Threshold Concepts and Their Use in Rangeland Management and Restoration: The Good, the Bad, and the Insidious

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1 Threshol Concepts an Their Use in Rangelan Management an Restoration: The Goo, the Ba, an the Insiious Branon T. Bestelmeyer 1,2 Abstract Ecological threshols escribe abrupt changes in ecological properties in time or space. In rangelan management, threshols reflect changes in vegetation an soils that are expensive or impossible to reverse. The threshol concept has catalyze important avances in rangelan management thinking, but it has also introuce two classes of rawbacks. First, the ambiguity of the term threshol an the esire for simplicity in its application has le to an overemphasis on classification threshols, such as vegetation cover values. Uncritical use of classification threshols may lea to the abanonment of management efforts in lan areas that woul otherwise benefit from intervention. Secon, it is possible that the invocation of threshols an irreversible egraation may eventually result in the wholesale conversion of lan areas that woul have been recoverable or serve important societal functions, such as bioiversity maintenance, that are not reflecte in threshol efinitions. I conclue with a recommenation to clarify the nature of threshols by efining the relationships among pattern, process, an egraation an istinguishing preventive threshols from restoration threshols. We must also broaen the attributes use to efine states an threshols. Key wors: bioiversity, exurban evelopment, grazing, lanscape ecology, rangelan health, state-an-transition moels. 1 USDA-ARS Jornaa Experimental Range an Jornaa Basin LTER, MSC 2 Aress corresponence to B. T. Bestelmeyer, bbestelm@nmsu.eu 3 JER Box 30003, New Mexico State University, Las Cruces, NM 88003, U.S.A. Journal compilation Ó 2006 Society for Ecological Restoration International No claim to original US government works Introuction Nature is full of threshols layere upon threshols. Wiens et al. (2002). The threshol concept has become a major theme in ecology an natural resources management (Groffman et al. 2006). Ecological threshols are use to escribe the nonlinear an persistent reorganization of ecosystem properties (i.e., states) in response to graual or iscrete changes in environmental patterns an rivers. Crossing threshols leas to loss or recovery of ecosystem functions an bioiversity. The significance of threshols for management has mae them a key emphasis in restoration ecology (Hobbs & Harris 2001), lanscape ecology (Turner 2005), an rangelan ecology (Walker 1993). The concept has been use to iscuss natural resource issues within the U.S. Senate (Watson 2003). In the Unite States, ieas about threshols are beginning to influence public lan management policies (e.g., USDI Bureau of Lan Management 2004) an to etermine feeral assistance provie to private lanowners (USDA Natural Resources Conservation Service 2003). In this essay, I relay my satisfaction an concerns with how the threshol concept is being (an coul be) use in rangelan management. Threshols have been incorporate into rangelan management via potential-base lan classification systems an associate state-an-transition moels (STMs; e.g., Brown 1994; Stringham et al. 2003). STMs (Westoby et al. 1989) synthesize informal knowlege an publishe ata to escribe alternative states an the nature of threshols between each state. For example, a transition from savanna to a shrub-encroache woolan state is precipitate by graual or episoic loss of grass ue to continuous grazing an rought, resulting in a loss of fuel connectivity an lack of fire isturbance. Without fire, grasses lose the avantage of fire tolerance an shrubs recruit an survive to aulthoo. The shrubs are increasingly able to monopolize resources formerly use by grasses. The shift in feebacks from one governe by fire isturbance to one governe by grazing an shrub grass competition is epicte as a biotic threshol. Moving back across the threshol woul require reuce grazing intensity an shrub removal. Dominance by shrubs, however, maintains bare areas, allowing accelerate erosion rates an surface soil egraation. With time, soils become egrae (or are lost) to the point that shrub removal an restoration actions such as grass seeing cannot be use to recover the original grasslan state. This point marks a secon (abiotic) threshol (Whisenant 1999). The increasing aoption of conceptual moels for rangelans that inclue or emphasize threshols has far-reaching implications for rangelan assessment an management SEPTEMBER 2006 Restoration Ecology Vol. 14, No. 3, pp

2 Threshol Concepts an Their Use policies in the Unite States. Because threshol-base moels are becoming increasingly integrate with proceures use to evaluate vast areas of public an private lans (e.g., Spaeth et al. 2003), we nee to evaluate critically the roles that threshol concepts play. Below, I consier positive, negative, an insiious consequences of the application of threshol ieas. I conclue with some recommenations for promoting the positive contributions of threshol concepts while minimizing the amage that they might cause. The Goo Recognition of states an threshols has been tremenously useful for lan evaluation an management. First, in regions where threshols are iscusse, managers often consier ecisions with threshols in min. Processes associate with threshols compel managers to consier a broaer array of ecosystem behaviors an attributes when evaluating the status of rangelans (e.g., Pyke et al. 2002; Tongway & Hinley 2004). Thousans of managers are now better equippe to anticipate an unerstan the changes they observe an the options that are available to them. Secon, states an threshols can be use to prioritize management an restoration efforts in management areas comprising tens of thousans of hectares (Suing et al. 2004). Lan areas that have crosse abiotic threshols that are unlikely to respon to restoration actions are consiere low priority. Similarly, areas that o not inicate egraation towar biotic threshols are low priority. Monitoring an restoration resources are then increasingly available to focus on the intermeiate states where relatively low-cost grazing management an restoration actions are most likely to be effective. Thus, consieration of threshols as a sorely neee triage element to public an private lans management (Hobbs & Kristjanson 2003). Finally, stakeholers are increasingly aware that threshol behavior is possible. Threshol concepts an STMs are increasingly important elements of scenario planning (cf Bennett et al. 2003). The evelopment of increasingly realistic scenarios involving government agencies, ranchers, an environmentalists can reuce unnecessary conflict (R. A. Alexaner, Bureau of Lan Management, 2004, personal communication). Threshols move the arguments from historically egrae an irrecoverable rangelans to areas where the to graze or not to graze controversy has meaning. The Ba Despite the benefits, successful application of the threshol concept has been limite by several problems. First, there is usually a lack of clarity in use of the term threshol, especially in terms of pattern process coupling. The often-use phrase crosse the threshol evokes a value of some variable beyon which ecosystem organization changes. This value is often represente in STMs using classification (or structural) threshols (Briske et al. 2005) base on cover, reflectance, or a multivariate characterization of plant composition. Within STMs being evelope by rangelan ecologists, the establishment of classification criteria for states is implicitly assume to reflect process (or functional) threshols that etermine the efforts require to reverse a transition. Mechanistic linkages between classification an process threshols, however, often are poorly evelope. Even when a pattern process relationship is escribe, it is likely to be inconsistent in time an space. For example, threshol behavior can be base on a small change in a pattern (vegetation cover) that results in a large change in a process rate (e.g., erosion; Davenport et al. 1998). Many spatially an temporally varying factors, however, conition the relationship between pattern an process (e.g., soil erosion potential an climate), so a single preictive threshol value seems unlikely to emerge even for a specific application (Muraian 2001; Huggett 2005). Furthermore, a process rate an the uration of time at a given rate rive the environmental changes unerlying physical (Groffman et al. 2006) or resource threshols (Aguiar & Sala 1999) that etermine the survival an establishment of particular species. Simple classification threshols currently use for the sake of management expeiency o not reflect the variable an hierarchical aspects of threshol phenomena an are inaequate inicators of possible future ecosystem behavior in many cases (Linenmayer & Luck 2005). The inaequate characterization of threshol phenomena leas to a secon problem: how lan parcels are manage base on lan evaluations relative to threshols. Classification threshols may overemphasize the consequences of threshols relative to the chronic vulnerability that permits a rapi transition, such as mismanage grazing an low grass cover (Staffor Smith 1996). Commonly use classification threshols in STMs (e.g., shrub ominance) are usually base not on the effects of vegetation pattern on process rates that coul be use to mitigate egraation but on the ultimate changes to structural attributes that are easily measure but recognize too late to prevent egraation. Moreover, once managers classify a lan area to a state, the classification asserts the existence of restoration barriers escribe in STMs. If the classification is flawe, then lan that might be recoverable towar a esire or healthy conition via simple ajustments (e.g., stocking strategies) might be mae a low priority because the costs of restoration are incorrectly assume to be too high (Bestelmeyer et al. 2003a; Briske et al. 2005). Once lan areas are juge to be past the threshol, the elaye management response or outright abanonment may permit continue egraation an a self-fulfilling prophecy of irreversible change. The Insiious The abanonment of active management in rangelans an their conemnation as irreversibly egrae via the threshol concept may have an insiious consequence for 326 Restoration Ecology SEPTEMBER 2006

3 Threshol Concepts an Their Use lan use an human welfare in the American West. We often fail to appreciate that the assertion of egraation base on comparisons with reference or assume pre- European vegetation types an associate agricultural uses (Bestelmeyer et al. 2003a) oes not account for other functions of the lan. Some components of bioiversity, for example, may be well represente in egrae vegetation types (James et al. 1999; Bestelmeyer et al. 2003b). The bioiversity in egrae lans ajacent to human communities, in turn, may be critical for maintaining human connections with nature that support quality of life an health (Miller 2005). Furthermore, ecosystems ajacent to urban or agricultural areas often have especially high value for bioiversity conservation (Scott et al. 2001) an woul be priorities for restoration from this perspective. Degrae public rangelans, however, may be at high risk for isposal to resiential or inustrial evelopment an private rangelans may be subivie an sol for the same purpose, especially near towns an cities (M. W. Brunson 2002, Utah State University, 2006, personal communication). With little hope for the restoration of agricultural uses, the relative value of the lan for evelopment shoul increase. Once evelope, former rangelans are unlikely to be reverte to other uses for long perios of time (Hansen et al. 2005). Development is clearly a persistent transition base on the linkage of socioeconomic an ecological processes (Walker & Meyers 2004). Figure 1. A classification of threshols an their relationships. Example measurements after the bullets coul be use to quantify position relative to a given threshol (if one exists). The arrows at the sies inicate whether pattern, process, or egraation threshol shoul be use to efine the two types of classification threshol. Overcoming the Ba an the Insiious There is abunant evience that some rangelan ecosystems exhibit threshols, but critics point out that some rangelans are more equilibrial an resilient than we give them creit for (Staffor Smith 1996; Valone et al. 2002). Proper interpretation of system behavior can be achieve by greater experimentation at appropriate scales, restrictions of inference to truly similar environmental omains, an longer-term observations of system behavior. A secon criticism is that we ten to focus on the consequences of threshol (catastrophic) shifts at the expense of the graual, eterministic processes that precee them (Watson et al. 1996). This problem can be remeie by istinguishing the types of threshols that ecologists have conceptualize an clarifying their roles in management. I suggest a classification of threshols an their linkages (Fig. 1). The classes link several existing threshol concepts an can be use to help conceptualize threshol research an management applications. The first class is the pattern threshol relate to percolation theory, connectivity, an self-organization concepts (e.g., Pascual & Guichar 2005). If isturbance rives a pattern such as grass cover, bare patch size, or fragmentation to a critical value, then the rate of a process, such as erosion or ispersal, may change nonlinearly. Feebacks between pattern values an process rates can create the nonlinearity. The pattern threshol leas to a process threshol that escribes the consequences to an environmental conition of an altere process rate, such as the effect of increasing erosivity or eroibility on changes to soil epth or quality (Okin et al. 2006). In animal ecology, reuce ispersal rates/ability can create nonlinear reuctions in habitat occupancy (With & King 1999). It is important to note that processes can also have linear relationships to environmental conitions. At a sufficient level of change in the environmental conition, a egraation threshol is reache whereby habitat becomes unsuitable to species that are use to recognize states, such as ominant plants, or species go regionally extinct. The relationships of egraation threshols to pattern an process threshols shoul, in the ieal worl, be use to efine classification threshols use to ientify states. In the real worl, we selom have ata on these relationships an so are force to make a best guess. There ought to be two kins of classification threshols base either on preventive management or restoration. Preventive management shoul focus on regulating changes to patterns that make systems vulnerable to eterministic or event-riven change. Bare patch size, connectivity, an relate inicators can be use in this way (Tischenorf 2001; Tongway & Hinley 2004; Herrick et al. 2005). Management failures result when egraation threshols are use as the primary inicators of a problem because it is too late to regulate the patterns preceing egraation. In contrast, restoration of egrae SEPTEMBER 2006 Restoration Ecology 327

4 Threshol Concepts an Their Use rangelans nees to aress egraation, process, an pattern threshols simultaneously. Barriers to the ominance of esire species must be overcome alongsie the stabilization of processes an recovery of patterns that preserve reestablishe habitat conitions (Whisenant 1999). Restoration failures result when egraation threshols are aresse but pattern an process threshols are not. Finally, we must broaen our consieration of relevant patterns, processes, an types of egraation. Applications of the threshol concept in rangelan ecology have been regare as parochial ue to an almost exclusive focus on plant composition an prouction. Bioiversity an sensitive species typically are ignore (for various reasons), an threshols in their responses to environment are unlikely to be reflecte in threshols base on prouction-relate attributes. This oversight has resulte in a schism between rangelan an willife/bioiversity managers working in the same lanscapes. The schism is unfortunate because many changes to animal habitat are ultimately governe by processes stuie by rangelan ecologists, espite ifferences in threshol efinitions. By linking threshol concepts relate to organismal behavior, emography, an iversity with those of a variety ecosystem functions, we will have a broaer perspective on lan management an restoration. Implications for Practice Ecological threshols an alternative states are capture in moels that are use to ecie when a management change or restoration practice is necessary. Consieration of threshols can help prioritize management an restoration efforts, but threshol concepts may also be inappropriately use to write off lan or abanon management where it is still neee. Threshol concepts woul be better use in rangelans if preventive versus restoration threshols were istinguishe. Rangelan managers also nee to consier a broaer range of attributes to efine states an threshols, especially those relate to bioiversity. 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