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 ecosystem 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 ochraceous, 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, tiger sharks, as well as bats and birds that assist with pollination.

Did the dodo bird make the cut, too?