Regime shift
Regime shifts are large, abrupt, persistent changes in the structure and function of ecosystems, the climate, financial systems or other complex systems. A regime is a characteristic behaviour of a system which is maintained by mutually reinforced processes or feedbacks. Regimes are considered persistent relative to the time period over which the shift occurs. The change of regimes, or the shift, usually occurs when a smooth change in an internal process or a single disturbance triggers a completely different system behavior. Although such non-linear changes have been widely studied in different disciplines ranging from atoms to climate dynamics, regime shifts have gained importance in ecology because they can substantially affect the flow of ecosystem services that societies rely upon, such as provision of food, clean water or climate regulation. Moreover, regime shift occurrence is expected to increase as human influence on the planet increases – the Anthropocene – including current trends on human induced climate change and biodiversity loss. When regime shifts are associated with a critical or bifurcation point, they may also be referred to as critical transitions.
History of the concept
Scholars have been interested in systems exhibiting non-linear change for a long time. Since the early twentieth century, mathematicians have developed a body of concepts and theory for the study of such phenomena based on the study of non-linear system dynamics. This research led to the development of concepts such as catastrophe theory; a branch of bifurcation theory in dynamical systems.In ecology the idea of systems with multiple regimes, domains of attraction called alternative stable states, only arose in the late '60s based upon the first reflections on the meaning of stability in ecosystems by Richard Lewontin and Crawford "Buzz" Holling. The first work on regime shifts in ecosystems was done in a diversity of ecosystems and included important work by Noy-Meir in grazing systems; May in grazing systems, harvesting systems, insect pests and host-parasitoid systems; Jones and Walters with fisheries systems; and Ludwig et al. with insect outbreaks.
These early efforts to understand regime shifts were criticized for the difficulty of demonstrating bi-stability, their reliance on simulation models, and lack of high quality long-term data. However, by the 1990s more substantial evidence of regime shifts was collected for kelp forest, coral reefs, drylands and shallow lakes. This work led to revitalization of research on ecological reorganization and the conceptual clarification that resulted in the regime shift conceptual framework in the early 2000s.
Outside of ecology, similar concepts of non-linear change have been developed in other academic disciplines. One example is historical institutionalism in political science, sociology and economics, where concepts like path dependency and critical junctures are used to explain phenomena where the output of a system is determined by its history, or the initial conditions, and where its domains of attraction are reinforced by feedbacks. Concept such as international institutional regimes, socio-technical transitions and increasing returns have an epistemological basis similar to regime shifts, and utilize similar mathematical models.
Current applications of the regime shift concept
During the last decades, research on regime shift has grown exponentially. Academic papers reported by ISI Web of Knowledge rose from less than 5 per year prior to 1990 to more than 300 per year from 2007 to 2011. However, the application of regime shift related concepts is still contested.Although there is not agreement on one definition, the slight differences among definitions reside on the meaning of stability – the measure of what a regime is – and the meaning of abruptness. Both depend on the definition of the system under study, thus it is relative. At the end it is a matter of scale. Mass extinctions are regime shifts on the geological time scale, while financial crises or pest outbreaks are regime shifts that require a totally different parameter setting.
In order to apply the concept to a particular problem, one has to conceptually limit its range of dynamics by fixing analytical categories such as time and space scales, range of variations and exogenous / endogenous processes. For example, while for oceanographers a regime must last for at least decades and should include climate variability as a driver, for marine biologists regimes of only five years are acceptable and could be induced by only population dynamics. A non-exhaustive range of current definitions of regime shifts in recent scientific literature from ecology and allied fields is collected in Table 1.
Table 1. Definitions of regime shifts and modifications used to apply the concept to particular research questions from scientific literature published between 2004 and 2009.
| Source | Definition | Modification |
| Collie et al. 2004 | "Three different types of regime shift are identified on the basis of different patterns in the relationship between the response of an ecosystem variable and some external forcing or condition. The smooth regime shift is represent by a quasi-linear relationship between the response and control variables. The abrupt regime shift exhibits a nonlinear relationship between the response and control variables, and the discontinuous regime shift is characterized by the trajectory of the response variable differing when the forcing variable increases compared to when it decreases " | ""Regime shifts" are considered here to be low-frequency, high-amplitude changes in oceanic conditions that may be especially pronounced in biological variables and propagate through several trophic levels" |
| Bakun 2004 | "persistent radical shift in typical levels of abundance or productivity of multiple important components of marine biological community structure, occurring at multiple trophic levels and on a geographical scale that is at least regional in extent" | |
| Walker & Meyers, 2004 | "A regime shift involving alternate stable states occurs when a threshold level of a controlling variable in a system is passed, such that the nature and extend of feedbacks change, resulting in a change of direction of the system itself. A shift occurs when internal processes of the system... have changed and the state of the system... begins to change in a different direction, toward a different attractor." | |
| Andersen et al. 2009 | "ecological regime shifts can be defined as abrupt changes on several trophic levels leading to rapid ecosystem reconfiguration between alternative states" | |
| Cumming & Norberg, 2008 | "the ability of a system to internally switch between different self reinforcing processes that dominate how the system functions" | |
| Brock, Carpenter and Scheffer 2008 | "Regime shifts, substantial reorganizations of complex systems with prolonged consequences... In environmental policy regime shifts raise the prospect that incremental stresses may evoke large, unexpected changes in ecosystem services and human livelihoods" | |
| Biggs et al. 2009 | "Ecological regime shifts are large, sudden changes in ecosystems that last of substantial periods of time... Regime shifts entail changes in the internal dynamics and feedbacks of an ecosystem that often prevent it from returning to a previous regime, even when the driver that precipitated the shift is reduced or removed... Regime shifts typically result from a combination of gradual changes in an underlying driving variable, combined with an external shock, such as a storm or fire" | "We defined a regime shift as the period over which the annual increase in the planktivore population exceeded 10%. In the model, regime shifts have a typical duration of ≈15 years, reflecting plausible limits on the growth rate of F" |
| Norström et al. 2009 | "Certain conditions may ultimately result in persistent alternative stable states, which are characterized by a different set of ecosystems processes, functions and feedback mechanisms..." | "we defined phase shifts as an extensive decreases in coral cover coinciding with substantial increases in some alternative benthic organism, due to a pulse or press disturbance, that have persisted >5yr. A minimum persistence time of 5 yr was used, as this is in accordance with the timeframe of studies describing cases of phase shifts from coral to macroalgal states..." |
| Scheffer | "a relatively sharp change from one regime to a contrasting one, where a regime is a dynamic 'state' of a system with its characteristics stochastic fluctuations and/or cycles" |
Theoretical basis
The theoretical basis for regime shifts has been developed from the mathematics of non-linear systems. In short, regime shifts describe dynamics characterized by the possibility that a small disturbance can produce big effects. In such situations the common notion of proportionality between inputs and outputs of a system is incorrect. Conversely, the regime shift concept also emphasizes the resilience of systems – suggesting that in some situations substantial management or human impact can have little effect on a system. Regime shifts are hard to reverse and in some cases irreversible. The regime shift concept shifts analytical attention away from linearity and predictability, towards reorganization and surprise. Thus, the regime shift concept offers a framework to explore the dynamics and causal explanations of non-linear change in nature and society.Regime shifts are triggered either by the weakening of stabilizing internal processes – feedbacks – or by external shocks which exceed the stabilizing capacity of a system.
Systems prone to regime shifts can show three different types of change: smooth, abrupt or discontinuous, depending on the configuration of processes that define a system – in particular the interaction between a system's fast and slow processes. Smooth change can be described by a quasi-linear relationship between fast and slow processes; abrupt change shows a non-linear relationship among fast and slow variables, while discontinuous change is characterized by the difference in the trajectory on the fast variable when the slow one increases compared to when it decreases. In other words, the point at which the system flips from one regime to another is different from the point at which the system flips back. Systems that exhibit this last type of change demonstrate hysteresis. Hysteretic systems have two important properties. First, the reversal of discontinuous change requires that a system change back past the conditions at which the change first occurred. This occurs because systemic change alters feedback processes that maintain a system in a particular regime. Second, hysteresis greatly enhances the role of history in a system, and demonstrates that the system has memory – in that its dynamics are shaped by past events.
Conditions at which a system shifts its dynamics from one set of processes to another are often called thresholds. In ecology for example, a threshold is a point at which there is an abrupt change in an ecosystem quality, property or phenomenon; or where small changes in an environmental driver produce large responses in an ecosystem. Thresholds are, however, a function of several interacting parameters, thus they change in time and space. Hence, the same system can present smooth, abrupt or discontinuous change depending on its parameters' configurations. Thresholds will be present, however, only in cases where abrupt and discontinuous change is possible.