J.A. Okello, N. Koedam, D. Di Nitto, F. Dahdouh-Guebas, T. Van der Stocken, J. Hugé, S. Fratini, S. Cannicci, C. Duncan, C. Golléty, A.J. Hamza, C. Macamo, D. Nicolau, L. Rasolofomanana, G. Savourey, V. Wang’ondu, Ena Suarez

‘Mangroves of the Western Indian Ocean’ is a regional ecosystem subgroup (level 4 unit of the IUCN Global Ecosystem Typology). This province spans 10 countries and includes the following marine ecoregions: Cargados Carajos/Tromelin Island, Delagoa, Mascarene Islands, Seychelles, Southeast Madagascar, East African Coral Coast, Northern Monsoon Current Coast, Bight of Sofala/Swamp Coast, Western and Northern and North-eastern Madagascar; however not every ecoregion has mangroves. The mangrove extent was 7,505 km2 in 2020, representing 5% of the global mangrove area. This province has predominantly terrigenous sedimentary ecosystems but also carbonate-type mangroves on oceanic islands. There are 10 species of true mangroves and several associated species. The ecosystem is threatened by catchment erosion and direct human pressures, including over-exploitation of mangrove-derived products, deforestation for conversion to other land use types (e.g., agriculture and aquaculture, or development infrastructure), pollution and climate-change. Oceanic islands with mangroves are threatened by sea-level rise and ocean surges, even where direct human impact is mostly absent. Today the Western Indian Ocean mangroves cover is ≈18% less than our broad estimation for 1970. However, the mangrove net area change has been positive since 1996. If this trend continues a global change of -8.3% is projected over the next 50 years. Furthermore, the Western Indian Ocean mangrove province is expected to be relatively resilient to even extreme sea-level rise scenarios, due to high sediment supply and vertical accretion, except for the carbonate-category of oceanic island mangroves. We estimate that 2% of the Western Indian Ocean mangroves are undergoing degradation. This value could rise to +7.5% % over a 50-year period based on decay of vegetation indexes. Overall, the Western Indian Ocean mangrove ecosystem is assessed as Least Concern (LC). However, for several sub-criteria, there is insufficient data. Therefore, it is recommended to update inputs to enhance the precision of the evaluation and to facilitate quantitative analysis of risks to the mangroves without precluding a potential change in status.