EFFECTS OF ALIEN PLANT INVASIONS ON NATIVE VEGETATION REMNANTS ON LA REUNION (MASCARENE-ISLANDS, INDIAN-OCEAN)

Although most of the lowland endemic forests of the mountainous Indian Ocean island of La Reunion have been converted by Man to other land-uses, relatively intact ecosystems still survive at higher elevations. Given the particular conservational value of these remnants of primary forest, detailed knowledge of the threats faced by these native ecosystems was urgently needed. Accordingly we conducted field surveys through the various plant communities on the island in order to answer particular questions regarding the vulnerability of native vegetation remnants to invasive, introduced plant species. Our results indicated that most invaders colonize human-disturbed sites most successfully. However, such alien plants are also frequent in the Reunion primary forest remnants, with at least 62 species penetrating into areas which have experienced no or little human disturbance. Nevertheless, the forest remnants were still mainly dominated by indigenous or endemic plant species and maintain what is, presumably, their original vegetational structure. The most threatening plant invaders are the trees Psidium cattleianum and Ligustrum robustum, the bramble Rubus alceifolius, the shrubs Fuchsia magellanica and Lantana camara, and the herbaceous Hedychium gardnerianum, Boehmeria macrophylla, and Boehmeria penduliflora. We found the early stages of primary succession on young volcanic sites to be dominated by aggressively expanding populations of alien species. It appears very unlikely that native species will replace the invaders as the succession proceeds, and furthermore the survival of many indigenous species probably hinges on the active control of alien species. In order for alien plant management efforts to be successful, we propose a priority ranking based on the current and potential future extent of specific infestation, the rate of spread, the difficulty with which a species can be controlled, and its ecological impact on the areas invaded. Finally, the long-term survival of the Reunion ecosystems depends largely upon the drawing up and implementation of an adequate overall conservation policy regarding human-caused invasions by alien plants in the native vegetation.