Widespread declines of formerly common species reshape local assemblages over time.

Despite dramatic human-driven changes across ecosystems, local-scale aggregate biodiversity metrics are often surprisingly stable, even as communities experience substantial reorganization of species identities and relative abundances. While rare species are often thought to be more vulnerable to these changes, emerging evidence suggests that common species may experience disproportionately greater declines. To evaluate these alternatives, we synthesize population trends for species initially rare or common across assemblages from 4748 sites within 129 studies spanning terrestrial, freshwater, and marine ecosystems. We quantify how population trends relate to initial abundances using hierarchical Bayesian models that account for regression to the mean and include realm, taxon, and environmental covariates. We find consistently stronger declines among formerly common species in marine and terrestrial assemblages, while freshwater assemblages show no clear directional pattern. Rare species present at the start also decline, but less so than common species, while colonists that were not present at the start of the time series compensate for losses within the rarest abundance category. Climate change and anthropogenic drivers modulate these trends, with increasing temperature and non-climatic human pressures attenuating losses of initially common species, particularly in marine systems. Overall, we find widespread declines in common species through time, with potential functional consequences.