Journal for Nature Conservation 34 (2016) 93–100 Contents lists available at ScienceDirect Journal for Nature Conservation j o ur nal homepage: www.elsevier.de/jnc Invasive alien plants progress to dominate protected and best-preserved wet forests of an oceanic island F.B.Vincent Florens a,b,∗ , Cláudia Baider c , Geneviève M.N. Martin b , Nooshruth B. Seegoolam b , Zeyn Zmanay b , Dominique Strasberg a a Université de La Réunion, UMR PVBMT (Peuplements Végétaux et Bioagresseurs en Milieu Tropical), 15, Avenue René Cassin, BP 7151, 97715 St Denis, La Réunion, France b Department of Biosciences, University of Mauritius, Réduit, Mauritius c The Mauritius Herbarium, Agricultural Services, Ministry of Agro-Industry and Food Security, Réduit, Mauritius a r t i c l e i n f o Article history: Received 7 February 2016 Received in revised form 21 September 2016 Accepted 29 September 2016 Keywords: Invasion Mauritius Mascarenes Oceanic island Psidium cattleianum Tropical lowland forest a b s t r a c t 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 m 2 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. © 2016 Published by Elsevier GmbH. 1. Introduction Invasive alien plants (IAP) pose significant impacts at the species, community and ecosystem levels (Vilà et al., 2011), and this is particularly true on islands compared to mainland (Pyˇ sek et al., 2012). Given the high level of endemism and degree of threats of insular biota, islands may offer particularly high returns per con- servation effort (Caujapé-Castells et al., 2010; Kier et al., 2009). Locally, much of each island’s native biota subsists in the least dis- turbed habitats, which for this reason, tend to be prioritised for protection. In this context, it is useful for invasion biologists and ∗ Corresponding author. Postal address: Faculty of Science, University of Mauritius, Réduit, Mauritius. E-mail addresses: Vin.Florens@uom.ac.mu (F.B.Vincent Florens), clbaider@gmail.com (C. Baider), umrc53@univ-reunion.fr (D. Strasberg). conservation managers to characterise invasions in these least- impacted and protected habitats that serve as last refuges for many endemic and threatened species. This can help gauging the level of risks faced by the biota which is benefiting from protection, and permit monitoring of trends through time, which itself may also help elucidate mechanisms and determinants of invasion. Against this background, the tropical oceanic island of Mauritius provides an interesting case study of worldwide relevance because it possesses attributes of human impacts which are becoming increasingly evident on most tropical oceanic islands worldwide, including high human population density, high extent of habitat destruction and fragmentation or advanced levels of invasion by alien species (Florens, 2013). Mauritius may thus serve as a “win- dow” into the future of many other islands. We selected five of the island‘s best preserved, not logged wet native forests, which have been receiving protection for many decades on the basis of their well-preserved state, to determine the extent and major http://dx.doi.org/10.1016/j.jnc.2016.09.006 1617-1381/© 2016 Published by Elsevier GmbH.