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The grass Agrostis stolonifera L. is the most successful introduced vascular plant at sub-Antarctic Marion Island. Since its accidental introduction, probably in the 1950s, it has spread over the northern half of the island, and presumably will eventually reach all parts of the island. It invades undisturbed native vegetation and reaches dominance in a range of habitats. It is most abundant on wet slopes and on river banks, where it replaces the rosaceous dwarf shrub Acaena magellanica communities by dense grassland. These communities show a reduction of 50% in the mean number of native plant species per sample plot, although more macroinvertebrate and more mite species were found where A. stolonifera was dominant. The invasion by A. stolonifera does not seem to pose an immediate threat to the survival of any of the native species on the island, but the changes induced in the drainage line communities significantly reduce the value of the island both from a conservation viewpoint and as a natural laboratory for fundamental ecological research. Therefore, it is important to keep Marion Island free of any further introductions. This is even more important for neighbouring Prince Edward Island, which, because of the negligible impact of aliens on this island, is of exceptional conservation value.