IUCN Red List of Ecosystems, Mangroves of the Western Indian Ocean

Abstract

‘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.