Underwater kelp forests remove 40 million tons of carbon from the atmosphere every year, and sustain a diverse range of organisms. Needless to say, the kelp is important. Yet hordes of sea urchins live in these pacific forests and eat the kelp, and would do so to depletion if left unchecked. In California, sea otters keep an entire ecosystem stable—and keep themselves fed—by hunting urchins. Without the sea otters, urchin populations would grow and consume the kelp that supports a delicate marine food web.