Ecological Resistance and Resilience in Rangelands

On the Ground Resistance and resilience are commonly used terms in discussions about agriculture and future ecological conditions. We provide a common understanding of these terms as they apply to the ecology of grazed systems. We discuss the relationship between ecological resistance and resilience, disturbances, and ecological processes. We present examples of how to make what is known about ecological resistance and resilience in grazed ecosystems accessible to planners and their clients.


Introduction
The terms resistance and resilience are used today by agricultural producers, government agency leaders, and policymakers.They are of ten inc luded in discussions about climate change, climate-smart agriculture, preparing for the next disturbance, and improving the sustainability of an agricultural business.Agricultural producers, grazing land managers, advisors, and policymakers should have accurate, scientifically based information to improve communication about ecological resistance and resilience.

Disturbances
Disturbances are changes in existing environmental conditions that cause ecosystem change. 9

Ecological resilience
Ecological resilience is the ability of an organism or defined area of land to return to the previous condition (i.e., within its ecological state following a specific external stress). 4 , 14,15cological resilience can be described by the amount of energy and disturbance that can be endured and still return to the previous condition. 3he time required to return may be an important component to include in descriptions of resilience.

Ecological resistance
][3] Ecological resistance is best measured by the amount of change in a specified attribute that the organism or area of land exhibits in response to a specific disturbance event. 3r paper provides a basis for a shared understanding of resistance and resilience and examples for making ecological resistance and resilience in rangelands easily accessible and helpful to grazing land specialists and their clients.

Ecological resistance and resilience in rangelands
Ecological resistance and resilience, defined in SideBar, operate at varying scales (e.g., individual plant, plant community, grazing unit, watershed), and exhibit complex interactions across scales. 1 It is important to recognize these variations and interactions.However, discussions with land managers and consideration of management options need to be presented at an appropriate scale for effective land management.We focus on ecological resistance and resilience at the ecological site, plant community, and species levels.
Much of ecological resistance and resilience in rangelands depends on the ability of the existing plants to survive, thrive, and grow while experiencing various disturbances.Examples of disturbances affecting plants directly include drought, herbivory, and wildfire.Each plant species or group of similar species has characteristics allowing them to access nutrients, moisture, and sunlight efficiently, respond to grazing events or damage from weather events, and compete with neighboring plants.These characteristics include root structure, leaf shape, growth patterns and timing, and location of growing points. 2or ease of planning and management, plant species characteristics related to resistance and resilience are often consolidated at the plant group (e.g., growth form) or community level.
Disturbances, defined in SideBar, that have effects at the plant community or ecological site scale include soil movement from wind or water, flooding or ponding, and water infiltration into the soil.While the effects of these disturbances are moderated by the existing plant community structure (e.g., plant stem rigidity and density, plant canopy, root density, and structure), site/soil characteristics are often major factors determining the degree of impact. 3he ecological site description (ESD) is used to describe plant communities'characteristics related to resistance and resilience.The ecological dynamics section of ESDs and the accompanying State and Transition Model (STM) convey information about plant community resistance and resilience to known disturbances.

Assumptions
There are three fundamental assumptions contained within the terms of ecological resistance and resilience -variabilit y, stabilit y, and attainabilit y.These assumptions create the framework within which ecological resistance and resilience operate.Clear descriptions of this framework will enable planners and managers to have a consistent understanding of the nature of ecological resistance and resilience.It is also important to relate the described ecological resistance or resilience to a specific disturbance.

Variability
Growing conditions in grazed landscapes are rarely the same from day to day or even hour to hour.These variations can occur in amount, timing, or rate.The degree to which these conditions vary must be c lear ly described.
While climate variability is often at the forefront of discussions on resilience and resistance, it is also important to recognize and describe variability in site and plant characteristics.Examples include: soil cover, which may vary from year to year due to variation in inputs and decay; and plant community composition, which may vary yearly due to growing conditions that favor one plant species over another.
In many ways, variabilit y and stabilit y are intertwined.Both must be considered and described to have a clear understanding of ecological resistance and resilience.It is crucial to describe the range of variability within the stable desired condition (described below).
Examples of variability include:

Stability
The current or other desired condition must be stable.In other words, the desired condition must be able to exist with minimal inputs.It also must be able to exist within the normal range of disturbances and variability expected for the area.
Examples of stability include: • The ability of perennial plants to remain alive through extended periods of extreme temperatures (i.e., cold, hot) or moisture conditions (i.e., flooding, drought); • The ability of the soil to absorb rainfall or snowmelt when it occurs; • The ability of annual plants to produce viable seeds as often as needed to replenish the seed bank; • The ability of plant communities to maintain species or functional group composition (i.e., dominance or relative proportion) within a defined boundary (see Variability above).

Attainability
The desired condition (i.e., ecological state) must be attainable from the existing situation.This assumes there are ways to reach the desired condition which are known and can be c lear ly described so a manager can use these methods to reach the desired condition.Attainability is communicated in an ESD's STM through the presence, absence, and direction of transition or pathway arrows and the various states and plant community phases portrayed.Transitions must be described in terms of processes (i.e., what is going to happen), metrics (i.e., measures of what is going to happen), and time (i.e., how long).
Examples of attainability include: • Soil temperatures are moderated by increasing soil cover through increased litter cover, supporting an existing plant community (i.e., increasing stability), or facilitating a plant community change.This is accomplished by allowing more plant growth to remain on-site and placed in contact with the soil through hoof action or snow load.This can happen within a single growing season.• The existing shallow-rooted plant community is replaced with deeper-rooted plants through seeding activities and subsequent management to support the seeded plants.It can take several growing seasons for seeded plants to become established.• The density or vigor of existing desired plant species is increased by altering the time, intensity, or duration of herbivory to support their physiological needs.This increase happens gradually over several growing seasons.Note: Adapted from Fu et al., 7 Girgis and Moseley, 9 and Lyons et al. 8 Note: A state's resilience is based on the transition or restoration pathways of an STM (depicted as 'arrows') that point away from the state.
• Existing woody species are reduced through the application of prescribed burning or brush management.This reduction occurs within one or two growing seasons.

Ecological resistance and resilience categories
Each state within an STM ( Fig. 1 ) is a recognizable set of plant communities or a single plant community that dif-fers in ecological structures and related functions from other plant communities on the same site. 4Ecological states are resilient and have varying abilities to maintain their characteristics in response to specific management actions or disturbance regimes.Ecological states have a range of variability in vegetation (often portrayed as plant community phases) and soil properties (both static and dynamic). 4 , 5"At-risk community phases" occur when the plant community composition changes to include new (i.e., invasive) species or plant species/group dominance begins to shift (i.e., typically caus-2023 ing attendant changes in ecological functions), and resistance of the state to a disturbance or threat decreases.With this decrease, the site may cross a threshold and undergo a transition to an alternative state.The rate of this change may be slow (e.g.., juniper invasion) or rapid (e.g., resulting from a fire).In either case, a transition to an alternative state results from a decrease in ecological resistance of a particular state to a defined disturbance.

Ecological state resistance and resilience
States within the STM are often described as existing within self-maintaining conditions.They are persistent without significant management inputs.States within the STM have distinct disturbance regimes and ecological reactions to these disturbances that differ from other states and make the state self-reinforcing.Thus, states (i.e., reference and alternative) have relatively high resilience to disturbances that are within the described disturbance regime and are resistant to change.This resistance to change is not absolute, as represented by the various states within the STM.At-risk community phases within a state represent the most vulnerable or least resistant.

Existing plant community ecological resistance and/or resilience
Plant communities may exhibit resistance and/or resilience to change due to disturbance events or other external pressures.The time frame for this category of resistance and/or resilience is often shorter than that associated with states.Plant communities will exhibit some variability in plant composition and/or production due to the natural variability of the ecosystem as described above.The resilience of a plant community to change may be low as some plants may become more dominant as time since disturbance increases (i.e., successional processes).An example of this low resilience is shrubs becoming more dominant over herbaceous plants (i.e., a change from one plant community phase to another within the state) as time since fire increases.A shrub-dominated community may become an at-risk community phase (i.e., at-risk of transitioning to another state) with very low resistance/resilience to change from the next disturbance.

Disturbances
The plant or defined ecosystem has varying degrees of resistance or resilience to any given disturbance.The duration, timing, and intensity of a disturbance are important to consider and may affect the degree to which the plant or ecosystem tolerates the disturbance.These characteristics need to be included in the disturbance regime description.

Plant characteristics
Ecological resistance and resilience often rely on the characteristics of individual plants. 2The individual plant's response to disturbances often conveys resistance and/or resilience to the disturbance.It is helpful to aggregate individual plant response(s) to convey the plant community's resistance or resilience to a disturbance.

Ecological processes
Both resistance and resilience must be linked to the cause and the ecological process(es) involved.The cause (e.g., disturbance or other external change in condition) creates a situation where ecological processes may be altered.The ability of the organism(s) or plant community to resist or accommodate the change in ecological processes determines the level of resistance or resilience the organism or plant community has.Knowing the ecological process(es) involved helps the planner and/or client focus on what needs to be protected or enhanced to ensure a short recovery time from the disturbance or other temporary change.
It is helpful and sometimes necessary to identify the weakest link.The weak link may be an ecological process that is most at risk of permanent change, a specific location within the defined area of land that is most susceptible to change in one or more ecological processes, a group of plants or plant species that are critical to the ability of the area to be resistant or resilient, or some combination of the above.These weak links represent a management opportunity to avoid transitioning from one state to another.Boundaries between two states are described as ecological thresholds.These thresholds are challenging to define precisely and even more difficult to measure and manage in a practical sense. 6They can best be thought of and expressed as the point in time when processes that impart resilience to a site (i.e., negative feedbacks) are overwhelmed by processes that drive change to a different state, resulting in lower resistance to change (i.e., positive feedbacks). 4

Table 1
Examples of ecological processes, ecological process components, site or plant characteristics related to the component, and the location or scale where the ecological process component occurs.Ecological resistance and resilience rely on the plant species and ecological process(es) response to disturbance.For example: • Is the site able to maintain site stability, or is soil movement increased when weather disturbance intensity increases?• Is water available for plant growth within the root zone after rain events?• Is there increased water flow off-site when rainfall exceeds expectations?• Can the plant community capture solar energy throughout the whole growing season given adequate soil moisture, growing temperatures, and available nutrients?• Is the root system of the existing plant community able to capture nutrients and moisture from throughout the soil profile?• Does the plant community have multiple age structures for all plant species?• Is there a functioning plant population recruitment and replacement process?
There are many ecological processes to consider ( Table 1 ).][9] Ecological processes operate at various scales, from the individual plant to plant communities to groups of ecological sites to watersheds.We limited the scale to the Ecological Site, ecological state, and plant growth form for ease of communication of this information and to indicate the lowest practical management level for ecological process function.

Ecological resistance and resilience information
Land managers are facing increased variability in disturbance patterns.Droughts, flooding, wildfire, and invasive species are some examples of such disturbances.1][12] Providing clear descriptions of ecological resistance and resilience information as related to disturbances can help land managers make better informed decisions about the ecological state they desire to have on their lands.
Easy access to clear descriptions of ecological resistance and resilience information will guide rangeland planners and managers as they decide which state or plant community phase to manage for, and which characteristic(s) to monitor.ESDs often contain information relative to resistance and resilience in the Ecological Dynamics section.Informational tables allow reports to be generated by the reader and provide improved consistency in the type of information displayed.While ESDs include resistance and resilience information, they often lack any description of the magnitude of such re-2023 sistance or resistance.The amount of resistance or resilience determines the ease or difficulty of transitioning from one state to another.Effectively managing and conserving plant communities relies on a thorough understanding of this principle.We propose that the degree of resistance or resilience be included in the tables qualitatively, ranging from high to medium to low.These categories should be relative to all ecological states described within a single ESD.These three categories can be subdivided into plus ( + ), neutral, and negative (-) groupings to distinguish more subtle but consistent resistance or resilience differences bet ween ecological states or plant community phases.
Consistent ratings are important and allow for valid comparisons across the various ecological sites and plant communities encountered on a ranch or other planning unit.
Existing data and expert opinion should be used to assign the categories (low, medium, high) of resistance and resilience.Ecological site developers will select, summarize, and group data about the climate, soils, landform, vegetation composition, and productivity within the ecological site descriptions as appropriate for the disturbance being considered.ESD technical teams will solicit expert opinion to consider the efficacy and value of the data to answer critical questions such as the time required for recovery, probability of responses depicted in each STM, and levels of intensity (i.e., energy) of the disturbances considered.To ensure consistent application, the range of values should extend across broad ecological regions such as Land Resource Regions (LRR). 13Chambers et al. 3 provides an example of this kind of approach used to define resistance and resilience categories for invasive plant introduction (e.g., Bromus tectorum ) disturbance.
Quantitative data should be used when possible.The categories can be defined and determined by the biophysical characteristics of the site, state, and plant community, their known responses to disturbances and management actions, and the 'intensity' of the disturbance required to switch states. 1 , 3 , 4 This information can be used by rangeland planners and managers to compare and contrast the resistance or resilience of various states or plant community phases to the disturbance of concern.The ecological processes most at risk can be identified and included in monitoring and decision-making.
Resistance and resilience ratings for various site characteristics and disturbances should be based on the biophysical

2023
characteristics of the ecological site, its known responses to disturbances, and the intensity of the disturbance ( Tables 2  and 3 ).Ratings of resistance and resilience need to be consistently applied within a given ecological site description.These rating categories can also be applied to many ecological sites, allowing for rating comparisons across multiple sites.Tables provide one way to enter important information about Ecological Resistance and Resilience and could be entered by range management specialists or ecological site specialists familiar with the ecological site and management activities possible for the state or plant community phase ( Table 4 ).Information is entered for each type of disturbance described for each pathway or transition indicated in the STM.
The information contained in these tables can be used to create summary reports based on parameters, selected by the planner/client.These parameters include ecological state, community phase, and disturbance.Specific summary report(s) provide selected resistance/resilience information to the client or planner without needing to read through the Ecological Dynamics section of the ESD for pertinent information.They contain information to help understand the effects of management decisions/actions on ecological processes.Not all disturbances or management actions result in the same ecological process response.These reports can help find an appropriate management action that balances the various responses while meeting the goals/objectives of the land manager.Summary report(s) are created as desired by the planner or land manager and do not need to be stored in the ESD database system since they are created from the information in the tables ( Table 4 ).Summary reports provide critical information to rangeland managers and planners as they decide which state or plant community phase to manage for, given the selected disturbance as well as what characteristic(s) to monitor.An example summary report ( Table 5 ) for the SR Cool 9-12 PZ ecological site shows information using "Fire" as the chosen disturbance/threat.

Summary
The ecological resistance and resilience of grazed lands depend on complex interactions between site characteristics, plant characteristics, and the surrounding landscape.Improving the ability of a site to absorb the effects of a disturbance relies on having a cushion within the plant community and within the site characteristics.Land managers can improve the resistance/resilience cushion by better understanding how the site, plant community, and individual plants respond to a disturbance.
Tables are a good way to consistently enter and display ecological resistance and resilience information within ESDs and make it easier for planners to find the desired information.Tables can be used to generate reports for specified threats or disturbances for use by land managers.
The information contained within the tables and reports will be useful to planners and clients as they consider how to remain in the desired state or plant community.It will pro-vide a clearer understanding of disturbances, affected ecological processes, and what activities increase the resilience of a state or plant community and return it to the desired condition following a disturbance.
The Ecosystem Dynamics Interpretive Tool (EDIT) can be modified to include these tables in the Ecological Dynamics section of each ESD which discusses the STM for each site. 14The report options contained within EDIT can be modified to include the suggested Ecological Resilience Summary Report, as well as other desired reports based on ecological resistance and resilience information.Our concepts about organizing and displaying resistance and resilience information will be useful when combined with other tools currently under development, such as the ARS Ecosystem Transition Sandbox. 15

Declaration of competing interest
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests.The authors certify that they have no financial interest in the subject matter discussed in the manuscript.B.G. and S.G are employees of USDA Natural Resources Conservation Service.

Figure 1 .
Figure 1.State and transition model for the SR Cool 9-12 PZ ecological site (R010XC030OR) showing state and plant community phase boxes with transition and pathway arrows.Legend: ARTRW8, Artemisia tridentata spp.wyomingensis ; FEID, Festuca idahoensis ; POSE, Poa secunda ; PSSPS, Pseudoroegneria spicata ssp.spicata ; R2A, Restoration pathway from State 2 to State 4; R3A, Restoration pathway from State 3 to State 4; T1A, Transition pathway from State 1 to State 2; T2A, Transition pathway from State 2 to State 3; T4A, Transition pathway from State 4 to State 3. Note: Adapted from Fu et al.,7 Girgis and Moseley,9 and Lyons et al.8 Note: A state's resilience is based on the transition or restoration pathways of an STM (depicted as 'arrows') that point away from the state.

Table 4
Proposed elements for the resistance and resilience table.

Table 3 )
Ecological Process Impacted by Response to Disturbance (see

Table 1 )
Description of Interaction of Disturbance and Ecological Process (narrative) Location of Resistance and Resilience (see

Table 1 )
Amount of Change when Resistance is Overcome (see

Table 5
Example of an ecological resilience summary report.