Turning back from the brink: Detecting an impending regime shift in time to avert it Reinette Biggsa,1, Stephen R. Carpentera,2, and William A. Brockb aCenter for Limnology and bDepartment of Economics, University of Wisconsin, Madison, WI 53706 Contributed by Stephen R. Carpenter, November 20, 2008 (sent for review July 24, 2008) Ecological regime shifts are large, abrupt, long-lasting changes in clear once the shift has occurred (1, 2). Regime shifts typically ecosystems that often have considerable impacts on human econ- result from a combination of gradual changes in an underlying omies and societies. Avoiding unintentional regime shifts is widely driving variable (or set of variables), combined with an external regarded as desirable, but prediction of ecological regime shifts is shock, such as a storm or fire (23, 24). Gradual changes in notoriously difficult. Recent research indicates that changes in underlying drivers usually have little or no apparent impact up ecological time series (e.g., increased variability and autocorrela- to a certain point, and then unexpectedly lead to a regime shift tion) could potentially serve as early warning indicators of im- when that threshold is crossed. Once an ecosystem is close to a pending shifts. A critical question, however, is whether such threshold, a shift is often precipitated by a shock that under indicators provide sufficient warning to adapt management to previous conditions had no dramatic consequences (1, 2). Slow avert regime shifts. We examine this question using a fisheries underlying drivers that push ecosystems toward thresholds often model, with regime shifts driven by angling (amenable to rapid go unnoticed and are frequently associated with increased reduction) or shoreline development (only gradual restoration is economic benefits. In the absence of known thresholds and possible). The model represents key features of a broad class of impacts, it is very difficult to constrain such drivers. Rising ecological regime shifts. We find that if drivers can only be demands on the world’s ecosystems are therefore expected to manipulated gradually management action is needed substantially increasingly push ecosystems toward ecological thresholds (3–5). before a regime shift to avert it; if drivers can be rapidly altered To avoid large-scale disruptions to human societies, there is aversive action may be delayed until a shift is underway. Large accordingly an urgent need to improve our ability to anticipate increases in the indicators only occur once a regime shift is and avert ecological regime shifts. initiated, often too late for management to avert a shift. To The need to better anticipate regime shifts has sparked much improve usefulness in averting regime shifts, we suggest that recent research (25–33). Most of this work has been based on research focus on defining critical indicator levels rather than mathematical models of ecosystems with multiple stable states detecting change in the indicators. Ideally, critical indicator levels (34, 35). The focus has been on generic changes in system should be related to switches in ecosystem attractors; we present behavior that might enable one to predict regime shifts across a a new spectral density ratio indicator to this end. Averting eco- diverse range of ecosystems, rather than experimental or model- logical regime shifts is also dependent on developing policy pro- based methods that focus on better understanding the mecha- cesses that enable society to respond more rapidly to information nisms of particular regime shifts in specific ecosystems. It turns about impending regime shifts. out that although little change may be evident in the average condition of an ecosystem as a regime shift is approached, there early warning indicator ͉ ecological threshold ͉ spectral density ratio may be detectable changes in other properties of monitoring data. Specifically, time series data may show increased variability cological regime shifts are large, sudden changes in ecosys- (25), changes in skewness (27), higher correlation through time Etems that last for substantial periods of time (1, 2). Regime (29, 33), and slower rates of recovery from disturbances (32) in shifts are a source of growing concern as rising human impacts advance of regime shifts. Such changes in the ways ecosystems on the environment are increasing the likelihood of ecological behave hold substantial promise as early warning indicators of regime shifts at local to global scales (3–5). Accumulating regime shifts, although there are as yet few empirical tests (25, evidence suggests that regime shifts can occur in diverse eco- 27, 29, 32, 33). However, given their potential use as early systems: They have been documented in oceans (6–8), freshwa- warning indicators, a critical question is: Would such changes in ters (2, 9, 10), forests (11), woodlands (12), drylands (13), ecosystem behavior provide sufficient advance warning to adapt rangelands (14–16), and agroecosystems (17–19). Ecological management to avoid a regime shift? Or are the lags and regime shifts are widely regarded as undesirable as they often momentum of change in the ecosystem so great that by the time have considerable impacts on human well-being. For example, these changes are detected the system is already committed to a the collapse of Canada’s Newfoundland cod fishery in the early shift? Answers to these questions clearly impact whether at- 1990s directly affected the livelihoods of some 35,000 fishers and tempts to employ such early warning indicators are worthwhile, fish-plant workers, led to a decline of over $200 million dollars and whether use of these indicators may be a viable strategy for per annum in revenue from cod landings (20) and had significant avoiding undesirable regime shifts. indirect impacts on the local economy and society (21). Ecolog- ical regime shifts are also undesirable because they may be very Author contributions: R.B., S.R.C., and W.A.B. designed research; R.B. and S.R.C. performed costly or impossible to reverse (1, 2, 22). Regime shifts entail research; R.B., S.R.C., and W.A.B. contributed new reagents/analytic tools; R.B. analyzed changes in the internal dynamics and feedbacks of an ecosystem data; and R.B., S.R.C., and W.A.B. wrote the paper. that often prevent it from returning to a previous regime, even when The authors declare no conflict of interest. the driver that precipitated the shift is reduced or removed (1, 2). 1To whom correspondence may be sent at the present address: Stockholm Resilience For instance, despite a moratorium on the Canadian cod fishery for Center, Stockholm University, Stockholm, Sweden 10691. E-mail: oonsie.biggs@ Ͼ15 years, the fishery has shown little sign of recovery (20). stockholmresilience.su.se. Avoiding unintentional regime shifts is widely regarded as 2To whom correspondence may be addressed. E-mail: [email protected]. desirable (3, 4). However, ecological regime shifts are notori- This article contains supporting information online at www.pnas.org/cgi/content/full/ ously difficult to predict. Most regime shifts come as surprises, 0811729106/DCSupplemental. and the conditions and mechanisms leading to them only become © 2009 by The National Academy of Sciences of the USA 826–831 ͉ PNAS ͉ January 20, 2009 ͉ vol. 106 ͉ no. 3 www.pnas.org͞cgi͞doi͞10.1073͞pnas.0811729106 Downloaded by guest on September 30, 2021 dW Adult Noise σ )( A D dt Piscivores ANTHROPOGENIC tion ha da (A) rv IMPACTS re es p ) (q t (c FA E) ) Planktivores JA (C (F) predation Angling pr eda tion (fast) (c Juvenile FORAGING ARENA FORAGING JF ) Piscivores (J) B E (slow) ) F Shoreline (v) (h) (D development modified modified exchange behavior- exchange Juvenile Planktivores Piscivores (F ) R (J ) REFUGE REFUGE R CF Fig. 1. The fisheries food web model used to explore the use of regime shift indicators in averting ecological regime shifts. Regime shifts are driven by angling or shoreline development. Angling directly affects the populations of adult piscivores through harvesting (qE). Shoreline development affects the amount and quality of refuge habitat, and thus the rate at which juvenile piscivores hide from predators (h). To examine these questions, we use a fisheries food web model to explore (i) how close an ecosystem can get to an ecological threshold and still avert a regime shift by implementing changes Fig. 2. The attainable proximity to a regime shift is much greater for driving in management, and (ii) which regime shift indicators might give variables that can be rapidly manipulated (angling) than for variables that can warning before this ‘‘point of no return.’’ Regime shifts in the only be manipulated gradually (shoreline development). Year 0 is defined as model can be triggered by 2 mechanisms: (i) angling, which is the year in which the switch from the lower (F ϭ 0.34) to the upper (F ϭ 76.92) amenable to relatively fast manipulation through management planktivore attractor occurs. (A) In the absence of policy action, a harvest- ϭ action, or (ii) shoreline development, which can only be manip- driven shift occurs in years 81 to 95. The switch in attractors occurs at qE 1.78. ulated gradually. This enables us to investigate the potential (B) The window for averting a regime shift by implementing harvest-reduction policy MS1 lasts to year 91, well within the regime shift. (C) However, insti- avoidance of regime shifts for fast versus slow management tuting MS1 just a year later cannot avert a regime shift. (D) In the absence of variables. We use a modeling approach to explore the use of policy action, a shoreline development-driven shift occurs between years 80 regime shift indicators under the most favorable possible con- and 95, and the switch in attractors occurs at h ϭ 3.7. (E) To avert a regime shift, ditions, where the signal from the indicators is not obscured by shoreline restoration policy MS2 has to be implemented substantially before environmental variability and management changes can be the shift, by year 35.