https://www.youtube.com/watch?v=qAgsOhS5g6U
Northern California bull-kelp forests collapsed after marine heatwaves, sea-star wasting disease, and a boom in purple sea urchins that graze kelp holdfasts to the rock.
Once an urchin barren forms, recovery is slow: urchins can persist on sparse food, and kelp spores struggle to recolonize without sustained grazing pressure reduction.