Purple Sea Stars, Part II

Last week I did a bit of digging into the carnivorous, brainless, neuro-sensory-cell-powered sea star of the tidepools of the Pacific Northwest.

These odd creatures are also responsible for the discovery of keystone species. Okay, the credit really goes to a scientist named Robert Paine, but the tidepools and sea stars provided him with the perfect natural laboratory. If you can believe it, prior to the 1960s, scientists though that food chains were regulated by the number of producers in an ecosystem. Further, predators were not thought to have any role in regulating species populations. It’s counterintuitive to us now, but we’re a long way from the 1960s.

Photo Credit – John Lloyd

The reason that Paine wanted a natural laboratory is because fellow scientists had questioned why, if above were true, why didn’t consumers simply devour all the plant material in a habitat? Why was there any green left? The challenge was finding an ecosystem they could control and manipulate. So when Paine discovered the tidepools, he knew he’d found the perfect place to run an experiment. He started by observing all the species in the tidepools and creating a food chain. That’s when he learned that the sea stars were the top predators in those systems. Then, to understand the role of the top predator, he removed them all from one tidepool and left them in others.

Within 18 months, the tidepools without the sea stars had changed. Within 8 years, the tidepools had but a single species left – mussels. Without the starfish to prey on the mussels, the mussels had taken over the tidepool and pushed all other species out. The mini ecosystem had collapsed without the sea stars. This was a groundbreaking discovery.

Borrowing from architecture, Paine coined the term “keystone species” that we still use today. Just as in a stone or brick arch, if the keystone is removed, everything collapses. Paine and colleagues soon also discovered that sea otters are a keystone species. In the following decades other keystone species were identified, including ones that were not top predators. These include ecosystem engineers like beavers, herbivores like bison, mutualists such as bees, and coral – a foundational species.