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Satellite imagery shows that the area covered by kelp forests off the coast of Northern California has dropped by more than 95 percent, with just a few small, isolated patches of bull kelp remaining. Species-rich kelp forests have been replaced by ‘urchin barrens,’ where purple sea urchins cover a seafloor devoid of kelp and other algae. A new study documents this dramatic shift in the coastal ecosystem and analyzes the events that caused it.