Ascension was essentially uninhabited until 1815, when Emperor Napoleon I was held captive on St Helena. A British naval garrison was positioned on Ascension to prevent French supporters of Napoleon gaining access to St Helena. With the arrival of human settlement, came the introduction of alien species, both deliberately and accidentally. Approximately 90% of plant species on Ascension are introduced. Unfortunately some of these, such as the Mexican thorn Prosopis juliflora and tree tobacco Nicotiana glauca, have become invasive, and outcompete the native flora. |