Global regime shift dynamics of catastrophic sea urchin overgrazing

TLDR

Sea urchin overgrazing drives a pronounced, widespread regime shift on temperate rocky reefs. The study empirically defines regime‑shift dynamics between macroalgal beds and urchin barrens. We compiled 10,901 quadrats from 13 reef systems and conducted 57 in‑situ manipulations to characterize the dynamics of this shift. The analysis reveals a catastrophic, hysteretic regime shift with a one‑order‑of‑magnitude difference in urchin biomass between overgrazing and recovery thresholds, asymmetric overgrazing versus recovery rates, strong feedbacks sustaining each state, and human stressors that erode macroalgal resilience and amplify the shift’s risk and irreversibility.

Abstract

A pronounced, widespread and persistent regime shift among marine ecosystems is observable on temperate rocky reefs as a result of sea urchin overgrazing. Here, we empirically define regime-shift dynamics for this grazing system which transitions between productive macroalgal beds and impoverished urchin barrens. Catastrophic in nature, urchin overgrazing in a well-studied Australian system demonstrates a discontinuous regime shift, which is of particular management concern as recovery of desirable macroalgal beds requires reducing grazers to well below the initial threshold of overgrazing. Generality of this regime-shift dynamic is explored across 13 rocky reef systems (spanning 11 different regions from both hemispheres) by compiling available survey data (totalling 10 901 quadrats surveyed in situ ) plus experimental regime-shift responses (observed during a total of 57 in situ manipulations). The emergent and globally coherent pattern shows urchin grazing to cause a discontinuous ‘catastrophic’ regime shift, with hysteresis effect of approximately one order of magnitude in urchin biomass between critical thresholds of overgrazing and recovery. Different life-history traits appear to create asymmetry in the pace of overgrazing versus recovery. Once shifted, strong feedback mechanisms provide resilience for each alternative state thus defining the catastrophic nature of this regime shift. Importantly, human-derived stressors can act to erode resilience of desirable macroalgal beds while strengthening resilience of urchin barrens, thus exacerbating the risk, spatial extent and irreversibility of an unwanted regime shift for marine ecosystems.