J. Range Manage.56: 106 -113 March 2003

State and transition modeling: An ecological processapproach

TAMZEN K. STRINGHAM, WILLIAM C. KRUEGER, AND PATRICK L. SHAVER

Authors are assistant professor and professor, Department of Rangeland Resources, Oregon State University, Corvallis, Ore. 97331.. and rangeland management specialist, USDA, Natural Resources Conservation Service, Grazing Land Technology Institute, Corvallis. Ore. 97331.

ResumenAbstract

State-and-transition models hold great potential to aid inunderstanding rangeland ecosystems' response to natural and/ormanagement-induced disturbances by providing a frameworkfor organizing current understanding of potential ecosystemdynamics. Many conceptual state-and-transition models havebeen developed, however, the ecological interpretation of themodel's primary components, states, transitions, and thresholds,has varied due to a lack of universally accepted definitions. Thelack of consistency in definitions has led to confusion and criti-cism indicating the need for further development and refinementof the theory and associated models. We present an extensivereview of current literature and conceptual models and point outthe inconsistencies in the application of nonequilibrium ecologyconcepts. The importance of ecosystem stability as defined by theresistance and resilience of plant communities to disturbance isdiscussed as an important concept relative to state-and-transitionmodeling. Finally, we propose a set of concise definitions forstate-and-transition model components and we present a concep-tual model of state/transition/threshold relationships that aredetermined by the resilience and resistance of the ecosystems'primary ecological processes. This model provides a frameworkfor development of process-based state-and-transition models formanagement and research.

Los modelos de estados-y- transicion presentan un granpotencial para ayudar a entender la respuesta de los ecosis-temas de pastizal a los disturbios naturales y/o inducidos por elmanejo al proveer una estructura para organizar elconocimiento presente de las dinamicas del potencial del ecosis-terns. Muchos model os conceptuales de estados-y-transicionhaD sido desarrollados, sin embargo, la interpretacion ecologi-ca de los componentes principales del modelo: estados, transi-ciones y umbrales haD variado debido a la carencia de deflni-ciones universalmente aceptadas. La faits de consistencia en lasdetiniciones ha conducido a confusion y critics indicando lanecesidad de un mayor desarrollo y retinamiento de la teorfa y!os model os asociados. Nosotros presentamos una revisionextensiva de la literatura actual y modelos conceptuales y pun-tualizamos las inconsistencias en la aplicacion de los conceptosde la ecologi'a de no equilibrio. La importancia de la estabilidaddel ecosistema, detinida como la resistencia y resilencia de lascomunidades vegetales a los disturbios, se discute como un con-cepto importante relativo al modelaje de estados-y- transicion.Finalmente, proponemos un grupo de detiniciones concisaspara los componentes del modelo de estados-y-transicion y pre-sentamos un modelo conceptual de las relaciones deestados/transiciones/umbrales que estan determinadas por laresilensia y resistencia de los principales procesos ecologicos delecosistema. Este modelo provee un marco para el desarrollo demodelos de estados-y-transicion basados en procesos paramanejo e investigacion.

Key Words: state, transition, threshold, modeling, ecological,process

Applied ecology disciplines, such as range management, arenecessarily organized around a response model based on theoreti-cal supposition. Thus, the litmus test for an ecological or mecha-nistic model is its ability to predict the consequences of naturaldisturbances and/or management activities with acceptable preci-sion over timescales relevant to management. Traditional theoriesof plant succession leading to a single climax community havebeen found to be inadequate for understanding the complex suc-cessional pathways of semi-arid and arid rangeland ecosystemsconsidering timescales important for making management adjust-ments (West 1979, Westoby 1980, Anderson 1986, Foran et al.

1986, Tausch et al. 1993). After 50 years of applying the quanti-tative climax model of Dyksterhuis (1949) to rangeland manage-ment its predictive capabilities have come under scrutiny. Theinability of the model to incorporate multiple pathways of changehas led some ecologists to abandon the model completely(Wilson 1984, Smith 1988). The recognition of this inadequacyhas generated a search for an alternative theory that more correct-ly reflects the observed dynamics of rangeland ecosystems. Asmany scientists were questioning the validity of the climaxmodel, Westoby et al. (1989) developed a foundational discus-sion and conceptual model based on non-equilibrium ecology.Numerous scientists have utilized these concepts as a basis forthe development of conceptual models of vegetation dynamicswhich incorporate multiple successional pathways, multiple

Authors wish to thank L.E. Eddleman, N.E. West, M. Westoby, W.A. Laycock,and G.L. Peacock for insightful review.

This article is submitted as Technical Paper No. 11874 Oregon AgriculturaJ

Experiment Station, Corvallis, Ore.Manuscript accepted 8 Oct. 2002.

106 JOURNAL OF RANGE MANAGEMENT 5612) March 2003

steady states, thresholds of change, anddiscontinuous and irreversible transitions(Archer 1989, Friedel 1991, Laycock1991, Fuhlendorf et al. 1996, Stringham1996, Rietkerk and Van de Koppel 1997,Davenport et al. 1998, Oliva et al. 1998,Petraitis and Latham 1999, Plant et al. 1999,West 1999, West and Young 2000,Stringham et al. 2001). However, the eco-logical interpretation of Westoby's modelhas varied due to a lack of universallyaccepted definitions of the key concepts.The lack of consistency in definitions hasled to confusion and criticism indicating theneed for further development and refine-ment of the theory and associated models(Iglesias and Kothmann 1997).

The USDA Natural Resources Conser-vation Service (NRCS) adopted the use ofstate-and-transition vegetation dynamics indescribing rangeland ecological sites. Theattempt to use this concept illustrated theinconsistency in the definitions and con-cepts. The NRCS recognizes the need forconsistency in the application of the con-cepts (USDA 1997). For management toutilize the non-equilibrium ecologicalmodel the definitions of model objectsmust be succinctly stated and validated.

Fig. 1. Broad applications of the state-and transition concepts. Derived from the Society forRange Management, Task Group on Unity in Concepts and Terminology (1995). Theplane labeled SCT (site conservation threshold) represents a change from 1 ecological siteto another and may also be considered a threshold between 2 states. The individual boxesor ovals represent plant communities or seral stages that exist within 1 site.

As defined, Friedel's thresholds mirrorWestoby et al.'s (1989) definition of per-sistent or irreversible transitions. However,the use of thresholds in current state-and-transition models has not been consistentnor clear on whether thresholds existbetween all states or only a subset of states.

Conceptual models, based on theseideas, have incorporated states and transi-tions but not always thresholds. As aresult, there have been both a broad inter-pretation of states, more or less separatedby thresholds, and a narrow interpretationof states that approximate seral stages orphases of vegetation development.Broadly applied, states are climate/soiU-vegetation domains that encompass a largeamount of variation in species composi-tion. Specifically a grassland state wouldinclude many seral stages of the overallgrassland community. These seral stagesare within the amplitude of natural vari-ability characteristic of the state and repre-sent responses to disturbances that do notforce a threshold breach. Westoby et al.(1989), Archer (1989), and Archer andSmeins (1991) provided examples of thisbroad definition of state where dominationof successional processes determine theboundary of the state (e.g. grass controlledsuccession versus shrub controlled succes-sion). The Society for Range Manage-ment, Task Group on Unity in Concepts

change the system does not stabilize untilthe transition is complete.

Quantitative fipproaches to ecologicalthresholds have been presented by May(1977), Wissel (1984) and Rietkerk andvan de Koppel (1997). Archer (1989)introduced the qualitative concept of atransitional threshold. He modeled theexpansion of a woodland community intoa grassland domain using a transitionalthreshold as the boundary between therespective grassland and shrub domains.Whisenant (1999) proposed a model ofdegradation based on the stepwise degra-dation concept of Milton et al. (1994).Whisenants model is similar to Archer's,which incorporates 2 transition thresholds,the first being controlled by biotic interac-tions and the second by abiotic limitations.The concept of a transitional threshold asused by both Archer and Whisenant is sim-ilar to the persistent transition as the suc-cessional processes shift from grass con-trolled to shrub controlled, however, inWhisenant's (1999) model the focus is onecological processes not vegetative groups.Friedel (1991) focused on the concept ofthresholds of environmental changebetween domains of relative stability. Shedefined a threshold as a boundary in spaceand time between 2 domains or states,which is not reversible on a practical timescale without substantial inputs of energy.

Background

Westoby et al. (1989) was the first toapply the use of state-and-transition termi-nology to non-equilibrium theory for thepurpose of producing a managementfocused model that describes vegetationdynamics in a non-linear framework as analternative to the linear continuum processincorporated in the quantitative climaxmodel. The authors defined a "state" as analternative, persistent vegetation commu-nity that is not simply reversible in the lin-ear successional framework. We interpretW estoby' s transitions as trajectoriesbetween states with the characteristic ofthe transition being either transient or per-sisting. Transitions between states areoften triggered by multiple disturbancesincluding natural events (e.g., climaticevents or fire) and/or management actions(grazing, farming, burning, etc.).Transitions may occur quickly, as in thecase of catastrophic events like fire orflood or slowly over an extended period oftime as in the case of a gradual shift inweather patterns or repeated stresses likefrequent fire. Regardless of the rate of

107JOURNAL OF RANGE MANAGEMENT 56(2) March 2003

+ ,

~

,Threshold

and Terminology (1995) developed agraphical depiction of the broad applica-tion of states with multiple vegetativestages diagrammed within one state (Fig.1). Milton et al. (1994) and Whisenant(1999) de-emphasized the species compo-nent of the ecosystem within their models,focusing instead on the functional integrityand self-repair thresholds of the site fordetermining state boundaries. In the broaddefinition of state the natural variabilitycharacteristic of plant communities withina site is the result of, and contributes to,the current functional integrity of the site's

primary ecological processes (hydrology,nutrient cycling, and energy capture).

The narrower interpretation of stateallows for far less variation in plant com-munity composition. States are typicallydepicted as seral stages or phases of vege-tation development. In the narrow applica-tion of. the model a state change does notnecessarily represent a movement across athreshold as envisioned by Friedel (1991).Figure 2 represents the narrow interpreta-tion of states as adapted from West (1999).Boxes represent states and arrows indicatethe transitions between states. Note thatmany of the transitions are reversible,however, the threshold indicates a persis-tent transition. Other examples of specificor narrow applications of states are pre-sented by Weixelman et al. (1997), Olivaet al. (1998), Allen-Diaz and Bartolome(1998), West (1999), and West and Young(2000). The specific approach to state-and-transition modeling may be the reason forstatements that such models are structural-ly similar to traditional linear climax-seralstage models. The significant differencebeing the description of communities asdiscrete entities as opposed to the continu-um concept of the quantitative climaxmodel (Iglesias and Kothmann 1997).

-

~

Fig. 2. Specific, or narrow, application of states with each state (box) representing 1 phaseor seral stage of vegetation development. Transitions between states are indicated byarrows and the dashed line represents a threshold. The dashed transitional line signifiesthe requirement of substantial energy input to move the state back across the threshold.Modified from West (1999) and West and Young (2000).

recover after it has been disturbed. thus,fully functioning ecosystems are bothresistant to change and resilient or able torecover without external energy inputsthereby maintaining stability while allow-ing for fluctuating combinations of plantspecies over time. States, by definition arerelatively stable (Westoby et al. 1989),therefore it follows that a state change isonly possible when a threshold is crossed.Accepting this concept points out the con-fusion that is apparent in the currentattempts to produce state-and-transitionmodels. The specific or narrow approachhas produced models, which depict statechanges occurring without having crosseda threshold. Often such changes are dia-grammed as reversible and perhaps occurwithout the input of managementresources (Fig. 2). Rather than considerthese vegetation dynamics as state changesit is more appropriate to consider them asphase shifts or plant community dynamicswithin a state. Therefore, within a statethere exists the potential for a large varia-tion in species composition, which ismerely a reflection of plant community

dynamics. A state change, on the otherhand, requires a shift across a boundary orthreshold, defined by a change in theintegrity of the site's primary ecologicalprocesses, resulting in a different potentialset of plant communities.

Rangeland Ecological Processes

Ecological processes functioning withina normal range of variation will support asuite of specific plant communities. Theimportant primary processes are (1)hydrology (the capture, storage, and redis-tribution of precipitation); (2) energy cap-ture (conversion of sunlight to plant andanimal matter); and (3) nutrient cycling(the cycle of nutrients through the physicaland biotic components of the environment(Pellant et al. 2000, Whisenant 1999).Pellant et al. (2000) defines the function-ing of an ecosystem by "the degree towhich the integrity of the soil, vegetation,water, and air, as well as the ecologicalprocesses of the rangeland ecosystem, are

Ecological Resistance andResilience

The concept of stability as defined bythe resistance and resilience of plant com-munities haye been discussed in the litera-ture for sometime and offer importantinsights for state-and-transition models(Margalef 1969, Verhoff and Smith 1971,Holling 1973, May 1977, Noy-Meir andWalker 1986). Resistance is defined as theability of the system to remain the samewhile external conditions change whereasresilience is the ability of the system to

1C1R JOURNAL OF RANGE MANAGEMENT 5612\ March 200:'\

balanced and sustained". Integrity isdefined as the "maintenance of the func-tional attributes characteristic of a locale,including normal variability" (Pellant etal. 2000). Degradation of an ecosystemoccurs when the integrity of the system isdamaged or lost. Maintenance of a func-tional site or repair of a damaged siterequires management focused on soil sta-bility, nutrient cycling, and the capture,storage and safe release of precipitation.Vegetation goals should be based on theconcept of vegetation as a tool for main-taining or repairing damaged ecologicalprocesses rather than predefined speciesgroups. Monitoring of species groups maybe a mechanism for evaluating or detectingchange in the site's ecological processes.

Temporal ScaleThe definition of threshold as presented

by Friedel (1991) indicates that once athreshold has been breached return to theprevious state is precluded within a timeframe relevant to management, withoutsubstantial inputs of energy. Ecologicalmanagement models should focus on thetime required to repair damaged ecologicalprocesses not on a time scale predicatedby management. Careful consideration ofthe threshold concept negates the need forincluding management timescales in thedefinition of ecological thresholds as thesethresholds represent a permanent changein the function of the state. Thus, restatingthe threshold definition, independent ofmanagement timescales, results in the con-clusion that once a threshold has been vio-lated return to the prior state is precludedwithout substantial inputs of energy.Therefore, under the current climatic con-ditions and without substantial inputs ofenergy, state changes are permanent. Thetemporal scale is defined by the perma-nence of the current climate regime.

. Vegetation Structure: a component

resulting from above ground communi-ties of living organisms, whose vitalattributes (Noble and Slatyer 1980)competitively capture and utilize thesystem's available energy, water, nutri-ents, and space.The interaction between the structural

attributes of soil and the vegetative commu-nities, through the processes of energy cap-ture, hydrology and nutrient cycling definesthe resilience and resistance of the state.

Resilience and ResistanceThe stability of a state is defined above

in terms of resilience and resistance.Resilience and resistance are inherentproperties of an ecosystem that are deter-mined by the physical components of thesystem and the functional capacity of theassociated ecological processes. Resiliencefocuses on how far a system can be dis-placed from equilibrium before return toequilibrium is precluded. The emphasis isplaced on the persistence of relationshipsas they affect the systems ability to adaptto change (Walker et al. 1981), therefore,resilience relates to the functioning of thesystem's ecological processes. Resistanceindicates the ability of a system to remainat or near its equilibrium condition bymaintaining control of its ecologicalprocesses. Thus, the strength of this con-trol determines a system's inherent resis-tance to change. Consequently, under anexisting climate, stability of a state is afunction of the combination of its inherentresilience and resistance.

Clarification of the Conceptsand Definitions

Spatial ScaleEcosystems are difficult to define or

delimit in space and time. Hierarchy theo-ry, as applied to ecological systems, sug-gests several levels of organization exist,i.e., organisms, populations, communities,ecosystems, landscapes (Archer andSmeins 1991). Each level of organizationencompasses one or more of the primaryecological processes that are operating atspecific spatial and temporal scales.Although landscape scale managementmay be the goal, our current understandingof organization function declines withincreasing spatial and temporal scale.

The ecological site concept has longbeen utilized as an organization level thatprovides an appropriate spatial scale forinventory, evaluation, and management ofrangelands (USDA 1997). Organisms,populations, and communities exist withinthis spatial scale and interact with oneanother through the flow of water andenergy, and the cycling of nutrients. Anecological site has evolved a kind of char-acteristic plant community such as coolseason shrub-grass or warm season grass-land. Within an ecological site numerousexpressions of the various developmentalstages of the characteristic plant communi-ty can occur. The concept and definitionof an ecological site fits the large-scaleinterpretation of the state-and-transitionmodel. We define the ecological site as theminimum scale for definition of a state.

StateA state is a recognizable, resistant and

resilient complex of 2 components, thesoil base and the vegetation structure. Thevegetation and soil components are neces-sarily connected through int~grated eco-logical processes that interact to produce asustained equilibrium that is expressed bya specific suite of vegetative communities.

Soil Base and Vegetation StructureThe base of any rangeland ecosystem is

the soil resource that has developedthrough time from a specific parent materi-al, climate, landscape position, and interac-tion with soil and terrestrial biota. Thesefactors are the primary determinants of theecological site's capability. The integrityof the soil resource, as reflected by sitehydrology and nutrient cycling, is directlyconnected to the composition and energycapture process of the above-ground vege-tative component. The interaction betweenthe soil resource and the associated vegeta-tive community determines the functionalstatus of the state's ecological processes.. Soil Base: a component that results from

the interaction of climate, abiotic soilcharacteristics, soil biota and topogra-phy that determines the hydrologic char-acteristic and biotic potential of the sys-tem.

Thresholds and TransitionsThresholds are points in space and time

at which one or more of the primary eco-logical processes responsible for maintain-ing the sustained equilibrium of the statedegrades beyond the point of self-repair.These processes must be actively restoredbefore the return to the previous state ispossible. In the absence of active restora-tion a new state, which supports a differ-ent suite of plant communities and a newthreshold, is formed. Thresholds: boundary in space and time

between any and all states, or along irre-versible transitions, such that one ormore of the primary ecological process-es has been irreversibly changed andmust be actively restored before returnto a previous state is possible.Transitions are trajectories of change

that are precipitated by natural events

JOURNAL OF RANGE MANAGEMENT 56(2) March 2003 109

the narrow application of the non-equilib-rium approach to states and transitions(Fig. 5). States are diagrammed as thelarge boxes and are bordered by thresh-olds. Thresholds are the boundaries of anyand all states, but may also occur duringthe transition between states. For a statechange to occur a threshold must bebreached. The small boxes within the stateare referred to as plant community phasesor seral stages and are joined by communi-ty pathways that flow in both directions.Transitions are reserved for a trajectory ofchange with the dashed line inside thestate indicating the portion of the transi-tion that is reversible with minimal inputfrom management. Figure 4 illustrates theprocess of a state change. Once the thresh-old is crossed the state has lost control ofits primary ecological processes, is nolonger able to self-repair and will transi-tion to a new equilibrium with a differentecological capability. The entire trajectoryfrom a vegetation phase in State I, acrossthe threshold to the formation of State 2 isconsidered a transition and represents adegradation of ecological capability. Theportion of the transition contained withinthe boundary of State 1 is reversible withremoval of the stressor(s), however, oncethe trajectory crosses the threshold it is notreversible without active restorationincluding substantial energy input.Additional thresholds may occur while thesystem is in transition, changing the direc-tion of the trajectory away from State 2towards State 3 (Fig. 4). State-and-transi-tion modeling efforts indicate the firstthreshold is forced by a change in the biot-ic component of the system whereas addi-tional thresholds would involve changes inthe soil resource (Westoby et al. 1989,

Milton et al. 1994, and Whisenant 1999).Plant community phase changes within

states, in addition to transitions of change,thresholds and multiple stable states areillustrated in Figure 5. The managementand natural mechanisms responsible forcommunity phase shifts and transition ini-tiation must be defined in terms of ecolog-ical processes and included in the modeldescription. For example, prolongeddrought or overgrazing leads to a reduc-tion in the perennial herbaceous understo-ry. The decrease in perennial understoryleads to a decrease in total energy captureand nutrient cycling. In addition, the plantcommunity's ability to protect the soilfrom raindrop impact and potential soilerosion declines. The mechanism (ormechanisms) of disturbance have led to achange in the 3 primary ecologicalprocesses and a phase shift as diagrammedby community phase pathway PI (Fig. 5).In the case of prolonged drought return tothe late seral sagebrush steppe phasewould gradually occur with a return to anormal or above normal precipitation peri-od (P2). Increased available moistureleads to an increase in biomass of theherbaceous understory that translates intoan increase in energy capture, nutrientcycling and an improvement in soil pro-tection and site hydrology. The degrada-tion mechanism of overgrazing wouldneed to be addressed through grazingmanagement with the goal of improvingthe function level of the primary ecologi-cal processes. Continued overgrazingwould further decrease the vigor of thenative herbaceous understory and furtherimpact the community's ability to main-tain control of the primary ecologicalprocesses. As the vigor of the native

and/or management actions which degradethe integrity of I or more of the states pri-mary ecological processes. Transitions areoften composed of 2 separate propertiesthat are defined by the state threshold. Thefirst property is reversibility and it occurswithin the state. The second property isirreversibility and it occurs once a thresh-old has been breached. Transitions arevectors of system change that will lead toa new state without removal of the stres-sor(s). The primary difference between thereversible and irreversible property of atransition is defined by the systems' abili-ty or inability to repair itself.. Transition: a trajectory of system change

away from the current stable state that istriggered by natural events, managementactions, or both.

- Reversible Property of the Transition:

trajectory of change that occurs within astate and indicates the system is movingtoward a threshold. Reversal requireselimination of the stress or stressesresponsible for triggering the transition.

- Irreversible Property of the Transition:

trajectory of change that occurs after athreshold has been breached. The sys-tem can no longer self-repair even withremoval of the stressor(s). The systemwill not come to rest until a new equilib-rium (i.e., new state) is established thatsupports a different suite of plant com-munities.

Model Structure

The conceptual model, illustrating theabove definitions, is represented inFigures 3 and 4. The model accommodatesboth the quantitative climax approach and

Objects within a State

a c~ Reversible transition b vThreshold of the state

Plant community phases orseral stages within a state ~

"4-

Community pathways..

Fig. 3. Conceptual model depicting the objects of 1 state. Note the linear response, retrogression-succession model may be modeled within thestate (i.e., a to b to c and vice-versa).

110 JOURNAL OF RAN~F MANA~FMFNT ~~{?\ M"'r"h ?nn~

Threshold.,..",.",.

~

;/State 1

State 2Threshold

,.,

Reversible transition

Community Pathway~

,../,/

Irreversible transition

State 3Community Phases orsera. stages within a state

t

Fig. 4. Conceptual state-transition model incorporating the concepts of community pathways between plant community phases within states,reversible transitions, multiple thresholds, irreversible transitions, multiple pathways of change, and multiple steady states.

herbaceous community declines, the site isopened up for invasion by annual species.The transition from State 1 towards State 2has begun and will continue without theremoval of the stress from improper graz-ing (Tla). At the point in time whereannuals dominate the herbaceous under-story and fire frequency intensifies, thestate has crossed a threshold and is transi-tioning to a new state (Tlb). During thistransition phase the plant community maystill retain a minor component of sage-brush; however, this is not representativeof a stable state and with increased firefrequency the brush will be eliminated andthe new equilibrium state formed. Thenew state is defined as a Bromus tectorum(cheatgrass) and/or Taeniatherum asperum(medusahead) dominated community witha fire frequency interval of 2 to 3 years.Energy capture has declined and the timeperiod for energy capture has beenreduced. Nutrient cycling in both the verti-cal and horizontal plane has decreasedwith the shift to a shallow rooted, primari-ly monoculture community. The hydrolo-gy of the site will be impacted through a

Conclusions

Definitions and model concepts as dis-cussed in this paper are being adopted bythe USDA Natural Resources Conserva-tion Service as the standard for describingvegetation dynamics in rangeland ecologi-cal site descriptions. State-and-transitionmodels hold great potential to aid in

understanding rangeland ecosystems'response to natural and/or management-induced disturbances by providing aframework for organizing understandingof potential ecosystem dynamics. Manystate-and-transition model applications areavailable in the literature, although thescale of interpretation of the concepts hasvaried. We have attempted to review andclarify a large amount of information intoa proposed conceptual model of state/tran-sition/threshold relationships that aredetermined by the resilience and resistanceof the systems' primary ecologicalprocesses. Most of the components pre-sented are not new; however, the proposedmodel attempts to clarify the definitions

reduction in the amount of organic materi-al being added to the soil and an increasein the potential for damage to soil surfacestructure from raindrop impact. Return toState 1 would be impossible without theuse of intensive management inputs. Thepracticality of this level of managementwould preclude its use. State 3 may be thepractical state of choice.

Although many scientists have recog-nized the short-comings of the quantitativeclimax model developed by Dyksterhuis(1949) there are ecosystems, generally ofmore mesic climates, where the linearmodel is appropriate. It is important to real-ize that any modeling approach is a best-fitsolution, not a perfect-fit solution.Therefore, the retrogression-successioncontinuum can be modeled within the statesto depict the situation where plant commu-nity phases do respond linearly. However,it is also possible for linear response mech-anisms to be pushed past an ecologicalthreshold, resulting in a state change.

JOURNAL OF RANGE MANAGEMENT 56(2) March 2003 111

Fig. 5. Modification of the West (1999) and West and Young (2000) specific sagebrush steppemodel (see Fig. 2) to illustrate the broad concept of state with plant community phases andcommunity pathways (i.e., PI and P2) within states. Tla and Tlb signify the reversible andirreversible properties of the transition between State 1 and State 2. For additional discus-sion of the mechanisms leading to community phase shifts see West (1999) and West andYoung (2000).

that the ecological site is the minimumscale associated with a state, understand-ing ecological processes at the landscapescale should be the target. This model con-tains the flexibility to accommodate land-scape level dynamics; however, furtherresearch is needed to clarify the ecologicalrelationships occurring at that scale. Thiseffort is not viewed as completed, butrather as another step in the process to fur-ther develop understanding of rangelandecosvstems.

and concepts and to link them togetherinto a process-based model for manage-ment and research. The management andnatural mechanisms responsible for com-munity phase shifts and transition initia-tion must be included in the modeldescription. The description of thesemechanisms should contain informationon their impact on the primary ecologicalprocesses and the resulting change in thebiotic community and system function.Further research is needed to identify indi-cators of change for ecological processesthat will allow manag~ment to interveneprior to a threshold change. Once a thresh-old has been crossed, the focus of manage-ment should be on restoration of the dam-aged ecological processes, not on reestab-lishing a specific plant community.Although this conceptual model suggests

Literature Cited

Anderson, J.E. 1986. Development and struc-ture of sagebrush steppe plant communities.In: P.J. Joss, P.W. Lynch, and O.B. Williams(eds.) Rangelands: A resource under siege -Proc. of 2nd Internat. Rangeland Congress.Aust. Acad. of Sci., Canberra, Aust.

Archer, S. 1989. Have southern Texas savan-nas been converted to woodlands in recenthistory? The Amer. Natur. 134:545-561.

Archer, S. and F .E. Smeins. 1991.Ecosystem-level processes, p. 109-139. In:K. Rodney and Jeffery W. Stuth (eds.)Grazing Management an EcologicalPerspective. Timber Press, Portland Ore.

Davenport, D. W., D.O. Breshears, B.P.Wilcox, and C.D. Allen. 1998. Viewpoint:Sustainability of pinon-juniper ecosystems -a unifying perspective of soil erosion thresh-olds. J. Range Manage. 51:231-240.

Dyksterhuis, E.J. 1949. Condition and man-agement of rangeland based on quantitativeecology. J. Range Manage. 2:104-115.

Foran, B.D., G. Bastin, and K.A. Shaw. 1986.Range assessment and monitoring in aridlands: the use of classification and ordinationin range survey. J. Environ. Manage. 22:67-84.

Friedel, M.H. 1991. Range condition assess-ment and the concept of thresholds: A view-point. J. Range Manage. 44(5):422-426.

Fuhlendorf, S. D., F. E. Smeins, and W. E.Grant. 1996. Simulation of a fire-sensitiveecological threshold: a case study of Ashejuniper on the Edwards Plateau of Texas,USA. Ecol. Modelling 90:245-255.

Holling, C. S. 1973. Resilience and stability ofecological systems. Ann. Review EcolSystematics 4:1-23.

Iglesias, R.M.R. and M.K. Kothmann. 1997.Structure and causes of vegetation change instate and transition model applications. J.Range Manage. 50(4):399-408.

Laycock, W.A. 1991. Stable States and thresh-olds of range condition on North Americanrangelands: A viewpoint. J. Range Manage.44(5):427-433.

Margalef, R. 1969. On certain unifying princi-ples in ecology. Amer. Natur. 97:357-374.

May, R. M. 1977. Thresholds and breakpointsin ecosystems with a multiplicity of stablestates. Nature 269:471-477.

Milton, S. J., W. R. J. Dean, M. A. duPlessis,and W. R. Siegfried. 1994. A conceptualmodel of arid rangeland degradation. Biosci.44:70-76.

Noble, I. R. and R. O. Slatyer. 1980. The useof vital attributes to predict successionalchanges in plant communities subject torecurrent disturbances. Vegetatio 43:5-21.

Noy-Meir, I. and B.H. Walker. 1986.Stability and resilience in rangelands. p.21-25. In: P.J. Joss, P.W. Lynch, and O.B.Williams. (eds.) Rangelands: a resourceunder siege - Proc. of the 2nd Internat.Rangeland Congress. Australian Acad. ofSci.. Canberra, Aust.

Oliva, G., A. Cibils, P. Borrelli, and G.Humano. 1998. Stable states in relation tograzing in Patagonia: a 10-year experimentaltrial. J. Arid Environ. 40:113-131.

Allen-Diaz B. and J.W. Bartolome. 1998.Sagebrush-grass vegetation dynamics: com-paring classical and state-transition models.Ecological Applications 8(3):795-804.

JOURNAL OF RANGE MANAGEMENT 56(2) March 2003

Stringham, T.K., W.C. Krueger, and P.L.Shaver. 2001. States, transitions, and thresh-olds: Further refinement for rangeland appli-cations. Agr. Exp. Station Special Rep. 1024.Oregon State Univ., Corvallis, Ore.

Tausch, R.J., P.E. Wigand, and J.W.Burkhardt. 1993. Viewpoint: plant commu-nity thresholds, multiple steady states, andmultiple successional pathways: legacy ofthe Quaternary? J. Range Manage.46:439-47.

USDA, Natural Resources ConservationService. 1997. National Range and PastureHandb. : USDA. Washington D.C.

Verhoff, F. H. and F. E. Smith. 1971.Theoretical analysis of a conserved nutrientecosystem. J. Theor. Bioi. 33:131-147.

Walker, B.H., D. Ludwig, C.S. Holling, andR.M. Peterman. 1981. Stability of semi-aridsavannah grazing systems. Ecol. 69:473-498.

Weixelman, D.A., D.C. Zamudio, K.A.Zamudio, and R.J. Tausch. 1997.Classifying ecological types and evaluatingsite degradation. J. Range Manage. 50(3):315-321.

West, N. E., 1979. Basic synecological rela-tionships of sagebrush-dominated lands inthe Great Basin and the Colorado Plateau. In:Anon, The sagebrush ecosystem: A sympo-sium. Utah State University, College ofNatural Resources, Logan, Utah.

West, N. E. 1999. Managing for biodiversity ofrangelands. p. 101-119. In: W. Collins, C.and Qualset (eds.) Biodiversity inAgroecosys-tems. CRC Press Washington,D.C.

West, N. E. and J. A. Young. 2000. Inter-mountain valleys and lower mountain slopesp. 256-284 In: Barbour, M. G. and W. D.Billings (eds.) North American TerrestrialVegetation, Second Edition. CambridgeUniv. Press, New York, N.Y.

Westoby, M. 1980. Elements of a theory ofvegetation dynamics in rangelands. Israel J.Bot. 28:169-194.

Westoby, M., B. Walker, and I. Noy-Meir.1989. Opportunistic management for range-lands not at equilibrium. J. Range Manage.42(4):266-274.

Whisenant, S. G. 1999. Repairing damagedwildlands: A process-oriented, landscape-scale approach. Cambridge Univ. Press. 312pp.

Wilson, A.D. 1984. What is range condition:development of concepts in Australia. Bull.Ecol. Soc. Amer. 65:171.

Wissel, C. 1984. A universal law of the charac-teristic return time near thresholds.Oecologia 65:101-107.

Pellant, M. P. Shaver, D. A. Pyke, and J. E.Herrick. 2000. Interpreting indicators ofrangeland health, ver. 3, Tech. Reference1734-6. USDI, BLM, Nat. Sci. and Tech.Center, Denver, Co10

Petraitis, P .S. and R. E. Latham. 1999. Theimportance of scale in testing the origins ofalternative community states. Eco1. 80(2):429-442.

Plant R.E., M.P. Vayssieres, S.E. Greco,M.R. George, and T .E. Adams. 1999. Aqualitative spacial model of hardwood range-land state-and-transition dynamics. J. RangeManage. 52:51-59.

Rietkerk, M. and J. van de Koppel. 1997.Alternate stable states and threshold effectsin semi-arid grazing systems. Oikos79:69-76.

Smith, E.L. 1988. Successional concepts inrelation to range condition assessment. p.113-133. In: P.T. Tue11er (ed.) Vegetationscience applications for rangeland analysisand management. K1uwer AcademicPublishers, Boston, Mass.

Society for Range Management, Task Groupon Unity in Concepts and Terminology.1995. New concepts for assessment of range-land condition. J. Range Manage.48:271-282.

Stringham, T .K. 1996. Application of non-equilibrium ecology to managed riparianecosystems. Ph.D. Diss.. Dept. of RangelandResour., Oregon State Univ., Corvallis, Ore.pp.156.

113JOURNAL OF RANGE MANAGEMENT 5612) March 2003