefore theoretically easier. The department is monitoring the processes and hopes to learn from their example,” Nqayi said.
Wanless, now seabird division manager of BirdLife South Africa, says his organisation “fully supports a push to getting a Marion mouse eradication going”. - Sunday Argus