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From chilly Alaska to sunny Baja California, on the exposed rocky shores between low and high tide, lives a group of marine organisms in delicate balance with one another. But one organism, the starfish Pisaster ochracues, is the pillar of the community on which the stability of the entire system depends.

If that single predator is removed, dramatic changes result in the varieties and population densities of all the other species in the community. Interestingly, no comparable changes appear when other "consumers" are removed from the biological fabric. That starfish is the "keystone" species governing biological diversity in the entire intertidal zone.footnote 1

The keystone species concept has been a mainstay of the ecological and conservation biology literature since its introduction by UW zoology professor Robert T. Paine in 1969. His seminal paper extended the conclusions of a field experiment published three years earlier. The research resulting in the keystone species concept was done on Makah Tribal lands on the outer coast of Washington State, with the Makahs' permission. It involved the sustained removal of a single predator species over a three-year interval and documentation of the resultant changes.

Keystone species are usually noticed when they are removed or they disappear from an ecosystem, resulting in dramatic changes to the rest of the community. The phenomenon has been observed in a wide range of ecosystems and for a wide range of organisms.

Put another way, a keystone species is one whose impacts on its community or ecosystem are large and greater than would be expected from its relative abundance or total biomass, explains Paine. Species that are known to play this role, besides the starfish mentioned above, include the sea otter, the freshwater bass, and the predatory whelk Concholepas (a kind of elongated sea snail). By contrast, trees, giant kelp, prairie grasses, and reef-building corals all have impacts that are large but not disproportionate to their total biomass, and therefore they are not keystone species.

A good illustration of Paine's keystone species concept is provided by the sea otter, which formerly occupied a range extending from the northern Japanese archipelago, through the Aleutian Islands, down the coast of North America as far south as Baja California. The return of the sea otter to southern California, for example, is restoring kelp beds and associated marine life there. That's because one of the favorite delicacies of the otter is the large sea urchin, which in turn feeds on kelp. As the sea otter returns to its native territories, scientists expect the population of invertebrates, like urchins and abalone, for example, to decrease as marine plant biomass increases. In fact, a decrease in sport and commercial abalone fisheries was reported following an influx of sea otters into areas of California, causing a controversy there.footnote 2

Paine's work has been cited by hundreds of researchers over the years, and it has been proposed as a foundation for management efforts to protect the biological diversity of the world's ecosystems.

"Its importance," says Paine, "is that it convinced managers and conservationists alike that the ecological impact of single species matters. That is, in order to manage, understand, and restore ecological assemblages, the roles of individual species have to be understood and considered."

The concept has provided a powerful model for understanding the forces that organize ecological communities, and it has influenced the thinking of managers and policy makers as they set priorities in their efforts to conserve species and habitats. Over the years, controversy in scientific circles has grown up around the term keystone species as a result of "overly expansive usage."footnote 2 Through a United Nations Environmental Programme workshop held in 1994, prominent researchers in the field reached a new consensus about a definition of the term and new guidelines for its expanding applications in the future.footnote 3


  1. "The Keystone-Species Concept in Ecology and Conservation," L. Scott Mills, Michael E. Soule, and Daniel F. Doak, BioScience, 43 (4), 219 (1993).
  2. "Sea Otters: Their Role in Structuring Nearshore Communities," Science, 185, 1058 (1974).
  3. "The Keystone Cops Meet in Hilo," Mary E. Power and L. Scott Mills, TREE, 10 (5), 182 (1995).

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