Climate change is causing species redistribution, potentially leading to ecosystem collapse. This study examines a catastrophic phase shift in kelp ecosystems due to the range extension of an overgrazing sea urchin (*Centrostephanus rodgersii*) in southeastern Australia. Early-warning signals, termed ‘incipient barrens,’ were detected well before collapse, showing poleward progression over 15 years. Projections indicate half of all kelp beds in the affected region will collapse by ~2030. Overgrazing was linked to deep boulder reefs and negatively associated with fished predatory lobsters and abalone. Equatorward observations provide a space-for-time substitution to detect early warnings, enabling proactive climate adaptation measures.

Scott D. Ling, John P. Keane