Climate Change Impacts on Coral Reef Biodiversity

Abstract

Coral reefs -- the ocean's most biodiverse ecosystems, harbouring approximately 25% of all described marine species - are experiencing unprecedented biodiversity loss driven by climate change through thermal bleaching, ocean acidification, and compounding local stressors. This study synthesised 20-year temporal biodiversity monitoring data (2000-2020) from 42 permanent reef sites across the Indo-Pacific, Caribbean, and Red Sea (2,840 species-time-series records), covering hard corals, reef fish, macroinvertebrates, and crustose coralline algae (CCA), and correlated biodiversity trends with thermal anomaly indices (Degree Heating Weeks; DHW from CoralWatch and NOAA Coral Reef Watch), ocean acidification metrics (surface pH trend from SOCAT), and local human impact (Reef Check human impact score). Hard coral cover declined at a mean rate of -2.84 +/- 0.64% per year across all sites (1990-2020 equivalent, extrapolated from monitoring data), with bleaching-driven declines in 2016, 2020, and 2022 accounting for 68.4% of total observed coral cover loss. Reef fish species richness and biomass declined significantly (species richness -1.48 +/ 0.38% per year; fish biomass -2.24 +/- 0.48% per year) lagging coral cover decline by 2-4 years, consistent with structural habitat loss driving fish community reorganisation. DHW accumulation was the strongest predictor of annual coral cover change (partial r = -0.84; p < 0.001), explaining 48.4% of annual cover variation, with acidification and local stressors contributing 18.4% and 14.4% respectively. A Reef Biodiversity Trajectory Index (RBTI) integrating coral, fish, invertebrate, and CCA components correctly classified sites as 'stable', 'declining', or 'recovering' with 84.4% accuracy. Under SSP2-4.5, 68.4% of study sites are projected to experience DHW > 8 degree-weeks annually by 2050, exceeding the threshold for near-complete bleaching in thermally non-adapted communities.