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Keystone Species: Species that has a disproportionately large effect on its environment relative to its abundance

• Keystone species play a critical role in maintaining the structure of an ecological community, affecting many other organisms in an ecosystem, and helping to determine the types and numbers of various other species in the community.
• Without keystone species, the ecosystem would be dramatically different or cease to exist altogether.
• The role that a keystone species plays in its ecosystem is analogous to the role of a keystone in an arch.
• The concept of the keystone species was introduced in 1969 by zoologist Robert T. Paine.
• A classic keystone species is a predator that prevents a particular herbivorous species from eliminating dominant plant species.
• Sea otters protect kelp forests from damage by sea urchins.
• Keystone predators may increase the biodiversity of communities by preventing a single species from becoming dominant.
• Keystones mutualists are organisms that participate in mutually beneficial interaction, the loss of which would have a profound impact upon the ecosystem as a whole.
• The beaver is a well-known ecosystem engineer and keystone species.
• The term has been applied widely in different ecosystems and to predators, prey, and plants (primary producers), inevitably with differing ecological meanings.
• The term has thus been given quite different meanings in different cases.
• The concept has been criticized by L. S. Mills and colleagues for oversimplifying complex ecological systems.