Keystone Species

Keystone species are plants or animals that have important effects on a particular ecosystem's biodiversity and function, despite having a relatively low abundance. The term keystone originated in the field of architecture to describe a centrally located wedge-shaped stone that supports an entire arch. Robert Paine, an American ecologist, first introduced the keystone species concept to ecology in 1966. The meaning of the term in ecology is analogous to its meaning in architecture; similar to the support function of a keystone within an arch, a keystone species is considered a main supporting component of an ecosystem, without which the system would fail. The loss of a keystone species could result in the loss of several dependent species and critical ecosystem functions.

Paine's original usage of the term keystone species referred specifically to the high trophic position and unique function of a particular predatory starfish species found in a rocky marine intertidal zone. He found that the starfish's activities clearly modified the species composition and physical appearance of the ecosystem, more than the activities of other predators. The actions of the starfish clearly served to regulate biodiversity in the ecosystem.

In the years since the introduction of the concept, the definition of the term has broadened to include any species with uniquely strong, positive effects on biodiversity regardless of its tropic level; however, the term still has different meanings to different people. A general, current definition of a keystone species encompasses plants, primary consumers, or even species that make important physical modifications to an ecosystem through its daily activities. Thresholds are particularly relevant to keystone species because when the density of a keystone falls below a given level, the biodiversity of the ecosystem is likely to decrease, affecting the balance and stability of the entire ecosystem.

Keystone species differ from foundation species. While foundation species serve essential ecosystem functions and affect biodiversity, their impact is primarily the result of their abundance. Keystone species, however, are generally not recognized as being abundant in the ecosystems in which they have a critical role sustaining a biologically diverse environment. Regardless of its classification, the loss of either foundation or keystone species can be detrimental to ecosystem functionality.