Purple Sea Star

Pisaster ochraceus

Two purple sea stars on rock surface

In the 1960's, the purple sea star was one of the first species recognized as a keystone. Its presence keeps tide pools in balance and prevents mussels from taking over the ecosystem.

Range map of Purple Sea Star

A tide pool habitat near an ocean with many purple sea stars and mussels

Healthy population of Purple Sea Stars

Makah Bay, Washington is where the story of keystone species begins. Ecologist Robert Paine was a young professor who designed an experiment to see what would happen in tide pools if he removed the top predator: a purple sea star known as Pisaster ochraceus.

Purple Sea Star, Pisaster ochraceus

One might expect that with the purple starfish gone, the species it preyed upon would thrive. Instead, most of the prey species disappeared.

Tide pool habitat with kelp, algae, and sea anemone partially submerged

Tide pool at low tide showing lots of diverse animals and algae. Photo credit: iStock/Getty Images Plus

Mussels exposed in the open air near a tide pool habitat

Mussels are one of the prey species of purple sea stars. Photo credit: iStock/Getty Images Plus

Ocean shoreline tide pool ecosystem dominated by mussels

Without the top predator, tide pool community is dominated by mussels. Photo credit: iStock/Getty Images Plus

The initial community had 16 species. One year after removing the star fish, the community was down to just eight, including a snail that had not been there to begin with. After 5 years, only 1 species of mussel remained. It had taken over the ecosystem. The loss of a single top predator caused an entire community to collapse.

Illustration of the tide pool food web before starfish removal. There are 16 total species and starfish are shown as the top predator. The prey species for starfish include gooseneck barnacles, chitons, limpets, barnacles, and mussels.

Food web of 16 species found in North Pacific tide pools at the beginning of Paine's experiments.

Illustration of the tide pool food web after one year of starfish removal. There are 8 total species and the main predators are sea snails, which eat barnacles and mussels, and anemones, which eat mussels.

Food web after removing starfish for 1 year. Only 8 of 16 original species remained.

Illustration of the tide pool food web after five years of starfish removal, only the mussel remains

Only one species remained in tide pools after removing star fish for 5 years.

Paine repeated the experiment with other species, but their removal did not have the same dramatic impact on the tide pool ecosystem. Paine dubbed the starfish a keystone, and created a new field of ecological study.