IELTS Reading: True / False / Not Given

Along much of the rocky coast of the North Pacific, from northern Japan to California, lies a habitat that few people ever see: the kelp forest. Kelp is a large brown seaweed, and in cool, nutrient-rich water some species can grow by as much as half a metre in a single day, forming canopies at the surface. Beneath them, long stalks rise like the trunks of trees from a root-like anchor called a holdfast, which grips the rock of the sea floor. These forests shelter a remarkable variety of life. Yet in many places their survival depends on a single animal, and not a large one: the sea otter.

Unlike seals and whales, sea otters have no layer of blubber to protect them from the cold. Instead they keep warm in two ways: with their fur, the densest of any animal, which traps a layer of air against the skin, and by burning energy at a very high rate. To fuel this, an adult must eat between a fifth and a quarter of its own body weight every day. Otters dive for sea urchins, crabs and clams, and they are among the few mammals known to use tools: floating on its back, an otter will place a flat stone on its chest and strike hard-shelled prey against it until the shell breaks.

Its appetite links the otter to the forest, because sea urchins, which graze on kelp, are among its favourite foods. Where otters are present, the urchins that survive tend to stay hidden in cracks in the rock, living largely on pieces of kelp that have broken off and drifted down to them. Where otters are absent, urchin numbers can rise very quickly and the animals move out into the open. They chew through the holdfasts, cutting the kelp loose from the sea floor so that whole plants drift away, and eat any young kelp that tries to grow in its place. The result is what ecologists call an urchin barren: a stretch of bare rock carpeted with urchins, where a forest once stood. This chain of effects, passing from predator to prey to plant, is known as a trophic cascade.

The fur that keeps the otter warm also made it extraordinarily valuable, and for much of the past three centuries its survival was far from certain. After Russian explorers returned from the North Pacific in the 1740s with sea otter pelts, hunters pushed along the Aleutian Islands and down the coast of North America, later joined by British and American traders who sold many of the skins in China. The hunt continued for more than 150 years. By the early twentieth century, a population that had once numbered in the hundreds of thousands had been reduced to perhaps no more than two thousand animals, scattered in small, isolated groups. In 1911 an international treaty finally banned the commercial hunting of the species.

The disappearance of the otters revealed what the animal had been doing all along. Where it vanished, urchins multiplied and kelp forests gave way to barrens that could persist for decades. The clearest evidence came in the early 1970s, when researchers compared islands in the Aleutian chain. Around islands where otters had recovered, divers found thick kelp, plenty of fish and only a few small urchins. Around an island where otters had never returned, the sea floor was dominated by urchins and there was almost no kelp at all.

Protection allowed the surviving groups to grow, slowly at first. One of them, off the central coast of California, was so small that for years it was believed to have died out altogether. Between the mid-1960s and the early 1970s, wildlife agencies also moved several hundred otters from Alaska to parts of the coast where the species had been wiped out, including British Columbia and Washington State. Not every attempt succeeded: animals released off Oregon failed to establish themselves, for reasons that are still debated. But where colonies took hold, the change could be rapid, and in some bays urchin barrens have turned back into kelp forest within a few years of otters moving in.

The return of the otter has not been welcomed by everyone. Because many of its favourite prey are also valuable to people, its recovery has brought it into conflict with shellfish fisheries. In parts of southeast Alaska, divers who harvest sea cucumbers, sea urchins and clams have reported steep falls in their catches where otters have returned, a serious loss for communities built around shellfish. Some fishers argue that what looks like the restoration of a natural balance is, from their point of view, simply the arrival of a very efficient competitor.

Supporters of the otter reply that the restored forests have value of their own. Young fish of many commercially important species use them as nurseries, the canopies reduce the force of waves reaching the shore, and the kelp absorbs carbon dioxide as it grows. Martha Ellery, a fisheries economist, weighed these gains against the losses on one stretch of coast and concluded that the benefits were likely to exceed the costs. Her critics note, however, that the gains are spread thinly across a whole region while the losses fall heavily on a few small communities.

The sea otter's story shows that an ecosystem does not always return to normal once a disturbance ends. A coast can settle into two quite different states, forest or barren, and a single species can determine which of them it is in. Protecting a predator, in other words, is rarely only about the predator. It is a decision about the whole shoreline, and about who will share in its gains and who will bear its costs.