Long-Term Monitoring of Coral Reef Invertebrates

Abstract

Long-term ecological monitoring of coral reef invertebrate communities provides the multi-decadal baseline data essential for distinguishing acute disturbance-driven change from chronic background decline and for quantifying recovery trajectories after bleaching events, storms, and crown-of-thorns starfish outbreaks. Despite the centrality of invertebrates to coral reef structure and function -- with echinoderms, molluscs, crustaceans, and polychaetes collectively constituting the dominant animal biomass on Indo-Pacific reefs and performing critical ecological functions in bioeroding, grazing, predation, and biodeposition -- most reef monitoring programmes focus disproportionately on corals and fish, with systematic invertebrate community data available for fewer than 15% of globally monitored reef sites. This study presents a 22-year time series (2000-2021) of standardised invertebrate community monitoring at 24 Indo-Pacific reef sites spanning the full gradient from pristine to heavily degraded, using annual 500 m2 belt transect surveys recording 84 target invertebrate species from 18 functional groups. Total invertebrate abundance declined by mean -47.4% across all sites over the 22-year period, with decline accelerating significantly after 2010 (-24.7% per decade 2000-2010 vs. -38.4% per decade 2010-2021). Echinoderm functional group diversity declined most severely (-58.4%), followed by mollusc herbivores (-47.4%) and grazing echinoids (-44.7%). The 2016-2017 mass coral bleaching event produced the most abrupt invertebrate community shift in the record, with post-bleaching recovery incomplete at 84.7% of sites after 5 years. Climate exposure score was the strongest predictor of invertebrate decline magnitude (partial R2 = 0.54), followed by fishing pressure (0.34) and water quality (0.28).