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alternative stable state
In ecology, the theory of alternative stable states (sometimes termed alternate stable states or alternative stable equilibria) predicts that ecosystems can exist under multiple “states” (sets of unique biotic and abiotic conditions). These alternative states are non-transitory and therefore considered stable over ecologically-relevant timescales. Ecosystems may transition from one stable state to another, in what is known as a state shift (sometimes termed a phase shift or regime shift), when perturbed. Due to ecological feedbacks, ecosystems display resistance to state shifts and therefore tend to remain in one state unless perturbations are large enough. Multiple states may persist under equal environmental conditions, a phenomenon known as hysteresis. Alternative stable state theory suggests that discrete states are separated by ecological thresholds, in contrast to ecosystems which change smoothly and continuously along an environmental gradient.
== Theory ==
Alternative stable state theory was first proposed by Richard Lewontin (1969), but other early key authors include Holling (1973), Sutherland (1974), May (1977), and Scheffer et al. (2001). In the broadest sense, alternative stable state theory proposes that a change in ecosystem conditions can result in an abrupt shift in the state of the ecosystem, such as a change in population or community composition. Ecosystems can persist in states that are considered stable (i.e., can exist for relatively long periods of time). Intermediate states are considered unstable and are, therefore, transitory. Because ecosystems are resistant to state shifts, significant perturbations are usually required to overcome ecological thresholds and cause shifts from one stable state to another. The resistance to state shifts is known as “resilience” (Holling 1973).
State shifts are often illustrated heuristically by the ball-in-cup model (Holling, C.S. et al. (1995) Biodiversity in the functioning of ecosystems: an ecological synthesis. In Biodiversity Loss, Ecological and Economical Issues (Perrings, C.A. et al., eds), pp. 44–83, Cambridge University Press). A ball, representing the ecosystem, exists on a surface where any point along the surface represents a possible state. In the simplest model, the landscape consists of two valleys separated by a hill. When the ball is in a valley, or a “domain of attraction,” it exists in a stable state and must be perturbed to move from this state. In the absence of perturbations, the ball will always roll downhill and therefore will tend to stay in the valley (or stable state). State shifts can be viewed from two different viewpoints, the “community perspective” and the “ecosystem perspective.” The ball can only move between stable states in two ways: (1) moving the ball or (2) altering the landscape. The community perspective is analogous to moving the ball, while the ecosystem perspective is analogous to altering the landscape.
These two viewpoints consider the same phenomenon with different mechanisms. The community perspective considers ecosystem variables (which change relatively quickly and are subject to feedbacks from the system), whereas the ecosystem perspective considers ecosystem parameters (which change relatively slowly and operate independent of the system). The community context considers a relatively constant environment in which multiple stable states are accessible to populations or communities. This definition is an extension of stability analysis of populations (e.g., Lewontin 1969; Sutherland 1973) and communities (e.g., Drake 1991; Law and Morton 1993). The ecosystem context focuses on the effect of exogenic “drivers” on communities or ecosystems (e.g., May 1977; Scheffer et al. 2001; Dent et al. 2002). Both definitions are explored within this article.
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