Sarah L. Robin, Joanna C. Ellison, Cyril Marchand, Mary Tahu, Ena Suarez

Mangroves of the Tropical Southwestern Pacific are a regional ecosystem subgroup (level 4 unit of the IUCN Global Ecosystem Typology). It includes the marine ecoregions of Central and Southern Great Barrier Reef, Coral Sea, Fiji Islands, New Caledonia, Tonga Islands, Torres Strait Northern Great Barrier Reef, and Vanuatu. The Tropical Southwestern Pacific province mapped extent in 2020 was 874.0 km2, representing 0.6% of the global mangrove area. The biota is characterized by 21 species of true mangroves (15 of them assessed in the IUCN Red List of Threatened Species (RLTS)).
The Tropical Southwestern Pacific mangroves flourish on various islands with distinct climate settings that shape their unique characteristics. These islands predominantly experience a tropical climate with pronounced dry and humid seasons, except for the west coast of New Caledonia, which is semi-arid. Mangrove trees in this region are typically small, rarely exceeding 10 m in height. The local populations depend heavily on these mangrove forests for food and wood, vital for their subsistence and economic activities. This province is home to 160 mangrove-associated animal species, with 99 of them being fish, according to the IUCN RLTS. Fishing is a crucial source of income for Pacific communities, and 85% of the species caught in mangrove areas are consumed by local populations.
Despite the relatively low population density, the mangroves of the Tropical Southwestern Pacific province face significant threats from both natural and anthropogenic pressures. In Fiji, the main anthropogenic threat is the conversion of mangrove forests to agricultural land, particularly for sugarcane cultivation. In New Caledonia, urban development, mining, and aquaculture are the primary drivers of mangrove degradation. Tonga experiences pressure from population migration from smaller islands to the main island, impacting mangrove areas. In Vanuatu, while anthropogenic pressures are minimal, natural threats like cyclones pose a significant risk to mangrove forests and may surpass anthropogenic impacts in this region.
Today the Tropical Southwestern Pacific mangroves cover ≈874 km² and the net area change has been 0.7% since 1996. Particularly in the recent decade, the Tropical Southwestern Pacific mangroves rate of area gain has increased. Extrapolating this recent trend over the next 50 years suggests a projected increase of 3.6% in mangrove extent. However, under a high sea-level rise scenario (IPCC RCP8.5) ≈-45.4% of the Tropical Southwestern Pacific mangroves would be submerged by 2060. Overall, the Tropical Southwestern Pacific mangrove ecosystem is assessed as Vulnerable (VU).