Resilience indicators: prospects and limitations for early warnings of regime shifts

Abstract

In the vicinity of tipping points—or more precisely bifurcation points—ecosystems recover slowly from small perturbations. Such slowness may be interpreted as a sign of low resilience in the sense that the ecosystem could easily be tipped through a critical transition into a contrasting state. Indicators of this phenomenon of ‘critical slowing down (CSD)’ include a rise in temporal correlation and variance. Such indicators of CSD can provide an early warning signal of a nearby tipping point. Or, they may offer a possibility to rank reefs, lakes or other ecosystems according to their resilience. The fact that CSD may happen across a wide range of complex ecosystems close to tipping points implies a powerful generality. However, indicators of CSD are not manifested in all cases where regime shifts occur. This is because not all regime shifts are associated with tipping points. Here, we review the exploding literature about this issue to provide guidance on what to expect and what not to expect when it comes to the CSD-based early warning signals for critical transitions.

Elevated nonlinearity as an indicator of shifts in the dynamics of populations under stress

“…For example, we found strong fluctuations in nonlinearity estimates, especially in the exploitation scenario (electronic supplementary material, figure S2f). Changes in variance and autocorrelation can also be unreliable in the presence of short time series [51], high levels of stochasticity [57], fast changing stress drivers [54,58] or due to portfolio effects [56] and life-history strategies [59]. Further research is needed to find if similar constraints hold for the elevated nonlinearity indicator we propose here.…”

“…CSD indicators can capture changes in these dynamics but only when it comes to stable equilibria in the presence of weak stochasticity [12]. Identifying transitions across chaotic attractors or, more generally, in systems with nonlinear dynamics may be difficult with CSD indicators [53,54]. By contrast, EDM-derived nonlinearity may be broader in its application as it can capture changes in dynamics beyond stable attractors typical of the dynamics encountered in natural systems.…”

Biodiversity and Resilience of Ecosystem Functions

“…89 There has been much semantic and theoretical treatment of the resilience concept, but 90 here we are concerned with identifying metrics for real world applications. An ecological 91 system can be defined by the species composition at any point in time [26] and there is a 92 rich ecological literature, both theoretical and experimental, that focusses on the stability of 93 communities [16,[27][28][29] with potential relevance to resilience. Of course, the species in a 94 community are essential to the provision of many ecosystem functions which are the 95 biological foundation of ecosystem services [3].…”

“…'critical slowing down' [53]. However, these properties only occur before critical transitions in a subset of cases and thus are likely to be poor general predictive indicators of resilience [91]. A focus on emergent properties of systems also ignores the mechanisms that underpin resilience and therefore has limited ability to inform management advice.…”

Body size distributions signal a regime shift in a lake ecosystem

“…In addition, some early warning signals may fail to predict regime shifts (e.g. increasing variance [41]), limiting their use in all ecosystems. Therefore, it may be advantageous to use methods that are shaped by spatio-temporal scaling processes, such as body size discontinuity analysis.…”