What Happens When a Keystone Species Goes Extinct?

When ecologist Robert Paine removed Ochre sea stars (*Pisaster ochraceus*) like these from one area of Mukkaw Bay in Washington, he observed a dramatic decrease in species diversity. Wikimedia Commons

Some of history's most persistent structures are the aqueducts built by ancient Romans to carry water from mountains to heavily populated areas. Many still operate today, more than 2,000 years after they began their service. What makes aqueducts so strong is the cascade of arches holding up the structure.

If you examine one of these arches, you'll see it consists of a series of bricks — what engineers call voussoirs — supported at the center by a keystone. The keystone gives the arch its strength and stability. When it's in position, the arch can stand indefinitely. Remove it, and the whole structure collapses.

In 1969, a zoologist named Robert T. Paine realized that certain species in an ecosystems function just like the keystone in a Roman arch, and he coined the term keystone species to describe them. Such a species plays an essential role in the structure, functioning or productivity of an ecosystem and, like its bridge counterpart, keeps the ecosystem from falling apart.

Keystone species don't earn their distinction because of abundance, but because of influence. They can be carnivores or herbivores, plant or animal, marine or terrestrial. They can tower over you, like an elephant, or fit in the palm of your hand, like a sea star.

It was, in fact, a particular species of sea star that led to Paine's development of the keystone concept. The sea star was Pisaster ochraceus, which lives in rocky intertidal communities in western North America and feeds on mussels. When Paine removed Pisaster from one area of Mukkaw Bay in Washington, he observed a dramatic decrease in species diversity. The mussel population, of course, exploded, but other species saw their numbers decline dramatically.

Of 15 species counted at the beginning of the experiment, only eight remained at the end. In a control area from which Pisaster wasn't removed, Paine didn't observe any changes in species diversity. Paine characterized Pisaster as a keystone species.

Soon ecologists and conservation biologists around the world were on the hunt to identify others. Like Paine, they used removal experiments — taking away a single species, recording the changes that occur and returning organisms to their habitats when the removal experiments concluded — to find them. Throughout the next three decades, the list of keystone species grew to include a wide variety of organisms, including sea otters, parasitic wasps, elephants and tiger sharks, as well as bats and birds that assist with pollination.

Did the dodo bird make the cut, too?

Was the Dodo Bird a Keystone Species?

Outside of removal experiments we described previously, natural conditions are driving keystone species close to extinction in certain parts of the world. This has led to significant changes in other interconnected populations.

The sea otter is a perfect example. As a keystone predator in the North Pacific, otters feed on sea urchins, which in turn feed on kelp. When sea otters thrive in their range, kelp forests remain thick and healthy. But over the last 30 years, killer whales have begun to include otters in their diets, as the whales' normal prey has declined. This has caused sea otter populations to plummet and sea urchin populations to increase rapidly. In these areas, kelp is almost nonexistent.

In a worst case scenario, if the sea otter were to go extinct, other species would surely follow, and the ecosystem would be changed forever.

Another case of a keystone species going extinct may have occurred on the island of Mauritius in the Indian Ocean. The species was the dodo, a large flightless bird that thrived until Dutch explorers discovered the island in the late 1590s. Then the dodo population began to fall sharply as settlers cut down forests and introduced rats and pigs, which raided the bird's nests for their eggs. In less than 100 years, the dodo was gone, left to languish in history books as a dumb animal unable to adapt.

However, ecologists have hypothesized for decades that the dodo may have been a keystone species and that it had what's called obligate mutualism with another endangered organism — a tree known as the tambalacoque. The last tambalacoque germinated just before 1700, suggesting that the tree's demise is somehow related to the dodo's extinction. The hypothesis suggests that the dodo ate the fruit of the tambalacoque and, through digestive processes, activated the seeds. With no dodos around to carry out this important function, the population of tambalacoque trees dwindled to just a few trees.

Many scientists, however, dispute the tambalacoque/dodo connection, but even if the bird's extinction is not tied to the tree's, the hypothesis underscores an important point about conservation biology: Connections between organisms are not always obvious and that the disappearance of even one species can have far-reaching effects. For these reasons, ecologists continue to focus on keystone species as a way to preserve the structure and function of a wide range of habitats.