Tens of millions of voracious purple sea urchins that have already chomped their way through towering underwater kelp forests in California are spreading north to Oregon, sending the delicate marine ecosystem off the shore into such disarray that other critical species are starving to death. A recent count found 350 million purple sea urchins on one Oregon reef alone — more than a 10,000% increase since 2014. And in Northern California, 90% of the giant bull kelp forests have been devoured by the urchins, perhaps never to return. Vast “urchin barrens” — stretches of denuded seafloor dotted with nothing but...