Sea otters are important members of the kelp forest community of America's Pacific Northwest coast. This “forest” is made up of algae called giant kelp, with stalks up to 30 meters long, and smaller types of kelp. The kelp forest provides a habitat for a variety of animals. Sea otters need a lot of energy to stay warm in cold water, so they eat large quantities of their favorite food: sea urchins. Sea urchins, in turn, feed on kelp.

The relationships along this food chain set the stage for a classic tale of population growth and decline. A century ago, otters were nearly eliminated by hunting. Sea urchin populations increased greatly, and kelp forests nearly disappeared. Why? Because the kelp was eaten down to the bare rock by hordes of sea urchins! The future of the kelp forests looked grim. Then, sea otters were declared an endangered species and were protected from hunting. With hunters out of the picture, otter populations recovered. Sea urchin numbers dropped dramatically. Kelp grew back. But now, some otter populations are shrinking again because otters are being eaten by killer whales. To better understand why populations such as these change as they do, we turn to the study of population biology.