Alternative stable state
Encyclopedia
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 threshold
s, in contrast to ecosystems which change smoothly and continuously along an environmental gradient
.
was first proposed by 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 (Beisner et al. 2003). 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., Law and Morton 1993; Drake 1991). 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.
s) in response to feedbacks from the system (i.e., they are dependent on system feedbacks), such as population densities
. This perspective requires that different states can exist simultaneously under equal environmental conditions, since the ball moves only in response to a state variable
change.
For example, consider a very simple system with three microbial species. It may be possible for the system to exist under different community structure regimes depending on initial conditions
(e.g., population densities or spatial arrangement of individuals) (Kerr et al. 2002). Perhaps under certain initial densities or spatial configurations, one species dominates over all others, while under different initial conditions all species can mutually coexist. Because the different species interact, changes in populations affect one another synergistically to determine community structure. Under both states the environmental conditions are identical. Because the states have resilience, following small perturbations (e.g., changes to population size
) the community returns to the same configuration while large perturbations may induce a shift to another configuration.
The community perspective requires the existence of alternative stable states (i.e., more than one valley) before the perturbation, since the landscape is not changing. Because communities have some level of resistance to change
, they will stay in their domain of attraction (or stable state) until the perturbation is large enough to force the system into another state. In the ball-and-cup model, this would be the energy required to push the ball up and over a hill, where it would fall downhill into a different valley.
, death rate
, migration, and density-dependent predation indirectly alter the ecosystem state by changing population density (a state variable). Ecosystem parameters are quantities that are unresponsive (or respond very slowly) to feedbacks from the system (i.e., they are independent of system feedbacks). The stable state landscape is changed by environmental drivers, which may result in a change in the quantity of stable states and the relationship between states.
By the ecosystem perspective, the landscape of the ecological states is changed, which forces a change in the ecosystem state. Changing the landscape can modify the number, location, and resilience of stable states, as well as the unstable intermediate states. By this view, the topography in the ball-and-cup model is not static, as it is in the community perspective. This is a fundamental difference between the two perspectives.
However, it should be noted that although the mechanisms of community and ecosystem perspectives are different, the empirical evidence
required for documentation of alternative stable states is the same. In addition, state shifts are often a combination of internal processes and external forces (Scheffer et al. 2001). For example, consider a stream-fed lake in which the pristine state is dominated by benthic vegetation. When upstream construction releases soils into the stream, the system becomes turbid. As a result, benthic vegetation cannot receive light and decline, increasing nutrient availability and allowing phytoplankton to dominate. In this state shift scenario the state variables changing are the populations of benthic vegetation and phytoplankton, and the ecosystem parameters are turbidity and nutrient levels. So, whether identifying mechanisms of variables or parameters is a matter of formulation (Beisner et al. 2003).
Hysteresis can occur via changes to variables or parameters. When variables are changed the ball is pushed from one domain of attraction to another, yet the same push from the other direction cannot return the ball to the original domain of attraction. When parameters are changed a modification to the landscape results in a state shift, but reversing the modification does not result in a reciprocal shift.
A real-world example of hysteresis is helpful to illustrate the concept. Coral reef
systems can dramatically shift from pristine coral-dominated systems to degraded algae-dominated systems when populations grazing on algae decline. The 1983 crash of urchin populations in Caribbean reef systems released algae from top-down (herbivory) control, allowing them to overgrow corals and resulting in a shift to a degraded state. When urchins rebounded, the high (pre-crash) coral cover levels did not return, indicating hysteresis (Mumby et al. 2007).
In some cases, state shifts under hysteresis may be irreversible. For example, tropical cloud forest
s require high moisture levels, provided by clouds that are intercepted by the canopy (via condensation). When deforested, moisture delivery ceases. Therefore, reforestation is often unsuccessful because conditions are too dry to allow the trees to grow (Wilson and Agnew 1992).
Resilience can change in stable states when environmental parameters are shifted. Often, humans influence stable states by reducing the resilience of basins of attraction. There are at least three ways in which anthropogenic forces reduce resilience (Folke et al. 2004): (1) Decreasing diversity and functional group
s, often by top-down effects (e.g., overfishing); (2) altering the physico-chemical environment (e.g., climate change
, pollution, fertilization); or (3) modifying disturbance regimes to which organisms are adapted (e.g., bottom trawling
, coral mining, etc.). When the resilience is decreased, ecosystems can be pushed into alternative, and often less-desirable, stable states with only minor perturbations. When hysteresis effects are present, the return to a more-desirable state is sometimes impossible or impractical (given management constraints). Shifts to less-desirable states often entail a loss of ecosystem service and function, and have been documented in an array of terrestrial, marine, and freshwater environments (reviewed in Folke et al. 2004).
(although this claim is refuted by Schröder et al. 2005). Also, Beisner et al. (2003) suggest that commercially-exploited fish populations can be forced between alternative stable states by fishing pressure due to Allee effects that work at very low population sizes. Once a fish population falls below a certain threshold, it will inevitably go extinct when low population densities make replacement of adults impossible due to, for example, the inability to find mates or density-dependent mortality. Since populations cannot return from extinction, this is an example of an irreversible state shift.
Most work on alternative stable states has been theoretical, using mathematical model
s and simulations to test ecological hypotheses. Other work has been conducted using empirical evidence from surveying, historical records
, or comparisons across spatial scales. There has been a lack of direct, manipulative experimental tests for alternative stable states. This is especially true for studies outside of controlled laboratory conditions, where state shifts have been documented for cultures of microorganisms. Empirical evidence for the existence of alternative stable states is vital to advancing the idea beyond theory. Schröder et al. (2005) reviewed the current ecological literature for alternative stable states and found 35 direct experiments, of which only 21 were deemed valid. Of these, 62% (14) showed evidence for and 38% (8) showed no evidence for alternative stable states. However, the Schröder et al. (2005) analysis required evidence of hysteresis, which is not necessarily a prerequisite for alternative stable states. Other authors (e.g., Scheffer et al. 2001; Folke et al. 2004) have had less-stringent requirements for the documentation of alternative stable states.
Although alternative stable state theory is still in its infancy, empirical evidence has been collected from a variety of biomes. The Sahara
region, once vegetated, suddenly collapsed into a desert about 5,000 to 6,000 years ago
due to climate change (Scheffer and Carpenter 2003). Recent alteration of disturbance states via fire suppression
has caused state shifts in fire-adapted ecosystems of the southeastern United States
(Peterson 2002).
In the northeastern United States
alternative stable states have been experimentally documented in hardwood forest food webs
(Schmitz 2004). Spiders exhibit top-down control on generalist herbivore grasshoppers, causing them to hide in and forage on a competitively dominant plant species. Herbivory on the superior plant competitor reduces dominance, thereby increasing plant diversity. Under experimental removal of spiders, grasshoppers were released from predation and consumed plants without preference for the competitively-superior species. This top competitor outcompeted other species, resulting in lower overall plant diversity. Furthermore, reintroductions of spiders failed to return the system to high plant diversity, indicating ecological thresholds.
Ponds, lakes, and other small bodies of water
are ideal for studying alternative stable states because they are relatively self-contained (Holling 1973). There has been great interest in state shifts in shallow temperate lakes between clear-water and turbid-water states. Nutrient loading can shift clear lakes to turbid systems by causing phytoplankton blooms (Scheffer 1997). Feedbacks maintaining the turbid state include shading out benthic macrophytes, which otherwise stabilize sediments and reduce rapid nutrient cycling. Hysteresis may occur if reductions in nutrient input do not return the clear-water state. Alternative stable states have also been shown for freshwater faunal communities. Initial conditions, such as densities of snails, can influence the movement of an ecosystem into one of several alternative stable equilibria (Chase 2003). Additionally, alternative stable states have been documented experimentally in coastal marine ecosystem
s. Arctic salt marshes of Hudson Bay
, Manitoba have been shown to shift from a vegetated state to an unvegetated state due to overgrazing by snow geese
(Handa et al. 2002). When geese were experimentally removed, recovery of the system only occurred under ideal soil conditions, indicating hysteresis.
Verifying the existence of alternative stable states carries profound implications for ecosystem management. If stable states exist, gradual changes in environmental factor
s may have little effect on a system until a threshold is reached, at which point a catastrophic state shift may occur. Understanding the nature of these thresholds will help inform the design of monitoring programs, ecosystem restoration, and other management decisions. Managers are particularly interested in the potential of hysteresis, since it may be difficult to recover from a state shift (Beisner et al. 2003). The mechanisms of feedback loops
that maintain stable states are important to understand if we hope to effectively manage an ecosystem with alternative stable states.
Ecology
Ecology is the scientific study of the relations that living organisms have with respect to each other and their natural environment. Variables of interest to ecologists include the composition, distribution, amount , number, and changing states of organisms within and among ecosystems...
, 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
Phase (waves)
Phase in waves is the fraction of a wave cycle which has elapsed relative to an arbitrary point.-Formula:The phase of an oscillation or wave refers to a sinusoidal function such as the following:...
or regime shift
Regime shift
Regime shifts is defined as rapid reorganizations of ecosystems from one relatively stable state to another. The idea of a regime shift was first used in investigations of freshwater and terrestrial systems. It is now been used for marine and estuarine systems to describe rapid shifts between two...
), 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
Hysteresis
Hysteresis is the dependence of a system not just on its current environment but also on its past. This dependence arises because the system can be in more than one internal state. To predict its future evolution, either its internal state or its history must be known. If a given input alternately...
. Alternative stable state theory suggests that discrete states are separated by ecological threshold
Ecological threshold
Ecological threshold can be described as the point at which a relatively small change in external conditions causes a rapid change in an ecosystem. When an ecological threshold has been passed, the ecosystem may no longer be able to return to its state...
s, in contrast to ecosystems which change smoothly and continuously along an environmental gradient
Environmental gradient
An environmental gradient is a gradual change in abiotic factors through space . Environmental gradients can be related to factors such as altitude, temperature, depth, ocean proximity and soil humidity....
.
Alternative stable state theory
Alternative stable state theoryState theory
State theory is a theory of absolute time in which the number of states from the first moment in time up to the current moment in time can be counted. This is in contrast to Einstein's theory of relativity, in which time is modelled using the real number system. A state is a moment in time, a...
was first proposed by 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 (Beisner et al. 2003). 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., Law and Morton 1993; Drake 1991). 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.
Perturbations to state variables: the community perspective
Ecosystems can shift from one state to another via a significant perturbation directly to state variables. State variables are quantities that change quickly (in ecologically-relevant time scaleTime scale
A time scale specifies divisions of time.*Time standard, a specification of either the rate at which time passes, or points in time, or both*Duration , a quantity of time...
s) in response to feedbacks from the system (i.e., they are dependent on system feedbacks), such as population densities
Population density
Population density is a measurement of population per unit area or unit volume. It is frequently applied to living organisms, and particularly to humans...
. This perspective requires that different states can exist simultaneously under equal environmental conditions, since the ball moves only in response to a state variable
State variable
A state variable is one of the set of variables that describe the "state" of a dynamical system. Intuitively, the state of a system describes enough about the system to determine its future behaviour...
change.
For example, consider a very simple system with three microbial species. It may be possible for the system to exist under different community structure regimes depending on initial conditions
Initial value problem
In mathematics, in the field of differential equations, an initial value problem is an ordinary differential equation together with a specified value, called the initial condition, of the unknown function at a given point in the domain of the solution...
(e.g., population densities or spatial arrangement of individuals) (Kerr et al. 2002). Perhaps under certain initial densities or spatial configurations, one species dominates over all others, while under different initial conditions all species can mutually coexist. Because the different species interact, changes in populations affect one another synergistically to determine community structure. Under both states the environmental conditions are identical. Because the states have resilience, following small perturbations (e.g., changes to population size
Population size
In population genetics and population ecology, population size is the number of individual organisms in a population.The effective population size is defined as "the number of breeding individuals in an idealized population that would show the same amount of dispersion of allele frequencies under...
) the community returns to the same configuration while large perturbations may induce a shift to another configuration.
The community perspective requires the existence of alternative stable states (i.e., more than one valley) before the perturbation, since the landscape is not changing. Because communities have some level of resistance to change
Change management
Change management is a structured approach to shifting/transitioning individuals, teams, and organizations from a current state to a desired future state. It is an organizational process aimed at helping employees to accept and embrace changes in their current business environment....
, they will stay in their domain of attraction (or stable state) until the perturbation is large enough to force the system into another state. In the ball-and-cup model, this would be the energy required to push the ball up and over a hill, where it would fall downhill into a different valley.
Perturbations to ecosystem variables: the ecosystem perspective
It is also possible to cause state shifts in another context, by indirectly affecting state variables. This is known as the ecosystem perspective. This perspective requires a change in environmental parameters that affect the behavior of state variables. For example, birth rateBirth rate
Crude birth rate is the nativity or childbirths per 1,000 people per year . Another word used interchangeably with "birth rate" is "natality". When the crude birth rate is subtracted from the crude death rate, it reveals the rate of natural increase...
, death rate
Mortality rate
Mortality rate is a measure of the number of deaths in a population, scaled to the size of that population, per unit time...
, migration, and density-dependent predation indirectly alter the ecosystem state by changing population density (a state variable). Ecosystem parameters are quantities that are unresponsive (or respond very slowly) to feedbacks from the system (i.e., they are independent of system feedbacks). The stable state landscape is changed by environmental drivers, which may result in a change in the quantity of stable states and the relationship between states.
By the ecosystem perspective, the landscape of the ecological states is changed, which forces a change in the ecosystem state. Changing the landscape can modify the number, location, and resilience of stable states, as well as the unstable intermediate states. By this view, the topography in the ball-and-cup model is not static, as it is in the community perspective. This is a fundamental difference between the two perspectives.
However, it should be noted that although the mechanisms of community and ecosystem perspectives are different, the empirical evidence
Empirical research
Empirical research is a way of gaining knowledge by means of direct and indirect observation or experience. Empirical evidence can be analyzed quantitatively or qualitatively...
required for documentation of alternative stable states is the same. In addition, state shifts are often a combination of internal processes and external forces (Scheffer et al. 2001). For example, consider a stream-fed lake in which the pristine state is dominated by benthic vegetation. When upstream construction releases soils into the stream, the system becomes turbid. As a result, benthic vegetation cannot receive light and decline, increasing nutrient availability and allowing phytoplankton to dominate. In this state shift scenario the state variables changing are the populations of benthic vegetation and phytoplankton, and the ecosystem parameters are turbidity and nutrient levels. So, whether identifying mechanisms of variables or parameters is a matter of formulation (Beisner et al. 2003).
Hysteresis effects
Hysteresis is an important concept in alternative stable state theory. In this ecological context, hysteresis refers to the existence of different stable states under the same variables or parameters. Hysteresis can be explained by “path-dependency,” in which the equilibrium point for the trajectory of “A → B” is different than for “B → A.” In other words, it matters which way the ball is moving across the landscape. Some ecologists (e.g., Scheffer et al. 2001) argue that hysteresis is a prerequisite for the existence of alternative stable states. Others (e.g., Beisner et al. 2003) claim that this is not so; although shifts often involve hysteresis, a system can show alternative stable states yet have equal paths for “A → B” and “B → A.”Hysteresis can occur via changes to variables or parameters. When variables are changed the ball is pushed from one domain of attraction to another, yet the same push from the other direction cannot return the ball to the original domain of attraction. When parameters are changed a modification to the landscape results in a state shift, but reversing the modification does not result in a reciprocal shift.
A real-world example of hysteresis is helpful to illustrate the concept. Coral reef
Coral reef
Coral reefs are underwater structures made from calcium carbonate secreted by corals. Coral reefs are colonies of tiny living animals found in marine waters that contain few nutrients. Most coral reefs are built from stony corals, which in turn consist of polyps that cluster in groups. The polyps...
systems can dramatically shift from pristine coral-dominated systems to degraded algae-dominated systems when populations grazing on algae decline. The 1983 crash of urchin populations in Caribbean reef systems released algae from top-down (herbivory) control, allowing them to overgrow corals and resulting in a shift to a degraded state. When urchins rebounded, the high (pre-crash) coral cover levels did not return, indicating hysteresis (Mumby et al. 2007).
In some cases, state shifts under hysteresis may be irreversible. For example, tropical cloud forest
Cloud forest
A cloud forest, also called a fog forest, is a generally tropical or subtropical evergreen montane moist forest characterized by a persistent, frequent or seasonal low-level cloud cover, usually at the canopy level. Cloud forests often exhibit an abundance of mosses covering the ground and...
s require high moisture levels, provided by clouds that are intercepted by the canopy (via condensation). When deforested, moisture delivery ceases. Therefore, reforestation is often unsuccessful because conditions are too dry to allow the trees to grow (Wilson and Agnew 1992).
Resilience
By their very nature, basins of attraction display resilience. Ecosystems are resistant to state shifts – they will only undergo shifts under substantial perturbations – but some states are more resilient than others. In the ball-and-cup model, a valley with steep sides has greater resilience than a shallow valley, since it would take more force to push the ball up the hill and out of the valley.Resilience can change in stable states when environmental parameters are shifted. Often, humans influence stable states by reducing the resilience of basins of attraction. There are at least three ways in which anthropogenic forces reduce resilience (Folke et al. 2004): (1) Decreasing diversity and functional group
Functional group
In organic chemistry, functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. The same functional group will undergo the same or similar chemical reaction regardless of the size of the molecule it is a part of...
s, often by top-down effects (e.g., overfishing); (2) altering the physico-chemical environment (e.g., climate change
Climate change
Climate change is a significant and lasting change in the statistical distribution of weather patterns over periods ranging from decades to millions of years. It may be a change in average weather conditions or the distribution of events around that average...
, pollution, fertilization); or (3) modifying disturbance regimes to which organisms are adapted (e.g., bottom trawling
Bottom trawling
Bottom trawling is trawling along the sea floor. It is also often referred to as "dragging".The scientific community divides bottom trawling into benthic trawling and demersal trawling...
, coral mining, etc.). When the resilience is decreased, ecosystems can be pushed into alternative, and often less-desirable, stable states with only minor perturbations. When hysteresis effects are present, the return to a more-desirable state is sometimes impossible or impractical (given management constraints). Shifts to less-desirable states often entail a loss of ecosystem service and function, and have been documented in an array of terrestrial, marine, and freshwater environments (reviewed in Folke et al. 2004).
Evidence for alternative stable states
State shifts via the community perspective have been induced experimentally by the addition or removal of predators, such as in Paine’s (1966) work on keystone predators (i.e., predators with disproportionate influence on community structure) in the intertidal zoneIntertidal zone
The intertidal zone is the area that is above water at low tide and under water at high tide . This area can include many different types of habitats, with many types of animals like starfish, sea urchins, and some species of coral...
(although this claim is refuted by Schröder et al. 2005). Also, Beisner et al. (2003) suggest that commercially-exploited fish populations can be forced between alternative stable states by fishing pressure due to Allee effects that work at very low population sizes. Once a fish population falls below a certain threshold, it will inevitably go extinct when low population densities make replacement of adults impossible due to, for example, the inability to find mates or density-dependent mortality. Since populations cannot return from extinction, this is an example of an irreversible state shift.
Most work on alternative stable states has been theoretical, using mathematical model
Mathematical model
A mathematical model is a description of a system using mathematical concepts and language. The process of developing a mathematical model is termed mathematical modeling. Mathematical models are used not only in the natural sciences and engineering disciplines A mathematical model is a...
s and simulations to test ecological hypotheses. Other work has been conducted using empirical evidence from surveying, historical records
History
History is the discovery, collection, organization, and presentation of information about past events. History can also mean the period of time after writing was invented. Scholars who write about history are called historians...
, or comparisons across spatial scales. There has been a lack of direct, manipulative experimental tests for alternative stable states. This is especially true for studies outside of controlled laboratory conditions, where state shifts have been documented for cultures of microorganisms. Empirical evidence for the existence of alternative stable states is vital to advancing the idea beyond theory. Schröder et al. (2005) reviewed the current ecological literature for alternative stable states and found 35 direct experiments, of which only 21 were deemed valid. Of these, 62% (14) showed evidence for and 38% (8) showed no evidence for alternative stable states. However, the Schröder et al. (2005) analysis required evidence of hysteresis, which is not necessarily a prerequisite for alternative stable states. Other authors (e.g., Scheffer et al. 2001; Folke et al. 2004) have had less-stringent requirements for the documentation of alternative stable states.
Although alternative stable state theory is still in its infancy, empirical evidence has been collected from a variety of biomes. The Sahara
Sahara
The Sahara is the world's second largest desert, after Antarctica. At over , it covers most of Northern Africa, making it almost as large as Europe or the United States. The Sahara stretches from the Red Sea, including parts of the Mediterranean coasts, to the outskirts of the Atlantic Ocean...
region, once vegetated, suddenly collapsed into a desert about 5,000 to 6,000 years ago
4th millennium BC
The 4th millennium BC saw major changes in human culture. It marked the beginning of the Bronze Age and of writing.The city states of Sumer and the kingdom of Egypt were established and grew to prominence. Agriculture spread widely across Eurasia...
due to climate change (Scheffer and Carpenter 2003). Recent alteration of disturbance states via fire suppression
Fire fighting
Firefighting is the act of extinguishing fires. A firefighter fights fires to prevent loss of life, and/or destruction of property and the environment...
has caused state shifts in fire-adapted ecosystems of the southeastern United States
Southeastern United States
The Southeastern United States, colloquially referred to as the Southeast, is the eastern portion of the Southern United States. It is one of the most populous regions in the United States of America....
(Peterson 2002).
In the northeastern United States
Northeastern United States
The Northeastern United States is a region of the United States as defined by the United States Census Bureau.-Composition:The region comprises nine states: the New England states of Connecticut, Maine, Massachusetts, New Hampshire, Rhode Island and Vermont; and the Mid-Atlantic states of New...
alternative stable states have been experimentally documented in hardwood forest food webs
Food chain
A food web depicts feeding connections in an ecological community. Ecologists can broadly lump all life forms into one of two categories called trophic levels: 1) the autotrophs, and 2) the heterotrophs...
(Schmitz 2004). Spiders exhibit top-down control on generalist herbivore grasshoppers, causing them to hide in and forage on a competitively dominant plant species. Herbivory on the superior plant competitor reduces dominance, thereby increasing plant diversity. Under experimental removal of spiders, grasshoppers were released from predation and consumed plants without preference for the competitively-superior species. This top competitor outcompeted other species, resulting in lower overall plant diversity. Furthermore, reintroductions of spiders failed to return the system to high plant diversity, indicating ecological thresholds.
Ponds, lakes, and other small bodies of water
Body of water
A body of water or waterbody is any significant accumulation of water, usually covering the Earth or another planet. The term body of water most often refers to large accumulations of water, such as oceans, seas, and lakes, but it may also include smaller pools of water such as ponds, puddles or...
are ideal for studying alternative stable states because they are relatively self-contained (Holling 1973). There has been great interest in state shifts in shallow temperate lakes between clear-water and turbid-water states. Nutrient loading can shift clear lakes to turbid systems by causing phytoplankton blooms (Scheffer 1997). Feedbacks maintaining the turbid state include shading out benthic macrophytes, which otherwise stabilize sediments and reduce rapid nutrient cycling. Hysteresis may occur if reductions in nutrient input do not return the clear-water state. Alternative stable states have also been shown for freshwater faunal communities. Initial conditions, such as densities of snails, can influence the movement of an ecosystem into one of several alternative stable equilibria (Chase 2003). Additionally, alternative stable states have been documented experimentally in coastal marine ecosystem
Marine ecosystem
Marine ecosystems are among the largest of Earth's aquatic ecosystems. They include oceans, salt marsh and intertidal ecology, estuaries and lagoons, mangroves and coral reefs, the deep sea and the sea floor. They can be contrasted with freshwater ecosystems, which have a lower salt content. Marine...
s. Arctic salt marshes of Hudson Bay
Hudson Bay
Hudson Bay , sometimes called Hudson's Bay, is a large body of saltwater in northeastern Canada. It drains a very large area, about , that includes parts of Ontario, Quebec, Saskatchewan, Alberta, most of Manitoba, southeastern Nunavut, as well as parts of North Dakota, South Dakota, Minnesota,...
, Manitoba have been shown to shift from a vegetated state to an unvegetated state due to overgrazing by snow geese
Snow Goose
The Snow Goose , also known as the Blue Goose, is a North American species of goose. Its name derives from the typically white plumage. The genus of this bird is disputed...
(Handa et al. 2002). When geese were experimentally removed, recovery of the system only occurred under ideal soil conditions, indicating hysteresis.
Verifying the existence of alternative stable states carries profound implications for ecosystem management. If stable states exist, gradual changes in environmental factor
Environmental factor
Environmental factor or ecological factor or ecofactor is any factor, abiotic or biotic, that influences living organisms.- Environmental factors inducing diseases :...
s may have little effect on a system until a threshold is reached, at which point a catastrophic state shift may occur. Understanding the nature of these thresholds will help inform the design of monitoring programs, ecosystem restoration, and other management decisions. Managers are particularly interested in the potential of hysteresis, since it may be difficult to recover from a state shift (Beisner et al. 2003). The mechanisms of feedback loops
Feedback
Feedback describes the situation when output from an event or phenomenon in the past will influence an occurrence or occurrences of the same Feedback describes the situation when output from (or information about the result of) an event or phenomenon in the past will influence an occurrence or...
that maintain stable states are important to understand if we hope to effectively manage an ecosystem with alternative stable states.