Resilience and resistance

Definition of Resistance: The capacity of a (ecological) system to weather a disturbance without loss of integrity and function (Lake 2013). This is the common definition for Resistance, other definitions can be discussed in the article

Definition of Resilience: (1) the capability to anticipate, prepare for, respond to, and recover from significant multihazard threats with minimum damage to social well-being, the economy, and the environment (sometimes called 'socio-ecological resilience')(Olsson et al. 2019); (2) the capability of a (socio-)ecological system to remain within a stability domain when subjected to environmental change, while continually changing and adapting yet remaining within critical thresholds (sometimes called 'general resilience') (Folke et al. 2010; Scheffer 2009; Brand and Jax 2007); (3) the capacity to experience shocks while retaining essentially the same function, structure, feedbacks, and therefore identity (sometimes called 'ecological resilience') (Brand and Jax 2007; DEFRA 2019), which is closely related to the concept of 'ecosystem resistance': the amount of disturbance that a system can withstand before it shifts into a new regime or an alternative stable state (Holling 1973; Gunderson 2000); (4) the capacity of an ecosystem to regain its fundamental structure, processes, and functioning (or remain largely unchanged) despite stresses, disturbances, or invasive species (e.g., Hirota et al. 2011; Chambers et al. 2014; Pope et al. 2014; Seidl et al. 2016), which can be measured by the time needed to recover its original state and which is sometimes called 'engineering resilience' (Lake 2013). This is the common definition for Resilience, other definitions can be discussed in the article

Introduction

Coastal and marine ecosystems are continuously subjected to environmental disturbances of all sorts at various spatio-temporal scales. The organisms inhabiting these systems are adapted to such disturbances, either by being tolerant of these conditions or by playing a role in one or more of the successional stages that follow during ecosystem recovery.

If all species in the system were tolerant to a particular perturbation, very little would change at the ecosystem level, and we could call the system resistant to this disturbance. However, it may happen that a disturbance, for example a temporary very low oxygen level, affects a substantial proportion of the organisms dramatically, either causing them to die, or forcing them to rapidly migrate to more favorable parts of the environment. Such an adverse disturbance could locally defaunate a certain volume in the pelagic or a certain area of hard or soft substrate. Such destruction at a local scale does not mean the end of local functioning. Usually organisms are available at a larger spatial scale that can re-colonize the affected area, according to their particular tolerances and abilities to favorably affect their local environment. Assessing the capacity of (eco)systems to survive disturbance has led to the notions resistance and resilience. These notions apply to the various scales of (eco)system organisation: individual, population, and community.

According to definitions (1) and (2), the concept of resilience refers not only to ecosystems, but more generally to socio-ecological systems. It is therefore also related to the concept of sustainability, where sustainability can be defined as the resilience of socio-ecological systems (Ruggiero, 2021).

The concepts resistance and resilience

The term resilience has been defined in different ways, as illustrated in the definition above. Resistance and resilience are intrinsic (eco)system properties which cannot be directly measured. According to a DEFRA report (2019) there is limited consensus in the literature about how resilience can be characterized and assessed. The term resilience is sometimes used to represent some kind of normative proposition about what kinds of ecosystem characteristics are desirable or necessary in the context of sustainable development, reflecting particular cultural and philosophical assumptions. However, it is also true that the resistance of an ecosystem (see the definition above) to changing conditions and the rate of recovery following some disruptive event are generally considered major components of resilience that can in principle be expressed in quantitative terms. An observable indicator of resistance is for example the unit increase in effect size per unit increase in the stressor. An observable indicator of resilience is the rate of change per unit time for ecosystem return to baseline conditions after disturbance.

Other attributes of resilience such as the capacity of ecosystems to transform and adapt in the face of environmental change are more difficult to translate to practice. According to Dawson et al. (2010), resilience concerns the response of ecosystems to changing environmental conditions and must be looked at alongside other additional dynamic features, namely durability, robustness and stability. These concepts can be defined as:

• Durability: ability to cope with a chronic stress, where the source of this stress is endogenous to the system;
• Robustness: ability to recover or maintain the systems' social-ecological functions in the face of an external and chronic driver;
• Stability: system’s tolerance to transient and endogenous shocks or disruptions.

Both resistance and resilience cause an ecosystem to remain relatively unchanged when confronted to a disturbance, but in the case of resistance alone no internal re-organization and successional change is involved. This can lead to collapse of the system when a disturbance threshold is exceeded. In contrast, resilience implies that the system is internally re-organizing while retaining essentially the same function, structure, identity and feedbacks, perhaps through a mozaic of patches that are at different stages of re-assembly (Scheffer and Carpenter, 2003). This self-organizing capacity prevents catastrophic collapse and allows for flexible adaptation to disturbance. The system may return after perturbation to its previous state but it may also shift to another ecosystem state. This behaviour can be analyzed in terms of nonlinear systems theory, that provides a mathematical framework for phenomena such as tipping points, bifurcations (regime shifts), hysteresis and multiple stable states (Nicolis and Prigogine, 1989; Fisher and Pruitt, 2020). Typical system responses to changing environmental conditions are displayed schematically in Figs. 1 and 2, corresponding to different resistance and resilience characteristics. See also the article Ecological thresholds and regime shifts. The behaviour sketched in these figures is strongly simplified. Resistance does not mean that an ecosystem is locked in a steady stable state. Due to the energy flow through the system (food, light, water motion), it moves cyclically or stochastically around stable states (so-called attractors), even without change in external environmental conditions.

Fig. 1. Schematic plot of different combinations of resistance and resilience, where the ecosystem state is represented by a black marble and the resistance-resilience characteristics by a fictitious energy potential. Low resistance: shallow basin of attraction; a small perturbation of the system state induces major change. High resistance: deep basin of attraction; a strong perturbation is needed to cause major system change. Low resilience: narrow basin of attraction; a small perturbation of the system state can cause irreversible change. High resilience: wide basin of attraction; the system returns to its initial state even after strong perturbation.

Fig. 2. Schematic plot of the (eco)system response to a temporal change in environmental conditions that exert a pressure (red) on the system. The time evolution of the ecosystem state is represented by blue lines corresponding to different resistance-resilience characteristics. The waviness of the blue lines mimics system state fluctuations, as generally observed in nature. Case of low resilience and low or medium resistance: the system collapses sooner or later. Case of high resistance: the system is insensitive to the exerted pressure. Case of high resilience: the system recovers with a hysteresis delay.

When considering the potential effect of a certain type of disturbance it is thus useful to ask two questions:

1. Will the species of this system be able to tolerate it (implying resistance), and if not,
2. Is recovery possible through a successional trajectory, back to the same, or at least a desirable, ecosystem state (implying resilience)?

Resistance breaks down when uni-directional environmental change acts faster than the organisms' ability to adapt their tolerances. If uni-directional change is an ongoing process (for example climate change), the system will also lose resilience after a certain time, as full recovery through succession will then not be possible. Recovery from sudden and local disturbance is often possible through recolonization, but the rate of recovery will depend crucially on the spatial extent of disturbance. For example, recovery from anoxia could take 5 to 8 months at the scale of square meters (Rossi et al. 2009), but could take 5 to 8 years at the scale of a whole bay (Diaz & Rosenberg, 1995).

According to definition (4), the speed at which an ecosystem returns to its former state following a (minor) disturbance can be considered a measure of resilience. The idea is that a system with a short return time is more resilient than one with a long return time. Such resilience measured as (1 / the return time to a stable equilibrium) has also been called engineering resilience. It has however a long history of use among ecologists (Pimm 1982, DeAngelis 1992, Vos et al. 2005, Alexander 2013). Resilience is also used in a way that more closely resembles the definition of resistance. Ecological resilience was defined as the amount of disturbance that an ecosystem could withstand without shifting to another ecological state characterized by another structure and other functions (definition 3).

Homeostasis

The word “homeostasis”, literally, indicates the absence of changes and an absolute maintenance of the status quo of an organism, but is generally understood as the coordinated physiological processes which maintain the steady functioning of the organism. Resistance and homeostasis are clearly related concepts. A homeostatic response to external perturbations may be selectively favored when the advantage offered by a reorganization of cell resources cannot be exploited. This is most likely to occur in the case of environmental perturbations of moderate intensity and short duration relative to the growth rate (Giordano, 2013). However, some authors define homeostasis as the self-regulation capacity of a system (organism) to resist or to adapt to external disturbances - a definition that includes both resistance and resilience.

Resistance-resilience trade-off

Intrinsic resistance and resilience are dictated by different processes and need not be positively correlated with one another (Patrick et al. 2022). Naturally dynamic and frequently disturbed ecosystems exhibit higher intrinsic resistance and/or resilience, whereas variation in intrinsic resistance and resilience among individual species is expected to be a function of traits including generation time and mobility. Traits that confer resistance to a species need not also enhance resilience and empirical evidence from multiple case studies even suggests that the opposite may be true. For example, seagrass meadows in the lower portion of the Chesapeake Bay dominated by fast-growing Ruppia maritima (widgeon grass) have higher resilience but lower resistance to episodic water quality declines than meadows dominated by Zostera marina (eelgrass). In contrast, eelgrass forms more highly resistant meadows that completely collapse in cases of exceptional heat-stress and recover only slowly (Patrick et al. 2021, Berger et al. 2024).

Short generation time in a species is hypothesized to enhance intrinsic resilience through rapid population growth, but this trait does not necessarily enhance resistance. For individual species, there is selection to excel at either resistance or resilience, but rarely both simultaneously. This is illustrated by the observation that resistance to cyclone effects is less an advantage for highly mobile species that have generation times substantially shorter than the local return interval of tropical cyclones. These species can rapidly recolonize and proliferate in the wake of disturbances that are infrequent relative to their life span, and strong selection would require substantial lasting changes to the gene pool as a consequence of the event (Patrick et al. 2022).

Trade-off between resistance and resilience also applies at ecosystem level. Tidal freshwater marshes are both resistant and resilient to natural pulsed brackish water intrusion but are not resistant to chronic salinization. Complete recovery after cessation of chronic salinization is possible, but not rapid. Though tidal freshwater marshes are able to resist periodic saline intrusions, prolonged events like sea level rise or extended drought may lead to ecosystem changes that have been shown to cause conversion from freshwater to brackish marsh (Thompson et al. 2024).

Field evidence suggests the following (Patrick et al. 2022):

• Responses to stressors that are completely or largely independent of control by biota should tend toward lower resistance.
• In contrast to resistance, resilience should hardly depend on stressor intensity. This indicates that while a larger perturbation to an ecosystem may require a longer recovery period than a smaller perturbation, the actual rate of recovery would be similar in both cases.

However, stressor-invariant resilience may not be the case if, for example, a very severe disturbance completely extirpates a taxon from a region. This would result in a slower rate of recovery (e.g., low resilience) if there are time lags until the species recolonizes from another source population, if there is displacement by a new opportunistic species, or if the habitat has changed.

Coastal defense systems are another example of trade-off between resistance and resilience, see the articles Dune erosion and Shoreline retreat and recovery. Hard coastal defense structures, for example a sea dike, are designed to resist against severe storm surges. But exceptional storm surges that exceed the design conditions can result in collapse of the structure, causing irreversible damage (and possibly many human victims). A sandy beach backed by a dune field that has evolved from long-term natural processes, can suffer great damage from severe storm surges, which take large amounts of sand from the beach and the dune far into to the sea. However, already in the waning of the storm, natural processes will be at work to bring eroded sand back to the beach and the dunes.

The above implies that managers seeking to simultaneously enhance both resistance and resilience in coastal systems may face an impossible task. In areas with infrequent disturbance, resistance may be better to manage for, whereas resilience may be preferable in areas experiencing frequent perturbations (Patrick et al. 2022).

Resilience and biodiversity

Resilience of coastal systems largely depends on biodiversity, which is a major requirement for allowing ecosystems to adapt to changing conditions (Loreau et al. 2001). The human impact on the environment through pollution, fisheries, sediment erosion / deposition and global climate change has brought about much faster change than would occur under natural conditions, putting severe stress on many ecosystems (Luypaert et al. 2020). Genetically diverse communities are more likely to contain a greater range of functional traits and environmental sensitivities. High diversity therefore entails opportunities for more efficient resource use as well as providing stability to ecosystem functions in variable environments and in the face of disturbance (Chapin et al. 1997). Alternatively, systems with species-poor communities are theoretically likely to be functionally poorer, less resistant (capacity to resist change) and resilient (capacity to recover from change) to disturbance than systems with species-rich communities (Strong et al. 2015) – see also the article Disturbances, biodiversity changes and ecosystem stability. Without genetic diversity, natural selection cannot occur and if natural selection is limited, adaptation is impossible. Preservation of biodiversity and, more specifically, genetic diversity is therefore of paramount importance for successful adaptation to our rapidly changing environments. According to present understanding, preservation of biodiversity does not necessarily imply conservation of the species community composition, because the maintenance of a particular species assemblage is not necessarily a requirement for the resilience of ecosystem functions (Gladstone-Gallagher et al. 2019). It has also become clear that biodiversity may not always protect ecosystems from major abiotic disturbances (Folke et al. 2004).

Resilience through recolonization

To understand resilience of ecosystems it is essential to understand what drives succession within ecosystems. Succession determines how, and how fast, communities return to their original state, or perhaps enter a new state. Many aspects of succession can be understood in terms of trade-offs between the ability to be either a good early (re)colonizer or a good competitor. Succession involves a gradual replacement of colonizer / competitor species according to the degree to which they tolerate, facilitate or inhibit certain environmental conditions and other species (Rossi et al. 2009). The extent to which processes of (re)colonization and succession can take place largely determines the recovery of ecosystems after major disruption and is therefore an essential characteristic of the resilience of ecosystems (Gladstone-Gallagher et al. 2019).

In this context, it is important to consider the spatial component of ecosystem resilience. Diversity of structurally and functionally connected landscapes, rich in resources and species, promotes the flow or movement of individuals, genes, and ecological processes. Below certain thresholds of connectivity the capacity to regain structure and function after perturbation is lost (Rappaport et al. 2015). Chambers et al. (2019), based on Allen et al. (2016), have therefore introduced the concept of 'spatial resilience', which is a measure of how spatial attributes, processes, and feedbacks vary over space and time in response to disturbances and affect the resilience of ecosystems. Self-organization through strong feedbacks at multiple scales and high levels of functional diversity and redundancy, stabilizes the system with respect to disturbances within the range of historic variability.

When creating Marine Protected Areas, the sources of populations at all stages of succession should be protected, to preserve 'ecological memory' to the fullest possible extent. This includes protecting not only 'high quality' habitats that harbour healthy mature communities, but also 'low quality' and disturbed habitats that are required for those species that contribute to early recovery of perturbed areas (Rossi et al. 2009). The selection of Marine Protected Areas thus involves evaluating the number, size, and spatial configuration of habitat fragments and degree of connectivity required to support restoration of ecosystems and conservation of focal habitats and species (Chambers et al. 2019; Olsson et al. 2019).

Resistance to long-term changes in abiotic and biotic factors

Community composition and ecosystem function may change very little under environmental change when the organisms can adapt to such change or tolerate it for some time (when the change is only temporary). However, all organisms have bounds to what they can temporarily or permanently tolerate, and when change exceeds some of these limits, the community composition and ecosystem functioning is likely to change.

It is unlikely that communities can be resistant to ongoing gradual change, such as global warming. Acclimation and phenotypic plasticity do not suffice to maintain the system as it is. Genetic adaptation could allow community members to track such abiotic environmental change, but it is more likely that the area where the community is functioning will be invaded by species that function well at higher temperatures. The original species will thus have to deal with new competitors and predators, in addition to the changed abiotic factor. To some extent the original community can track the preferred temperature range, by moving spatially to greater depths or to alternative geographic areas. But these new areas are likely to differ in other ecological aspects such as water pressure, light climate and perhaps speeds of water flow etc.

Adaptation and the consequences of mortality at different trophic levels

External disturbance interacts with internal mechanisms that shape community structure. To understand how an increased mortality of top-predators will affect the entire food chain, it is essential to understand how processes of mutual adaptation within food chains already give shape to existing patterns such as trophic structure (how biomass in ecosystems is partitioned between trophic levels) – see the article Trophic cascade.

Abundances at different trophic levels (such as algae, herbivores, carnivores and top-predators) and their responses to increased mortality (as under environmental change) depend critically on different mechanisms of adaptation within food chains and on the importance of population density at each of these trophic levels. However, different types of adaptation to living in a food chain context (balancing the need to acquire resources with the need to avoid predation) can often have similar consequences. For example, micro-evolution of behaviour, species replacement and induced defenses at a middle trophic level may all have similar effects on trophic level abundances in disturbed food chains (Abrams and Vos 2003).

Assessment of resilience

The foregoing considerations can be summarized in a number of resilience characteristics (Walker et al. 2004) – see also Fig. 1:

1. Latitude: the maximum amount a system can be changed before losing its ability to recover within the same state (before crossing a threshold), represented by the width of the basin of attraction;
2. Resistance: the ease or difficulty of changing the system, represented by the width of the resistance domain; deep basins of attraction indicate that greater disturbances are required to change the current state of the system;
3. Precariousness: how close the current state of the system is to a limit or threshold that, if crossed, makes reorganisation difficult or impossible;
4. Cross-scale interactions ('panarchy'): the resilience of a system at a particular spatial scale will depend on the influences from states and dynamics at spatial scales above and below.

Implications for ecosystems are (Gladstone-Gallagher et al. 2019):

• For ongoing (chronic) changes and changes on large space scales (such as climate change), resilience is largely determined by the resistance of particular species: biological or behavioral traits that increase a species' resistance (tolerance) to disturbances. Resilience is likely best secured in genetically diverse and functionally redundant ecosystems: ecosystems in which many different species share the same combinations of functional (or resource use) characteristics.
• For short-term hefty disruptions, resilience is largely determined by spatial resilience (recolonization potential): the ability of populations to supply recruits to a disturbed patch and the ability of new recruits to colonize new space. Resilience is likely best secured in diverse connected ecological systems that are capable to change, adapt, and transform in response to stresses and strains through feedbacks across multiple spatial and temporal scales.

Related articles

Ecological thresholds and regime shifts

Disturbances, biodiversity changes and ecosystem stability

Biodiversity and Ecosystem function

Vulnerability and risk

Thresholds and Marine Policies

Species extinction

Trophic cascade

Integrated Coastal Zone Management (ICZM)

Sustainability indicators

Measurements of biodiversity

References

The main authors of this article are Vos, Matthijs and Job Dronkers Please note that others may also have edited the contents of this article. Citation: Vos, Matthijs; Job Dronkers; (2024): Resilience and resistance. Available from http://www.coastalwiki.org/wiki/Resilience_and_resistance [accessed on 24-11-2024] For other articles by this author see Category:Articles by Vos, Matthijs For other articles by this author see Category:Articles by Job Dronkers For an overview of contributions by this author see Special:Contributions/Matthijs For an overview of contributions by this author see Special:Contributions/Dronkers J