Keystone Species

Origins: Paine's starfish experiments

Charles Elton (1927) explained community structure through food chains and the pyramid of numbers, but it was unclear which species mattered most. In the rocky intertidal zone of Washington State, Robert Paine (1966) repeatedly removed the predatory starfish Pisaster ochraceus. Mussels then monopolized the rock, crowding out other species competing for space, and species diversity fell from fifteen to eight. The predator had been suppressing a dominant competitor and allowing many species to coexist. Paine (1969) called such species, which hold up community structure, keystone species, after the keystone at the top of an arch.

Trophic cascades and classic cases

Estes and Palmisano (1974) compared Aleutian islands and found that where sea otters were present, sea urchins were scarce and kelp forests flourished, whereas where otters were absent, urchins multiplied and produced urchin barrens almost devoid of kelp. The transmission of a top predator's influence down through several trophic levels is called a trophic cascade. Estes et al. (1998) reported that when killer whales began eating more sea otters, otters declined and kelp forests deteriorated, linking oceanic and nearshore ecosystems. Ripple and Beschta's (2004) ecology of fear work, describing recovery of streamside vegetation as elk behavior and numbers shifted after wolves were reintroduced to Yellowstone in 1995, is widely known, though the size of the effect is debated. Estes et al. (2011) warned of the trophic downgrading of planet Earth, as worldwide declines in large predators simplify ecosystems.

Refinement and critique

Mills, Soulé and Doak (1993) criticized the term for being used so broadly that its meaning had blurred. Power et al. (1996) defined a keystone species as one whose impact is disproportionately large relative to its biomass and proposed a community importance index to measure that impact quantitatively. Menge et al. (1994) showed that the same species can play a keystone role in some places and conditions but not others. Jones, Lawton and Shachak's (1994) concept of ecosystem engineers refers to species such as beavers that modify the physical environment and create habitat for others, a concept that overlaps with but is distinct from keystone species. Simberloff (1998) asked whether single-species conservation strategies remain useful in an era of landscape-scale conservation, and Cottee-Jones and Whittaker (2012) critically reappraised the concept. Even so, together with metapopulation theory and island biogeography, the keystone concept is widely used in setting conservation priorities and in ecosystem restoration research.

FAQ

What is a keystone species in simple terms?

It is a species that, though not necessarily numerous, holds an ecosystem together like the keystone of an arch, so that its loss changes the whole system.

What are examples of keystone species?

Commonly cited examples include the starfish in Paine's experiment, sea otters that protect kelp forests by eating urchins, and large predators such as wolves whose effects cascade through ecosystems.

Sources reviewed: 2026-10-26

References

1. Elton, C. S. (1927). Animal Ecology. London: Sidgwick & Jackson.
2. Paine, R. T. (1966). Food web complexity and species diversity. American Naturalist, 100(910), 65–75.
3. Paine, R. T. (1969). A note on trophic complexity and community stability. American Naturalist, 103(929), 91–93.
4. Estes, J. A., & Palmisano, J. F. (1974). Sea otters: Their role in structuring nearshore communities. Science, 185(4156), 1058–1060.
5. Estes, J. A., Tinker, M. T., Williams, T. M., & Doak, D. F. (1998). Killer whale predation on sea otters linking oceanic and nearshore ecosystems. Science, 282(5388), 473–476.
6. Mills, L. S., Soulé, M. E., & Doak, D. F. (1993). The keystone-species concept in ecology and conservation. BioScience, 43(4), 219–224.
7. Power, M. E., Tilman, D., Estes, J. A., Menge, B. A., Bond, W. J., Mills, L. S., Daily, G., Castilla, J. C., Lubchenco, J., & Paine, R. T. (1996). Challenges in the quest for keystones. BioScience, 46(8), 609–620.
8. Jones, C. G., Lawton, J. H., & Shachak, M. (1994). Organisms as ecosystem engineers. Oikos, 69(3), 373–386.
9. Menge, B. A., Berlow, E. L., Blanchette, C. A., Navarrete, S. A., & Yamada, S. B. (1994). The keystone species concept: Variation in interaction strength in a rocky intertidal habitat. Ecological Monographs, 64(3), 249–286.
10. Simberloff, D. (1998). Flagships, umbrellas, and keystones: Is single-species management passé in the landscape era? Biological Conservation, 83(3), 247–257.
11. Ripple, W. J., & Beschta, R. L. (2004). Wolves and the ecology of fear: Can predation risk structure ecosystems? BioScience, 54(8), 755–766.
12. Estes, J. A., Terborgh, J., Brashares, J. S., et al. (2011). Trophic downgrading of planet Earth. Science, 333(6040), 301–306.
13. Cottee-Jones, H. E. W., & Whittaker, R. J. (2012). The keystone species concept: A critical appraisal. Frontiers of Biogeography, 4(3), 117–127.