Invasion of alien woody plants on the islands of Mahé and Silhouette, Seychelles

Abstract

Abstract. A method is presented for a quick and easily repeatable evaluation of the state of invasion of alien woody plants and the ecological status (prominence value, diversity, singularity and rejuvenation) of potentially important biodiversity sites in the Seychelles. The usefulness of this method has been tested in a survey on the islands of Mahé and Silhouette. The survey revealed 34 species as invasive alien species, the most prominent being Cinnamomum zeylanicum, Psidium littorale, Adenanthera pavonina and Tabebuia pallida. The greatest intensity of invasion was recorded in Intermediate Forests on moderate or gentle slopes and in stream ravines protected from desiccating winds and insolation. ‘Mountain Mist Forests’, ‘Palm Forests’ and ‘Glacis’ are the least invaded vegetation types. The difference in prominence values of alien and native plant species between leeward and windward sides is significant. Glacis habitats do not show a decline in protection values with decreasing altitude. Prominence values and percentage frequency of invaders are exceeded by the endemic palm Phoenicophorium borsigianum which is able to maintain itself in areas suffering from forest destruction and topsoil erosion.