Scientists say fast-spreading sea urchins have eaten away at huge underwater kelp forests in California and Oregon. The invasion by tens of millions of the sea creatures is causing the destruction of important forms of sea life. A recent count found 350 million purple sea urchins on one Oregon reef alone. That is more than a 10,000 percent increase since 2014. In Northern California, 90 percent of large kelp forests have been swallowed up by the urchins. Scientists have reported finding so-called “urchin barrens,” or huge stretches of the seafloor that contain nothing but sea urchins. Such growth has spread from California to waters off the state of Oregon, where kelp forests were once so thick that boats had trouble passing through.