Keystone Species


A keystone species is essential to an ecosystem or food web. If you remove it, like the keystone in an arch, the whole ecosystem collapses.


Keystones can be identified by removing certain species from a food web and observing the effects this has on the ecosystem, as seen in Robert Paine’s studies in which starfish were removed to determine their role in rocky shore ecosystems. He found that after a few months mussels, which also lived on the shore, had outcompeted all other species, and only the mussels, and barnacles which lived on their shells, remained. It was therefore thought that the starfish predated on the mussels, limiting the population and so without a natural predator the mussels outcompeted all other species. Without the starfish, the ecosystem could not function normally and therefore starfish were identified as a keystone species in this ecosystem.


Keystone species tend to occupy a niche meaning that they perform a unique function in their ecosystem, that no other organism would be able to do. Without the keystone species, an ecosystem becomes radically changed or is unable to function. Consequently, the conservation of keystone species is vital to the preservation of an ecosystem.

Figure 1 – A rocky coast starfish

‘800px-Starfish,_Oregon_coast’, August 2011, Graphic, Steven Pavlov. Source:,_Oregon_coast.jpg (accessed on 9/1/19)

Key Terms:

Ecosystem – A community of interdependent organisms and the physical/chemical environment in which they inhabit.

Food webs – The food interactions between all the species of a certain ecosystem (see Food Webs and Chains article for more information).

Natural Predator– The organism which automatically preys on a certain prey item.

Niche – The way in which an organism specifically fits into an ecological system under specific environmental conditions (see Niches article for more information).

Population – A group of individuals of the same species living in the same area at the same time.

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