Citation for this excerpt: Miller, M.E., and L. Cudlip. 2002. Conceptual models. Chapter III in Evenden, A., M. Miller,

M. Beer, E. Nance, S. Daw, A. Wight, M. Estenson, and L. Cudlip. 2002. Northern Colorado Plateau Vital Signs Network and Prototype Cluster, Plan for Natural Resources Monitoring: Phase I Report, October 1, 2002. [Two volumes]. National Park Service, Northern Colorado Plateau Network, Moab, UT. 138 p. plus appendices.

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III. CONCEPTUAL MODELS

Introduction and Approach The body of this chapter is organized in four sections corresponding with the monitoring themes identified by the Northern Colorado Plateau Prototype Cluster – ecosystem structure and function, invasive exotic species, and species of concern (Figure 11, see Chapter IV). Water quality, currently identified as a network theme, is integrated with the prototype monitoring themes as a key component affecting the structure and function of aquatic, riparian and wetland ecosystems. In the first section of this chapter, an overall theoretical framework is outlined to guide the development of system- and issue-specific conceptual models, and to provide insights applicable to the prioritization and selection of vital signs. This framework consists of two components: 1) a simple, general model describing factors and processes controlling the structure, function, and sustainability of ecosystems, and 2) a set of corollary hypotheses concerning key aspects of ecosystem dynamics with particular implications for vital-signs monitoring.

Figure 11. Monitoring themes of the Northern Colorado Plateau Prototype Cluster.

In the second section, a set of detailed conceptual models is introduced to describe hypotheses concerning the mechanisms by which natural drivers and anthropogenic stressors affect change in NCPN ecosystems. In the third section of the chapter, these ecosystem-based conceptual models are supplemented with models focused specifically on invasive exotic species. The final section of the chapter presents models developed to describe pathways from stressors to effects on particular species or populations of concern. This chapter remains under development, and sections described above are in various stages of completion. Conceptual models will continue to be developed and revised during fall 2002 following peer review, consultation with subject-matter experts, and on-going literature review.

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Theoretical Ecosystem Framework and Key Concepts of Ecosystem Dynamics

Rationale Ecosystems, including the full suite of abiotic and biotic components and processes that they encompass, are fundamental resources of NCPN parks. A premise of the NCPN vital-signs monitoring program is that the many species and landscapes valued by NPS staff, visitors, and society at large cannot be conserved in the absence of an ecosystem focus. This perspective is based on practical as well as theoretical considerations. Walker (1995:748) noted that “Given our inadequate understanding and knowledge of how many and which kinds of species occur in an ecosystem, the best way to approach the problem of conserving them all is to ensure that the system continues to have the same overall structure and function”—a practical view shared by many conservation biologists (e.g., Noss 1990, Franklin 1993, Noon et al. 1999). Contemporary ecological theory further suggests that conservation should emphasize the maintenance of ecosystem processes because ecosystems and ecosystem components are inherently dynamic both in space and in time and thus cannot be conserved as static entities (Pickett et al. 1992, Christensen et al. 1996). The process-based perspective described for ecosystems is equally important to other levels of organization including populations, species, and landscapes. Ecosystems are connected with other ecosystems by flows of materials, energy, and organisms in spatially structured landscape mosaics (Turner et al. 2001). Thus landscape-level considerations are encompassed in the ecosystem approach of the NCPN. Ecosystems and landscapes can be represented conceptually in many different ways along continua of complexity and specificity. Conceptual models are “caricatures of nature” (Holling et al. 2002) designed to describe and communicate ideas about how nature works. For purposes of vital signs monitoring, it is useful to begin with a simple, general model that summarizes ideas about ecosystem sustainability. This general model and a set of corollary hypotheses provide a theoretical framework for aspects of the monitoring plan related to ecosystem structure and function.

Interactive Controls of Ecosystem Sustainability Jenny (1941, 1980) proposed that soil and ecosystem processes are determined by five state factors – climate, organisms, relief (topography), parent material, and time since disturbance. Jenny’s state-factor approach has been widely applied as a framework for examining temporal and spatial variations in ecosystem structure and function (e.g., Walker and Chapin 1987, Vitousek 1994, Seastedt 2001). Chapin and colleagues (1996) recently extended this framework to develop a set of ecological principles concerning ecosystem sustainability. They defined “...a sustainable ecosystem as one that, over the normal cycle of disturbance events, maintains its characteristic diversity of major functional groups, productivity, and rates of biogeochemical cycling” (Chapin et al. 1996:1016). These ecosystem characteristics are determined by a set of four “interactive controls”–climate, soil-resource supply, major functional groups1 of organisms, and disturbance regime–and these interactive controls both govern and respond to ecosystem

1 Functional groups are groups of species that have similar effects on ecosystem processes (Chapin et al. 1996). This concept is generally synonymous with functional types. Díaz and Cabido (2001) noted that two categories of functional types are equally important for ecosystem dynamics: (1) functional effect types – a set of organisms with similar effects on ecosystem functions such as productivity, nutrient cycling, flammability, and resistance / resilience, and (2) functional response types – organisms with similar responses to environmental factors such as temperature, resource availability, and disturbance.

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attributes (Figure 12). Interactive controls are constrained by the five state factors, which determine the “constraints of place” (Dale et al. 2000).

Figure 12. Relationship (a) between Jenny’s (1941) state factors and ecosystem processes, and (b) among state factors, interactive controls, and ecosystem processes. The circle represents the boundary of the ecosystem (from Chapin et al. 1996).

By substituting water quality and quantity for soil resources in the model, the interactive-control model can be applied to aquatic as well as terrestrial ecosystems (Chapin et al. 1996). This extends the utility of the model, and it suggests further clarifications. Soil, water, and air are the media from which primary producers acquire resources. As the abiotic matrix that supports the biota, they form the foundation of ecosystems. These media also are characterized by condition attributes (e.g., temperature, stability) that affect the physiological performance of organisms. Water and air qualities are accepted concepts with legislative standards. No legislative standards exist for the comparable concept of soil quality, and the concept itself was defined only recently. Karlen and colleagues (1997:6) defined soil quality as “the capacity of a specific kind of soil to function, within natural or managed ecosystem boundaries, to sustain plant and animal productivity, maintain or enhance water and air quality, and support human health and habitation.” Soil quality can be regarded as having 1) an inherent component defined by the soil’s inherent soil properties as determined by Jenny’s (1941) five factors of soil formation, and 2) a dynamic component defined by the change in soil function that is influenced by human management of the soil (Seybold et al. 1999). In terms of the interactive-control model, the concepts of water quality and soil quality will be used interchangeably with the more descriptive concepts of water resources and conditions and soil resources and conditions, respectively. With respect to climate as it is represented in the interactive-control model, the broader concept of atmospheric resources and conditions is more precise, encompassing climatic conditions such as temperature, resources such as precipitation and CO2, and stressors such as airborne pollutants. This is an important clarification in the context of global environmental changes. Figure 13 illustrates the modified version of the interactive-control model, the array of stressors affecting NCPN parks, and the first-order pathways linking stressors to NCPN ecosystems. Complex, higher order effects occur as the four major controls interact via ecosystem processes.

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Figure 13. Modified version (a) of the interactive-control model that serves as the general ecosystem model for the NCPN, and (b) the array of stressors affecting NCPN ecosystems arranged in the model in relation to their first-order effects.

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For vital signs monitoring, a key aspect of the interactive-control model is the associated hypothesis that interactive controls must be conserved for an ecosystem to be sustained. Large changes in any of the four interactive controls are predicted to result in a new ecosystem with different characteristics than the original system (Chapin et al. 1996). For example, major changes in soil resources (e.g., through erosion, salinization, fertilization, or other mechanisms) can greatly affect productivity, recruitment opportunities, and competitive relations of plants, and thus can result in major changes in the structure and function of plant communities and higher trophic levels. Changes in vegetation structure can affect the ecosystem’s disturbance regime (e.g., through altered fuel characteristics). These factors and processes in combination can result in a fundamentally different type of ecosystem. Under some circumstances, effects of land uses such as grazing even can affect regional atmospheric resources and conditions through alterations of vegetation and soil conditions that alter ecosystem-atmosphere exchanges of water and energy (e.g., Bryant et al. 1990, Eastman et al. 2001). Additions or losses of species with traits that have strong effects on ecosystem processes also can result in an ecosystem with fundamentally different characteristics – potentially affecting the persistence of previous ecosystem components. Species that affect soil-resource regimes, disturbance regimes, or functional-group structure are those most likely to have profound effects on ecosystem characteristics following their introduction or loss from a system (Vitousek 1990, Chapin et al. 1997). Examples with particular relevance to vital signs monitoring include invasive exotic species that alter ecosystem disturbance regimes (D’Antonio and Vitousek 1992, Mack and D’Antonio 1998) and/or ecosystem resource regimes (Vitousek et al. 1987, Simons and Seastedt 1999). In these cases, the first-order ecosystem change was the introduction of a new functional group. The simple model presented in Figure 13, including the associated hypothesis concerning ecosystem sustainability, is the general ecosystem model adopted by the NCPN for guiding the development of system-specific conceptual models and the consideration of vital signs. In Figure 14, elements of the ecosystem-structure-and-function theme are specified in relation to the general model.

Figure 14. Elements of the ecosystem-structure-and-function monitoring theme of the Northern Colorado Plateau Prototype Cluster.

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Ecological Stability, Thresholds, and Resilience The general ecosystem model describes factors and processes affecting ecosystem structure, function, and sustainability. The notion of ecosystem sustainability is associated with the broader concept of ecological stability. Grimm and Wissel (1997) found 163 definitions of 70 concepts related to ecological stability in the literature. They reduced this chaos to six stability properties:

1. Constancy – staying essentially unchanged. 2. Resilience – returning to the reference state (or dynamic) after a temporary disturbance. 3. Persistence – persistence through time of an ecological system. 4. Resistance – staying essentially unchanged despite the presence of disturbances. 5. Elasticity – speed of return to the reference state (or dynamic) after a temporary

disturbance. 6. Domain of attraction – the whole of states from which the reference state (or dynamic)

can be reached again after a temporary disturbance. Sustainable ecosystems, as defined by Chapin and colleagues (1996), are persistent. Inherent in the notions of ecosystem sustainability and persistence is the hypothesis that ecosystems can be caused to cross thresholds and switch from one state (or dynamic) to an alternative state (or dynamic). Of greatest concern from a conservation perspective are alternative states characterized by irreversibly degraded ecosystem structure and function. Ecosystems that have been driven across thresholds of degradation cannot be restored to previous conditions simply by removing the stressor. Costly, manipulative restoration efforts are required (Hobbs and Norton 1996, Whisenant 1999). The success of such restoration efforts usually is uncertain. Arid-land ecosystems and aquatic ecosystems are the most-frequently-cited examples of systems characterized by multiple alternative states (Rapport and Whitford 1999). Questions concerning ecological thresholds and multiple ecosystem states are at the forefront of theoretical ecology (e.g., Gunderson and Holling 2002), but threshold issues are not only theoretical. Because of the conceptual link between thresholds and ecosystem sustainability, threshold issues are fundamental to many current questions in applied ecology (e.g., Davenport et al. 1998, Archer and Stokes 2000), including ecosystem management, assessment, and monitoring (Dale et al. 1998, Paine et al. 1998, Andreasen et al. 2001, Herrick et al. 2002, Whitford 2002). The ball-and-cup heuristic provides a useful means of describing concepts of ecosystem stability (Figure 15). In this scheme, the ball represents the ecosystem and the cup represents the domain (or basin) of attraction of the ecosystem. The basin of attraction is analogous to the natural range of variation in the ecosystem. The likelihood of a stochastic perturbation driving the ecosystem across a threshold into a basin of attraction of another state depends on characteristics of the perturbation as well as on the shape of the basin (Scheffer et al. 2001). Resilience in Figure 15 refers to the size and shape of the basin of attraction, and these correspond to the maximum perturbation that can be absorbed by the ecosystem without resulting in a shift to an alternative state. This definition of resilience follows that of Holling (1973) and encompasses both the properties of resilience and resistance as described above by Grimm and Wissel (1997). The ball-and-cup model presented in Figure 15 can also be referred to as a stability landscape (Scheffer et al. 2001).

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Figure 15. Integration (a) of the general ecosystem model with the ball-and-cup heuristic. In (b), resilience is illustrated as the magnitude of perturbation that the system can absorb without crossing a threshold from the original dynamic state to a new dynamic state. (Derived from concepts of Scheffer et al. 2001).

Stability landscapes describing the resilience of a particular ecosystem to a particular stochastic perturbation are not static but can be altered as a consequence of gradual changes in environmental conditions that affect ecosystem attributes (Scheffer et al. 2001). In Figure 16a, configuration of the stability landscape can be seen to vary in relation to environmental conditions. Stated alternatively, ecosystem resilience to the perturbation is condition dependent. For example, fluctuating or gradually changing climatic conditions may alter the stability landscape of an ecosystem and increase the likelihood that a land-use perturbation is capable of driving the ecosystem across a threshold (Figure 16b; Tausch et al. 1993). Conversely,

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accumulated stresses attributable to past and on-going land-use activities may alter the stability landscape of an ecosystem and reduce its resilience to unpredictable (but expected) climatic episodes (Figure 16c). Ecosystems can be driven across thresholds abruptly (catastrophically) as resilience erodes under the influence of incremental, cumulative stresses. In arid and semiarid lands, catastrophic shifts among ecosystem states often are preconditioned by impacts of land-use activities on soils and plant-soil interactions (Schlesinger et al. 1990, van de Koppel et al. 1997, Whitford 2002).

Figure 16. Application of the ball-and-cup heuristic to illustrate how shifts among ecosystem states can be facilitated by environmental conditions that alters system resiliency to perturbations. In (a), the resiliency of an ecosystem to a generalized perturbation is shown to vary in relation to environmental conditions. Examples indicate how ecosystem resiliency to land-use perturbations can vary in relation to climatic conditions (b), and vice versa (c). (Adapted from Scheffer et al. 2001 and concepts of Tausch et al. 1993.)

Scheffer et al. (2001:596) noted that the notion of ecological resilience has important implications for resource management:

“Efforts to reduce the risk of unwanted state shifts should address the gradual changes that affect resilience rather than merely control disturbance. Stability domains typically depend on slowly changing variables such as land use, nutrient stocks, soil properties and biomass of long-lived organisms. These factors may be predicted, monitored, and modified. In contrast, stochastic events that trigger state shifts (such as hurricanes, droughts or disease outbreaks) are usually difficult to predict or control. Therefore, building and maintaining resilience of desired ecosystem states is likely to be the most pragmatic and effective way to manage ecosystems in the face of increasing environmental change.”

Theoretical discussions of ecological stability often address ecosystems in very broad terms, but the application of stability concepts requires greater precision. Grimm and Wissel (1997) developed a checklist of six characteristics of an ecological situation, which must be specified to bound statements or hypotheses concerning ecological stability (Table 24). The checklist is

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equally useful as a framework for posing specific questions to be addressed by vital signs monitoring. Table 24. A checklist specifying six characteristics of an ecological situation that bound statements or hypotheses concerning ecological stability (modified from Grimm and Wissel 1997). Characteristics of the ecological situation Checklist question for this feature Example answers 1. Level of description On what level of description is the

stability property examined? Individual, population, community, ecosystem, landscape, ...

2. Variable of interest Which ecological variable of interest is being considered?

Biomass, population size, age structure, nutrient cycling rate, soil stability, spatial patterns, ...

3. Reference state or reference dynamic, respectively

What is the reference state or dynamic of the variable of interest without external influences?

Equilibrium, trend, cycles, high or low, spatial or temporal variability, ...

4. Disturbance [or stress]

What does the disturbance [stress] look like? What is being disturbed, and what are the characteristics of the disturbance?

Disturbance of the state variable or of a system parameter, lasting disturbance or short-term effect, intensity of the disturbance, frequency of the disturbance, ...

5. Spatial scale To which spatial scale does the stability statement refer?

Size of the researched area, ability of the researched species to spread, typical lengths in the spatial heterogeneity of the research area, ...

6. Temporal scale To which temporal scale does the stability statement refer?

Time horizon of the statement, longevity of the examined organisms, temporal structure in the environmental heterogeneity, ...

Summary: Theoretical Framework The theoretical framework for vital signs monitoring of ecosystem structure and function can be summarized as follows: • General model:

- Factors governing ecosystem structure and function; - Controls of ecosystem sustainability; and - Changes that can lead to alternative ecosystem 'states.’

• Corollary hypotheses: - Episodic events can cause ecosystems to cross thresholds between alternative states

abruptly; and - The probability of sudden state shifts is affected by declines in system resilience.

Supplementary Models Relevant to Concepts of Ecological Thresholds Ideas related to ecological thresholds are represented in a variety of existing conceptual models. The most common approach for modeling threshold phenomena in relation to management is through state-and-transition models. State-and-transition models are management-oriented tools for organizing information and posing hypotheses about ecological thresholds, irreversible transitions among states, and effects of management activities on transition probabilities (Westoby et al. 1989, Stringham et al. 2001a, Jackson et al. 2002, Bestlemeyer et al. in press). In the application of state-and-transition models used here (Figure 17), a state is defined as “a recognizable, resistant and resilient complex of two components, the soil base and the vegetation

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structure” (Stringham et al. 2001a:4). These two ecosystem components interactively determine the functional status of the primary ecosystem processes of energy flow, nutrient cycling, and hydrology (water capture, retention, and supply). A threshold is defined as “a boundary in space and time between any and all states, or along irreversible transitions, such that one or more of the primary ecological processes has been irreversibly changed and must be actively restored before return to a previous state is possible” (Stringham et al. 2001a:5). Thus states and thresholds are defined with respect to the functioning of primary ecosystem processes. Transitions are defined as “trajectories of change that are precipitated by natural events and/or management actions which degrade the integrity of one or more of the state’s primary ecological processes” (Stringham et al. 2001a:5). In terms of resistance and resilience, a threshold is crossed when the capacities for resistance and recovery of one or more primary processes are exceeded. After the threshold is crossed, the transition is irreversible under current climatic conditions without substantial inputs of energy by management (Stringham et al. 2001a). In this type of application, a specific state-and-transition model is developed for a specific ecological site2. For monitoring applications, state-and-transition models should be accompanied by mechanistic models describing how stressors affect key ecosystem components and processes (e.g., biotic functional groups, disturbance regimes, and soil/water resources and dynamics) and influence transition probabilities.

Figure 17. State-and-transition model–a management-oriented tool for organizing information and posing hypotheses about ecological thresholds and irreversible transitions among dynamic states. Dynamics within the dark-lined boxes (i.e., within a dynamic state) are within the natural range of variability and include normal successional changes (adapted from Stringham et al. 2001a).

2 An ecological site is defined as “a kind of land with specific physical characteristics which differs from other kinds of land in its ability to produce distinctive kinds and amounts of vegetation and in its response to management” (Society for Range Management, Task Group on Unity in Concepts and Terminology 1995:279). Ecological sites are land units defined and recognized on the basis of climate, landscape position, and inherent soil properties (texture and mineralogy by depth); typically they are described or named on the basis of the dominant vegetation. Ecological sites are basic land units for resource management and analysis by the Bureau of Land Management and the USDA Natural Resource Conservation Service. The concept is synonymous with “ecological types” of the USDA Forest Service (Society for Range Management, Task Group on Unity in Concepts and Terminology 1995).

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Whisenant (1999) presented a process-based conceptual model that identified two types of thresholds in relation to restoration and management (Figure 18). As in the application of state-and-transition models described above, primary ecological processes in his model include water capture and retention, nutrient cycling, and energy capture and flow. Whisenant’s approach is based in part on earlier work by Archer (1989) and Milton et al. (1994), and it is closely allied with concepts of rangeland health and landscape function (National Research Council 1994, Ludwig et al. 1997, Ludwig and Tongway 2000, Pellant et al. 2000, Rosentreter and Eldridge 2002). The fundamental hypothesis underlying these approaches is that health and sustainability of arid-land ecosystems are dependent on maintaining the capacity of these systems to capture and retain water and nutrients (Whitford 2002).

Figure 18. Conceptual model illustrating the application of threshold concepts to restoration and management (from Whisenant 1999). In this framework, “primary processes” include water capture and retention; nutrient capture, cycling and retention; and energy capture.

System-Specific Models Draft conceptual models presented in this section are organized in relation to the three major ecosystem groups outlined in Table 11 and described in Chapter II – arid-semiarid shrubland, grassland, and woodland ecosystems; montane shrubland, coniferous woodland, and forest ecosystems; and riparian, wetland and aquatic ecosystems. Models for arid-semiarid ecosystems are also generally applicable to sparsely vegetated ecosystems listed in Table 11. A unifying feature of ecosystems in these two categories is the open-canopy structure of vascular vegetation, which varies along a continuum from sparsely vegetated badland systems to late-successional pinyon-juniper woodlands. Conceptual models for caves and orchards will eventually be developed (or adopted from the literature) to guide monitoring of those systems.

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Arid-Semiarid Ecosystems

Key Components, Functions, and Attributes The key ecosystem components, functions, and attributes that control the functional status of primary ecosystem processes in arid-semiarid ecosystems of the NCPN are catalogued in Table 25. Primary ecosystem processes are defined as water capture and retention, nutrient cycling, and energy capture and flow (Whisenant 1999, Whitford 2002). Of these, the retention of water and nutrients are most important for the sustainability of arid-land ecosystems (Whitford 2002); energy flow cannot be maintained in the absence of water and nutrients stored and supplied by soil. The physical structure of arid-land ecosystems–consisting of both abiotic and biotic features – is essential for capturing and retaining soil resources (Ludwig et al. 1997, Ludwig and Tongway 2000, Whitford 2002). Specific types of arid-land ecosystems, including those listed in Table 11, each are characterized by particular spatiotemporal patterns of soil resources that are determined jointly by climate, topography, and strong feedbacks between vegetation and soil (Whitford 2002). Other organisms such as ants and rodents (Cammeraat et al. 2001, Brown and Heske 1990) can modify soil-resource heterogeneity directly due to effects on soil or indirectly due to effects on vegetation structure. Consistent with the general model in Figure 13, the spatiotemporal distribution of soil resources has been proposed as a fundamental characteristic of arid-land ecosystems (Schlesinger et al. 1990, Schlesinger et al. 1996, Ludwig et al. 1997, Schlesinger and Pilmanis 1998, Whitford 2002). The spatial and/or temporal redistribution of soil resources, whether by natural disturbances, anthropogenic stressors, or both, can cause significant and potentially long-lived changes in the structure and function of arid-land systems (Whitford 2002). Primary ecosystem functions listed in association with ecosystem components in Table 25 emphasize those functions that are most strongly related to the capture, retention, and spatiotemporal distribution of soil resources. A further clarification aids interpretation of Table 25. White and Pickett (1985:7) defined disturbance as “...any relatively discrete event in time that disrupts ecosystem, community, or population structure and changes resources, substrate availability, or the physical environment.” For purposes of monitoring, disturbances are considered to be those events that are within the evolutionary history of the ecosystem (Whitford 2002). These are differentiated from stressors, which may fit the disturbance definition but are outside the range of disturbances naturally experienced by the ecosystem and typically are anthropogenic in origin (Whitford 2002). Precipitation and precipitation variability (among years, among seasons, within seasons, and across the landscape) are fundamental natural drivers of ecosystem processes in arid-land ecosystems (Whitford 2002, Table 25). Temporal coupling between precipitation and ecosystem processes depends on precipitation seasonality, amount and intensity; ecosystem component (e.g., surface-dwelling biological soil crusts vs. deep-rooted shrubs), landscape position; and depth of soil-water storage (Ehleringer et al. 1991, 2000; Whitford 2002). Wind is an important force for redistributing soil resources within the ecosystem (e.g., from interspace to canopy patch) and for transferring resources to other ecosystems.

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Table 25. Primary functions and attributes associated with natural components of arid-semiarid ecosystems of the NCPN. Components are organized in relation to the four interactive controls of ecosystem structure and function. Components with the greatest influence over primary ecosystem processes of water capture and retention, nutrient cycling, and energy capture in arid-semiarid NCPN ecosystems (in the absence of anthropogenic stressors) are underlined in bold. INTERACTIVE CONTROLS* COMPONENTS PRIMARY ECOSYSTEM FUNCTIONS ATTRIBUTES

Precipitation

Water inputs; driver of water-limited ecosystem and population processes (e.g., nutrient cycling, C and N fixation, seed germination); erosive force for detachment, entrainment, and overland redistribution and export of soil, litter, and propagules; driver of fire disturbances due to effects of interannual variability on fuel production and flammability; enhance resistance and resilience of biotic and biotically structured ecosystem components (e.g., soil) to natural disturbances and anthropogenic stressors.

Seasonality; quantity; intensity (amt. per event and per unit time), duration, temporal frequency; temporal variability (among seasons, within seasons, among years), spatial variability, form (rain vs. snow).

Wind Soil, litter, and propagule redistribution and export (transfer among patches and among systems); effects on ecosystem-atmosphere gas-exchange (CO2 intake and evapotranspiration); energy-balance modification (transfer of sensible and latent heat).

Average sustained and peak velocities and direction (and frequency-magnitude distributions of these by season), seasonal and diurnal variability, spatial variability.

Radiation Energy inputs for photosynthesis and heat; effects on ecosystem-atmosphere gas-exchange (CO2 uptake and evapotranspiration).

Maximum, minimum, and average values by season (heat), spectral characteristics, intensity; temporal variability (seasonal and diurnal), spatial variability (horizontally and vertically).

CO2 Carbon inputs. Atmospheric concentration

Local atmospheric resources and conditions

Dust & other airborne constituents Mineral nutrient inputs. Quantity, chemical composition, temporal distribution (seasonality), spatial

distribution.

Drought

Drives change in ecosystem structure and function (1) by altering competitive relations and inducing selective, potentially widespread, mortality – resulting in persistent dominance shifts among vegetative functional groups; (2) by affecting resistance and resilience of biotic and biotically structured ecosystem components (e.g., soil) to other natural disturbances (e.g., subsequent extreme precipitation events or wind storms) and anthropogenic stressors; and (3) by altering the likelihood of other natural disturbances such as wind storms or fire.

Seasonality, intensity, duration, frequency, timing in relation to extreme precip. and wind events.

Extreme precip. events / floods

Drives change in ecosystem structure and function (1) by inducing selective establishment episodes (or less commonly, selective mortality) of vegetative functional groups – resulting in persistent dominance shifts; and (2) due to extreme erosive forces for detachment, entrainment, and redistribution and export of soil and soil resources, potentially inducing geomorphic change.

Seasonality, intensity, duration, frequency, timing in relation to drought and the amount of time required for biotic and biotically structured ecosystem components and functions to recover from drought.

Episodic climatic events

Wind storms Drives change in ecosystem structure and function due to extreme erosive forces for detachment, entrainment, and redistribution and export of soil and soil resources.

Seasonality, intensity, duration, frequency, timing in relation to drought and the amount of time required for biotic and biotically structured ecosystem components and functions to recover from drought.

Fire

Drives change in ecosystem structure and function by (1) directly altering vegetation structure (differential resistance and resilience to fire), including spatial heterogeneity, (2) altering the forms, bioavailability, and spatiotemporal distribution of soil resources, (3) increasing exposure and erosion susceptibility of soil and soil resources and reducing ecosystem capacity to retain soil resources (including water).

Intensity, spatial extent and pattern, frequency, timing in relation to other disturbances such as extreme precipitation and wind events.

Herbivory

Drives change in ecosystem structure and function by (1) altering competitive relations among palatable and unpalatable plant taxa, (2) altering vegetation resistance and resilience to drought, other disturbances, and stressors, and (3) potentially affecting primary productivity and litter deposition. In combination, these can alter functional group structure, including spatial heterogeneity, and ecosystem capacity to capture and retain soil resources. Defecation and urination further alters the spatiotemporal distribution of resources.

Intensity, selectivity, spatial extent and pattern, frequency, timing in relation to other disturbances such as drought and the amount of time required for biotic and biotically structured ecosystem components and functions to recover from drought.

Digging / burrowing Alters soil structure and function (creation of macropores potentially increase water capture and retention), alters spatiotemporal distribution of soil resources, generates patch structure / heterogeneity, potentially alters structure of vegetative functional groups due to resource alteration and creation of establishment opportunities.

Spatial distribution and extent, frequency, depth, timing in relation to other disturbances such as extreme wind and precipitation events.

Disturbance regime

Trampling

Destabilizes soil and decreases resistance of soil to erosion and redistribution by wind and water; compacts soil (alters soil structure and function), alters structure and function of biological soil crusts; alters vegetation structure due to trampling of vegetation or via effects of altered soil function on resistance and resilience of vegetation to drought. Together, these decrease ecosystem capacity to capture and retain soil resources.

Intensity, spatial extent and pattern, frequency, timing in relation to other disturbances such as extreme wind and precipitation events, drought, and the amount of time required for biotic and biotically structured ecosystem components and functions to recover from drought.

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Table 25 (continued). INTERACTIVE CONTROLS COMPONENTS PRIMARY ECOSYSTEM FUNCTIONS ATTRIBUTES

Predators Regulation of (or response to) prey populations, including granivores and herbivores. May also impact ecosystem structure and function by digging / burrowing (see above).

Composition, quantity, population structure and dynamics; physiological condition.

Herbivores See Herbivory, above. May also impact ecosystem structure and function by digging / burrowing (see above).

Composition, quantity, population structure and dynamics; physiological condition.

Granivores Alteration of vegetation structure (composition and spatial heterogeneity) due to selective collection, consumption, burial, and redistribution of propagules. May also impact ecosystem structure and function by digging / burrowing (see above).

Composition, quantity, population structure and dynamics; physiological condition.

Small trees Shrubs Dwarf shrubs Perennial grasses

Energy capture and conversion, biomass production, litter deposition (soil protection and inputs to nutrient cycles), nutrient retention (intraplant cycling), environmental modification (reducing soil temperatures and evaporative rates via shading and litter deposition; generating resource heterogeneity via uptake, litter deposition, and capture of airborne and windborne materials), obstruction to wind and overland water flow (reducing erosive energy and enhancing capture and retention of soil resources), rainfall interception and redistribution via stemflow (reducing erosive energy and enhancing capture and retention of soil resources). In combination, these functions contribute to resistance / resilience of soil functions to disturbance by trampling and erosive forces of wind and water. Provide fuel for fire and habitat structure for vertebrates and invertebrates.

Composition, quantity (cover and biomass), population structure and dynamics, vertical structure, spatial distribution / heterogeneity, photosynthetic pathway, litter quantity and quality (e.g., C:N), productivity; physiological activity and condition; resistance & resilience of structure and function to dominant natural disturbances and anthropogenic stressors.

Biological soil crusts (photoautotrophs)

Soil stabilization and soil-surface protection; energy capture and conversion; nutrient capture, retention, and cycling (N fixation, capture of airborne minerals in dust); obstruction to overland water flow (increased surface roughness enhances capture and retention of soil resources); environmental modification (albedo & soil temperature, ); soil-temperature increases (decreased albedo); habitat creation (due to long-term soil stabilization).

Composition, quantity (cover and biomass), spatial distribution and contiguity, microtopographic heterogeneity / surface roughness; physiological activity and condition; productivity; resistance & resilience of structure and function to dominant natural disturbances and anthropogenic stressors.

Roots Soil stabilization, nutrient and water acquisition and transport, water redistribution in soil profile, organic-matter inputs (exudates & tissues), macropore creation, rhizospere acidification (release CO2 and organic acids).

Morphology, density, horizontal and vertical distribution, spatial and temporal patterns of physiological activity, productivity.

Biotic functional groups

Soil biota (heterotrophs)

Litter decomposition and nutrient cycling, N fixation, symbiotic relations with vascular plants (symbiotic enhancement of nutrient and water delivery to vascular plants may increase resistance / resilience of these plants to drought or other disturbances).

Composition, quantity (biomass), spatial distribution vertically and horizontally, temporal distribution, spatial and temporal patterns of physiological activity, productivity; resistance & resilience of structure and function to dominant natural disturbances and anthropogenic stressors.

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Soil mineral matrix Soil organic matter Soil water Soil air

Nutrient storage, supply, and cycling; water storage and supply; medium for plant growth; habitat for soil biota involved in nutrient cycling; positive effects on resistance / resilience of vegetative functional groups to drought, herbivory, and trampling.

Inherent properties (relatively insensitive to change): Mineralogy and texture by depth, spatial heterogeneity in these properties, depth. Dynamic properties (subject to change): Aggregate stability and bulk density (structure), organic-matter quantity and quality (e.g., C:N), depth (often considered an inherent property, but subject to change over decadal time scales), erosion rate, infiltration rate, biotic activity, surface crusting (biotic or physicochemical), surface roughness, spatial heterogeneity of these properties, resistance & resilience of structure and function to dominant natural disturbances and anthropogenic stressors.

Soil resources and conditions

Soil temperature Regulates physiological activity of autotrophic and heterotrophic soil biota, including roots. Maximum, minimum, and average values by season; temporal variability (seasonal and diurnal), spatial variability (horizontally and vertically).

Primary sources: Whitford (2002), Herrick et al. (2002), Belnap and Lange (2001), Ehleringer et al. (2000), Seybold et al. (1999), Whisenant (1999), and Ludwig et al. (1997). * Interactive controls are constrained by the five state factors—(1) global/regional atmospheric resources and conditions, (2) topography, (3) parent material, (4) potential biota, and (5) time (Jenny 1980, Chapin et al. 1996).

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Episodic climatic events are major disturbances in the evolutionary history of arid-land ecosystems (Walker 1993, Whitford 2002). Drought, extreme precipitation events and floods, and wind storms can induce long-term changes in ecosystem structure and function by causing mortality or enabling establishment of long-lived plants that are structural dominants. The erosive energy of rainstorms and windstorms can result in massive transport and redistribution of soil resources, potentially inducing geomorphic changes. Event sequencing (e.g., timing of flood in relation to drought) is an important factor that affects the resistance of particular ecosystem components, attributes, and functions to episodic storm events. Episodic, event-driven change is an important feature of many ecosystems (Holling 1996, Scheffer et al. 2001), but it is particularly characteristic of arid-land ecosystems (Whitford 2002). The significance of other natural disturbances such as herbivory, trampling, and fire varies among the specific arid-semiarid ecosystems listed in Table 11. From the end of the Pleistocene to the time of European settlement, large native ungulates were uncommon in arid-semiarid ecosystems of the Intermountain West in comparison with the Great Plains region east of the Rocky Mountains (Mack and Thompson 1982, Grayson 1994). Thus herbivory and trampling by large mammals were minor components of pre-European disturbance regimes relative to episodic climatic events. Of the arid-semiarid ecosystems in Table 11, fire is most significant as a natural disturbance at the upper end of the moisture/fuel gradient where sagebrush steppe, grasslands, and pinyon-juniper-dominated ecosystems intergrade in relation to soil, landscape position, and fire history (West and Young 2000). In the NCPN, these intergrading arid-semiarid systems are most characteristic of DINO and ZION. Natural fire-return intervals in sagebrush steppe varied from 20-30 years in mountain big sagebrush systems (Artemisia tridentata var. vaseyana) to 50-100 years in Wyoming big sagebrush systems (A. tridentata var. wyomingensis) (Miller et al. 1994). Natural fire-return intervals in pinyon-juniper woodlands are poorly understood in part because of the tremendous structural diversity of systems broadly categorized as pinyon-juniper woodlands. Floyd and colleagues recently estimated the natural fire-turnover time (amount of time required for repeated fires to cumulatively burn over an entire landscape) in pinyon-juniper woodlands at Mesa Verde NP to be approximately 400 years (Floyd et al. 2000). Intensive, stand-replacing fires are naturally characteristic of this Mesa Verde system, but other pinyon-juniper systems experienced relatively frequent, low-intensity surface fires during pre-European times (Floyd et al. 2000). In sparse pinyon-juniper “woodlands” found on rocky, skeletal soils in most NCPN units, fire (other than single tree lightning strikes) was probably uncommon in pre-European times due to an absence of continuous fuels. Key biotic functional groups with the greatest influence over primary ecosystem processes are small trees, shrubs, dwarf shrubs, perennial grasses, biological soil crusts, and soil biota (Whitford 2002, Table 25). In addition to conducting photosynthesis, aboveground structures of vascular plants protect soils from erosive raindrops, obstruct erosive wind and overland water flow, and enhance the capture and retention of soil resources. Litter from plants further reduces the erosive impacts of rainfall on soil surfaces and provides inputs to soil organic matter for nutrient cycling. Aboveground structures of plants also modify the physical environment by shading and litter deposition, strongly affecting spatial and temporal patterns of soil-resource availability to other organisms. Roots stabilize soils, are conduits for resource acquisition and redistribution, and provide soil organic matter. The major vascular functional types (generally synonymous with life form in this context) vary considerably in their canopy structures, basal

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diameters, root morphologies, and litter characteristics. Thus shifts in relative dominance among these functional types strongly alter ecosystem function–including the capacity for capturing and retaining soil resources in the system (Breshears and Barnes 1999, Whitford 2002). Biological soil crusts stabilize soils (Williams et al. 1995a,b), and they create soil-surface heterogeneity that can obstruct overland water flow and increase water infiltration in relatively fine-textured soils (Warren 2001). They are important sources of carbon and nitrogen inputs to arid-land nutrient cycles (Evans and Lange 2001). They affect vegetation structure directly due to effects on seedbed characteristics and safe-site availability, and indirectly through effects on soil temperature and on water and nutrient availability (Belnap et al. 2001). As the medium for storage, cycling, and supply of water and nutrients to primary producers, soil is the foundation of the ecosystem. Dynamic properties (those subject to change by disturbance, management, and/or climatic variations) are particularly important for maintaining soil functions such as nutrient cycling and soil-water storage and supply (Karlen et al. 1997, Seybold et al. 1999). These properties include aggregate stability and bulk density (structural characteristics strongly affected by organic-matter inputs from vascular plants and biological soil crusts), as well as soil-organic-matter quantity and quality, biotic activity, surface roughness, and spatial heterogeneity. The functional status of hydrologic processes associated with dynamic soil properties can affect resistance and resilience of vascular plants to drought (Thurow 2000).

Human-Caused Transitions Among States Lockwood and Lockwood (1993) proposed that the dynamics of arid-semiarid ecosystems could be described by catastrophe theory – a framework designed for study of discontinuous phenomena. Concepts of catastrophe theory have been applied to studies of arid-land ecosystem dynamics in a variety of contexts (e.g., van de Koppel et al. 1997, Davenport et al. 1998). Features of arid-land ecosystems that correspond with properties of catastrophe systems include (from Whitford 2002:282):

1. Distinct conditions or states of existence; 2. Conditions that are very unstable; 3. Relatively rapid movement between states; 4. Hysterisis (i.e., processes associated with degradation or recovery which are not readily

reversible by inverting the sequence of events); and 5. Relatively small changes in initial conditions can result in dramatically different

outcomes with time. State-and-transition models represent a useful conceptual approach for modeling the dynamics of such ecosystems (Whitford 2002). Figure 19 presents a general state-and-transition model for describing dynamics of arid-semiarid ecosystems of the NCPN in response to anthropogenic stressors. Irreversible shifts among states may be caused by stressors directly, or they may be caused indirectly due to effects of anthropogenic stressors on the resistance of ecosystem components and functions to natural disturbances such as episodic climatic events.

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Figure 19. General state-and-transition model (a) for arid-semiarid ecosystems of the NCPN, and (b) landscape-level application of the model. The ecosystem in State 1 (S1) is characterized by primary processes functioning within the normal range of variability (energy flow; and the capture, retention, and cycling of nutrients and water). In S1, structure and function of soil and vegetative functional groups vary in relation to climatic conditions and natural disturbances, and the resistance and resilience of ecosystem functions to natural disturbances are retained. Transitions between states have a reversible phase (dashed) and a phase that is irreversible (solid) under the current climatic regime without substantial management inputs. Alternative states are characterized by irreversibly altered soil-resource regimes (S2), irreversibly altered functional-group structure (S3), or both (S4). Transition probabilities are affected by anthropogenic stressors and management actions. Given strong feedbacks between soil-resource regimes and vegetative functional groups, S2 and S3 are hypothesized to be relatively less stable than S4, and may be viewed as intermediate stages in the eventual development of S4. In (b), Landscape Units A, B, and C are shown to be connected by flows of soil resources, biotic functional groups (potentially including exotics), disturbances, and stressors. Thus probabilities of state transitions in particular ecosystems are influenced by resources, organisms, disturbances, and stressors transferred from ecosystems elsewhere in the landscape. (Expanded from concepts of Stringham et al. 2001a and Whitford 2002).

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A basic set of three alternative states is proposed in Figure 19a. Relative to the resistant and resilient ecosystem with primary processes functioning within the natural range of variability (S1), the three alternative states are characterized by an irreversibly altered soil-resource regime (S2), irreversibly altered functional-group structure (S3), or both (S4). Although specific state-and-transition models for the several arid-semiarid ecosystems in Table 11 would reveal a large number of variations in the number and soil-vegetation attributes of alternative states, states S2 through S4 in Figure 19a represent the basic set of alternatives. Likewise, transition probabilities (e.g., T1.2 –the probability of an irreversible ecosystem shift from S1 to S2) will vary among different ecosystems exposed to the same suite of anthropogenic stressors. Figure 19b applies the general state-and-transition model to landscape-level processes, illustrat-ing how transition probabilities in a particular ecosystem are affected by the functional condition of other ecosystems in the landscape due to among-system transfers of soil resources, biotic functional groups, disturbances, and stressors. For example, altered soil-resource regimes associated with transitions T1.2 or T1.4 in Landscape Unit A could accelerate soil-resource transfers to Landscape Unit C, thereby affecting ecosystem conditions and transition probabilities there. Similarly, transitions T1.2 or T1.4 in terrestrial ecosystems could accelerate transfers of sediment, nutrients, and fluvial energy to riparian, wetland, and/or aquatic ecosystems elsewhere in the watershed. Table 26 summarizes potential effects of selected anthropogenic stressors on key ecosystem components and functions, and presents hypotheses concerning effects of these stressors on transition probabilities of arid-semiarid NCPN ecosystems. Consistent with Table 25, informa-tion in Table 26 emphasizes effects of stressors on the functioning of primary ecosystem processes. Soil degradation is the most common cause for irreversible changes in the structure and function of arid-land ecosystems (Whitford 2002), which is reflected Table 26. Trampling of soils by park users and livestock provides a good example of the effects of an anthropogenic stressor on transition probabilities. Effects of trampling are numerous and can strongly affect the capacity of arid-land ecosystems to capture and retain soil resources directly and/or indirectly through a large number of interacting mechanisms (described in Table 26). If trampling of soils and damage to biological soil crusts exceeds capacity for recovery, a T1.3 transition can occur. In arid-semiarid systems where biological soil crusts play important roles in soil stabilization and nutrient cycling, the irreversible loss of this key functional group is likely to inevitably trigger a T3.4 transition associated with an irreversible change in soil-resource conditions. Examples of some of the transitions described in Figure 19a and Table 26 can be found in NCPN units. As a consequence of past livestock grazing, trampling, and vehicular activities, several native grasslands and shrubsteppe ecosystems at CANY have been irreversibly converted to systems dominated by Bromus tectorum (cheatgrass). These systems are now in states S3 or S4. Also at CANY, a native grassland with soils and biological soil crusts relatively unimpacted by anthropogenic stressors is now in state S3 due to an episodic increase in cheatgrass abundance associated with an El Niño event (Belnap and Phillips 2001). Strong effects of cheatgrass on soil-resource dynamics suggest that this system already is on a T3.4 trajectory (Evans et al. 2001). At GOSP, land-use activities that preceded NPS management resulted in vast alterations of ecosystem structure and function. Like the impacted grasslands at CANY, many GOSP ecosystems are in states S3 or S4 due to dominance by invasive exotic annuals. Similar examples can be found at CARE and probably other NCPN units

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Table 26. Potential effects of selected anthropogenic stressors on key components, functions, and transition probabilities of arid-semiarid NCPN ecosystems.

Increased transition

probabilities* Stressors Stress mechanisms

Potential effects on ecosystem components, attributes and key ecosystem functions (energy flow, nutrient capture and retention, water capture and retention) T1.2 T T T1.3 1.4 2.4 T3.4

Trampling of soil and vegetation

Damaged biological soil crusts, decreased N fixation by biological crusts, decreased soil-surface roughness, enhanced recruitment opportunities for exotic plants, decreased recruitment opportunities for crust-adapted native plants; altered vegetation structure, decreased soil protection by biological crusts, decreased soil aggregate stability, decreased soil stability, decreased resistance of soil to erosion by wind and water, increased bulk density, decreased infiltration, increased overland flow of water, decreased soil-water availability for plant growth, soil biotic activity, and nutrient cycling; decreased root growth and soil-organic-matter inputs, decreased plant growth, decreased resistance and resilience of plants to drought, increased redistribution and export of soil, litter, nutrients, and water; decreased capacity of ecosystem to capture and retain soil resources; multiple cascading effects due to feedbacks among soil functions, soil-resource retention, resource heterogeneity, and vegetation structure.

X X X X X Park users

Introduction of exotic plants See exotics, below. X XTrampling of soil and vegetation See trampling, above. X X X X X

Excessive herbivory

Altered competitive relations of plants, altered vegetation structure (e.g., dominance shift from perennial grasses to unpalatable shrubs and/or from palatable native plants to unpalatable exotics), reduced plant canopy cover, reduced plant-canopy protection of soil, reduced vegetative obstruction of overland water flow, reduced capture and retention of soil resources, reduced litter deposition and litter-protection of soil, reduced soil-organic-matter inputs, reduced soil aggregate stability, decreased resistance and resilience of soil to trampling, decreased root growth, decreased resistance and resilience of grazed plants to drought; multiple cascading effects due to feedbacks among vegetation structure, soil-resource retention, resource heterogeneity, and soil functions.

X X

Defecation and urination Nutrient losses (N volatilization), nutrient immobilization in dung pats, increased spatial and temporal heterogeneity of nutrients, eventual alteration of vegetation structure, facilitation of exotic-plant invasion where nutrients locally enriched (see exotics, below).

X X

Livestock

Introduction of exotic plants See exotics, below. X X

Competition with native plants Altered vegetation structure, eventual alteration of soil-resource dynamics and/or heterogeneity due to vegetation-soil feedbacks. X X

Altered soil-resource dynamics

Altered nutrient dynamics (e.g., exotic characterized by different tissue chemistry, and/or by different spatiotemporal patterns of nutrient uptake and litter deposition than native plants), altered soil-water dynamics (e.g., exotic characterized by different spatiotemporal patterns of water use than native plants), eventual alteration of vegetation structure due to soil-vegetation feedbacks.

X XExotic plants

Altered disturbance regime Increased frequency and extent of fire (facilitated by increased quantity, flammability, and/or spatial continuity of fuels); eventual alterations of vegetation structure and soil-resource availability due to strong feedbacks among fire, vegetation structure, and resource availability.

X X

Trespass livestock See livestock, above. See livestock, above. Introduction of exotic plants See exotics, above. X XAdjacent land-use

activities Accelerated transfers of soil, nutrients, and water

Soil-resource enrichment, eventual alteration of vegetation structure, facilitation of exotic-plant invasion where resources enriched (see exotics, above), increased overland water flow, increased redistribution and export of soil and nutrients.

X X X X X

Nitrogen deposition Soil-resource enrichment, eventual alteration of vegetation structure, facilitation of exotic-plant invasion (see exotics, above). X X

Air pollutants Ozone Altered competitive relations between ozone-sensitive and ozone-tolerant plants, altered vegetation

structure X X

*Transition notation: T1.2 = transition from State 1 to State 2. State 1: structure and function unaltered by anthropogenic stressors. State 2: irreversibly altered soil-resource regime. State 3: irreversibly altered functional-group structure. State 4: irreversibly altered soil-resource regime and functional-group structure.

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Resistance and Resilience to Disturbance and Stress For purposes of the vital signs monitoring program, it is important to monitor the functional status of key ecosystem components and processes in relation to their capacities to resist and recover from 1) prevailing anthropogenic impacts and 2) unpredictable but expected natural disturbances (Scheffer et al. 2001). Irreversible thresholds in Figure 19 are defined on the basis of whether primary ecosystem processes (and the ecosystem components that determine them) still retain their capacities for resistance and recovery. According to Seybold and colleagues (Seybold et al. 1999:229), “The most important factors controlling the recovery of degraded soils are floral and faunal activity, including microbial activity, and climate.” Recovery processes and mechanisms associated with key soil functions (Table 27) are strongly affected by vegetative inputs of organic matter that helps support the formation of soil aggregates (improved soil structure) and the activity of soil biota. Because of these plant-soil feedbacks, human impacts to vegetation also affect soil resilience. Table 27. Soil functions and associated recovery processes or mechanisms (from Seybold et al. 1999).

Soil function Recovery processes or mechanisms Nutrient cycling Biological activity, biological diversity, plant growth Productivity Carbon sequestration, aggregation processes, nutrient cycling, biological diversity Water storage Carbon sequestration, aggregation processes, biological activity Decomposition Biological activity Absorbing and detoxifying pollutants Biological activity, carbon sequestration, biological diversity, mineral weathering

Nutrient supplying capacity Biological activity, mineral weathering, nutrient cycling Based on case studies from Australia, Walker and Abel (2002) provided several examples of biotic and abiotic factors that determined the resistance and resilience of arid-semiarid rangeland ecosystems subjected to drought and livestock grazing (Table 28). Table 28. Examples of abiotic and biotic factors conferring resistance and resilience of arid-semiarid Australian rangelands to disturbance by drought and stress by livestock grazing (modified from Walker and Abel 2002). Abiotic factors Rainfall with low erosivity and soils with low erodibility

Soils that maintain infiltration rates under impacts of grazing and trampling

Soils that otherwise resist degradation

Soils that maintain stability of vegetation structure – e.g., heavy clay soils or soils with shallow hardpans that do not allow encroachment of woody vegetation and persistent alteration of vegetation structure

Biotic factors Strong vegetation control over water and nutrient flows, maintaining spatial pattern and retaining resources under

fluctuating rainfall Plant species with high root:shoot ratios and high seed production and dispersal

Plant communities with high species richness within functional groups, ensuring a variety of responses to differing environmental disturbances

High genetic variability within species, ensuring a diversity of response capabilities

Banks of seeds and other propagules that retain memory and influence ecosystem recovery following disturbances

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The resistance and resilience of biological soil crusts to trampling varies in relation to soil characteristics (texture, depth, rock cover), vegetation structure (plant canopy spacing and cover), and effective precipitation (Belnap and Eldridge 2001, Figure 20). Effective precipitation varies spatially in relation to climate and microclimate (Figure 20), and seasonally in relation to evaporative demand. Trampling effects on the physiological performance and structural integrity of biological soil crusts also reduce crust resistance to erosion by wind (Williams et al. 1995a, Belnap and Gillette 1998) and water (Williams et al. 1995b).

Figure 20. Vulnerability and recoverability of biological soil crusts in relation to gradients of site stability, effective precipitation, and disturbance regimes. Crusts at sites with the greatest surficial stability, greatest effective precipitation, and lowest disturbance frequency or intensity will be less impacted (dark shading) than sites with lower stability, less effective precipitation, and higher disturbance frequency or intensity (light shading). Recovery time is faster (dark shading) in areas of low vulnerability, and slower (light shading) where vulnerability is higher. (Modified from Belnap and Eldridge 2001.)

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Potential Indicators Tables 25 and 26 together provide a starting point for the selection of vital signs for monitoring structure, function and sustainability of arid-semiarid ecosystems. In particular, most attributes identified in Table 25 represent quantifiable properties related to the functional status of key ecosystem processes or to the status of phenomena such as precipitation that are important drivers of variation and change in ecosystem properties. Seybold and colleagues (1999) emphasized three categories of indicators important for monitoring programs–indicators of 1) the capacity to function, 2) the capacity to resist change when exposed to anthropogenic stressors and/or natural disturbances, and 3) the capacity to recover following the stress and/or disturbance (Figure 21). They also presented several potential indicators specifically for assessing and monitoring soil resilience (Table 29).

Figure 21. Representation of the concept that protocols for assessment and monitoring of ecosystem function in relation to disturbance and stress should include indicators of the capacity to function, the capacity of functions to resist change, and the capacity of functions to recover (from Seybold et al. 1999). The concept presented here for soil also is applicable to other key functional components of ecosystems, such as vegetation and biological soil crusts.

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Table 29. Potential indicators for soil resilience (from Seybold et al. 1999, citing Bezdicek et al. 1996).

Soil structure Microaggregates Soil water Retention and transmission properties Cation exchange capacity Exchangeable cations Soil organic matter content Transformations Nutrient-supplying capacity Soil pH Rooting depth Soil biodiversity Soil faunal activity Microbial activity

Indicators applicable to assessment and/or monitoring health and sustainability of arid-semiarid ecosystems should meet the following criteria (from Whitford 2002:311):

1. Reflect the status of a critical ecosystem process, important ecosystem property, or an economic-social value;

2. Be unambiguous (i.e., the trajectory of the measure is unidirectional in response to ecosystem stressors of increasing intensity);

3. Be applicable to the range of ecosystems encountered in the arid and semiarid landscapes; and

4. Be readily and inexpensively measured. Herrick and colleagues (2002:3) suggested guidelines for the development of integrated soil-vegetation monitoring programs for arid-land ecosystems:

1. Identify a suite of indicators that is consistently correlated with the functional status of one or more critical ecosystem processes.

2. Base indicator selection on site- or project-specific resource concerns and inherent soil and site characteristics.

3. Use spatial variability in developing and interpreting indicators to make them more representative of ecological processes.

4. Interpret indicators in the context of an understanding of dynamic, nonlinear ecological processes.

One set of indicators for consideration as potential arid-land vital signs is presented in Table 30. Indicators in the table are associated with assessment and monitoring of soil stability and hydrologic function (capacity of the to capture and retain soil resources), nutrient cycling, and energy flow.

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Table 30. Quantitative indicators for assessing and monitoring the condition or health of arid and semiarid ecosystems–including the capacity of ecosystems to perform selected functions and maintain those functions (due to resistance and resilience) following natural disturbance and/or anthropogenic stress (from Whitford 2002 and Herrick et al. 1995).

Ecosystem function Indicator

1. Total vegetation cover and average height of vegetation 2. Size of unvegetated patches (mean, median, skewness of distribution, weighted skewness (skewness x mean)) 3. Spatial distribution and orientation of unvegetated patches 4. Surface stability (soil aggregate stability by slake test, Herrick et al. 2001) 5. Biological soil crust cover 6. Litter and rock cover 7. Infiltration capacity (single-ring infiltrometer, unconfined) 8. Size and spatial distribution of litter patches 9. Soil penetration resistance (compaction) (cone penetrometer, Herrick and Jones 2002) 10. Root density and depth based on species composition and cover of the vegetation 11. Soil disturbance by animals (percent surface area disturbed, volume moved per unit area, soil horizon origin of ejected soil) 12. Predictability of annual plants (rainfall seasonality) 13. Ratio of long-lived grasses to short-lived grasses

Soil stability and hydrologic function (indicators related to wind and water erosion potential, and to ecosystem capacity for capturing and retaining soil resources)

14. Ratio of seed-reproducing grasses to vegetatively-reproducing grasses Indicator 1 15. Rainfall-use efficiency (Le Houérou 1984) (based on biomass production estimates by harvest or dimension analysis)

Primary productivity (energy flow)

16. C3/C4 plant cover ratio vs. rainfall seasonality Nutrient cycling Indicators 1, 2, 3, 8, 12, 14

Montane Shrubland, Woodland, and Forest Ecosystems During fall 2002, tabular and diagrammatic conceptual models for montane ecosystems of the NCPN will be developed parallel to those presented above for arid-semiarid ecosystems.

Riparian, Wetland and Aquatic Ecosystems This section presents conceptual models for rivers and intermittent streams, springs, seeps and hanging gardens, and pothole ecosystems. As with the models presented above for arid-semiarid terrestrial ecosystems, models in this section will be revised following peer review, consultation with subject-matter experts, and on-going literature review during fall 2002. Threshold concepts are particularly applicable to riparian (Stringham et al. 2001b) and aquatic ecosystems (Scheffer et al. 1993), and will be incorporated in the models as appropriate.

Rivers and Intermittent Streams Rivers and streams are dynamic systems dependent on the watershed in which they reside for inputs of energy (Figure 22). Like energy, nutrients flow through the system and are not necessarily cycled within an immediate stream reach. The watershed, including vegetative cover, geology, precipitation (temporal and amount), temperature and soil structure are but a few of the elements which control the dimension, profile, patterns, water quality and associated biota of rivers and streams.

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Figure 22. Conceptual model illustrating mechanisms by which anthropogenic stressors can affect water quality of riverine aquatic ecosystems (partially based on Karr and Chu 1999).

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In the arid and semiarid regions of the Northern Colorado Plateau, few large rivers dominate. Instead, the landscape is riddled with intermittent and ephemeral streams. The Colorado and Green rivers in Canyonlands, the Virgin River in Zion National Park, and the Fremont River in Capitol Reef National Park comprise the large rivers of the network. Here the importance of understanding the spatial and temporal scale will help define a monitoring effort. In the short term, the shape of river channels is unchanging–only discharge, temperature, sediment transport, and chemistry change daily. Over an intermediate time frame, the channel profile, dimension, and pattern approach a quasi-dynamic equilibrium state between water and sediment transport, and the longer-term controlling factors of gradient, sedimentology, and paleochannel features (Allan 1995). Energy and nutrients flow through these systems, a consequence simply of gradient and movement. The River Continuum Concept (Vannote et al. 1980), although a contested theory that expresses general concepts about streams across biomes, continues to help frame questions (Allan 1995). These questions anticipate answers regarding water quality and biotic functional groups based on the size of the river under examination. Intermittent streams differ greatly from rivers, in that flow does not always occur in these systems–the flow of energy and nutrients is interrupted. Since many of the streams in the network are intermittent, assessing stresses must occur at a level timed when water flows, and even then what may appear to be a water quality problem, e.g. high turbidity, is only a reflection of the natural system. Stressors such as visitation and all types of development can change the runoff rates, storage of water in the floodplain, the energy and nutrient flows in the system, evapotranspiration, and biogechemical cycling. Dams on rivers, for example, serve as reset mechanisms decreasing stream temperatures, reducing sediment load, and changing water quality (Stanford and Ward 1981). As a result, the biota changes. Contamination of water from wastewater, mining, landfills, non-point sources, and human fecal matter are reflected in changes in water quality and the biota as well. The objective of the Clean Water Act is to restore and maintain the chemical, physical, and biological integrity of the waters of the United States. Further, the Act recognizes state primacy in managing and regulating the nations water quality. As such, each state protects waters at various levels with assigned classes and standards. NCPN park’s means of determining whether thresholds have been exceeded is to match water quality in the parks to those standards. Those waters not meeting the standards are listed as impaired, and it is the state’s responsibility to address the problem. However, the parks through the Government Performance and Results Act of 1993, are required to assist in the clean-up effort. When water quality standards are exceeded, we recognize that the water is impaired. Parks, however, may want to anticipate exceedances, by monitoring and acknowledging when levels approach standards. Preventative management efforts may take place; however, in many cases the impact or stress occurs outside of the park boundaries. Remediative action will take place only when the water quality does not meet the standard.

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Springs Springs are point sources of water delivery resulting in a fluvial (stream) regime (May no date) (Figure 23). This flow of water from a spring creates erosional and depositional patterns similar to creeks and rivers, but on a much smaller scale. The water picks up rocks and soil particles and drops its bedload as an alluvial deposit. The source of water for springs is ground water which surfaces when an aquitard impedes flow in the aquifer, causing it to surface. Springs may produce large or small amounts of water. Regardless, development atop the aquifer, which supports the flow of water from a spring, can impact that spring. Contaminants, such as saline water from oil and gas development, may enter the aquifer, move through it and exit. Infiltration of other contaminants including automobile associated products, wastewater, landfill runoff, pesticides and mining runoff will also contaminate spring water. Finding contaminated water at the spring source is an indication that the aquifer is contaminated, and therefore directs management to review basin-wide impacts. However, the efficacy of monitoring spring water quality relates to its immediate use by humans and other organisms. Although too late to prevent the problem, monitoring will allow management to curtail culinary consumption. Contamination of spring water in the immediate area by visitation, grazing, and wildlife is possible. Giardiasis, a gastro-intestinal disease caused by drinking contaminated water, arises when fecal contamination from human and other mammals enters the water. In addition to monitoring core parameters and fecal contamination, metals may be most important, although costly. Visitation and grazing (non-native and native) leads to the trampling of vegetation in the riparian zone associated with the spring. As a result, loss of soil, increased erosion, increased runoff, and a number of other phenomena may occur. Monitoring for changes in aquatic biota, particularly, taxa richness and percentages of individuals belonging to tolerant taxa vary consistently with human influence (Karr and Chu 1999). Implementation of biomonitoring in some parks and continuation of this effort in the Southeast Utah Group parks is warranted.

Seeps and Hanging Gardens Hanging gardens are isolated mesophytic communities physically and biologically distinct from terrestrial environments including riparian areas (Figure 24). They exist in a J-shaped archipelago distributed within the drainage system of the Colorado Plateau. The sandstone aquifers of the plateau provide the necessary hydrologic and geologic conditions to control the existence of hanging gardens (May et al. 1995). These are ground water driven systems, whereby the seep is the necessary condition for hanging garden development. Other important controlling features are a lack of fluvial processes that are found in spring systems, and protection from sun and wind (May et al. 1995). Seeps are horizontal exit lines of water where water moving through an aquifer intercepts an impermeable layer or aquitard. Through chemical and mechanical weathering, loosened material falls away from the rock face, producing a protective overhang. Colluvial material falls downslope, and the hanging garden develops therein.

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Figure 23. Conceptual model illustrating mechanisms by which anthropogenic stressors can affect water quality of aquatic ecosystems associated with springs (partially based on Karr and Chu 1999).

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Figure 24. Conceptual model illustrating mechanisms by which anthropogenic stressors can affect water quality of aquatic ecosystems associated with seeps and hanging gardens (partially based on Karr and Chu 1999).

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The relatively small amount of water produced at these seeps presents a dilemma in water quality assessment. From a basin-wide approach, contamination of the aquifer may lead to acute or chronic contamination of waters supporting the hanging garden habitat. Contaminants may include high saline levels from oil and gas development or other non-point sources. Detection of poor water quality at the seep implies that the aquifer has already been degraded, and that the threshold level has already been attained. Monitoring ground water near development sites may be more predictive of early problems. Other basin-wide effects, which may degrade the seep and thus the hanging garden, are development, which impedes infiltration to and through the aquifer. The result is cessation of flow at the seep and loss of the hanging garden habitat. Here again, monitoring the development and recharge to ground water may be more important as a predictor than monitoring flow at the seep. At the seep or hanging garden, visitation may cause erosion of the protective cap to the seep and thus, the hanging garden. Monitoring here for the effects of visitation are important, and notation of excessive erosion (i.e., loss or change of vegetation, invertebrate colonization, and colluvial material) are the indicators that signify a need for changes in management practices. Again, these systems are ground water fed, and can predict the extent to which the associated aquifer is contaminated. They are the-end-of-the-line systems. If the water quality at these points is contaminated or flow has stopped, management has already lost the opportunity to measure the threshold, since the site for monitoring is the aquifer itself near the area of development.

Potholes and Tinajas These water resources within the Northern Colorado Plateau Network are generally confined to desert environments where depressions in bedrock provide catchment and storage areas for water. These water sources are particularly important for wildlife and aquatic organisms. Cattle and humans also find them key to survival in the desert environment. Potholes are typically smaller pockets of water, whereas tinajas, or waterpockets, can be deep and wide. A conceptual model for this type of water resource begins with the upland terrestrial environment (Figure 25). The topography of the sedimentary rock generally dictates where these potholes and tinajas occur. Steep gradients on slickrock above a flattened area comprise the watershed for tinajas. Generally a lack of vegetation on the slickrock ensures rapid runoff and collection in tinajas. Any materials collected with the runoff including may enter the tinaja. Potholes, however, since they are smaller and don’t have a watershed area associated with them, are generally affected by precipitation. In the case of tinajas, the upland geology, soils, and vegetation dictates functions such as capture and retention of water, biogeochemical cycling, evaporation rates from the potholes and tinajas, infiltration, and diurnal temperature changes. Disturbances such as excessive recreation (swimming or trampling the local environment) can impact the tinajas, by increasing or decreasing the runoff to tinajas, and polluting water with fecal material and chemicals. The aquatic biota can also be disturbed through increased siltation or changes in water chemistry. They are unique catchments of water for wildlife and aquatic organisms; they are, however, highly ephemeral and dependent upon precipitation events, temperature and evaporation rates.

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Changes in these controlling mechanisms may immediately reduce the number of tinajas that hold water.

Figure 25. Conceptual model illustrating mechanisms by which anthropogenic stressors can affect water quality of insular aquatic ecosystems associated with slickrock potholes, waterpockets, and tinajas (partially based on Karr and Chu 1999).

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Since the State of Utah (where most tinajas are found) does not regularly assess these waters, water quality is not well known. Monitoring core parameters such as pH, dissolved oxygen and specific conductance are important, but it is more important to monitor the level of water with respect to precipitation, evaporation rates, and changes in surrounding landscape. Perhaps most important, is maintenance of the mechanisms which control flow of water and sediment to these systems, in light of their function in supporting wildlife and aquatic organisms. Presence or absence of water, exceedance of standards for protection of warm water systems, and presence of an aquatic assemblage typical to tinajas may be the trigger for management actions in these systems.

Invasive Exotic Species Conceptual models designed to aid in the development of monitoring protocols for invasive exotic species will be developed cooperatively by NCPN staff and USGS scientists during fall 2002. Several types of models are envisioned: A general theoretical model developed for purposes of prediction and hypothesis

generation; Specific models for particular species and/or functional types in relation to particular

ecosystems and ecosystem attributes (linked with the theoretical model); and Strategic management models for integrating various aspects of exotic species

management–early detection, predictive monitoring (including climate and ecosystem condition), treatment, effectiveness monitoring, and adaptive-management feedbacks.

Hobbs and Humphries (1995) proposed four principle components of plant invasions (Figure 26). They noted that most emphasis by land managers has been on characteristics and control of the invasive plant itself, with relatively little consideration of demographic processes underlying population spread, ecosystem characteristics influencing invasion vulnerability, and human actions facilitating introduction and spread. The NCPN model will address these additional components of plant invasions.

Figure 26. Components of plant invasions (from Hobbs and Humphries 1995).

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NCPN and USGS partners will emphasize the integration of exotic species models and exotic species management with ecosystem models and ecosystem management. For example, Davis and colleagues (2000) recently reemphasized the ecological truism that an ecosystem becomes more susceptible to invasion when there is an increase in the amount of resources that otherwise limit invasion (Figure 27). Consistent with this simple theory, anthropogenic stressors that affect the spatiotemporal distribution of soil resources in a particular arid-semiarid ecosystem (see conceptual models above) may facilitate invasion of that system or of the connected system that receives accelerated inputs of resources. Management focus on the exotic species alone may result in treatment of the symptom rather than the ultimate ecosystem dysfunction. Strategic management models and protocols will be tailored to address the symptom and the dysfunction, as well as the various phases of exotic invasion (Figure 28).

Figure 27. Conceptual model illustrating the theory that an ecosystem becomes more susceptible to invasion when there is an increase in the amount of resources that otherwise limit invasion. Resource availability can increase due to a pulse in supply (A→B), a reduction in resource uptake (A→C), or both (A→D) (from Davis et al. 2000).

Figure 28. Phases of weed invasion (from Hobbs and Humphries 1995).

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Species of Special Concern Conceptual models designed to aid in the development of monitoring protocols for species of special concern (emphasizing threatened and endangered plants) will be developed cooperatively by NCPN staff and USGS scientists during fall 2002. As with exotic-species models, NCPN and USGS partners will emphasize integration of sensitive-species models with ecosystem models and ecosystem management.

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