THE ROLE OF DISTURBANCE IN THE ESTABLISHMENT OF INDIGENOUS AND ALIEN PLANTS AT INACCESSIBLE AND NIGHTINGALE ISLANDS IN THE SOUTH-ATLANTIC OCEAN

Disturbances play an important role in the establishment of vegetation at Inaccessible and Nightingale Islands in the South Atlantic Ocean. Type, altitude and age of disturbance all influenced the species richn ess, life-form and abundance of seedlings. A few indigenous species ap pear to be dependent on small disturbances, such as the clearings arou nd albatross nests, for their perpetuation, e.g. Acaena sarmentosa and Gnaphalium thouarsii. Alien forbs and grasses, e.g. Conyza albida, Ho lcus lanatus, Pseudognaphalium luteo-album and Rumex obtusifolius, ten ded to dominate large recent disturbances such as soil-slips and depre ssions, and to colonize man-modified sites. Alien plants were less com mon on the plateau at Inaccessible Island than on the lowlands. Their success appears to be enhanced by large-scale disturbances. The highes t density and diversity of seedlings occurred on large soil-slips wher e mineral soil was exposed and vegetation cover was minimal.