<P> A keystone species is a species that has a disproportionately large effect on its environment relative to its abundance . Such species are described as playing 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 . A keystone species is a plant or animal that plays a unique and crucial role in the way an ecosystem functions . Without keystone species, the ecosystem would be dramatically different or cease to exist altogether . </P> <P> 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 . It became a popular concept in conservation biology . Although the concept is valued as a descriptor for particularly strong inter-species interactions, and it has allowed easier communication between ecologists and conservation policy - makers, it has been criticized for oversimplifying complex ecological systems . </P>

A species that has an essential role in maintaining ecosystem structure is called a