As our understanding of ecosystems has deepened, so has our appreciation of the role of keystone species. A keystone is a wedge-shaped piece of stone at the crown of an arch that locks all the pieces in place. A keystone species holds an ecosystem together. Without such a species, an entire ecosystem would radically change and often collapse altogether.
Keystone species perform in various ways. There are apex predators, which keep a balance among many other populations. When these top predators are removed, ecosystem imbalance follows. Creatures they once hunted multiply rapidly, pushing out other organisms and reducing community diversity. Sea otters are apex predators. They maintain the health of coastal kelp forests — forests that provide food, shelter, nursery habitat, and hunting grounds for an abundance of fish, marine mammals, and other species. When otters were hunted to near extinction, the population of sea urchins, their principal food, exploded. The result was urchins consuming the giant kelp to the point of destruction. Decimation of these forests exposes our coastline to the ravages of violent storms.
As we killed off wolves, ecosystems changed and went into decline. When wolves were reintroduced into Yellowstone National Park, scientists were amazed at how rapidly they restored a biodiverse community. In our coastal tide pools, the sea star is the species that controls diversity, for when they are absent, mussels crowd out 15 or more other tide-pool dwellers, including algae, limpets, anemones, and sponges.