Fig. 2: Space-time dynamics of urchin invasion and catastrophic phase-shift from kelp to barrens.

Box plots of a C. rodgersii abundance, and b barrens coverage by period (blue = 2001–02; red = 2016–17) as assessed by divers, sites numbered north to south along coast (n = 3 subsites per site, with 4 transects within each subsite); box plots show median (50th percentile) as a horizontal bar, with upper and lower bounds of box defined as the 75th and 25th percentiles, whiskers extend to 1.5 times this interquartile range, outliers are shown as individual points occurring beyond >1.5 times interquartile range. c phase-shift dynamics between total kelp cover and C. rodgersii density through time as measured at 5 m² quadrat scale. Source data are provided as a Source Data file. c downward and upward dashed-arrows indicate tipping-points of overgrazing (i.e., ~2.2 urchins.m⁻²) and kelp recovery (i.e., ~0.36 individ.m⁻²)¹⁷; solid lines are simplified mean trendlines fitted to all data in 2001/02 (blue), y = −19.82x + 100.31, R² = 0.56, n = 1600 quadrats; and 2016/17 (red), y = −33.58x + 99.85, R² = 0.65, n = 1600 quadrats (red downward arrow indicates collapse of kelp through time); histogram on right of panel projects the frequency distribution of quadrats within 20% bins of kelp cover, showing increasing bimodality between high cover kelp beds and collapsed urchin barrens through time, 2001/02 (blue) to 2016/17 (red).