The Otter, the Urchin and the Kelp: Reading a Food Web from the Top Down

The Otter, the Urchin and the Kelp

[1]Just off the rocky coast of central California, tall brown seaweeds called giant kelp rise from the sea floor toward the light. Giant kelp is one of the fastest-growing organisms on Earth; a single strand can lengthen by tens of centimeters in a day. Like a land plant, kelp is a producer. It captures sunlight and uses that energy to build sugars from carbon dioxide dissolved in seawater, taking the other nutrients it needs from the water around it. A healthy kelp bed is a forest, with a canopy at the surface and a shaded floor below.

[2]Surprisingly little of that forest is eaten while it is alive. Snails and some fish nibble the blades, and purple and red sea urchins chew at the base of each strand, where it anchors to the rock. But most kelp matter follows another route. Storms tear off blades and whole strands, which sink to the sea floor or wash onto beaches. There, bacteria and other decomposers break the dead kelp down, releasing carbon dioxide and nutrients back into the water, where living kelp can take them up again. The matter in a kelp blade is never used up; it changes hands.

[3]The animal most people picture in a kelp forest is the sea otter, floating on its back and cracking shellfish on its chest. Otters eat sea urchins, along with crabs, clams and snails. Unlike seals, an otter has no thick layer of fat to keep it warm, so it burns food fast in cold water, eating roughly a quarter of its own body mass every day. Most of the energy in those urchins is used by the otter's cells and leaves its body as heat that warms the ocean by a tiny amount. Only a small share is ever stored as new otter.

[4]For much of the last three hundred years, otters were missing from most of the California coast. Hunters prized their dense fur, and by the early 1900s the animals had been nearly wiped out; a small group survived near Big Sur. Where otters disappeared, the sea floor changed. Sea urchins, no longer eaten, multiplied until they covered the rocks. They chewed through kelp faster than it could grow, and in many places the forest became bare rock carpeted with spiny urchins, a landscape scientists call an urchin barren.

[5]Notice that otters never eat kelp. The change in urchins was a direct effect: fewer otters meant fewer urchins eaten. The change in kelp was an indirect effect, passed along through the urchin population in between. In the 1970s, scientists tested this explanation by comparing islands in Alaska. On islands where otters lived, urchins were small and scarce and the kelp grew thick; on nearby islands without otters, urchins were large and abundant and kelp was rare. Comparing places that differed mainly in whether otters were present let the scientists link the predator to the plants.

[6]The energy story explains why there are so few otters compared with urchins, and so few urchins compared with kelp. At each feeding step, most of the energy in the food is used for living and leaves as heat, or stays locked in parts that are never eaten. As a rough average, only about one tenth of the energy at one level becomes energy stored in the next. A kelp forest must capture a great deal of sunlight to support a modest number of urchins, and those urchins can support only a few otters. The exact fraction varies from place to place, but the shrinking pattern does not.

[7]Since the mid-1900s, otters have spread slowly along the central California coast, and in many bays the kelp has returned with them. Yet the simple chain from otter to urchin to kelp does not explain everything. Starting in 2013, a disease killed huge numbers of sunflower sea stars, another animal that eats urchins, and unusually warm water in the following years weakened the kelp. Along parts of the northern coast, where otters had not returned, kelp forests shrank sharply. A food web is a model, and a model with only three arrows leaves out a great deal. The more arrows scientists add, the better their predictions become.