Human activities, ecosystem disturbance and plant invasions in subantarctic Crozet, Kerguelen and Amsterdam Islands

Recent floristic surveys of the French islands of the southern Indian Ocean (Ile de la Possession, in the Crozet archipelago, Iles Kerguelen and Ile Amsterdam) allow a comparison of the status of the alien vascular plant species in contrasted environmental and historical situations. Four points are established: (1) the current numbers of alien plant species are almost the same on Amsterdam (56) and La Possession (58), slightly higher on Kerguelen (68); (2) some of these species are common to two or three islands but a high number of them are confined to only one island (18, 28 and 28 on La Possession, Kerguelen and Amsterdam, respectively); (3) all the alien plant species are very common species in the temperate regions of the northern hemisphere and belong to the European flora; and (4) a high proportion of the introduced species are present on the research stations or their surroundings (100, 72 and 84% on La Possession, Kerguelen and Amsterdam, respectively). These results are discussed in term of propagule pressure (mainly attributed to ships visiting these islands), invasibility of such ecosystems (in relation to climatic conditions and degree of disturbance by previous or current human activities such as sheep farming or waste deposits) and invasion potential of alien plant species. (C) 2001 Elsevier Science Ltd. All rights reserved.