Coral reefs, for example, may be directly affected by increased CO 2 in the atmosphere as well as by the rising temperatures it brings. Biologists once thought that marine species were practically immune to extinction because currents spread their drifting larvae over huge areas, making it hard to wipe them out. Recent evidence, however, suggests that many marine species range much less widely over the oceans than previously realized, and so are more vulnerable. And even widely distributed marine species have been driven to extinction. Losing certain 'keystone' species can have cascading effects.
Sea otters like to eat sea urchins, which in turn eat kelp and other seaweeds.
As otter populations have crashed in the Aleutian Islands, urchins have proliferated and are eating their way through species-rich kelp forests. Killer whales, unable to find enough of the seals and sea lions they prefer, seem to have switched to eating otters. And scientists speculate that the seals and sea lions have declined because their food has been decimated by commercial fishing. There is much to learn at even a basic level. We now realize that the deep sea is highly diverse, though thought lifeless only a century and a half ago.