What If Gone

Species → Ochre sea star

Ochre sea star

Pisaster ochraceus

Not assessed by the IUCN. Density decline of 85 to 90% within a few…
Extinction risk Not Evaluated
Keystone speciesRocky shoreMarine environment
Ochre sea star (Pisaster ochraceus)
Photo: D. Gordon E. Robertson, Wikimedia Commons (CC BY-SA 3.0)

What if it disappeared?

Impact Major
3 years

The number of species on the intertidal shore drops from 15 to 8: without a predator, the mussel Mytilus californianus takes over the available space and crowds out the species that competed with it for rock attachment sites.

Documented
Detail

Paine, R.T. (1966). Food Web Complexity and Species Diversity. The American Naturalist, 100(910), 65–75.

By experimentally removing the ochre sea star from plots of rocky intertidal shore and comparing them to control plots, Paine showed that the mussel, the sea star's main prey, comes to monopolize the available space within three years, crowding out seven other rock-attached species. This is the founding experiment behind the keystone species concept.

10 years

The shore becomes a mussel monoculture — a single species occupies nearly all the space once shared among several.

Documented
Detail

Lafferty, K.D. et Suchanek, T.H. (2016). Revisiting Paine's 1966 Sea Star Removal Experiment, the Most-Cited Empirical Article in the American Naturalist. The American Naturalist, 188(4), 365–378.

This revisit of Paine's experimental site, several decades later, confirms that over the long term, with the sea star durably absent, the mussel eventually forms an almost continuous mat — even though, when counting the species living within the mussel bed itself (worms, small crustaceans), the site's total diversity otherwise remains high.

Since 2013

The real-world decline of the sea star across large portions of the Pacific coast has not been the subject of published monitoring as complete as Paine's controlled experiment.

Insufficient
Detail

Wasting disease caused sea star density and biomass to drop by 59 to 90% depending on the site between 2013 and 2014, but recovery has been very uneven across regions (better in Oregon and northern California than in Alaska and Washington). No one has confirmed whether the mussel takeover Paine observed at small scale has occurred at the regional scale of this real-world epidemic.

Documentation

Last verified: August 14, 2026