Since its emergence, roughly 2.5 million years ago, the island of La Réunion, situated 800 km from the east coast of Madagascar in the Indian Ocean, has been frequently submitted to major natural disturbances dues to the vocanism, cyclones and landslides. In the last three centuries, human activities have caused an accelerating loss of biological diversity principally due to the destruction and fragmentation of native tropical forests and the introduction of alien organisms. Alt least sixty alien plant species are now known to actively colonize native forest communities. The large part of what remains of the original lowland rain forest is to be found in the south-east corner of the island in a landscape naturally fragmented by lava flows. In this study, tree species diversity and the spatial structure of these forest remnants have been analyzed at the scale of 1-ha plot on a lava flow at least four centuries old. The colonisation dynamics of these plant communities have been studied at different spatial scales on a chronosequences of recent lava flows. The long term survival of these habitats is threatened by the consequences of fragmentation combined with alien plant invasions which have considerably modified the process of primary succession in this system. A major factor that has excerbated the detrimental effect of the invading species is the reduced seed dispersal capacity of native plant species relative to the introduced species.