Mangrove Ecosystem Diversity Under Anthropogenic Stress

Abstract

Mangrove ecosystems -- intertidal forests of salt-tolerant trees and shrubs at tropical and subtropical coastlines -- provide exceptional ecosystem services including coastal protection, carbon storage (blue carbon; 800-3,000 Mg C/ha in soil + biomass), nursery habitat for commercially important fish and invertebrates, and extraordinary biodiversity of associated species (fish, invertebrates, birds, reptiles), yet mangroves are among the most threatened ecosystems globally, having lost approximately 50% of their historical extent since 1940 from aquaculture conversion, urban development, and climate-change-driven sea level rise. This study quantified mangrove ecosystem diversity (tree species composition, associated biodiversity, structural complexity, and carbon stocks) across 42 mangrove forest sites in three regions (West Africa: 14; Southeast Asia: 14; Central America: 14) representing a gradient of anthropogenic stress (from intact pristine mangroves to heavily degraded fragments adjacent to shrimp aquaculture ponds; 2,840 species-site records; 2019-2022). Mangrove tree species richness was highest in Southeast Asia (mean 8.4 +/- 1.8 species per ha) and lowest in West Africa (3.4 +/- 0.8), reflecting the centre-of-diversity effect of the Indo-Pacific mangrove biogeography. However, associated biodiversity (fish, birds, invertebrates) showed no significant regional difference in species richness (mean 124 +/- 28 species per site) when controlled for forest structural complexity. Anthropogenic stress (composite index of aquaculture proximity, deforestation rate, and sedimentation from upstream land use) significantly reduced both tree diversity (r = -0.74) and associated biodiversity (r = -0.68; all p < 0.001) and was the primary driver of carbon stock decline (r = -0.84; p < 0.001). A Mangrove Ecosystem Integrity Index (MEII) integrating tree diversity, structural complexity, carbon stocks, and associated species richness correctly classified sites as intact, degraded, or severely degraded with 88.4% accuracy. Restoration of mangroves at degraded sites through community-based replanting predicted MEII recovery to > 0.60 within 12 years, with highest recovery rates in sites with active tidal exchange and access to propagule sources.