Taxonomic Diversity Trends Over Five Decades

Abstract

Understanding how taxonomic diversity has changed across major organismal groups over the past five decades -- a period spanning the acceleration of global biodiversity loss through habitat destruction, pollution, invasive species, overexploitation, and climate change -- is essential for calibrating the scale of the biodiversity crisis, tracking progress toward global conservation targets, and projecting future trajectories under alternative policy scenarios. This study presents the most comprehensive multi-taxon analysis of taxonomic diversity trends over 1970-2024 yet published, synthesising 52,847 population time series from 8,470 species across 18 major taxonomic groups from the BIOCHANGE global database (DBA paper 260) supplemented with specialised monitoring programme data from 84 countries. Geometric mean abundance declined by mean 52.4% (1970-2024; all taxonomic groups combined) -- a substantially larger decline than previously reported, attributable to improved coverage of invertebrate and freshwater taxa that show the steepest declines. Taxonomic group trends were highly heterogeneous: freshwater invertebrates showed the steepest decline (-73.4%; p < 0.001); marine mammals showed the strongest recovery (+84.7%; driven by post-whaling population rebound); and birds showed intermediate decline (-38.4%). The geographic pattern was stark: temperate regions showed mean -38.4% abundance decline while tropical regions, where monitoring data are most sparse, showed a modelled -67.4% decline from occupancy model extrapolation. A taxonomic vulnerability index (TVI) integrating abundance trend; range change; functional trait rarity; and phylogenetic isolation predicted IUCN threat status with AUC = 0.91, substantially outperforming abundance trend alone (AUC = 0.74).