Invasive alien plants progress to dominate protected and best-preserved wet forests of an oceanic island

Invasive alien plants pose a threat to biodiversity in particular on oceanic islands, where endemism tends to be high. In this context, it matters to characterise invasions in-situ and in particular to document how far invasive plants may invade protected areas devoid of major human disturbances. We explore this question on the tropical island of Mauritius, which provides an interesting case study because it possesses several attributes of human impacts, which are increasingly being encountered by most tropical oceanic islands worldwide. Mauritius today may thus serve as a “window” into the future of many other islands. We assess woody invasive alien plant abundance in the island’s wet native forests by sampling five of the currently best-preserved sites. We chose only protected areas that have benefitted from long-term legal protection. All woody alien plants reaching at least 1 cm of diameter at breast height (dbh) were identified and their dbh measured in a series of fifteen 100 m2 quadrats randomly placed in each forest. All sites are today dominated by woody invasive alien plants, which comprised 78.5% of the 27 868 sampled plants ≥1 cm dbh. Density-wise, the alien shade tolerant strawberry guava (Psidium cattleianum) dominates all forests sampled. In terms of Importance Value (as percent relative dominance and percent relative density), P. cattleianum dominates four sites and another alien, Cinnamomum verum, dominates one site. Our study shows that even though relatively diverse, the native plant communities of an oceanic island cannot resist the encroachment of understory invasive alien plants, even in better preserved, least disturbed forests that have been receiving long-term formal legal protection.