Keystone species

Lore & Background

Paine developed the concept to explain his observations and experiments on the relationships between marine invertebrates of the intertidal zone, including starfish and mussels. He removed the starfish from an area, and documented the effects on the ecosystem.

Reader's Guide

The keystone species concept became popular in conservation biology, alongside flagship and umbrella species. Although the concept is valued as a descriptor for particularly strong inter-species interactions, and has allowed easier communication between ecologists and conservation policy-makers, it has been criticized for oversimplifying complex ecological systems. The role that a keystone species plays in its ecosystem is analogous to the role of a keystone in an arch: while the keystone is under the least pressure of any of the stones in an arch, the arch still collapses without it. Similarly, an ecosystem may experience a dramatic shift if a keystone species is removed, even though that species was a small part of the ecosystem by measures of biomass or productivity. Examples include sea otters protecting kelp forests from sea urchins, wolves as apex predators in Yellowstone, and the ochre starfish controlling mussel populations in intertidal zones.

Did You Know?

• Paine's primary example was the ochre starfish (Pisaster ochraceus) and the mussel (Mytilus californianus).
• When Paine removed the ochre starfish, the number of rock-clinging species fell from 15 to 8 in three years, and to largely one species (mussels) after ten years.
• The jaguar is considered both a keystone species and an umbrella species, flagship species, and wilderness quality indicator.