M-STEP Practice Test 5

Guardians of the Kelp Forest

¹Off the coast of central California, just beyond the breaking waves, forests grow underwater. Giant kelp, a kind of large brown algae, rises from the rocky seafloor toward the sunlit surface and can grow as much as two feet in a single day. These forests shelter hundreds of species, from tiny snails to rockfish to harbor seals. Yet the health of this underwater world depends, to a surprising degree, on one furry animal that floats on its back at the surface: the sea otter.

²Sea otters are among the smallest marine mammals in North America, but they have enormous appetites. Unlike whales and seals, otters have no thick layer of blubber to keep them warm in the cold Pacific. Instead, they rely on two things: the densest fur of any animal on Earth, with up to a million hairs in a single square inch, and a fast metabolism that burns energy quickly. To fuel that metabolism, an adult otter must eat about a quarter of its body weight every day. Much of that food comes from the seafloor, where otters dive for crabs, clams, and, above all, sea urchins.

³Sea urchins are spiny, round grazers that feed on kelp. In a healthy forest, most urchins hide in cracks in the rocks and eat bits of kelp that drift down to them. When urchins have no predators to fear, however, they behave differently. They leave their hiding places and move across the seafloor in large groups, chewing through the holdfasts, the rootlike structures that anchor kelp to the rocks. Once its holdfast is cut, a towering kelp plant breaks loose and drifts away. A forest that took years to grow can disappear in a few seasons. Scientists call what is left behind an "urchin barren," and it can stay that way for many years.

⁴Scientists learned how important otters are partly because of a terrible loss. Beginning in the 1700s, hunters killed sea otters by the hundreds of thousands for their valuable fur. By the early 1900s, only about one or two thousand otters were left, in scattered groups across a range that had once stretched from Japan to Mexico. Along coastlines where otters had vanished, urchin populations exploded and kelp forests collapsed. Where small groups of otters survived, the forests remained.

⁵That contrast gave researchers a natural experiment. In the 1970s, scientists compared islands in Alaska that had otters with islands that did not. Around the otter islands, kelp grew thick and fish were plentiful. Around the islands without otters, the seafloor was covered with urchins and held very little kelp. The difference was so sharp that it changed how many ecologists thought about food webs. An animal near the top of a food web, they realized, could shape an entire ecosystem by controlling the animals below it. Ecologists call such an animal a keystone species, after the central stone at the top of an arch that holds the other stones in place.

⁶Protecting otters, then, also protects kelp forests, and the benefits do not stop there. Kelp forests provide nurseries for young fish that people later catch for food. Their thick canopies calm waves before they reach shore, which can reduce erosion. Kelp also absorbs carbon dioxide as it grows, and some of that carbon is stored when pieces of kelp sink into the deep ocean. Some researchers have estimated that, in parts of the North Pacific, the return of otters has helped kelp forests store far more carbon than they could in areas without otters.

⁷The story is not entirely simple, however. Sea otters also eat clams, crabs, and abalone that people harvest, and in some places fishers have worried that growing otter populations will compete with them. Communities along the coast have had to weigh the value of a healthy forest against the value of a shellfish catch. In addition, otters face newer threats, including oil spills, which ruin the fur they depend on for warmth, and diseases carried into the ocean by runoff from land.

⁸Since sea otters gained legal protection, beginning with an international treaty in 1911, their numbers have recovered in many areas. In other places, including most of the coasts of Oregon and Northern California, they have still not returned. Scientists and community groups are now studying whether otters could someday be brought back to those coasts, and what such a return would mean for the people who live and work there.

⁹The lesson of the sea otter reaches beyond one animal. In an ecosystem, the loss of a single species can set off changes no one predicted, and its return can bring back far more than itself. Watching an otter crack open an urchin on its belly, a visitor might see only a playful animal eating lunch. A scientist sees the gardener of an entire forest.