"They're purple, spiky and voracious, and just off the West Coast, there are more of them than you can count.
Purple sea urchins have exploded in recent years off California, covering the ocean floor in what divers describe as a "purple carpet." And they devour kelp: the once-lush forests of seaweed that hugged the coastline are disappearing. Since 2014, 95 percent of the kelp have vanished across a large part of Northern California, most of it bull kelp.
Kelp forests provide a crucial ecosystem for a broad range of other marine life and animals, so their demise threatens the ecology across the entire stretch of California coast. The kelp's abrupt decline is being driven by warming waters, and it's a case of how climate change is helping push already-stressed ecosystems over the edge."