Quantifying resilience

The biosphere is under unprecedented pressure, reflected in rapid changes in our global ecological, social, technological and economic systems. In many cases, ecological and social systems can adapt to these changes over time, but when a critical threshold is surpassed, a system under stress can undergo catastrophic change and reorganize into a different state. The concept of resilience, introduced more than 40 years ago in the ecological sciences, captures the behaviour of systems that can occur in alternative states. The original definition of resilience forwarded by Holling (1973) is still the most useful. It defines resilience as the amount of disturbance that a system can withstand before it shifts into an alternative stable state. The idea of alternative stable states has clear and profound implications for ecological management. Coral reefs, for example, are high-diversity systems that provide key ecosystem services such as fisheries and coastal protection. Human impacts are causing significant, ongoing reef degradation, and many reefs have shifted from coral- to algal-dominated states in response to anthropogenic pressures such as elevated water temperatures and overfishing. Understanding and differentiating between the factors that help maintain reefs in coral-dominated states vs. those that facilitate a shift to an undesired algal-dominated state is a critical step towards sound management and conservation of these, and other, important social–ecological systems. Resilience has gained popularity among both academicians and laypeople, as a term meant to describe a systems’ ability to withstand disturbance. Resilience has become a buzzword in the last decade, as shown by its increasing appearance in calls for research proposals and scientific citation data bases. The term resilience has in many cases lost the clarity of the original definition and in fact is frequently used in a manner in direct opposition to the original definition. Many current uses of the concept are loose and incorrect. The term is becoming increasingly used in a normative sense (Brand when a system is in a desirable state and managers wish to enhance resilience, or when the system is in an undesirable state and managers wish to erode resilience and foster a transformation to an alternative state. Fostering or eroding resilience is a process. When a system is perturbed but resilience is not exceeded, then the recovery can be measured as a rate. Several frameworks to operationalize resilience have been proposed. A decade ago, a special feature focused on quantifying resilience was published in the journal Ecosystems (Carpenter, Westley Spears et al. 2015). Ideas such as ‘resilience thinking’ have utility in helping stakeholders conceptualize their systems, but provide little guidance on how to make resilience useful for ecosystem management, other than suggesting an ambiguous, Goldilocks approach of being just right (e.g. diverse, but not too diverse; connected, but not too connected). Here, we clarify some prominent resilience terms and concepts, introduce and synthesize the papers in this special feature on quantifying resilience and identify core unanswered questions related to resilience. To progress towards quantification of resilience, it is necessary to have a common vocabulary. Prior to introducing the broad approaches that have been developed to assess and quantify resilience and discussing the contributions of this special feature, we provide common definitions of terms commonly used in resilience science and theory. Ecological resilience is a measure of the amount of change needed to change an ecosystem from one set of processes and structures to a different set of processes and structures (Holling 1973). Resilience is an emergent property of ecosystems and other complex systems and recognizes that systems operate in multiple basins of attraction. From a human perspective, it is crucial because it implies a predictable, although variable, delivery of expected ecosystem services. Understanding the dynamics of resilience is critical to achieving sustainable human interactions with their supporting ecosystems. An ecosystem with high ecological resilience requires a substantial amount of energy to transition to an alternative state, whereas a low resilience system would transition with a relatively small amount of energy. Ecological resilience as a systemic phenomenon can be measured through an assessment of mutually non-exclusive attributes, including scales, alternative states, feedbacks and thresholds. Cross-scale resilience posits that resilience in ecological systems is enhanced when functional diversity and traits are diverse within scales and reinforced across scales. This model explicitly considers the compartmentalization of ecological patterns and processes by spatial and temporal scales. The cross-scale resilience model was established for operationalizing and quantifying ecological resilience (Peterson, Allen & Holling 1998), and especially for understanding the relationship between biodiversity, scale and resilience. Cross-scale resilience can be assessed by evaluating the distribution of functional traits of species within and across scales. Several approaches, including discontinuity analysis and time-series or spatial modelling tools, are available for assessing cross-scale resilience (Angeler et al. 2014). Engineering resilience focuses on the return of structural and functional attributes of systems to predisturbance conditions following a disturbance. Rapid return times are interpreted as reflecting high engineering resilience (Pimm 1991). The unit of measurement is time of recovery. This definition assumes that systems are characterized by a single equilibrium and therefore fails to account for the potential for alternative states of the same system. In a management context, engineering resilience incorrectly assumes that an ecosystem always recovers from a degraded state to a previous or more desirable state, and therefore, the only measure of interest is return time. This term is analogous to engineering resilience or ‘bounce back’ and refers explicitly to the capacity of a system to return to its initial state following disturbance. Although this definition does not explicitly exclude the existence of thresholds (e.g. the physical property of a material to return to its original shape or position without exceeding its elastic limit), multiple equilibria are not considered in this definition. More clear terms for this behaviour include return time, recovery and engineering resilience. Based on current empirical and theoretical knowledge of spatial resilience, a tractable ‘shorthand’ definition of spatial resilience is: the contribution of spatial attributes to the feedbacks that generate resilience in ecosystems and other complex systems and vice versa. This definition allows for the operationalization of spatial resilience in management and is consistent with the foundational aspects of spatial resilience described by Nyström & Folke (2001). It builds upon the spatially relevant aspects of complexity, including asymmetries, networks, connectivity, dispersal and information processing and how these facets change over time. A general and generic property of systems, the broad ability of a system to cope with disturbances without changing state. It does not define the part of the system that might cross a threshold and the kinds of shocks the system needs to deal with, and it copes with uncertainty in all ways (Folke et al. 2010). Managing for general resilience is complex given that multiple patterns and processes operating at distinct spatiotemporal scales need to be accounted for. However, this complexity may create opportunities for re-evaluating present and learning from past situations, boosting novelty and innovation and triggering social and policy change. That is, understanding general resilience may create new possibilities for adaptive or transformative change to swiftly changing social–ecological baselines. Resilience of a system or component of a system to a known, or anticipated disturbance. The resilience of what, to what (Carpenter et al. 2001). A management focus on specified resilience can become problematic because increasing resilience of particular parts of a system to specific disturbances may cause the system to lose resilience in other ways. For instance, international travel in Europe became increasingly focused on developing air travel, while international ground and water transportation was deemphasized. The volcano eruption on Iceland in 2010 uncovered the low resilience of this transportation system to the extensive cloud of ash in the air that interfered with the operation of aircrafts. This term has been used within ecological stability research to characterize the property of communities or populations to remain unchanged when subject to disturbance. The inverse of resistance is sensitivity (Grimm & Wissel 1997). As is the case with most ecological stability definitions, relevant patterns and processes are considered from a single equilibrium point of view and do not account for the existence of thresholds. Walker et al. (2004) broadened the use of this concept to consider resistance to be a component of ecological resilience. Stability is a complex and multifaceted concept, including components such as variability, resistance, resilience, persistence and robustness (Donohue et al. 2013). Despite this complexity, concepts related to stability are reductionist in a sense that they are less integrative of the dynamic and complex system components from which resilience emanates. Simply, stability refers to the ability of a system to remain unchanged in the face of perturbation, and to return to the initial state quickly when perturbation alters system parameters. That is, stability concepts generally ignore that ecological patterns and processes are compartmentalized by distinct spatial and temporal scales or operate in different attractor domains. From a management perspective, stability is often desirable while resilience, on the other hand, considers variability to be a desirable system property, and one that through adaption and evolution infers greater ecological resilience. Functional redundancy refers to the existence of more than one species or process delivering the same ecological function, or trait. Functional redundancy is often studied without explicitly accounting for scaling relationships in ecological systems (Truchy et al. 2015). In terms of cross-scale resilience, redundancy is considered in two forms: (i) Redundancy existing within a scale, whereby identical functions would be redundant in the strict sense. (ii) Redundancy that occurs in functionally similar species that exploit their environment at different scales. In this case, the more appropriate term is cross-scale reinforcement because it accounts for the fact that species and processes operating at other scales can maintain a function impacted by a disturbance at a single scale. Rather than focusing on the redundancy of a specific functional trait across scales, this concept emphasizes the variation in responses to environmental change by species within a functional group within scales (Elmqvist et al. 2003). Response diversity considers the functional make-up of a species accounting for multiple traits that modulate species responses to disturbances through, for instance, distinct colonization, growth, competition and dispersal abilities. Variability in combinations of multiple traits increases the adaptive capacity to cope with and respond to disturbances, maintaining functional patterns and processes in ecosystems. The behavioural or evolutionary response of species or systems to a particular stress. In ecology, adaptation has a single-species focus. 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A potential alternative in terms of and function and of a system. states are in ecological resilience. A stable state is by stable processes and by thresholds processes and feedbacks between alternative states. attributes characterize the of attraction. The terms state and are often used However, refers to the processes and feedbacks that dynamic to a given state of a system. A change in the process of a system in a state change. In ecological systems feedbacks from the set of interactions between and an by and being by the process which to A or these while negative feedbacks have the and negative feedbacks generally do not of regarding the of the or feedbacks are of most interest in resilience these help maintain ecological function and processes in specific alternative states or A shift in is a change in the function and mutually reinforced processes or feedbacks of an ecosystem et al. 2001). 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Many of these approaches have in but have not the social and social–ecological system For instance, Angeler et al. how discontinuity can current management approaches, a for ecological management and how discontinuity approaches can help managers measure and the resilience of ecosystems. how discontinuity can be used in other of ecological management, including the of critical scales of and time in ecological systems, the of and understanding and and Resilience has developed across multiple scientific However, the of has our ability to and between the ecological and social and policy to our understanding and management of social–ecological systems. resilience has to have two ecological which are often but with important A between social and ecological resilience research is the application of for resilience. have been made in years in ecology, with many approaches, including discontinuity and other modelling and spatial for quantifying attributes of resilience. of the social resilience research and the of approaches is because of the of to make complex system dynamics tractable et al. existing knowledge is the for scientific and the made in help questions in social resilience Despite in the of resilience as approaches to quantify resilience are often and to the scale of ecosystems. approaches often focus on specific which assessing specific resilience that might not be of other or the ecosystem at multiple resilience measurement approaches across including the assessment of functional traits and might in a understanding of the general resilience of ecosystems. approaches in ecosystems need to be with to into relationships in resilience. resilience research on approaches have from the analysis of However, assessment of the resilience of is by the of data that the relevant scales of and time. 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