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The Structure and Functioning of Dryland

Ecosystems—Conceptual Models to Inform

Long-Term Ecological Monitoring

By Mark E. Miller

Prepared in cooperation with the National Park Service

Scientific Investigations Report 2005-5197

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

P. Patrick Leahy, Acting Director

U.S. Geological Survey, Reston, Virginia: 2005

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Suggested citation:

Miller, M.E., 2005, The Structure and Functioning of Dryland Ecosystems—Conceptual Models to Inform Long-Term 

Ecological Monitoring: U.S. Geological Survey Scientific Investigations Report 2005-5197, 73 p.

Keywords: arid ecosystems, conceptual model, degradation, disturbance, ecosystem dynamics, ecosystem management, ecosystem structure and function, indicators, monitoring, resilience, resistance, stressors.
iii

Contents

Abstract...........................................................................................................................................................1

Acknowledgments.........................................................................................................................................1

Introduction and Background......................................................................................................................1

Purpose and Content of This Report..................................................................................................1

Vital Signs Definition ............................................................................................................................4

Purposes of Conceptual Models........................................................................................................4

Geographical and Ecological Scope .................................................................................................4

Structure and Functional Relationships.....................................................................................................5

Background: The Jenny-Chapin Model of Ecosystem Sustainability..........................................5

General Model.......................................................................................................................................7

Regional Climatic and Atmospheric Conditions .....................................................................7

Precipitation Regime..........................................................................................................7

Wind....................................................................................................................................15

Soil Resources............................................................................................................................15

Inherent Edaphic Heterogeneity ....................................................................................15

Soil Functions and Soil Quality .......................................................................................15

Resource Limitations........................................................................................................15

Plant-Soil Feedbacks .......................................................................................................16

Major Functional Groups..........................................................................................................17

Background .......................................................................................................................17

Vegetation ..........................................................................................................................18

Soil Biota ............................................................................................................................19

Biological Soil Crusts .......................................................................................................19

Invertebrate and Vertebrate Consumers ......................................................................22

Natural Disturbance Regimes .................................................................................................27

Background .......................................................................................................................27

Extreme Climatic Events ..................................................................................................27

Fire.......................................................................................................................................27

Insect and Disease Outbreaks .......................................................................................30

Herbivory and Trampling..................................................................................................30

Landscape-Level Relationships ..............................................................................................31

Predominant Anthropogenic Stressors ..........................................................................................33

Visitor Use...................................................................................................................................33

Livestock and Other Large Herbivores...................................................................................33

Fire-Regime Alteration..............................................................................................................33

Invasive Exotic Plants ...............................................................................................................33

Adjacent Land use Activities ...................................................................................................34

Air Pollutants ..............................................................................................................................34

Global Atmospheric Changes ..................................................................................................34

Ecosystem Dynamics ..................................................................................................................................37

Background—Alternative Ecosystem States ................................................................................37

Common Pathways and Processes of Dryland Degradation ......................................................37
iv

Figures

1. Map showing mean annual precipitation and mean seasonal precipitation for

National Weather Service Cooperative Network stations located at or near

NPS units of the Southern and Northern Colorado Plateau Networks ...............................7

2. Map showing location of NCPN and SCPN parks in relation to ecoregions......................9

3. Modified version (A) of the Jenny–Chapin model that serves as the general

ecosystem model for the SCPN and NCPN............................................................................10

4. General conceptual model depicting the structure and functioning of dryland 

ecosystems..................................................................................................................................11

5. Distribution of precipitation events (based on daily values) by size class

at the Needles District of Canyonlands National Park, 1965–1998.....................................13

6. Map depicting regional variations in the estimated maximum intensity of

precipitation (mm/hr) that can be expected to occur over a 5-min period with

50 percent probability during any given year at NWS stations located at or near

NCPN and SCPN units ...............................................................................................................15

7. Magnetic susceptibility (mean ± 1 SE) of surficial sediments at a formerly

grazed grassland site in the Island in the Sky District of Canyonlands

National Park, a currently grazed grassland site outside the Park, and of

Navajo Sandstone bedrock.......................................................................................................26

8. Examples of landscape-level processes and attributes important for

ecosystem monitoring................................................................................................................32

9. Conceptual model depicting major types of degraded alternative states........................38

10. Conceptual model illustrating the processes by which excessive grazing, fire 

suppression, and climate interactively can lead to increasing dominance by

woody plants ...............................................................................................................................40

11. Conceptual model illustrating the processes by which excessive grazing,

soil-surface disturbances, off-site soil-surface disturbances, and

climate interactively can lead to accelerated erosion/redistribution of

soil resources by wind...............................................................................................................43

12. Conceptual model illustrating the processes by which excessive grazing,

soil-surface disturbances, off-site hydrologic function, and climate

interactively can lead to accelerated erosion/redistribution of soil

resources by water ....................................................................................................................43

Transition 1—Exotic Plant Invasion........................................................................................39

Transition 2—Woody Plant Encroachment/Thickening ......................................................40

Transition 3—Soil Erosion/Redistribution..............................................................................42

Transition 4—Soil Erosion/Redistribution..............................................................................45

Transition 5—Exotic Conversion.............................................................................................46

Transition 6—Exotic Conversion.............................................................................................47

Transition 7—Soil Erosion/Redistribution .............................................................................48

Ecosystem Susceptibility to Change ...............................................................................................48

Ecosystem Exposure .................................................................................................................48

Ecosystem Resistance and Resilience ..................................................................................49

Implications for Ecosystem Classification.............................................................................49

Conclusion.....................................................................................................................................................51

References....................................................................................................................................................51
v

13. Example of site where accelerated erosion has led to the significant loss of soil

resources and persistent alteration of ecosystem structure and functioning.................44

14. Conceptual model illustrating the processes by which increasing woody-plant

dominance, excessive grazing, soil-surface disturbances, off-site soil

disturbances, and climate interactively can lead to accelerated

erosion/redistribution of soil resources by wind...................................................................45

15. Conceptual model illustrating the processes by which increasing woody-plant

dominance, excessive grazing, soil-surface disturbances, off-site hydrologic

function, and climate interactively can lead to accelerated erosion/redistribution of

soil resources by water.............................................................................................................46

16. Conceptual model illustrating the processes by which excessive grazing,

soil-surface disturbances, management-prescribed fire and mechanical thinning,

wildfire, atmospheric nitrogen deposition, concentrated livestock activites,

landscape-level soil-resource transfers, and climate interactively can lead to

increasing dominance (and site conversion) by exotic annual grasses...........................47

17. Conceptual model illustrating processes by which increasing woody-plant

dominance, soil-surface disturbances, insect pests, climate, off-site fuel/fire

conditions, wildfire, and management-prescribed fire interactively can lead to

increasing dominance (and site conversion) by exotic annual grasses...........................48

18. Conceptual model illustrating processes by which increasing site dominance by

exotic annual grasses, excessive grazing, soil-surface disturbances, off-site

soil-surface disturbances, and climate interactively can lead to accelerated

erosion/redistribution of soil resources by wind...................................................................49

19. Conceptual model illustrating processes by which increasing site dominance by

exotic annual grasses, grazing, soil-surface disturbances, off-site hydrologic

function, and climate interactively can lead to accelerated erosion/redistribution of

soil resources by water.............................................................................................................50

20. Example of an annualized ecosystem at Arches National Park where

climate-driven fluctuations in ground cover and persistent soil instability likely

contribute to accelerated rates of wind erosion...................................................................50

Tables

1. Selected characteristics of NPS units included in the Southern and

Northern Colorado Plateau Networks.......................................................................................3

2. Compensatory growth of plants in response to tissue removal .........................................24

3. Structure, disturbance dynamics, distribution patterns, current status,

and restoration needs of three contrasting types of piñon–juniper ecosystems

in western North America.........................................................................................................41

Appendix

Appendix Table A. Primary functions and attributes/measures associated with

components of dryland ecosystems........................................................................................66

Appendix Table B. Potential effects of major anthropogenic stressors on structure

and functioning of dryland ecosystems..................................................................................69

Appendix Table C. Climatic conditions, inherent ecosystem properties, and dynamic

ecosystem properties hypothesized to confer resistance and resilience to

selected ecosystem attributes in relation to particular disturbances or stressors ........70
Abstract

The National Park Service (NPS) has undertaken the task 

of developing long-term ecological monitoring programs for 

32 ecoregional networks of parks that have significant natural 

resources. As part of this effort, each network will select 

“vital signs” that are intended to represent the overall health 

or condition of park ecosystems, known or hypothesized 

effects of stressors, or elements that have important human 

values. As an aid to the consideration and selection of vital 

signs for long-term monitoring in NPS units of the Colorado 

Plateau region, this report presents a literature synthesis and 

a set of diagrammatic conceptual models that summarize 

existing knowledge and hypotheses concerning the structure 

and functioning of dryland ecosystems. These generalized 

conceptual models describe functional relationships among 

key structural components of dryland ecosystems; effects of 

climate, natural disturbances, and anthropogenic stressors 

on the functioning of selected ecosystem processes; and how 

these drivers of ecosystem change interact to produce particular patterns of ecosystem dynamics. Because NPS policies 

dictate that monitoring data will be used to help sustain the 

integrity of park ecosystems, conceptual models in this report 

emphasize common processes and pathways of dryland degradation—with the intent that this emphasis will best highlight 

ecosystem attributes that should be monitored for purposes of 

sustaining ecosystem integrity. Although developed to support 

the identification of long-term monitoring needs in NPS units, 

information in this report is broadly applicable well beyond 

NPS management boundaries and should be of use to other 

organizations or persons involved in management, conservation, and monitoring of dryland ecosystems in the Colorado 

Plateau or bordering ecoregions. 

Acknowledgments

This work was supported by the National Park Service 

(Southern Colorado Plateau and Northern Colorado Plateau 

Inventory & Monitoring Networks), the U.S. Geological Survey 

The Structure and Functioning of Dryland Ecosystems—

Conceptual Models to Inform Long-Term

Ecological Monitoring

By Mark E. Miller

(the Southwest Biological Science Center and the Status and 

Trends Program of the Biological Resources Discipline and the 

Earth Surface Dynamics Program of the Geologic Discipline), 

and the U.S. Bureau of Land Management (Grand StaircaseEscalante National Monument). Conversations with Jayne 

Belnap, Jeff Herrick, Brandon Bestelmeyer, Joe Truett, Lisa 

Thomas, Richard Reynolds, Jason Neff, Marith Reheis, and 

Thom O’Dell contributed to ideas presented herein, although 

all of the errors and oversimplifications are solely mine. Review 

comments from Craig Allen, David Breshears, Greg Eckert, 

Denny Fenn, and David Pyke significantly improved the quality 

of the manuscript. Neil Cobb, George San Miguel, and Tamara 

Naumann assisted with figure captions. I thank Tina Kister, 

Craig Brunstein, and Christy Parry for their attention to detail in 

editing the manuscript, and Aneth Wight for map preparation. 

Finally, I thank Shelley Pistorius for the tremendous, wideranging support she provides to the staff of the Canyonlands 

Research Station, and Ann Welshko for her continued good 

humor, encouragement, and tolerance.

Introduction and Background

Purpose and Content of This Report

This report presents conceptual ecological models 

describing the structure and functioning of dryland ecosystems

 of the Colorado Plateau and bordering ecoregions of 

North America. These models and the associated literature 

review were prepared in support of the Southern Colorado 

Plateau Network (SCPN) of the National Park Service’s 



An ecosystem is a spatially explicit unit of the Earth that includes 

all of the organisms, along with all components of the abiotic environment within its boundaries (Likens, 1992, cited by Christensen 

and others, 1996, p. 670). Ecosystem structure refers to the types, 

amounts, and spatial arrangement of biotic and abiotic components of 

an ecosystem. Ecosystem functioning refers to the flow of energy and 

materials through biotic and abiotic components of an ecosystem, and 

includes processes such as primary production, trophic transfer from 

plants to animals, nutrient cycling, water dynamics, and heat transfer 

(Díaz and Cabido, 2001).
Inventory and Monitoring Program (NPS I & M Program). In 

conjunction with the I & M Program, ecoregional networks 

of NPS units have been tasked with the identification of “vital 

signs” to be monitored for the purpose of tracking long-term 

trends in the “health” or condition of park ecosystems. This 

report was developed for SCPN policymakers and researchers to help inform them in the consideration and selection of 

vital signs for dryland ecosystems in 19 NPS units located in 

the Colorado Plateau region of Utah, Arizona, Colorado, and 

New Mexico (table 1). The SCPN and the Northern Colorado 

Plateau Network (NCPN; consists of 16 NPS units) are working together closely to coordinate planning and implementation of their respective monitoring programs. As a consequence, this document also is intended to support the NCPN 

in the development of its monitoring plan and to facilitate the 

coordinated development of monitoring across both networks 

in the Colorado Plateau region. Although developed to support the identification of long-term monitoring needs in NPS 

units, information in this report is broadly applicable well 

beyond NPS management boundaries and should be of use 

to other organizations or persons involved in management, 

conservation, and monitoring of dryland ecosystems in the 

Colorado Plateau or bordering ecoregions. 

This report begins with background information regarding the vital-sign concept, intended purposes of conceptual 

models in relation to ecosystem monitoring, and the geographical and ecological scope of the report. Following the 

background information section, a general conceptual model 

and literature review are presented to characterize important 

functional relationships among biotic and abiotic components 

of dryland ecosystems. In the third section, several additional 

models are presented to describe particular patterns and 

processes of dryland ecosystem dynamics that have significant implications for ecosystem monitoring and management. 

Models in this section emphasize common processes and 

pathways of dryland degradation—with the intent that this 

emphasis will best highlight ecosystem attributes that should 

be monitored for purposes of sustaining ecosystem integrity. 

Supplementary material is presented in accompanying appendix tables A–C. 

USGS plant ecologists evaluating sampling methods in support of the National Park Service Inventory and Monitoring 

Program. San Juan County, Utah. Photograph by Lisa Thomas, NPS.

 Structure and Functioning of Dryland Ecosystems
Network/Park Code State Size (ha) Altitude (m) Ecoregion

Southern Colorado Plateau Network

Aztec Ruins National Monument AZRU NM 130 1705–1764 Colorado Plateau

Bandelier National Monument BAND NM 13,254 1626–3081 Southern Rocky Mts.

Canyon de Chelly National Monument CACH AZ 37,448 1687–2336 Colorado Plateau/Arizona-New 

Mexico Mts.

Chaco Culture National Historic Park CHCU NM 14,090 1832–2096 Colorado Plateau

El Malpais National Monument ELMA NM 46,559 1950–2554 Arizona-New Mexico Mts.

El Morro National Monument ELMO NM 518 2183–2304 Arizona-New Mexico Mts.

Glen Canyon National Recreation Area GLCA AZ/UT 505,868 930–2319 Colorado Plateau

Grand Canyon National Park GRCA AZ 493,050 348–2798 Colorado Plateau

Hubbell Trading Post National Historic Site HUTR AZ 65 1920–1946 Colorado Plateau

Mesa Verde National Park MEVE CO 21,093 1833 – 2613 Colorado Plateau

Navajo National Monument NAVA AZ 146 1658–2294 Colorado Plateau

Petrified Forest National Park PEFO AZ 38,024 1618–1891 Colorado Plateau

Petroglyph National Monument PETR NM 2,915 1519–1838 Arizona-New Mexico Mts.

Rainbow Bridge National Monument RABR UT 65 1129–1492 Colorado Plateau

Salinas Pueblo Missions National Monument SAPU NM 433 1815–2058 Arizona-New Mexico Mts.

Sunset Crater Volcano National Monument SUCR AZ 1,227 2076–2441 Arizona-New Mexico Mts.

Walnut Canyon National Monument WACA AZ 1,456 1896–2106 Arizona-New Mexico Mts.

Wupatki National Monument WUPA AZ 14,350 1304–1744 Colorado Plateau

Yucca House National Monument YUHO CO 14 1767–1805 Colorado Plateau

Total 1,189,205

Northern Colorado Plateau Network

Arches National Park ARCH UT 30,966 1206–1725 Colorado Plateau

Black Canyon of the Gunnison National Park BLCA CO 12,159 1636–2752 Southern Rocky Mts.

Bryce Canyon National Park BRCA UT 14,502 2000–2777 Utah High Plateaus

Canyonlands National Park CANY UT 136,610 1140–2189 Colorado Plateau

Capitol Reef National Park CARE UT 97,895 1182–2730 Colorado Plateau

Cedar Breaks National Monument CEBR UT 2,491 2461–3247 Utah High Plateaus

Colorado National Monument COLM CO 8,310 1411–2160 Colorado Plateau

Curecanti National Recreation Area CURE CO 17,433 1982–2898 Southern Rocky Mts.

Dinosaur National Monument DINO CO/UT 85,097 1442–2747 Utah-Wyoming Rocky Mts.

Fossil Butte National Monument FOBU WY 3,318 2012–2466 Wyoming Basins

Golden Spike National Historic Site GOSP UT 1,107 1317–1613 Great Basin

Hovenweep National Monument HOVE CO/UT 318 1548–2056 Colorado Plateau

Natural Bridges National Monument NABR UT 3,009 1702–2019 Colorado Plateau

Pipe Spring National Monument PISP AZ 16 1495–1559 Colorado Plateau

Timpanogos Cave National Monument TICA UT 101 1669–2452 Utah-Wyoming Rocky Mts.

Zion National Park ZION UT 59,900 1112–2661 Colorado Plateau

Total 474,709

Table 1. Selected characteristics of NPS units included in the Southern and Northern Colorado Plateau Networks. 

Ecoregion designations follow The Nature Conservancy and NatureServe (Groves and others, 2002), as modified from 

Bailey (1995).

Introduction and Background 
Vital Signs Definition

As defined by the NPS (see http://science.nature.nps.

gov/im/monitor/vsm.htm#Definitions), vital signs are a subset 

of physical, chemical, and biological elements and processes 

of park ecosystems that are selected to represent the overall 

health or condition of park resources, known or hypothesized 

effects of stressors, or elements that have important human 

values. The elements and processes that are monitored are a 

subset of the natural resources that park managers are directed 

to preserve “unimpaired for future generations,” including 

water, air, geological resources, plants and animals, and the 

various ecological, biological, and physical processes that act 

on those resources. Vital signs may occur at any level of the 

ecosystems including landscape, community, population, or 

genetic level, and may be compositional (referring to the variety of elements in the system), structural (referring to the organization or pattern of the system), or functional (referring to 

ecological processes). Although vital signs are used to support 

long-term monitoring in NPS units, they have relevance for 

other monitoring efforts useful for ecosystem management.

Purposes of Conceptual Models

In the vital-sign selection process, conceptual models are 

used to summarize existing knowledge and hypotheses concerning the structure and functioning of park ecosystems. An 

important goal of the models is to depict how natural drivers 

(for example, climate) and anthropogenic stressors affect ecosystem structure and functioning. The ability of the monitoring program to detect the ecological effects of anthropogenic 

stressors is dependent upon interpreting trends in resource 

condition against the backdrop of natural ecosystem variation. 

Hypotheses concerning the effects of anthropogenic stressors 

on ecosystem structure and function must be grounded in an 

understanding of the relationship between natural drivers and 

the structure, functioning, and dynamics of ecosystems. Ecosystems can be characterized on the basis of far more structural 

and functional attributes than can be monitored affordably. 

Thus another important goal of the models is to guide the identification of a few important attributes that provide information 

about multiple aspects of ecosystem status (Noon, 2003). 

No single conceptual model can satisfy all needs. Spatially explicit applications, such as ecological resource assessments, monitoring design, and landscape-level ecological 

modeling, ultimately require site-specific models, but monitoring programs also require generalized ecological models 

Ecosystem management is the process of land use decision making and land-management practice that takes into account organisms 

and processes that characterize and comprise the ecosystem and is 

based on the best understanding currently available as to how the 

ecosystem works. Ecosystem management includes a primary goal 

of sustainability of ecosystem structure and function, recognition that 

ecosystems are spatially and temporally dynamic, and acceptance of 

the dictum that ecosystem function depends on ecosystem structure 

and diversity (Dale and others, 2000, p. 642).

to facilitate communication among scientists, managers, and 

the public regarding ecosystems and how they are affected by 

human activities and natural processes. Together, the NPS and 

USGS have adopted an iterative approach of first developing general conceptual models for broadly defined ecosystem 

types, and then adapting and refining those models with sitespecific data concerning abiotic constraints, land use history, 

current condition, and specific patterns of ecosystem dynamics. Models presented in this report are generalized because of 

the great diversity of dryland ecosystems found in SCPN and 

NCPN parks. 

Previous NPS reports established a framework for the 

conceptual modeling effort associated with the NCPN and 

SCPN monitoring programs (see Evenden and others, 2002; 

Thomas and others, 2003; Miller and others, 2003). The 

conceptual framework presented in these earlier documents 

provides a basis for the material included in this report, but it 

is not repeated here in detail. 

Geographical and Ecological Scope

Given the distribution of parks included in the NCPN 

and SCPN, the geographic scope of this report extends from 

northern Utah and southwestern Wyoming southward to northcentral Arizona and New Mexico (fig. 1). This region is centered on the Colorado Plateau physiographic province, but the 

region also includes parts of the Wyoming Basin, the Southern and Central Rocky Mountains, and the Basin and Range 

physiographic provinces (Hunt, 1974). Park altitudes range 

from 348 m at the lower end of the Colorado River corridor 

in the Grand Canyon to 3,247 m at Cedar Breaks National 

Monument (NM) in southwestern Utah (table 1). Mean annual 

precipitation (MAP) at National Weather Service Cooperative 

Network (NWS) stations located in or near NCPN and SCPN 

parks ranges from 162 mm at Page, Arizona, to 752 mm at 

Cedar Breaks NM (fig. 1). Because this report focuses on 

characteristics of dryland ecosystems, it is generally restricted 

to those areas where MAP is less than 450 mm—the upper 

limit for piñon–juniper vegetation (pygmy woodlands and 

forests dominated by Pinus edulis and Juniperus osteosperma) 

at Mesa Verde National Park (NP; Floyd, Colyer, and others, 

2003). With the exception of the dry, high-elevation Gunnison 

Basin where Curecanti National Recreation Area (NRA) is 

located, areas receiving less than 450 mm in this region are 

generally found below 2,100–2,300 m altitude. Approximately 

90–95 percent of NCPN park lands and 85–90 percent of 

SCPN park lands fall below these precipitation and altitude 

zones (estimated from data presented by Evenden and others, 

2002, and Thomas and others, 2003). [See reports by Evenden 

and others (2002) and Thomas and others (2003) for additional 

park-specific information.] Dryland ecosystems within the 

scope of this report are characterized by mixtures of pygmy 

conifers (Juniperus and Pinus spp.), shrubs, dwarf shrubs, 

herbaceous plants, and biological soil crusts. Excluded from 

this report are montane shrublands (or “petran chaparral”) 

and forests that are typically above the piñon–juniper zone. 

 Structure and Functioning of Dryland Ecosystems
Characteristics and dynamics of these systems are addressed 

in a separate report (Vankat, 2004). 

Across the geographic region included in the report, there 

is tremendous diversity in the composition, structure, and functioning of dryland ecosystems. This diversity is attributable to 

regional- and local-scale variations in climate, landforms, soils, 

and land use history. Variations in biogeographic history are 

reflected in the fact that the region also encompasses several 

distinct floristic areas (McLaughlin, 1986, 1989). As a consequence of these factors, SCPN and NCPN parks are found in 

seven broadly defined ecoregions (table 1, fig. 2). 

Numerous schemes have been used to classify plant 

communities, ecosystems, and ecological land units of the 

region (for example, Küchler, 1964; Brown, 1982; Dick-Peddie, 1993; Spence and others, 1995; Creque and others, 1999; 

NatureServe, 2003; Comer and others, 2003). These schemes 

are based on various combinations of floristic, physiognomic, 

edaphic, topographic, and climatic parameters (Spence and 

others, 1995; Comer and others, 2003). Because this report 

focuses on ecosystem structure and function, it adopts a functional approach that generally corresponds with physiognomy 

rather than a particular classification scheme. On the basis of 

conceptual models presented below, this report concludes by 

offering recommendations concerning the type of classification 

best suited for the design of long-term monitoring in drylands. 

Piñon-Juniper woodland in Mesa Verde National Park, Colorado. Photograph by Mark Miller, USGS.

Structure and Functional Relationships

This section begins with a brief overview of the ecosystem framework adopted by the SCPN and NCPN. Following this overview, a general conceptual model and literature 

review are presented to characterize important functional 

relationships among biotic and abiotic components of dryland 

ecosystems. 

Background: The Jenny–Chapin Model 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 (for example, Walker and 

Chapin, 1987; Vitousek, 1994b; Seastedt, 2001). Chapin and 

others (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 

Structure and Functional Relationships 
Green R iver

C olorado R iver

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Ki lometers

N o r t h e r n C o l o r a d o N o r t h e r n C o l o r a d o

P l P l a t e a u N e t w o r k a t e a u N e t w o r k

N o r t h e r n C o l o r a d o N o r t h e r n C o l o r a d o

P l P l a t e a u N e t w o r k a t e a u N e t w o r k

SS o u t h e r n C o l o r a d o o u t h e r n C o l o r a d o

P l P l a t e a u N e t w o r k a t e a u N e t w o r k

SS o u t h e r n C o l o r a d o o u t h e r n C o l o r a d o

P l P l a t e a u N e t w o r k a t e a u N e t w o r k

103

122

 Structure and Functioning of Dryland Ecosystems
Figure 1 (facing page). Map showing mean annual precipitation

(symbol diameter) and mean seasonal precipitation (symbol 

segments) for National Weather Service Cooperative Network 

stations located at or near NPS units of the Southern and 

Northern Colorado Plateau Networks. Shaded zone approximates 

the mean northwestern extent of summer monsoon moisture 

(from Mitchell, 1976). Precipitation data were acquired from the 

Western Regional Climate Center (see http://www.wrcc.dri.edu/

climsum.html). See table 1 for key to four-letter park codes.

biogeochemical cycling” (Chapin and others, 1996, p. 1016). 

These ecosystem characteristics are determined by a set of 

four interactive controls—climate, soil-resource supply, major 

functional groups of organisms, and disturbance regime—and 

these interactive controls both govern and respond to ecosystem attributes. Interactive controls are constrained by the five 

state factors, which determine the constraints of place (Dale 

and others, 2000). The SCPN and NCPN have adopted a modified version of the Jenny–Chapin model as a general ecosystem 

framework for guiding the development of additional conceptual models, the consideration and selection of vital signs, and 

the design of long-term monitoring (fig. 3). 

For long-term monitoring associated with goals of sustaining the health or integrity of ecosystems, a key aspect 

of the Jenny–Chapin 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 and others, 

1996). For example, major changes in soil resources (for 

example, through erosion or other mechanisms) can greatly 

affect productivity, recruitment opportunities, and competitive relationships of plants, and thus can cause significant 

change to the structure and functioning of plant communities 

and higher trophic levels. Changes in vegetation structure can 

affect the ecosystem’s disturbance regime through altered 

fuel characteristics. These factors and processes in combination can result in an altered system which is fundamentally 

different from the original system in terms of structure, 

functioning, and dynamics. 

Functional groups are groups of species that have similar effects 

on ecosystem processes (Chapin and others, 1996). This concept is 

generally synonymous with functional types. 

Ecosystem health is a metaphor pertaining to the assessment and 

monitoring of ecosystem structure, function, and resilience in relation 

to the notion of ecosystem sustainability (following Rapport, 1998, 

and Costanza and others, 1998). A healthy ecosystem is sustainable.

Ecological integrity is a concept that expresses the degree to 

which the physical, chemical, and biological components (including composition, structure, and process) of an ecosystem and their 

relationships are present, functioning, and capable of self-renewal. 

Ecological integrity implies the presence of appropriate species, populations and communities and the occurrence of ecological processes 

at appropriate rates and scales as well as the environmental conditions that support these taxa and processes (http://science.nature.nps.

gov/im/monitor/Glossary.htm). 

General Model

Expanding on the framework of the Jenny−Chapin 

model, figure 4 serves as a general conceptual model describing structural components and functional relationships that 

characterize dryland ecosystems. Climatic and atmospheric 

conditions, soil resources, major functional groups, and 

disturbance regimes characteristic of dryland ecosystems are 

reviewed in this section. (Appendix table A provides a tabular 

supplement to fig. 4.) 

Regional Climatic and Atmospheric Conditions

Precipitation Regime

Precipitation regime is the most important climatic factor 

defining the characteristics of dryland ecosystems because of 

the importance of precipitation inputs for driving water-limited ecological processes such as primary production, nutrient 

cycling, and plant reproduction (Noy-Meir, 1973; Comstock 

and Ehleringer, 1992; Whitford, 2002). Precipitation seasonality (that is, timing in relation to the annual cycle of potential 

evapotranspiration) and form (that is, snow versus rain) are 

particularly important attributes because they strongly control 

the partitioning of precipitation among various compartments 

of the hydrologic budget—evaporation, transpiration, runoff, 

drainage, and soil-water storage. Because of their effects on 

hydrologic partitioning, precipitation seasonality and form 

are major determinants of ecosystem dominance by different 

vegetative life forms and functional groups (Comstock and 

Ehleringer, 1992). 

In the area encompassed by this report, precipitation 

seasonality varies regionally due to the influence of the Arizona monsoon (Mitchell, 1976; Peterson, 1994). The Arizona 

monsoon (also referred to as the “southwest monsoon” or the 

“Mexican monsoon”) is recognized by climatologists as the 

northernmost part of an extensive summer monsoon region that 

extends to central Mexico and the western slopes of the Sierra 

Madre Occidental (Higgins and others, 1998). Areas affected 

by monsoon circulation receive greater amounts of summer 

precipitation from moist air masses derived from the Gulfs of 

Mexico and California. Notably, the mean northwestern extent 

of summer monsoon moisture is approximated by a band which 

cuts across the Colorado Plateau (fig. 1). Areas that are well 

northwest of this band are dominated by cool-season precipitation (for example, Golden Spike National Historic Site, 

Timpanogos Cave NM, and Fossil Butte NM), whereas areas 

that are southeast of this band (for example, Bandelier NM, El 

Malpais NM, and Salinas Pueblo Missions NM) receive higher 

amounts of summer monsoon precipitation from convective 

thunderstorms. Areas close to the band are generally characterized by a bimodal precipitation regime, with summer monsoon 

precipitation that is highly variable from year to year. 

Ehleringer and others (1999) hypothesized that effects 

of global change on atmospheric circulation patterns and precipitation may be seen relatively early in the Colorado Plateau 

region because of the presence of this significant climatic 

Structure and Functional Relationships 
Green R iver

C olorado R iver

Great Basin

Colorado Plateau

Wyoming

Basins

Southern

Rocky

Mountains

Southern

Rocky

Mountains

Arizona− Arizona−New Mexico Mountains untains

Utah−Wyoming

Rocky Mtns

Utah−Wyoming

Rocky Mtns

Utah

High

Plateaus

River

Interstate Highway

State Boundary

Park Boundary

Network Boundary 0 50 100Kilometers

 Structure and Functioning of Dryland Ecosystems
Figure 2 (facing page). Map showing location of NCPN and SCPN 

parks in relation to ecoregions. Ecoregion designations follow The 

Nature Conservancy and NatureServe (Groves and others, 2002), 

as modified from Bailey (1995). See table 1 for key to four-letter 

park codes.

boundary. In an analysis of regional precipitation trends for the 

central Colorado Plateau, Spence (2001) found that there has 

been a weak trend towards increasing winter precipitation but 

no evidence for significant changes in monsoon precipitation 

patterns since the 1960’s. Annual minimum temperatures were 

found to have increased significantly across the region during 

the same time period (Spence, 2001). 

Regional precipitation patterns are affected by globalscale fluctuations in sea-surface temperatures, atmospheric 

pressure, and atmospheric circulation patterns that vary at two 

different time scales (Hereford and others, 2002). Short-term 

interannual variations in precipitation are related in part to the 

occurrence of El Niño and La Niña conditions—the two contrasting phases of the El Niño−Southern Oscillation (ENSO) 

phenomenon that is driven by variations in sea-surface 

temperatures (SSTs) in the eastern tropical Pacific Ocean 

(Hereford and Webb, 1992; Cayan and others, 1999; Hereford and others, 2002). Hereford and others (2002) recently 

reported their analyses of 20th century precipitation patterns 

on the Colorado Plateau in relation to ENSO phases. They 

found that the detailed relationships were complex, but that 

strong El Niño episodes generally increased the variability of 

warm-season precipitation or the frequency of above-normal 

cool-season precipitation. In contrast, strong La Niña episodes 

tended to cause normal, low-variability warm-season precipitation and below-normal cool-season precipitation. Whether 

characterized by dry or wet conditions, extreme years can have 

long-lasting consequences for ecosystem structure and functioning by causing episodes of plant mortality or establishment 

(Ehleringer and others, 1999).

Over decadal-scale time periods, spatial and temporal 

patterns in precipitation across the conterminous United 

States are related to phenomena known as the Pacific Decadal 

Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO; Mantua and Hare, 2002; McCabe and others, 

2004). The AMO is an index of SST variations over the North 

Atlantic Ocean, whereas the PDO reflects SST variations over 

the North Pacific Ocean (Mantua and Hare, 2002; McCabe 

and others, 2004). The PDO and AMO are quasiperiodic and 

may be caused by internal variations in oceanic circulation 

patterns and associated patterns of heat transport (McCabe and 

others, 2004). In their analyses of 20th-century data, McCabe 

and others (2004) found that 52 percent of the spatial and 

temporal variance in multidecadal drought frequency across 

the conterminous United States was attributable to variations 

in these two SST indices. Hereford and others (2002) analyzed 

20th-century precipitation patterns on the Colorado Plateau 

and found evidence for three relatively distinct precipitation 

regimes that appeared to be in phase with the PDO. The first 

of these was a period of relatively wet conditions from 1905 

to 1941. Notably, the onset of this wet period corresponds 

well with the pulse of ponderosa pine establishment that was 

documented throughout much of the Southwest during the 

first decades of this century (Cooper, 1960; Savage, 1991). 

Following this wet period, two other distinctly recognizable 

precipitation regimes occurred from 1942 to 1977 (dry) and 

from 1978 to 1998 (wet). The marked shift to dry conditions 

that began in 1999 and continues through the present suggests 

a transition to the dry PDO phase that could continue for the 

next 2–3 decades (Hereford and others, 2002). This has important implications for ecosystem management and monitoring 

in the region due to the effects of precipitation patterns on 

disturbance regimes (Swetnam and Betancourt, 1998) and on 

the capacity of ecosystems to resist or recover from natural 

disturbances and human land use activities (Ehleringer and 

others, 1999; Whitford, 2002). 

In addition to temporal variability, spatial variability is 

another defining attribute of dryland precipitation regimes 

(Noy-Meir, 1973; Whitford, 2002). Topography and storm 

type are two factors that control spatial variability in precipitation. On a local scale, precipitation tends to increase with 

increasing elevation due to orographic effects of topography (precipitation caused by adiabatic cooling of rising air 

masses), but rain shadows also can develop on the lee side of 

massive topographic features. Such rain-shadow effects can 

produce high-elevation zones of aridity like the Gunnison 

Basin in western Colorado. As for storm type, summer precipitation derived from convective thunderstorms is characterized 

by greater spatial variability than winter precipitation from 

frontal storms (Noy-Meir, 1973; Whitford, 2002). 

The size of precipitation events also is an important attribute of dryland precipitation regimes (Noy-Meir, 1973; Sala 

and Lauenroth, 1982; Lauenroth and Sala, 1992; Ehleringer 

and others, 1999; Whitford, 2002; Austin and others, 2004; 

Loik and others, 2004). Event size and timing (seasonal, 

diurnal, and in relation to antecedent environmental conditions) in combination are important for determining ecological 

responses to precipitation due to effects on hydrologic partitioning. A significant characteristic of dryland precipitation 

regimes is the predominance of small events (for example, see 

fig. 5). Depending on environmental conditions, small events 

(< 5 mm) may trigger soil-surface processes such as nutrient mineralization/volatilization, whereas larger events may 

be required to initiate seed germination, plant physiological 

processes such as photosynthesis, and hydrologic processes 

such as soil-water recharge (Ehleringer and others, 1999). The 

capacity to respond to precipitation events of different sizes 

and timing is an important ecophysiological characteristic of 

plants that determines their ability to persist under particular 

precipitation regimes (Sala and Lauenroth, 1982; Ehleringer 

and others, 1999). 

Precipitation intensity (amount per unit time period) also 

affects hydrologic partitioning of precipitation. Precipitation intensity, soil characteristics (for example, texture and 

antecedent moisture conditions), and soil-surface features (for 

Structure and Functional Relationships 
Ecosystem

Processes

(geo-hydro-bio)

TOPOGRAPHY

GLOBAL

CLIMATE

TIME

(since disturbance)

POTENTIAL

BIOTA PARENT

MATERIAL

Regional Atmospheric

Resources & Conditions

Soil / Water Resources

& Conditions

Disturbance

Regime

Functional

Groups

Increasing CO2 concentrations Increasing temperatures Changing precipitation patterns Increasing O3 concentrations Other airborne pollutants (NH4, NOx, SO2, dust)

Tramp

Tr

Trampling by livestock & visitors Recreational pollutants (e.g., vehicles) Excretion by livestock & visitors Water diversion for agriculture Agricultural & industrial pollutants River regulation

Ecosystem

Processes

(geo-hydro-bio)

Regional Atmospheric

Resources & Conditions

Soil / Water Resources

& Conditions

Disturbanc

Regime

Functiona

Groups

A B

Management control

Management control 

Figure 3. Modified version (A) of the Jenny-Chapin model that serves as the general ecosystem model for the SCPN and NCPN (redrawn with permiNaturalist and The University of Chicago Press; Chapin and others, 1996) and (B) the array of stressors affecting SCPN/NCPN ecosystems arranged in their first-order effects. Complex, higher order effects occur as the four major controls interact via ecosystem processes. The circle represents the b
SOIL RESOURCES

MINERALS, WATER, AIR, ORGANIC

MATTER, ORGANISMS

NUTRIENT & HYDROLOGIC CYCLING

BIOLOGICAL SOIL CRUSTS

CYANOBACTERIA, LICHENS, MOSSES

PRIMARY PRODUCTION,

NITROGEN FIXATION

VEGETATION

SMALL TREES, SHRUBS, GRASSES, 

BELOW-GROUND MUTUALISMS

PRIMARY PRODUCTION,

EPISODIC REGENERATION &

MORTALITY, FACILITATION, COMPETITION

VERTEBRATES

HERBIVORES, GRANIVORES,

PREDATORS / INSECTIVORES

SECONDARY PRODUCTION,

COMPETITION, PREDATION

INVERT

POLLINATORS

GRANIVORES

SECONDARY 

COMPETITIO

resource availability

soil & water retention, resource uptake

microclimatic modification,

N & organic-matter inputs

resource availability soil & water retention, resource uptake, microclimatic modification, organic-matter inputs

altered composition,

structure, productivity

fuel availability &

flammability

safe-site availability microclimatic modification

energy & water balance,

C&N inputs 

food / habitat availability

herbivory, granivory, seed dispersal

trampling & digging pollination, herbivory, granivory

food / habitat availability

decomposition & bioturbation

predation food availability

resource / habitat availability

trampling & bioturbation

resource / habitat availability

energy & water balance

energy & water balance

resource transformation & redistribution

energy balance,

transpiration, wind

modification

precipitation (with nutrients), energy balance, wind, resource redistribution, dust deposition (nutrients and fine soil particles)

evaporation, CO2 & N emission COtranspiration, 2 & N emission Extreme wind, precipitation and temperature events

REGIONAL CLIMATIC

& ATMOSPHERIC CONDITIONS

PRECIPITATION, WIND, RADIANT ENERGY, CO2, N2

PRECIPITATION SEASONALITY,

EVENT FREQUENCY & MAGNITUDE / INTENSITY,

INTERANNUAL & DECADAL VARIABILITY,

SPATIAL VARIABILITY, LONG-TERM TRENDS

DISTURBANCE REGIMES

EXTREME CLIMATIC EVENTS, FIRE,

INSECT/DISEASE OUTBREAKS

FREQUENCY, MAGNITUDE / INTENSITY,

SPATIAL PATTERNING & EXTENT energy & water balance, C&N inputs

Figure 4. General conceptual model depicting the structure and functioning of dryland ecosystems. Ovals indicate major drivers of ecosystem chanas well as important attributes of drivers. Boxes indicate major structural components and important processes. Arrows indicate functional relationsand line weights indicate relative importance of relationships. The model is constrained by global climatic and atmospheric conditions, topography, pbiota, and time. 
Summer rainstorm on the Colorado Plateau, Grand County, Utah. Photograph by Brad Wilcox, Texas A&M 

University.

USGS meteorological station established to monitor climate (temperature, precipitation, wind direction and 

strength) and geologic processes (erosion and deposition) for studies of land use and landscape change on 

the Colorado Plateau (USGS Earth Surface Dynamics Program, http://climweb.cr.usgs.gov/info/sw/clim-met/). 

San Juan County, Utah. Photograph by Rich Reynolds, USGS.

12  Structure and Functioning of Dryland Ecosystems
0-5 5-10 10-15 15-20 20-25 25-30 30-35

Precipitation size class (mm)

0

10

20

30

40

50

60

70

80

P

erc

ent of total

Frequency

Contribution to total precipitation

Figure 5. Distribution of precipitation events (based on daily values) by size class at the Needles District of Canyonlands 

National Park, 1965-1998. 

example, soil-surface roughness; amount, type, and distribution of ground cover) together determine whether precipitation events result in infiltration or runoff (Whitford, 2002; 

Breshears and others, 2003). If precipitation intensity exceeds 

the soil infiltration rate, runoff will be generated—increasing 

the potential for soil erosion. In the Colorado Plateau region, 

precipitation intensity tends to increase with increasing elevation and decreasing latitude (fig. 6), because these variables 

together generally indicate relative exposure to high-intensity 

convective thunderstorms. Results of regression analyses 

indicate that about two-thirds of the variation in precipitation 

intensity across the region can be explained by elevation and 

latitude (F = 2, 29; R2 = 0.66, p < 0.001). 

 The intensity measure used in this analysis was the estimated 

maximum intensity of precipitation (mm/hr) expected to occur over 

a 5-minute period with 50 percent probability during any given year 

at NWS stations located at or near NCPN and SCPN units. Data 

were acquired in December 2003 from the National Oceanic and 

Atmospheric Administration’s Precipitation Frequency Data Server 

(http://hdsc.nws.noaa.gov/hdsc/pfds/). 

Studies conducted in some dryland ecosystems have 

found time lags in the response of above-ground net primary 

production (ANPP) to interannual variations in precipitation (Lauenroth and Sala, 1992; Oesterheld and others, 2001; 

Wiegand and others, 2004). For example, analyses of a 

52-year-long data set from a short-grass steppe ecosystem in 

Colorado found that only 39 percent of the interannual variation in ANPP could be explained by current-year precipitation 

(Lauenroth and Sala, 1992). The best model, which accounted 

for 60 percent of ANPP variability, included precipitation from 

the current year and from the two previous years (Oesterheld 

and others, 2001). Due to this lag or carryover effect, ANPP 

fluctuations are buffered if wet and dry years alternate but 

amplified if multi-year sequences of dry and wet years occur 

(Oesterheld and others, 2001; Wiegand and others, 2004). 

Preliminary analyses of vegetation-monitoring data from 

Canyonlands NP suggest that production-precipitation lags 

also may occur in Colorado Plateau systems (J. Belnap, unpub. 

data, 2005). 

Structure and Functional Relationships 13
Green R iver

C olorado R iver

146

231

0 50 100 200

Ki lometers

N o r t h e r n C o l o r a d o N o r t h e r n C o l o r a d o

P l P l a t e a u N e t w o r k a t e a u N e t w o r k

N o r t h e r n C o l o r a d o N o r t h e r n C o l o r a d o

P l P l a t e a u N e t w o r k a t e a u N e t w o r k

SS o u t h e r n C o l o r a d o o u t h e r n C o l o r a d o

P l P l a t e a u N e t w o r k a t e a u N e t w o r k

SS o u t h e r n C o l o r a d o o u t h e r n C o l o r a d o

P l P l a t e a u N e t w o r k a t e a u N e t w o r k

14  Structure and Functioning of Dryland Ecosystems
Figure 6 (facing page). Map depicting regional variations in 

the estimated maximum intensity of precipitation (mm/hr) that 

can be expected to occur over a 5-min period with 50 percent 

probability during any given year at NWS stations located at or 

near NCPN and SCPN units. Shaded zone approximates the mean 

northwestern extent of summer monsoon moisture (from Mitchell, 

1976). Data were acquired from NOAA’s Precipitation Frequency 

Data Server (see http://hdsc.nws.noaa.gov/hdsc/pfds/). See table 1

for key to four-letter park codes.

Wind

Wind is another atmospheric phenomenon that can 

have important effects on the structure and functioning of 

dryland ecosystems. As a driver of near-surface air circulation, wind strongly affects evapotranspiration rates and thus 

can modify the energy and water balances of plants and soils 

(Larcher, 1995). Like precipitation, wind also is an important force driving the redistribution of soil resources both 

within and among ecosystems (Whicker and others, 2002). 

In a recent comparison of wind- and water-driven erosion 

(off-site redistribution of soil resources) and horizontal 

transport (within-site redistribution of soil resources) in 

dryland ecosystems, Breshears and others (2003) estimated 

that wind erosion exceeded water erosion by about 33 times 

at a shrubland site and by about five times at a forest site. 

At a grassland site, the authors estimated that water erosion 

was about three times greater than wind erosion. Horizontal 

transport by wind was greater than that by water in all three 

systems, including a 2,200-fold difference in the shrubland. 

Soil texture, vegetation structure, and ground cover were 

important variables affecting among-site differences in the 

relative importance of wind and water erosion and transport. 

The frequency-intensity distributions of process-driving wind 

and precipitation events are climatic factors that also affect 

the importance of wind versus water erosion and transport on 

particular sites (Breshears and others, 2003). Finally, wind 

also interacts with topography to influence fire behavior 

and is a major factor determining whether a wildfire can be 

contained or whether it will spread explosively across the 

landscape (see Peters and others, 2004). 

Soil Resources

Inherent Edaphic Heterogeneity

Soil resources, including mineral nutrients, organic matter (including litter), water, and soil biota, are fundamental 

determinants of ecosystem structure and function (Jenny, 

1980; Vitousek, 1994b; Reynolds and others, 2003). A 

characteristic feature of soils in the Colorado Plateau region 

is tremendous spatial heterogeneity attributable to combined 

effects of geology, topography, and geomorphic processes 

(Brotherson and others, 1985; Norton and others, 2003). Due 

to low rates of weathering and pedogenic processes in dryland 

environments, the relative importance of parent material as 

a factor determining soil properties generally increases with 

aridity (Jenny, 1941). Geologic and climatic features of Colorado Plateau drylands have produced weakly developed soils 

with physical and chemical characteristics that closely match 

the shales, sandstones, limestones, and igneous materials from 

which they were derived. Geomorphic processes, such as 

erosion and deposition, have built upon this geologic template 

to generate abrupt or gradational juxtapositions of landforms 

and soils differentiated on the basis of soil depth, particlesize distributions, mineralogy, and degree of profile development (Schimel and others, 1985; Daniels and Hammer, 1992; 

Birkeland, 1999; Norton and others, 2003). Effects of human 

activities (Amundson and Jenny, 1991) and aeolian dust inputs 

(Reynolds and others, 2001) are superimposed on this mosaic. 

Whereas disturbances such as fire often are the primary factors 

responsible for generating landscape patterns in comparatively 

mesic environments (Clark, 1991), inherent edaphic heterogeneity is a primary cause of landscape patterns in dryland parts 

of the Colorado Plateau region. 

Soil Functions and Soil Quality

Soils perform several ecological functions pertinent 

to ecosystem management and monitoring. Soils regulate 

hydrologic processes and the cycling of mineral nutrients. As 

a medium for storage and delivery of water and nutrients, soils 

also sustain the existence and productivity of plant and animal 

populations. The capacity of a specific kind of soil to perform 

these functions is described by the concept of soil quality

(Karlen and others, 1997; Herrick and others, 2002; Norfleet and others, 2003). Soil quality and soil functioning are 

determined by relatively static soil properties, such as texture, 

depth, and mineralogy, and by relatively dynamic soil properties, such as organic-matter content, aggregate stability, soilsurface roughness, and structure (Seybold and others, 1999). 

Compared with soil properties that are relatively static over 

management time frames, dynamic soil properties are more 

subject to change under the influence of climatic fluctuations, 

land use activities, natural disturbances, and management 

actions. But even soil properties such as depth and texture can 

change as a consequence of erosion (Neff and others, 2005). 

Within a given climatic region, soil is the most important 

factor affecting the structure of terrestrial ecosystems because 

of its role in mediating the bioavailability of water and mineral 

nutrients (Whitford, 2002). This is particularly true of dryland ecosystems because small differences in soil physical 

and chemical traits can have relatively large effects on water 

and nutrient bioavailability (Comstock and Ehleringer, 1992; 

McAuliffe, 2003). 

Resource Limitations

In general, water has been described as the soil resource 

that most commonly limits the productivity of dryland ecosystems (Noy-Meir, 1973; Ehleringer and others, 1999). But 

there is increasing recognition that nutrients also can limit 

Structure and Functional Relationships 15
dryland productivity, particularly during periods when water 

is abundant relative to evapotranspirational demands (Havstad and others, 2000; Archer and Bowman, 2002; Whitford, 

2002; James and others, 2005). Water and nutrient uptake 

are not independent (Barber, 1995; Marschner, 1995), and 

Chapin (1991) has argued that the effects of low soil moisture 

on nutrient availability to plants may be as significant as the 

direct effects of water stress on plant performance. Following 

a review of fertilization experiments in dryland ecosystems, 

Hooper and Johnson (1999) concluded that there was no 

evidence for a shift from water to nutrient limitation along a 

geographic gradient of increasing water availability. Instead, 

their review supported the hypothesis that water and nitrogen 

(N) generally are co-limiting in dryland ecosystems. In addition to N, field studies on the Colorado Plateau suggest important roles for phosphorus (Miller and others, 2001; J.C. Neff, 

unpublished data, 2005) and potassium (Belnap and Phillips, 

2001) in structuring dryland ecosystems of the region. 

The retention of water and nutrient resources is essential for sustaining the structure and functioning of dryland 

ecosystems (Ludwig and Tongway, 1997, 2000; Whitford, 

2002). Dynamic soil properties important for water and nutrient retention include soil structure, infiltration capacity, soilsurface roughness, organic-matter content, soil aggregate stability, and soil biotic activity (Herrick and others, 2002). Other 

ecosystem components that promote the capture and retention 

of soil resources include vegetation, biological soil crusts, and 

other soil-stabilizing features such as litter and rocks (Warren, 

2003; Whitford, 2002). 

Plant-Soil Feedbacks

Coupled spatial patterning of vegetation and soil 

resources is a common characteristic of dryland ecosystems 

because of strong interactions between plants and soils (Charley and West, 1975; Schlesinger and others, 1990, 1996). Vegetation affects the spatial distribution of soil resources because 

of processes such as nutrient uptake, above- and below-ground 

litter deposition, and microclimatic modification, as well 

as vegetation interactions with hydrologic processes and 

Wind-driven erosion of unstable soils. San Juan County, Utah. Photograph by Jason Neff, USGS.

16  Structure and Functioning of Dryland Ecosystems
water- and wind-driven erosional processes (Whitford, 2002). 

Vegetation patterns in turn are strongly influenced by spatial 

patterns of resource availability, resulting in a classic positive-feedback relationship (Aguiar and Sala, 1999). Because 

the spatial patterning of soil resources is so closely linked 

with several important biotic and abiotic processes, changes in 

resource distributions both within (canopy vs. interspace) and 

among ecosystems can indicate significant changes in ecosystem functioning (Ludwig and Tongway, 1997, 2000). Some 

workers have suggested that indices of soil-resource patterning 

may prove useful as early warning indicators of ecosystem 

degradation in dryland environments (Herrick and Whitford, 

1995; Havstad and others, 2000). 

Major Functional Groups

Background

Chapin and others (1996) identified biotic functional 

groups (hereafter described as functional types) as one of the 

four interactive controls of ecosystem sustainability because of 

the capacity of dominant functional types to shape the structure and functioning of whole ecosystems. Associated with 

efforts to model ecological consequences of global change, a 

vast literature has developed concerning different approaches 

to deriving or classifying functional types—particularly with 

respect to vegetation (for example, Smith and others, 1997). 

Identification and use of a particular functional-type scheme 

Oblique aerial photograph of the Escalante River drainage in Grand Staircase-Escalante National 

Monument showing heterogeneous patterns of soil and vegetation attributable to effects of geology, 

topography, and geomorphic processes. Garfield County, Utah. BLM aerial photograph.

Structure and Functional Relationships 17
depend on the level or scale of analysis and on the function(s) 

of interest. For example, within a taxonomic group such as 

nematodes, different functional types may be identified on the 

basis of feeding preferences (Coleman and Crossley, 1996). 

The emphasis in this report is on broadly defined ecosystemlevel functional types. Several workers have proposed that the 

most important functions in dryland ecosystems are those that 

control the retention of water and nutrient resources because 

productivity and diversity cannot be sustained in systems 

that fail to retain resources (Ludwig and Tongway, 1997; 

Whisenant, 1999; Whitford, 2002). Functions affecting the 

cycling and retention of water and nutrient resources will be 

emphasized here, but other functions will not be excluded. For 

purposes of this report, it is less important to adopt a specific 

functional-type classification scheme than it is to take a broad 

functional perspective when considering the biotic components of dryland ecosystems. 

Without adopting a particular classification scheme, it 

remains useful to identify two general categories of functional 

types that are equally important for ecosystem dynamics. 

These are (1) functional effect types—organisms with similar 

effects on ecosystem functions such as primary production, 

nutrient cycling, and soil stabilization, and (2) functional 

response types—organisms with similar responses to environmental factors such as climate, resource availability, natural 

disturbances, and land use activities (Walker, 1997; Walker 

and others, 1999; Díaz and Cabido, 2001; Díaz and others, 

2002). The distinction between these two types is important 

for considering how biotic composition affects the resistance 

and resilience of ecosystems to climatic fluctuations and 

changes, natural disturbances, and anthropogenic stressors 

(Walker and others, 1999). Redundancy among organisms in 

terms of their effects on ecosystem processes, and diversity

among organisms in terms of their responses to environmental factors, both can contribute to the buffering of ecosystem 

functioning in relation to changing environmental conditions (Díaz and Cabido, 2001). Although some workers have 

emphasized the importance of overall functional diversity for 

sustaining ecosystem processes (Tilman and others, 1997), the 

functional effect-response distinction suggests that the longterm persistence of ecosystem functioning may be most likely 

when different functional response types are nested within the 

same functional effect type (Walker and others, 1999; Díaz 

and Cabido, 2001). Detailed consideration of functional effect 

types and functional response types probably is most useful 

to ecosystem managers when applied on a relatively site- or 

system-specific basis. Because of the generality of this report, 

Resistance refers to the capacity of a particular ecosystem attribute or process to remain essentially unchanged from its reference 

state or dynamic despite exposure to a disturbance and/or stressor. 

Resilience refers to the capacity of a particular ecosystem attribute 

or process to recover to its former reference state or dynamic after 

exposure to a temporary disturbance and/or stressor (adapted from 

Grimm and Wissel, 1997). Resistance and resilience are dynamic 

properties that vary in relation to environmental conditions (Scheffer 

and others, 2001). 

the functional effect-response distinction is not carried forth 

through the following discussion of biotic functional types. 

Vegetation

Broadly speaking, vegetation generally is recognized 

as the dominant functional type in terrestrial ecosystems. In 

addition to conducting photosynthesis, above-ground 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 soil stabilization 

and nutrient cycling. Plants and litter intercept solar radiation and precipitation, thereby mediating spatial and temporal 

patterns of soil water content and temperature (Breshears and 

others, 1997), and strongly affecting soil-resource conditions 

experienced by other organisms. Roots stabilize soils, are 

conduits for resource acquisition and redistribution, and provide organic-matter inputs to soil food webs. Vegetation also 

provides fuel for fire, as well as resources and habitat structure 

for below-ground and above-ground organisms ranging from 

fungi and bacteria to birds and large mammals (Whitford, 

2002; Wardle, 2002). Finally, carbon storage and the mediation of earth-atmosphere energy/water balances are additional 

vegetation functions that are increasingly emphasized by 

researchers investigating global-change processes (Breshears 

and Allen, 2002; Asner and others, 2003). 

A large number of vegetation attributes affects the manner and extent to which these many functions are performed. 

Size, biomass, photosynthetic rate, relative and absolute 

growth rates, tissue chemistry, basal cover, canopy cover, 

canopy structure, spatial arrangement and contiguity, leaf area, 

leaf longevity, and life span are some of the more important 

vegetation attributes for ecosystem functioning (Chapin, 

1993). Root distribution, photosynthetic pathway, and phenology are additional functional attributes of vegetation that are 

particularly important in dryland ecosystems (Ehleringer and 

others, 1999; Whitford, 2002). With respect to disturbance 

interactions, important functional attributes include palatability, flammability, and mode of post-disturbance regeneration. 

Small trees, shrubs, dwarf shrubs, and perennial grasses 

are the vegetative life forms with the greatest effects on the 

structure and functioning of dryland ecosystems (Whitford, 

2002). In some dryland systems, annual grasses (typically 

exotic) also can have significant effects on ecosystem structure 

and function (for example, Billings, 1990; Belnap and Phillips, 2001; Evans and others, 2001). The two most important 

tree genera in drylands of this region are Juniperus and Pinus. 

Important genera of shrubs and dwarf shrubs include members 

of the Asteraceae (Artemisia, Chrysothamnus, Ericameria, 

Gutierrezia, Tetradymia, and Xylorhiza), the Chenopodiaceae 

(Atriplex, Grayia, Sarcobatus, and Krascheninnikovia), the 

Rosaceae (Cercocarpus, Purshia, Coleogyne, and Fallugia), the Fagaceae (Quercus), and the Ephedraceae (Ephedra). Important genera of perennial grasses include those 

18  Structure and Functioning of Dryland Ecosystems
characterized by the C4

 photosynthetic pathway (Bouteloua, 

Muhlenbergia, Aristida, Pleuraphis, and Sporobolus) and the 

C3 photosynthetic pathway (Acnatherum, Hesperostipa, Poa, 

and Festuca). In general, the relative importance of C3 versus 

C4 perennial grasses in dryland ecosystems increases regionally with latitude and locally with elevation. Genera of annual 

grasses include Bromus, Festuca, and Schismus. 

Many of the functional attributes described above differ 

greatly among vegetative life forms. For example, there are 

relatively large differences among dryland trees, shrubs, and 

perennial grasses in terms of canopy architecture and spatial 

arrangement, as well as in their responses to climate, fire, and 

herbivory. As a consequence, ecosystems characterized by 

different proportions and spatial patterns of trees, shrubs, and 

grasses can be expected to differ greatly in terms of associated ecosystem processes including nutrient cycles, hydrologic 

cycles, disturbance regimes, and wildlife-habitat relationships. 

Likewise, temporal shifts in the relative abundance and spatial 

configuration of vegetative life forms can significantly affect 

the functioning of numerous ecosystem processes. 

Soil Biota

Soil biota represent another broadly defined group of 

organisms that is a major contributor to the structure and 

functioning of dryland ecosystems. Most of the ecosystem 

processes described above in relation to soil resources (that 

is, nutrient cycling, water infiltration and storage, soil aggregate stability) are mediated by soil organisms (Skujins, 1984; 

Whitford, 1996, 2002; Lavelle, 1997; Wardle, 2002). Although 

the general significance of soil biota for ecosystem processes 

(particularly nutrient cycling) has long been acknowledged, 

there is increasing recognition that this diverse group of organisms must be considered much more explicitly in order to 

develop a better understanding of the structure and functioning of terrestrial ecosystems (Bever and others, 1997; Wardle, 

2002; Reynolds and others, 2003). Because of their intimate 

association with other components of dryland ecosystems, soil 

biota in figure 4 are included in components identified as soil 

resources, vegetation, biological soil crusts, and invertebrates. 

Wardle (2002) noted that most terrestrial species occur in 

soil and that the tremendous (and poorly understood) diversity 

of this group has added to the logistical difficulties posed by 

studying below-ground organisms and processes. Soil biota 

include microfloral components (bacteria, algae, and fungi), 

microfaunal components (nematodes, microarthropods, and 

protozoans), and macrofaunal components (earthworms, ants, 

termites, and larval stages of several insect families) that are 

involved in a variety of processes essential for litter decomposition and nutrient cycling. Functioning of these below-ground 

processes is dependent on the amounts and types of organicmatter inputs from vegetation and on soil conditions such 

as moisture availability, soil structure, soil aeration, and soil 

 There also is significant functional variation within each of these 

life forms that must be considered with respect to the structure and 

functioning of specific ecosystems on a site-specific basis.

temperature (Coleman and Crossley, 1996; Whitford, 1996, 

2002; Wardle, 2002). 

Soil biota also include mycorrhizal fungi that form symbiotic associations with roots of many plant species. The mycorrhizal symbiosis is one in which the fungal partner provides 

nutritional benefits to the host plant, and the plant provides 

carbohydrates to the fungi (Smith and Read, 1997). Roots 

colonized by mycorrhizal fungi acquire phosphorus, zinc, and 

possibly copper and N more efficiently than uncolonized roots. 

There is also evidence that mycorrhizae can increase water 

uptake in plants due to the greater soil volume accessed by 

colonized roots (Smith and Read, 1997). Due to their role in 

enhancing resource availability to colonized plants, mycorrhizae have been found to mediate competitive relations between 

plants with differing responses to mycorrhizal colonization 

(Allen and Allen, 1990; Marler and others, 1999). Recent work 

also has shown that the diversity of mycorrhizal fungi can 

determine the diversity and productivity of vascular plant communities (van der Heijden and others, 1998). 

Some species in most of the plant families common to 

dryland ecosystems have been identified as mycorrhizal when 

inspected by botanists (Trappe, 1981). Families with a high 

frequency of mycorrhizal colonization among inspected species include the Asteraceae, Fabaceae, Rosaceae, Poaceae, 

and Solanaceae. Frequency of colonization among inspected 

species in the Chenopodiaceae and Cactaceae is much lower, 

although mycorrhizal species have been found. The Brassicaceae stands out as a common dryland plant family in which 

most inspected species are nonmycorrhizal (Trappe, 1981). 

Another important symbiotic relationship involving soil 

biota is that between plants and N-fixing bacteria. Several 

shrubs that are locally common or abundant throughout the 

Colorado Plateau region are capable of forming a symbiotic 

association with N-fixing actinomycetes in the genus Frankia. 

Actinorhizal shrub genera of the region include Cercocarpus

and Purshia (Rosaceae), Shepherdia (Elaeagnaceae), and 

Ceanothus (Rhamnaceae; Baker and Schwintzer, 1990; cited 

by Schwencke and Carú, 2001). The frequency of actual 

actinorhizal colonization in these genera and the overall contributions of this relationship to dryland N cycling are poorly 

understood. However, it is significant that all of the actinorhizal shrub genera in the region are important forage resources 

both for wild herbivores and for domestic livestock, suggesting that the actinorhizal relationship may be a factor affecting 

forage preferences.

Biological Soil Crusts

Biological soil crusts (BSCs) are biotic communities 

composed of cyanobacteria, algae, microfungi, mosses, and 

lichens that occur on and within the upper few millimeters 

of the soil surface (Belnap, Büdel, and Lange, 2003). These 

diverse communities are characteristic biotic components of 

ecosystems where environmental conditions limit the development of closed-canopy vascular plant communities or thick 

layers of surface litter (Belnap, Büdel, and Lange, 2003). 

They are particularly prominent features in drylands of the 

Structure and Functional Relationships 19
Colorado Plateau region where much pioneering work has 

been conducted concerning their environmental relationships, 

disturbance responses, and ecological functions (for example, 

Rosentreter and Belnap, 2003; Belnap, 2002; Bowker and 

others, 2002). 

In addition to their major contributions to biological 

diversity, BSCs perform several functions in dryland ecosystems. The presence and physiological activity of BSC organisms aggregate soil particles, thereby increasing soil stability 

and reducing the susceptibility of soil to erosion by wind and 

water (Williams and others, 1995a,b). Surface characteristics 

and stability of BSCs also favor the capture and retention of 

aeolian sediments that can be significant sources of mineral 

nutrients in dryland ecosystems (Belnap, Prasse, and Harper, 

2003; Reynolds and others, 2001). BSCs similarly can capture 

and enhance ecosystem retention of windborne and waterborne 

organic matter and seeds (Belnap, Prasse, and Harper, 2003). 

Hydrologic effects of BSCs are complex, with few generalizations possible due to variations in relation both to soil 

factors and BSC factors. [The following synthesis is mostly 

drawn from the recent review by Warren (2003).] Aboveground structures of BSCs intercept rainfall, thereby reducing 

raindrop impact and erosivity and reducing the detachment of 

soil particles that can clog soil pores and inhibit infiltration. In 

this function, cyanobacteria and algae are less effective than 

mosses and lichens. Based on available evidence, BSCs on 

sandy soils (> 80 percent sand content) with inherently high 

infiltration capacities tend to reduce infiltration relative to soils 

of similar texture without BSCs because BSC organisms block 

pore spaces near the soil surface. However, adverse effects of 

BSCs on infiltration in sandy soils may be mitigated by effects 

of BSCs on soil-surface roughness. In cool-desert regions such 

as the Colorado Plateau, frost heaving can result in BSCs with 

pinnacled or rolling soil-surface microtopography (Belnap, 

2003a). Where these microtopographic features significantly 

enhance soil-surface roughness, the velocity of overland flow 

may be reduced, thereby increasing the residence time of 

runoff on hillslopes, enhancing slope retention of water and 

waterborne sediment, and facilitating water infiltration (Belnap, 2003a). 

On relatively fine-textured soils, BSC effects on hydrologic processes can differ from those found on sandy soils—

although the soil-roughness hypothesis still applies (Warren, 

2003). Compared with sandy soils, soils with a significant percentage of clay-size particles are characterized by lower porosity and lower inherent infiltration capacities. Organic carbon 

produced by BSC organisms can contribute to the formation of 

stable soil aggregates that increase the ratio of macropores to 

micropores and thus enhance infiltration. Due to enhanced soil 

aggregate stability, fine-textured soils with BSCs are likely to 

Colorado Plateau vegetation consisting of shrubs (Artemisia tridentata and Krascheninnikovia lanata), 

perennial grasses (Acnatherum hymenoides and Hesperostipa comata), and juniper (Juniperus

osteosperma). The La Sal Mountains are in the background. San Juan County, Utah. Photograph by 

Mark Miller, USGS.

20 Structure and Functioning of Dryland Ecosystems
have higher infiltration rates than soils of similar texture without BSCs. However, in silty soils where a vesicular A horizon

subtends the BSC layer and controls soil infiltration capacity, 

the presence of BSC organisms is unlikely to enhance infiltration except through the hypothesized soil-roughness effect (for 

example, Dobrowolski, 1994). 

In addition to enhancing soil stability and the retention 

of nutrients in sediment and organic matter, BSCs contribute 

to dryland nutrient cycles in other ways. Mosses, cyanobacteria, green algae, and lichens all are photosynthetic and 

thus are significant sources of carbon in dryland ecosystems, 

particularly in interspaces among vascular plants where soil 

crusts can attain 100 percent cover (Lange, 2003). Cyanobacteria (for example, Microcoleus, Nostoc, and Scytonema) and 

cyanolichens (for example, Collema and Peltigera) also are 

capable of fixing atmospheric N into a mineral form that can 

be used by vascular plants (Evans and Lange, 2003; Belnap, 

2002). Although actinorhizal shrubs also are likely contribu-

 Vesicular A horizons as much as several mm thick are common at 

the surface of silty desert soils. Vesicular structure is characterized by 

a large volume of spherical soil voids that appear to form when soil 

air is entrapped in wetted soil. Presence of a vesicular horizon greatly 

reduces soil hydraulic conductivity (Hillel, 1998; Birkeland,1999). 

tors to N cycles in the region (see above), the ubiquity of 

BSCs relative to actinorhizal shrubs suggests that BSCs are 

the major N contributors in the region (Evans and Ehleringer, 

1993; Belnap, 2002). Soil-crust organisms are significant 

food sources for other soil biota, particularly enhancing the 

activity of soil food webs in interspaces among vascular 

plants (Belnap, 2003b). BSCs also affect the nutrient cycling 

activities of soil food webs through their effects on near-surface moisture availability, soil structure, soil aeration, and soil 

temperature (Belnap, 2003b).

BSCs influence vascular plants in a variety of ways. 

Effects of BSCs on soil-surface stability and roughness result 

in seed-bed characteristics that differ greatly from those of 

soils without BSCs. Seed catchment and retention are generally enhanced by the presence of BSCs. Belnap, Prasse, and 

Harper (2003) reviewed the literature concerning effects of 

BSCs on plant establishment and found that research results 

varied depending on BSC characteristics, the plant species 

investigated, methods of study, and environmental conditions. 

BSCs are best viewed as environmental sieves that favor or 

discourage plant establishment, depending on environmental 

conditions and on species-specific attributes such as propagule 

morphology and germination requirements. If a generalization 

Biological soil crusts fill the interspaces among perennial grasses (Hesperostipa comata), shrubs (Atriplex 

canescens and Ephedra viridis) and cacti (Opuntia sp.) in Canyonlands National Park, San Juan County, Utah. 

Photograph by USGS.

Structure and Functional Relationships  21
is possible, it is that BSCs tend to inhibit establishment of 

annuals and species without specialized burial mechanisms 

such as twisting awns (Belnap, Prasse, and Harper, 2003). In 

southeastern Utah, Howell (1998) found that BSCs inhibited 

establishment of the exotic grass Bromus tectorum. Following 

the establishment phase, plants growing in soils with BSCs 

generally have been found to have greater biomass and lower 

root:shoot ratios than comparable plants growing in soils 

without BSCs, suggesting greater availability of soil resources 

in the presence of BSCs (Belnap, Prasse, and Harper, 2003). 

Relative to plants growing in soils without BSCs, plants growing in association with N-fixing cyanobacteria and cyanolichens consistently have greater N concentrations in tissues 

and usually have higher concentrations of the plant-essential 

nutrients potassium, magnesium, copper, and zinc (Harper and 

Belnap, 2001). In contrast, plants growing in soils with BSCs 

commonly have lower concentrations of phosphorus and iron 

than plants growing in soils without BSCs, suggesting that 

plants and BSCs may compete for these elements. Nutritional 

differences between plants grown in soils with and without 

BSCs are greatest in shallow-rooted herbaceous species, probably because they are rooted in near-surface soils that are most 

directly influenced by BSCs (Harper and Belnap, 2001). 

Invertebrate and Vertebrate Consumers

Vertebrate and invertebrate consumers are significant 

contributors to the biological diversity of dryland ecosystems 

in the region (for example, Nelson, 1994; Griswold and others, 1997; Hanna and Graham, 2003; San Miguel and Colyer, 

2003). There are numerous ways in which above-ground 

consumers can directly or indirectly affect the structure and 

functioning of dryland ecosystems (Whitford, 2002). Activities associated with granivory and herbivory are among those 

that have the greatest ecosystem-level consequences because 

of their many effects on vegetation structure and soil processes. Processes of competition and predation likewise can 

have important ecosystem-level consequences by altering the 

structure of consumer food webs, but these processes are not 

reviewed here. 

Granivory is one of the most-studied processes in dryland 

ecosystems, although most research on this topic has been 

conducted outside the region encompassed by this report. 

Organisms that can be important granivores in dryland ecosystems of the region include harvester ants, beetles, rodents, 

frugivorous mammals, and birds (Chambers and others, 1999; 

Whitford, 2002). Through selective harvesting, consumption, 

and dispersal via caching and defecation, granivores can have 

Grains of sand held by cyanobacterial filaments found in biological soil crusts. San Juan County, Utah. 

Photograph by Mark Miller, USGS.

22  Structure and Functioning of Dryland Ecosystems
strong effects on the abundance, composition, and spatial 

distribution of the seed bank (Whitford, 2002). Through time, 

these seed-bank effects can be reflected in the composition 

and spatial structure of plant communities. For example, seed 

hoarding and caching behaviors of corvids such as Steller’s 

jays (Cyanocitta stelleri) and Clark’s nutcrackers (Nucifraga 

columbiana) have been implicated in the upslope migration of 

piñon in the Southwest (Chambers and others, 1999). In desert 

grasslands of southeastern Arizona, Brown and Heske (1990) 

proposed that selective seed harvesting and soil disturbance 

by kangaroo rats (Dipodomys sp.) were responsible for major 

changes observed in vegetation structure over a 12-year period. 

However, on the basis of subsequent studies, Whitford (2002) 

questioned whether the hypothesized mechanisms were sufficient to explain the observed vegetation changes. Whitford 

(2002) also suggested that effects of granivorous rodents and 

ants on soils (that is, resource enrichment through excretion 

and soil disturbance through burrowing) could have greater 

impacts on dryland vegetation structure than granivory itself. 

Like granivory, herbivory can have numerous direct and 

indirect effects on ecosystem properties. Native herbivores in 

dryland ecosystems of the region include insects (grasshoppers, 

moth and butterfly larvae, bark beetles, and many others) and 

mammals such as woodrats (Neotoma spp.), desert cottontails 

(Sylvilagus audubonii), black-tailed jackrabbits (Lepus californicus), bighorn sheep (Ovis canadensis), pronghorn (Antilocapra americana), and mule deer (Odocoileus hemionus). In 

some locations, elk (Cervus elaphus) use of dryland systems 

also can be significant, particularly in winter (Allen, 1989). 

Herbivorous insects and small to medium-size mammals can 

have significant effects on vegetation structure and ecosystem 

processes such as nutrient cycling. With the exception of infrequent insect outbreaks (discussed below as a disturbance), the 

greatest ecosystem-level consequences of herbivory are those 

that can be caused by high densities of large-bodied browsers 

and grazers such as mule deer, elk, and domestic livestock. 

Large herbivores can affect individual plants both directly 

and indirectly through a variety of mechanisms. Direct 

impacts include altered physiological function and morphology attributable to defoliation and trampling (Briske, 1991; 

Briske and Richards, 1995). Defoliation and trampling by 

large herbivores may indirectly influence plant performance as 

a consequence of altered microenvironmental conditions, soil 

properties (Thurow, 1991), mycorrhizal relations (Bethlenfalvay and Dakessian, 1984), competitive relations, and through 

effects on ecosystem processes such as nutrient cycling and 

hydrology. Seed dispersal is yet another indirect mechanism 

by which large herbivores and other animals may affect vegetation structure. Through time, combined direct and indirect 

impacts can result in altered plant population dynamics (for 

example, altered rates of reproduction, recruitment, and mortality) and consequent changes in plant community composition, structure, and distribution (Crawley, 1983; Archer and 

Smeins, 1991; Archer, 1994; Miller and others, 1994; Bich 

and others, 1995). Because of strong interactions of vegetation with nutrient cycling, hydrologic processes, disturbance 

regimes, and geomorphic processes, herbivore-driven changes 

in vegetation structure can have cascading effects on multiple 

ecosystem processes and properties. 

Considerable debate has occurred concerning the consequences of herbivory for plant productivity (for example, 

McNaughton, 1983, 1986, 1993; Belsky, 1986, 1987; Painter 

and Belsky, 1993; Dyer and others, 1993). This debate has 

centered on the phenomenon of “compensatory growth” in 

defoliated plants. Compensatory growth generally is defined as 

a positive response of plants to injury by defoliation (Belsky, 

1986). McNaughton (1983, p. 329) described the phenomenon 

as follows:

“Compensatory growth in plants subjected to herbivory may alleviate the potential deleterious effects of 

tissue damage, whether to vegetative or reproductive 

organs. Tissue destruction is rarely, if ever, translated monotonically into a proportional reduction of 

final yield. Internal mechanisms of compensation 

involve modifications of plant metabolism; external 

mechanisms of compensation involve modifications 

of the plant environment that are favorable to plant 

growth and yield.”

Belsky (1986) clarified the discussion by subdividing 

the term “compensation” into three separate terms defined by 

the productivity of a defoliated plant in relation to that of an 

undefoliated plant (see table 2).

Numerous physiological (internal) and/or microenvironmental (external) mechanisms may contribute to the occurrence of compensatory growth in plants (Crawley, 1997). For 

example, remaining foliage may experience increased relative 

growth rates due to diminished shading and increased light 

levels following grazing (a microenvironmental mechanism), 

or remaining foliage of a grazed plant may experience greater 

above-ground growth due to the reallocation of resources from 

root growth (a physiological mechanism). In general, the potential for and significance of compensatory growth is believed to 

vary in relation to the timing of herbivory in relation to plant 

phenology, the frequency and intensity of herbivory, the availability of resources both within the plant and in the surrounding soil environment (water and mineral nutrients), as well 

as the presence and competitive ability of neighboring plants 

(Briske and Richards, 1994, 1995; Crawley, 1997). Given the 

importance of resource availability for the occurrence and 

significance of compensatory growth, DeAngelis and Huston 

(1993) and Bartolome (1993) reasoned that overcompensation 

was most likely to occur in productive, intensively managed 

systems rather than in resource-poor environments characteristic of dryland ecosystems. In a review concerning the physiological responses of plants to grazing, Briske and Richards 

(1994, p. 169) concluded that (1) compensatory mechanisms 

only infrequently increase the total growth of defoliated plants 

above that of undefoliated plants (that is, “overcompensation” 

is rare), and (2) application of the concept to management of 

wildland vegetation is unwarranted because the phenomenon is 

so complex and poorly understood. 

Structure and Functional Relationships  23
Although compensatory growth of individual plants may 

occur, alteration of competitive relations among defoliated or 

differentially defoliated plants is of greater consequence for 

plant populations and communities (Briske, 1991; Archer and 

Smeins, 1991; Briske and Richards, 1994; Crawley, 1997). 

Plants that possess a greater capacity for regrowth following 

defoliation experience a competitive advantage over defoliated competitors that possess a lesser capacity for regrowth. 

Similarly, plants that are defoliated less frequently or less 

intensively experience a competitive advantage relative to 

plants that are defoliated more frequently or more intensively 

due to relative differences in accessibility or palatability. For 

an individual plant, the most significant benefit arising from 

herbivory is the relative advantage gained when a neighboring 

plant has been reduced in size and competitive ability by an 

herbivore (for example, Caldwell and others, 1987). Through 

time, altered competitive relations eventually can be expressed 

in population dynamics and plant community structure 

(Briske, 1991). 

Large herbivores also can affect the productivity and 

composition of plant communities through numerous indirect 

and direct effects on nutrient cycling (Archer and Smeins, 

1991). Herbivore-driven shifts in plant community structure 

can affect nutrient cycles by altering the capacity of vegetation to capture and retain soil and water resources (Whitford, 

2002) and by altering the quantity and quality of organic-matter inputs (Bardgett and Wardle, 2003; Reeder and others, 

2004). Herbivory removes foliage and directly diverts nutrients 

from litter and physiological processes of intra-plant cycling. 

Nutrients acquired from foliage may be incorporated in animal 

biomass or spatially redistributed across the landscape in urine 

and dung. Where excreta are deposited, productivity may be 

enhanced if nutrients contained in the excreta are accessible to 

nearby plants. In other portions of the landscape, productivity 

may be reduced due to the removal of nutrients in foliage. 

Significant losses of nutrients from ecosystems may 

occur as a consequence of herbivory (Archer and Smeins, 

1991; West 1991). A significant proportion of the N consumed 

in foliage is lost to the atmosphere in gaseous products of 

digestion and through volatilization of ammonia from dung 

and urine. In ecosystems where dung beetles are not present to bury feces and cycle nutrients through soil food webs 

(including much of the Colorado Plateau region), as much as 

80 percent of fecal N may be lost to volatilization, with greatest losses expected from sandy, high-pH soils (Woodmansee, 

1979). Schimel and others (1986) found lower rates of N loss 

from cattle urine, possibly because urine enters the soil more 

readily than dung. These researchers reported higher N losses 

from cattle urine in coarse soils (12–27 percent) than from 

fine-textured soils (0–2 percent). Nutrients incorporated in animal biomass are removed from the ecosystem entirely when 

livestock are removed. 

In dryland ecosystems where components of biological 

soil crusts are the predominant sources of N inputs, trampling 

by large herbivores also can have direct effects on nutrient 

cycling. Evans and Ehleringer (1993) found that N-fixing cyanobacteria and cyanolichens in biological soil crusts were the 

primary sources of N inputs to a piñon–juniper ecosystem in 

southern Utah. They hypothesized that trampling disturbance 

of biological soil crusts could eventually result in long-term 

ecosystem degradation due to diminished N inputs by damaged or eliminated N-fixing crust organisms. In southeastern 

Utah (Canyonlands NP), Evans and Belnap (1999) compared 

soil N dynamics between an ungrazed grassland and an adjacent grassland that experienced intermittent winter grazing 

by livestock prior to 30 years ago. Cover of N-fixing cyanolichens was 5.5 percent in the ungrazed grassland and less than 

1 percent in the formerly grazed grassland. Compared with 

the formerly grazed grassland, the ungrazed grassland was 

characterized by soil-crust N fixation rates that were 2.5 times 

greater and soil N content that was 1.35–2.5 times greater. The 

authors concluded that soil disturbance had resulted in longterm consequences for N cycling and storage in the dryland 

ecosystem they studied. Thus, although some plants may benefit from localized resource enrichment attributable to herbivore 

excreta, the literature suggests that the long-term consequence 

of herbivory and trampling by large ungulates in dryland 

ecosystems could be a gradual ecosystem-level decline in N 

Possible Plant

Responses to Defoliation

Productivity of Defoliated Plant in Relation 

to that of Undefoliated Plant Growth Compensation (%)

1. Overcompensation Defoliated > Undefoliated >100

2. Exact compensation Defoliated = Undefoliated 100

3. Undercompensation Defoliated < Undefoliated <100

a. Partial compensation Defoliated < Undefoliated 1-99

b. No compensation Defoliated < Undefoliated 0

c. Damage Defoliated < Undefoliated <0

Table 2. Compensatory growth of plants in response to tissue removal (modified from Belsky 1986).

24  Structure and Functioning of Dryland Ecosystems
USGS geologist measuring soil magnetic properties to support studies relating 

vegetation patterns to geomorphic processes affected by climate and land use. 

San Juan County, Utah. Photograph by Mark Miller, USGS.

Magnetic susceptibility meter used for measuring magnetic properties of soils and rocks. San Juan County, 

Utah. Photograph by Mark Miller, USGS.

Structure and Functional Relationships  25
In Park Outside Park Navajo Ss

0.00

0.02

0.04

0.06

0.08

0.10

0.12

n=39

n=101

n=30

Magnetic Susceptibility (10-6 SI units) 

Figure 7. Magnetic susceptibility (mean ± 1 SE) of surficial sediments at a formerly grazed grassland 

site in the Island in the Sky District of Canyonlands National Park (ungrazed for 30 years), a currently 

grazed grassland site outside the Park, and of Navajo Sandstone bedrock—the principal parent 

material for soils at both sites. Surface soils at both sites are classified as Begay fine sandy loams 

(coarse-loamy, mixed, mesic Ustollic Camborthid). The higher magnetic susceptibility value for soils at 

the formerly grazed site indicates greater soil content of dust-borne magnetite, clay- and silt-size soil 

particles, and rock-derived mineral nutrients relative to soils at the currently grazed site outside of the 

park, thus suggesting loss of accumulated nutrients at the grazed site. The value for Navajo Sandstone 

indicates a baseline measure of surficial sediments without aeolian magnetite.

fertility. Following a review of mechanisms by which aboveground and below-ground herbivores affect nutrient cycling, 

Bardgett and Wardle (2003) concluded that positive effects of 

herbivory on soil processes and soil biota are most common in 

ecosystems with high soil fertility and relatively high consumption rates, whereas adverse effects are most common in 

low-fertility ecosystems with relatively low consumption rates. 

In addition to impacts of trampling on N inputs where 

biological soil crusts are present, repeated trampling by large 

herbivores can destabilize soils and result in the depletion of 

rock-derived nutrients through the aeolian loss of fine soil 

particles. For example, Neff and others (2005) found lower 

contents of fine soil particles and rock-derived nutrients at two 

formerly grazed sites relative to an ungrazed site at the Needles District of Canyonlands NP. On the basis of soil magnetic 

properties which indicate the presence of far-traveled aeolian 

dust in these sandstone-derived soils, the authors attributed 

soil differences between the ungrazed and grazed sites to 

accelerated wind erosion caused by destabilizing effects of 

repeated trampling at the grazed sites. Because of the strong 

positive relationship between soil magnetic susceptibility and 

several lab-based measures pertaining to the bioavailability of 

rock-derived nutrients in sandstone-derived soils (for example, 

cation exchange capacity, silt content, phosphorus and potassium bioavailability), magnetic susceptibility shows promise 

as an indicator of long-term soil-surface stability in some 

Colorado Plateau landscapes (for example, fig. 7). 

Some workers have hypothesized that trampling by 

large herbivores has beneficial impacts on infiltration 

(Savory and Parsons, 1980; Savory, 1988). However, an 

extensive amount of hydrologic research has failed to support this hypothesis (Spaeth and others, 1996; Holechek 

and others, 2000), indicating instead that trampling tends 

to result in lower infiltration rates where it destroys stable 

26  Structure and Functioning of Dryland Ecosystems
soil aggregates and leads to a deterioration of soil structure 

(Thurow, 1991). Hydrologic impacts of trampling by large 

herbivores vary in relation to soil type (for example, texture 

and the presence of a vesicular A horizon), soil water content, seasonal climatic conditions, vegetation type, and the 

magnitude of trampling (Thurow, 1991). 

Natural Disturbance Regimes

Background

A disturbance is “...any relatively discrete event in time 

that disrupts ecosystem, community, or population structure 

and changes resources, substrate availability, or the physical 

environment” (White and Pickett, 1985, p. 7). For purposes 

of ecosystem management and monitoring, disturbances are 

considered to be ecological factors that are within the range of 

conditions naturally experienced by the ecosystem (for example, drought). These are differentiated from stressors, which 

may fit the definition of disturbance but are outside the range of 

disturbances naturally experienced by the ecosystem and typically are anthropogenic in origin (Whitford, 2002). Implicit in 

the “natural” aspect of these definitions is the need to identify 

ecosystem-specific reference conditions which are framed with 

respect to a particular time period and place (White and Walker, 

1997; Landres and others, 1999). In addition to these temporal 

and spatial bounds, ecosystem managers should explicitly identify associated goals, assumptions, and value judgments when 

adopting a particular set of reference conditions as the “natural” standard for management and monitoring (Truett, 1996; 

Landres and others, 1999). Late-Holocene, pre-European conditions often are identified as the standard for defining natural 

disturbance regimes, although it is important to recognize the 

potentially important role of pre-European human populations 

in shaping disturbance regimes and ecosystem conditions found 

at the time of European contact (Betancourt and Van Devender, 

1981; White and others, 1999). 

In addition to climatic fluctuations, disturbances are 

major drivers of ecosystem change and variability. Disturbance 

is a factor affecting the spatial and temporal dynamics of all 

ecosystems, but disturbances differ greatly among ecosystems 

in relation to climate, topography, substrate, and ecosystem 

attributes such as vegetation structure. Disturbances can be differentiated on the basis of several descriptors which, taken in 

combination, characterize the disturbance regime. Descriptors 

of disturbance regimes include the kind of disturbance (for 

example, surface fire versus windthrow), spatial characteristics 

(for example, patch size, spatial extent and patterning), temporal characteristics (for example, frequency, timing, return interval), specificity (for example, selective effects by species, age 

class, or landscape unit), magnitude (for example, intensity, 

severity), and synergisms (for example, interactions with other 

disturbances ; Sousa, 1984; White and Pickett, 1985; White 

and others, 1999). Most ecosystems experience more than one 

type of disturbance, and interactions among disturbances commonly occur because particular disturbance events can affect 

the capacity of ecosystem components or processes to resist or 

recover from subsequent disturbances. Similarly, interactions 

can occur between natural disturbances and anthropogenic 

stressors (White and others, 1999; Archer and Stokes, 2000). 

Because disturbances play such an important role in shaping 

the structure and functioning of ecosystems (strongly interacting with soil resources and vegetation structure), disturbanceregime alteration is one of the most common ways by which 

human activities affect the functioning and sustainability of 

ecosystems (Chapin and others, 1996). 

Extreme Climatic Events

Episodic severe climatic events are major disturbances in 

dryland 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 widespread mortality or enabling establishment 

of long-lived plants that are structural dominants. The erosive 

energy of extreme precipitation and wind events also can 

result in massive transport and redistribution of soil resources, 

potentially inducing geomorphic changes that fundamentally 

alter site conditions. Event sequencing (for example, timing of 

flooding in relation to drought) is an important factor that can 

affect ecosystem resistance and resilience to episodic climatic 

events. Although episodic, event-driven change is an important 

feature of many ecosystems (Holling, 1996; Scheffer and others, 2001), it is particularly characteristic of dryland ecosystems (Whitford, 2002). Examples from this region include the 

1950’s drought that affected the structure of piñon–juniper 

woodlands throughout much of the Southwest (Betancourt and 

others, 1993; Allen and Breshears, 1998), as well as the recent 

severe drought (the “early 21st century drought” of Webb and 

others, 2004) that has caused region-wide mortality in piñon 

(USDA Forest Service, 2003) and sagebrush (Artemisia spp.). 

Climatic conditions and events can affect ecosystem susceptibility to other disturbances such as fire and insect outbreaks 

(Swetnam and Betancourt, 1998), as well as affect ecosystem 

resistance and resilience to anthropogenic stressors (Archer 

and Stokes, 2000; Scheffer and others, 2001). 

Fire

Fire is another type of natural disturbance that can have 

many direct and indirect effects on the structure and functioning of dryland ecosystems. One of the most significant direct 

effects of fire is the alteration of vegetation composition 

and structure due to the selective damaging or elimination 

of fire-intolerant life forms or age classes (Whelan, 1995). 

Specific effects of fire on vegetation structure vary in relation 

to fire-regime characteristics (for example, frequency, intensity, seasonality, and spatial patterning) and fire responses of 

dominant vegetative life forms. Fire-regime characteristics 

are strongly influenced by the vegetation itself, as well as by 

preceding and coincident weather conditions that affect fuel 

availability, fuel flammability, and fire behavior. In some 

Structure and Functional Relationships  27
dryland ecosystems, low-intensity surface fires play a role 

in thinning or eliminating fire-intolerant woody vegetation 

and favoring the relative dominance of relatively fire-tolerant 

graminoids (Jameson, 1962; Wright, 1980). In systems where 

repeated surface fires are required to maintain grassland- or 

savanna-like physiognomic structure, the necessary fire 

frequency depends on the amount of time needed for predominant woody taxa to (1) develop features that confer resistance 

to surface fires (for example, thick bark and elevated canopies) 

or (2) competitively exclude herbaceous taxa that provide fine 

fuels to support surface fires. Most effects of fire on wildlife 

also are mediated through effects on vegetation structure and 

nutritional quality of forage (Smith, 2000).

Effects of fire on vegetation structure have multiple 

ecosystem-level consequences because of strong vegetation 

interactions with soil, hydrology, and geomorphic processes. 

Fire-caused reductions in ground cover and vegetation structure can result in significant erosional losses of soils, nutrients, 

and organic matter by water and wind (Johansen and others, 

2001; Whicker and others, 2002). Soil hydrophobicity caused 

by fire is another factor which can result in accelerated waterdriven erosion because of decreased infiltration and increased 

runoff (Johansen and others, 2001; MacDonald and Huffman, 

2004). In a study conducted in burned and unburned Chihuahuan desert shrublands, Whicker and others (2002) found that 

aeolian soil movement measured by passive soil collectors was 

70 times greater at the burned site relative to the unburned site 

during high-velocity wind events shortly after fire occurrence 

and was 3 times greater over the entire 8-month study period. 

Fire also has significant ecosystem-level consequences 

due to effects on nutrient cycles (Raison, 1979; Blank and 

others, 1994a, b). Depending on the type and intensity of fire 

events, fire can (1) increase nutrient bioavailability on a shortterm basis due to ash deposition and accelerated rates of nutrient cycling, and (2) deplete total nutrient stocks due to gaseous 

losses (particularly N) and off-site transfers of ash (Raison, 

1979). Nutrient losses in gases and ash are generally proportional to heat generated and organic-matter consumed by fire 

(Raison, 1979; Schlesinger, 1997). Depending on fire severity, other soil characteristics can be affected by fire, including 

pH (typically increased by ash deposition), cation exchange 

capacity and infiltration capacity (both typically decreased by 

organic-matter losses and transformations; Raison, 1979), and 

erodibility (Whicker and others, 2002). 

Natural fire regimes vary greatly among dryland ecosystems of the Colorado Plateau region. Fire generally is insignificant as a natural disturbance in sparse desert shrublands 

dominated by taxa such as Atriplex confertifolia, A. corrugata, 

Coleogyne ramosissima, and Sarcobatus vermiculatus, as 

well as in semidesert grasslands dominated by taxa such as 

Acnatherum hymenoides, Hesperostipa comata, Aristida spp., 

Sporobolus spp., and Pleuraphis jamesii. These grasslands 

have been described as “galleta—three-awn shrub steppe” 

(Küchler, 1964; West and Young, 2000) and are characteristic 

Soils susceptible to erosion following fire in a piñon-juniper woodland, Dinosaur National 

Monument, Moffat County, Colorado. Photograph by Mark Miller, USGS.

28  Structure and Functioning of Dryland Ecosystems
of Canyonlands NP, Capitol Reef NP, Arches NP, Glen Canyon NRA, and Chaco Culture National Historic Park. 

In contrast to these fuel-limited systems where fire is of 

minimal importance, fire is a significant natural disturbance in 

several other dryland ecosystems of the region. These include 

sagebrush shrublands and shrub steppe (for example, Dinosaur 

NM, Fossil Butte NM, and Black Canyon of the Gunnison 

NP), relatively productive semidesert grasslands and juniper 

savannas (for example, Wupatki NM; Jameson, 1962; Johnsen, 1962), and piñon–juniper woodlands and forests (for 

example, Mesa Verde NP and Zion NP). Miller and others 

(1994) estimated that natural fire-return intervals in sagebrush 

steppe varied from 20–30 years in mountain big sagebrush 

systems (Artemisia tridentata ssp. vaseyana) to 50–100 years 

in Wyoming big sagebrush systems (A. tridentata ssp. wyomingensis) prior to the introduction of livestock and effective 

fire suppression. However, caution is warranted regarding 

generalizing from these estimates since they were derived 

from observations made in Great Basin ecosystems rather 

than Colorado Plateau ecosystems. Fire-history studies in 

sagebrush systems also are hampered by the absence of plant 

species that record fires by means of datable scars. 

Natural fire regimes in ecosystems vegetated by various 

species of piñon and juniper are poorly understood. Contributing factors include the tremendous structural and functional 

diversity of systems broadly categorized as piñon–juniper vegetation types (for example, Harper and others, 2003) and the 

relative lack of area-specific fire-regime studies conducted in 

such systems across the region (Baker and Shinneman, 2004). 

In a systematic review of the natural role of fire in piñon–

juniper ecosystems, Baker and Shinneman (2004) concluded 

that (1) spreading, low-severity surface fires probably were 

Prior to a prescribed fire conducted by NPS resource managers, vegetation on Iron Springs Bench in Dinosaur National Monument 

was dominated by Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis). Post-fire vegetation is dominated by native 

grasses including western wheatgrass (Pascopyrum smithii), bluebunch wheatgrass (Pseudoroegneria spicata), muttongrass 

(Poa fendleriana), and needle-and-thread grass (Hesperostipa comata). Dinosaur National Monument, Moffat County, Colorado. 

Photograph by Mark Miller, USGS.

Structure and Functional Relationships  29
uncommon in such ecosystems prior to EuroAmerican settlement, and (2) much additional area-specific research is urgently 

needed to provide a foundation for science-based management 

and restoration efforts. In this latter conclusion, they are in 

agreement with other workers who have warned ecosystem 

managers not to rely uncritically on fire-regime generalizations derived from studies conducted elsewhere (for example, 

Veblen, 2003; Romme, Floyd-Hanna, and Hanna, 2003). 

Some of the most extensive work on piñon–juniper fire 

regimes on the Colorado Plateau has been conducted at Mesa 

Verde NP. Floyd and others (2000, 2004) recently estimated 

that the natural fire rotation time (amount of time required 

to burn an area equal in size to the entire area under consideration) in piñon–juniper ecosystems at Mesa Verde NP 

is approximately 400 years (Floyd and others, 2000, 2004; 

Floyd, Romme, and Hanna, 2003; Romme, Floyd-Hanna, and 

Hanna, 2003). No fire scars indicative of surface-fire occurrence have been located at Mesa Verde NP despite extensive 

searching. Instead, minimum stand ages ranging from 200 

to over 400 years indicate that the natural piñon–juniper fire 

regime at Mesa Verde NP is one characterized by infrequent, 

stand-replacing canopy fires. In a study conducted on the 

Uncompahgre Plateau north of Mesa Verde NP, Eisenhart 

(2004) also concluded that (1) there was no evidence to support the idea that frequent, low-severity surface fires were 

common in any piñon-juniper ecosystems in her study area, 

and (2) the predominant natural disturbance regime in these 

ecosystems is infrequent, stand-replacing canopy fires. 

In an attempt to clarify issues associated with piñonjuniper fire regimes, Romme, Floyd-Hanna, and Hanna 

(2003) proposed a framework describing a set of hypotheses 

concerning variations in piñon-juniper structure, function, 

and status (see table 3 on p. 41). They identified three types 

of piñon–juniper systems that are fundamentally different in 

these respects. Of the three types, the authors hypothesized 

that only the piñon–juniper grass savanna is characterized 

by a natural fire regime consisting of frequent, low-severity surface fires (table 3). Grasslands and juniper savannas 

found in and around Wupatki NM exemplify this type of 

system (Jameson, 1962; Johnsen, 1962), as did significant 

proportions of the landscape at Bandelier NM prior to the 

introduction of livestock in the 19th century (Allen, 1989). 

The piñon–juniper shrub woodland is a vegetation type with a 

natural disturbance regime that involves moderately frequent 

stand-replacing fire carried by shrub and tree canopies (table 

3; Romme, Floyd-Hanna, and Hanna, 2003). The authors 

hypothesize that the natural post-fire successional sequence in 

this type proceeds from herb dominance, to shrub dominance, 

to a mixed shrub-tree community (with young trees) before 

the sequence is reset by a stand-replacing fire. Evidence 

exists to suggest that this type of dynamic may be naturally 

characteristic of some sagebrush ecosystems on the Colorado 

Plateau (for example, Rowlands and Brian, 2001; Harris 

and others, 2003). Finally, the studies conducted at Mesa 

Verde NP (Floyd and others, 2004) and on the Uncompahgre 

Plateau (Eisenhart, 2004) provide evidence for the third type 

of system—piñon–juniper forests—which is characterized by 

infrequent, stand-replacing fires carried by tree canopies. 

Insect and Disease Outbreaks

The roles of insect and disease outbreaks in dryland ecosystems are poorly understood. In montane and boreal forests, 

insects are recognized as major agents of disturbance (for 

example, Veblen and others, 1991; Logan and others, 2003). 

It is believed that the occurrence of insect outbreaks in treedominated ecosystems is linked with climatic conditions that 

diminish the vigor and insect resistance of host plants and/or 

affect life cycles and dispersal patterns of insect herbivores 

(Swetnam and Betancourt, 1998; Logan and others, 2003). In 

piñon–juniper ecosystems, outbreaks of the bark beetle Ips confusus (piñon ips) can be triggered by drought conditions that 

weaken host-tree populations (Leatherman and Kondratieff, 

2003). Drought and ips beetle outbreaks in combination can act 

as broad-scale disturbances in these systems, as reflected by the 

massive mortality that has occurred recently in piñon populations across the southern Colorado Plateau region in response 

to the early 21st century drought (USDA Forest Service, 2003). 

As with fire, insect outbreaks thus can interact with climate to 

generate long-term changes in vegetation structure (Allen and 

Breshears, 1998). Insect- or pathogen-generated changes in 

vegetation structure can have multiple ecosystem-level consequences due to vegetation interactions with nutrient cycles, 

hydrologic processes, and geomorphic processes. 

Herbivory and Trampling

Herbivory and trampling are additional natural disturbances in dryland ecosystems of the region. Potential ecosystem-level effects of herbivory and trampling were discussed 

above, but actual effects will vary in relation to ecosystem 

characteristics (for example, climatic conditions, soil properties, and vegetation structure and composition) and disturbance characteristics. Similar to other disturbances such 

as fire, a key consideration for ecosystem management and 

monitoring is the regime of herbivory and trampling that is 

considered natural for particular ecosystems. 

Predominant types of herbivores and the spatial patterning, temporal patterning, magnitude, selectivity, and synergistic relationships of herbivory and trampling likely varied 

among dryland ecosystems prior to European settlement. At 

the time of contact, large native ungulates such as elk and 

bison (Bison bison) were uncommon in most dryland ecosystems of the Intermountain West in comparison with the 

Great Plains region east of the Rocky Mountains (Mack and 

Thompson, 1982; Grayson, 1994). Probable factors affecting 

regional-scale variations include landscape characteristics (for 

example, availability of forage and perennial water, presence 

and extent of topographic barriers, proximity to other favorable habitats), nonhuman predation, and hunting activities of 

pre-European human populations (Truett, 1996). Truett (1996) 

proposed that water scarcity and hunting by pre-European 

peoples were primary factors explaining low numbers of large 

30 Structure and Functioning of Dryland Ecosystems
ungulate herbivores encountered by early Europeans throughout much of the American Southwest (see Allen, 1996). 

Archaeological findings suggest that small numbers of bison 

may have been present occasionally in some dryland locations on the Colorado Plateau during late-Holocene times (for 

example, Mead and others, 1991). But overall, herbivory and 

trampling by large ungulates probably were relatively minor 

disturbances in most dryland ecosystems of the region during 

the period prior to European settlement. 

Landscape-Level Relationships

This report has focused on functional relationships among 

components of dryland ecosystems, but for purposes of ecosystem management and monitoring it also is important to recognize functional relationships among ecosystems in landscapes 

(Wiens and others, 2002). A landscape is a spatially structured 

mosaic of different types of ecosystems interconnected by 

ecological flows of materials (for example, water, sediments), 

energy, and organisms. Included in this definition is the notion 

that some of these flows may represent disturbances or stressors (fig. 8). A landscape perspective is essential for management and monitoring of bounded reserves such as NPS units 

where ecosystems within parks can be significantly affected 

by activities occurring well outside park boundaries. Consideration of landscape-level processes also can be important for 

assessing whether ecological conditions or events in one park 

location might generate ecological consequences elsewhere 

within the park (Reiners and Driese, 2001). 

Several aspects of landscape structure must be considered when assessing functional relationships among ecosystems in landscapes. These include ecosystem (or patch) type, 

ecosystem context, ecosystem condition, and ecosystem size 

(Groves and others, 2002; Wiens and others, 2002). Ecosystem context refers to the spatial configuration and connectivity of ecosystems (fig. 8). Connectivity is the degree 

to which ecological flows can occur between ecosystems in 

a landscape. Connectivity depends on spatial configuration, 

but it also is strongly dependent on the process of interest 

and the degree of contrast between ecosystems. For example, 

two adjacent ecosystems may be connected hydrologically 

due to their location within a particular watershed, but differences in vegetation structure may cause the same two 

adjacent ecosystems to be disconnected with respect to the 

movement patterns of particular organisms or the transmission of disturbances such as fire. In this example, the degree 

of cross-boundary contrast does not affect hydrologic connectivity but it strongly affects boundary permeability to 

Gallery of a cedar bark beetle (Phoeosinus sp.) on Utah juniper (Juniperus osteosperma) that subsequently 

was burned in a wildfire, Mesa Verde National Park, Colorado. Photograph by Mark Miller, USGS.

Structure and Functional Relationships  31
ECOSYSTEM/PATCH C

SIZE & LANDSCAPE C

C

Resources Disturbances

Ecosystem

Processes

Organisms

Landscape Unit 1

Soil & water resourcesOrganismsDisturbancesStressors

Soil & water resourcesOrganismsDisturbancesStressors

LANDSCAPE STRUCTURE:

FUNCTIONAL RELATIONSHIPS

AMONG LANDSCAPE UNITS

A

Landscape Unit 2

Resources Disturbances

Ecosystem

Processes

Organisms

Landscape Unit 3

Resources Disturbances

Ecosystem

Processes

Organisms

Landscape Unit 1:

DEGRADED CAPACITY

TO RETAIN RESOURCES

Resources Disturbances

Ecosystem

Processes

Organisms

La

E

Reso

Landscape Unit 3

RESOURCE

ENRICHMENT

Resources Disturbances

Ecosystem

Processes

Organisms

Acceleratedtransfers of soil& water resources

Soil & water resourcesOrganismsDisturbancesStressors

ECOSYSTEM/PATCH CONDITION

& LANDSCAPE CONTEXT B

Landscape Unit 3

Resources Disturbances

Ecosystem

Processes

Organisms

Landscape Unit 2

RESOURCE

ENRICHMENT

Resources Disturbances

Ecosystem

Processes

Organisms

Landscape Unit 1:

DEGRADED CAPACITY

TO RETAIN RESOURCES

Resources Disturbances

Ecosystem

Processes

Organisms

Figure 8. Examples of landscape-level processes and attributes important for ecosystem monitoring. In A, landscape units are functionally consoil and water resources, organisms, disturbances, and stressors. In B, degraded conditions in Unit 1 are shown to cause resource enrichment the importance of landscape context. In C, degraded conditions in Unit 1 are propagated to Unit 3 due to increased size of Unit 1 and decreased 
organisms and disturbance. From an ecosystem perspective, 

condition refers to the functioning of ecological processes 

required to sustain ecosystem characteristics including abiotic 

and biotic structure, productivity, rates of biogeochemical 

cycling, and natural disturbance regimes. The condition of 

ecosystems in the landscape can affect among-system flows 

and the ecological consequences of among-system flows. For 

example, a diminished capacity for soil and water retention 

in one ecosystem can result in accelerated resource transfers 

to connected ecosystems (fig. 8B). Consequences of resource 

enrichment in the receiving ecosystem are dependent on conditions there. If invasive species are present in the receiving 

ecosystem, resource enrichment may result in rapid population expansion (Davis and others, 2000; With, 2002). When 

considered with respect to individual species, ecosystem 

condition is equivalent to the notion of habitat quality (Wiens 

and others, 2002). 

The meaning of ecosystem size is intuitive, and ecosystem size (or changes in ecosystem size) can affect the type, 

magnitude, and consequences of among-system flows (fig. 

8C). For ecosystems characterized by large-scale disturbance 

regimes (for example, extensive fire), it may be necessary to 

expand the notion of ecosystem size to encompass the amount 

of area required for the ecosystem to persist under the influence of characteristic large-scale disturbances. This area has 

been referred to as the minimum dynamic area (Pickett and 

Thompson, 1978; Peters and others, 1997; Groves and others, 

2002). 

Predominant Anthropogenic Stressors

This section briefly describes predominant anthropogenic stressors affecting the structure and functioning of 

dryland ecosystems in SCPN and NCPN units. Additional 

information on the significance of these stressors in particular 

parks can be found in associated NPS reports (Evenden and 

others, 2002; Thomas and others, 2003; Miller and others, 

2003). Details concerning potential ecological effects of these 

stressors are summarized in tabular format in appendix table 

B and in conceptual models presented in the subsequent section of this report. 

Visitor Use

Park use by terrestrial recreationists has been identified 

as a primary stressor of concern in many NCPN units and, 

to a lesser degree, in some SCPN units. Most parks experienced rapid growth in the number of annual recreational visits 

between the mid-1980’s and the mid-1990’s (Evenden and 

others, 2002; Thomas and others, 2003). These changes are 

exemplified by visitation patterns at Canyonlands NP, which 

averaged 16 percent annual growth in visitation between 1985 

and 1995. During the year 2000, Canyonlands received seven 

times the number of recreational visits that it received in 1980. 

Zion NP—the most visited park in the NCPN—recorded 

over two million recreational visits in 2000, twice the number 

recorded in 1980. Grand Canyon NP, consistently the mostvisited SCPN park, has recorded over four million visitors 

every year since 1992 (Thomas and others, 2003). Potential 

resource impacts associated with terrestrial recreational activities include trampling of soils and vegetation (for example, 

Cole, 1990), dispersal of invasive exotic species, direct interactions with and disturbances of wildlife, and increased levels 

of water and air pollutants. 

Livestock and Other Large Herbivores

Livestock use is permitted in parts of one SCPN park 

(Glen Canyon NRA) and four NCPN parks (Dinosaur NM, 

Capitol Reef NP, Curecanti NRA, and Black Canyon of the 

Gunnison NP). Seasonal livestock trailing is permitted in Fossil Butte NM and Bryce Canyon NP, and several other parks 

repeatedly experience trespass livestock. Most other parks 

were grazed by domestic livestock at one time, and contemporary soil and vegetation conditions in many parks reflect 

persistent legacies of past livestock grazing and livestock-management practices (for example, Sydoriak and others, 2000; 

Neff and others, 2005). Herbivory and trampling by elk (Bandelier NM; Allen, 1996), bison (Capitol Reef NP and Grand 

Canyon NP), and feral burros (Grand Canyon NP) also occur. 

Fire-Regime Alteration

Ecological legacies of altered fire regimes are significant resource-management issues in many parks (Evenden 

and others, 2002; Thomas and others, 2003). Altered fire 

regimes attributable to past livestock grazing (fuel removal) 

and fire-suppression efforts have caused significant changes 

in vegetation structure and functioning of associated ecosystem processes. Mediated by changes in vegetation structure, 

ecosystem-level consequences of altered fire regimes can 

include diminished hydrologic functioning and increased 

erosion rates (for example, Wilcox and others, 2003; Davenport and others, 1998; Jacobs and Gatewood, 1999), as well 

as increased ecosystem susceptibility to drought and other 

disturbances and/or stressors.

Invasive Exotic Plants

Invasive exotic plants are recognized as an important 

component of global environmental change (Vitousek, 1994a; 

Mooney and Hofgaard, 1999), and they represent a significant resource-management challenge in the Colorado Plateau 

region. For example, NPS staff have estimated that approximately 24 percent of the total land area of Glen Canyon NRA 

may be infested with invasive plant species (Thomas and 

others, 2003). Significant proportions of grasslands found in 

Canyonlands NP and Arches NP have been converted to dominance by cheatgrass (Bromus tectorum; Evenden and others, 

2002), and cheatgrass constitutes approximately 85 percent 

Structure and Functional Relationships  33
of the dominant understory vegetation at Yucca House NM 

(Thomas and others, 2003). 

Ecological effects of exotic species invasions vary 

depending on the characteristics of the invader and the invaded 

ecosystem, but they can include major changes in community 

composition (Bock and others, 1986), competitive displacement of native species, and alterations of ecosystem-level 

properties such as disturbance regimes (D’Antonio and 

Vitousek, 1992; Mack and D’Antonio, 1998) and soil-resource 

regimes (Vitousek, 1990; Evans and others, 2001). Many 

invasive plant species also possess physiological traits that will 

enable them to benefit from aspects of global change such as 

increased levels of atmospheric CO2 and warmer minimum 

temperatures during winter (Alward and others, 1999; Dukes 

and Mooney, 1999; Smith and others, 2000). 

Adjacent Land use Activities

Ecological effects of adjacent land use activities also 

represent a uniform concern among SCPN and NCPN units 

(Evenden and others, 2002; Thomas and others, 2003). Particular activities of concern include livestock grazing, forest 

management, urban/exurban development, and emissions of 

industrial and agricultural pollutants. Potential resource issues 

associated with adjacent land use activities include altered 

habitat structure for wide-ranging wildlife species; increased 

transfers of soil and water resources; emissions of airborne 

and waterborne pollutants; introductions of exotic plant and 

USGS geologists investigating soil properties responsible for spatial patterns of sagebrush 

(Artemisia tridentata; shrubs in background) and invasive cheatgrass (Bromus tectorum; 

reddish grass in foreground). San Juan County, Utah. Photograph by Mark Miller, USGS.

animal species; diminished quality of soundscapes, nightskies, 

and viewsheds; and water diversion and/or regulation. 

Air Pollutants

Airborne particulates, tropospheric ozone, and nitrogen 

deposition are concerns at several SCPN and NCPN parks 

(Evenden and others, 2002; Thomas and others, 2003). Acid 

deposition may be an issue at Mesa Verde NP (Romme, 

Oliva, and Floyd, 2003). Nitrogen deposition in particular has 

potential implications for numerous ecological patterns and 

processes including ecosystem susceptibility to exotic species 

invasions (Asner and others, 1997; Galloway and others, 2003; 

Fenn, Baron, and others, 2003). Although current rates of N 

deposition generally are low across most of the western United 

States, there is very little information available for areas 

immediately downwind of emissions sources (Fenn, Haeuber, 

and others, 2003; Fenn, Baron, and others, 2003). Notably, 

modeling indicates potential “hot spots” of N deposition in the 

vicinity of Mesa Verde NP and Zion NP (Fenn, Haeuber, and 

others, 2003). 

Global Atmospheric Changes

Global atmospheric changes attributable to anthropogenic 

emissions of CO2 and other greenhouse gases are expected 

to have significant environmental consequences during this 

century (Houghton and others, 2001). Increasing levels of 

34  Structure and Functioning of Dryland Ecosystems
Invasive exotic plants can diminish ecosystem resilience to wildfire even in systems 

where fire is a natural disturbance process. Wildfire in this piñon–juniper woodland 

in Mesa Verde National Park led to significant increases in the relative abundance of 

invasive exotics including cheatgrass (Bromus tectorum) and musk thistle (Carduus 

nutans). Here these exotics surround Schmoll’s milkvetch (Astragalus schmolliae; 

plant with cream-colored flowers in center of photograph), a rare endemic plant 

restricted solely to Mesa Verde National Park and the adjacent Ute Mountain Ute 

Tribal Park. Photograph by Mark Miller, USGS.

Structure and Functional Relationships  35
Air-quality monitoring station at Bryce Canyon National Park, Garfield County, Utah. Photograph 

by Mark Miller, USGS.

Urban development directly adjacent to the boundary of Colorado National Monument 

(represented by the heavy yellow line). Width of photograph is 1,400 m on the ground. Mesa 

County, Colorado. NPS aerial photograph.

36  Structure and Functioning of Dryland Ecosystems
atmospheric CO2

, increasing soil and air temperatures, and 

altered precipitation patterns (including a potential increase in 

the frequency of extreme events) are likely to affect physiological processes and competitive relationships of vascular 

plants, nutrient cycles, hydrologic processes, and disturbance 

regimes—all of which have the potential to greatly alter the 

structure and functioning of dryland ecosystems (for example, 

Alward and others, 1999; Ehleringer and others, 1999; Smith 

and others, 2000; Weltzin and others, 2003) and the sensitivity of these systems to other anthropogenic stressors. Despite 

modeling efforts, there is great uncertainty about how global 

atmospheric changes will affect temperature and precipitation 

patterns in particular regions such as the Colorado Plateau. 

Uncertainty regarding regional climatic consequences and 

ecological outcomes of global-change processes greatly compounds the challenges associated with ecosystem management 

and monitoring. 

Ecosystem Dynamics

In this section, conceptual models are presented to 

describe hypotheses about how and why dryland ecosystems 

change through time. These models do not describe characteristic natural dynamics of the many specific dryland ecosystems 

found in the region (that is, dynamics driven solely by natural 

disturbances and short-term climatic fluctuations). Instead, 

they emphasize dynamics associated with a very generalized 

set of processes and pathways of degradation10 that have been 

described for dryland ecosystems. The intent is that an emphasis on degradational dynamics will best highlight ecosystem 

attributes that should be monitored for purposes of sustaining 

ecosystem integrity. The models are intended to depict general ways in which natural drivers (for example, climate and 

disturbance) interact with stressors and/or management actions 

to affect the likelihood of these degradational changes. Because 

of the generalized nature of these models, they will require 

refinement for site-specific applications. Following the models, 

this section ends with a brief discussion of factors affecting the 

susceptibility of dryland ecosystems to change. 

Background—Alternative Ecosystem States

Sustainable ecosystems, as defined by Chapin and others 

(1996), are persistent. Through the typical pattern of dynamics 

driven by natural disturbances and climatic fluctuations, such 

ecosystems maintain their characteristic diversity of major 

functional groups, productivity, and rates of biogeochemical 

10

 For purposes of this report, degradation is defined as an anthropogenic reduction in the capacity of a particular ecosystem or ecosystem component to perform desired functions such as the conservation 

of soil and water resources or the maintenance of native biodiversity. 

Degradation also can be interpreted as an anthropogenic reduction 

in the capacity of a system to maintain itself within the natural range 

of variability or within the range of desired conditions specified by 

ecosystem managers. 

cycling (Chapin and others, 1996). Inherent in the notions 

of ecosystem sustainability and persistence is the hypothesis 

that ecosystems can be caused to shift from one state to an 

alternative state.11 Though capable of existing on the same 

site, these alternative ecosystem states are distinguished by 

relatively large differences in structure and in rates of ecological processes such as erosion, nutrient cycling, and disturbance 

regime. Such differences in structure and processes typically 

are matched by great differences in ecosystem dynamics. A 

transition from one state to an alternative state may occur 

gradually or relatively rapidly as the result of natural processes 

(for example, extreme climatic disturbances) or human actions 

(for example, land use activities). Frequently, human actions 

and natural processes interact to cause persistent transitions 

among states (Westoby and others, 1989; Bestelmeyer and 

others, 2003; Stringham and others, 2003; Bestelmeyer and 

others, 2004). 

Although some transitions may be caused solely by 

natural processes, of greatest concern from a conservation 

perspective are anthropogenic transitions which result in 

degraded ecosystem states. Ecosystems that have been driven 

across thresholds of degradation generally cannot be restored 

to previous conditions simply by removing the stressor. Costly, 

manipulative restoration efforts are typically required (Hobbs 

and Norton, 1996; Whisenant, 1999; Sydoriak and others, 

2000; Suding and others, 2004). The success of such restoration efforts usually is uncertain, and in some cases complete 

restoration may be impractical due to financial and technical 

constraints. Dryland ecosystems and aquatic ecosystems are 

the most frequently cited examples of systems characterized 

by multiple alternative states (Rapport and Whitford, 1999; 

Whitford, 2002; Scheffer and Carpenter, 2003). 

Common Pathways and Processes of

Dryland Degradation

The conceptual framework of alternative states is a useful 

one for identifying monitoring needs associated with goals of 

sustaining the health or integrity of dryland ecosystems. This 

alternative-state framework is depicted graphically in figure 

9. In this framework, the reference state (state A, defined on 

a site-specific basis by climate, soil, and landscape setting) 

is represented by a dynamic ecosystem in which the characteristic abiotic and biotic components and processes are 

present and functioning within the natural range of variability, 

including processes that confer resistance and resilience to 

11 As applied here, state is a dynamic concept that (1) encompasses 

ecological processes such as natural disturbance and nutrient cycling, 

and (2) recognizes that plant community composition and structure 

on a particular site can vary spatially and temporally due to disturbance/successional processes and short-term climatic fluctuations. In 

contrast with these within-state dynamics, between-state dynamics 

reflect persistent changes in structure and processes that are not easily 

reversed through typical successional processes. For in-depth discussion of these topics, see Stringham and others (2003); Briske and 

others (2003); and Bestelmeyer and others (2003, 2004).

Ecosystem Dynamics  37
A. REFERENCE STATE/DYNAMIC

Major functional groups,

soil-resource conditions,

and ecological processes

(including natural disturbance

regime) are intact. System is 

resistant/resilient to

disturbances & stressors.

Global change?

E. SEVERELY ERODED

STATE/DYNAMIC

Significant loss (and/or redistribution)

of soil resources has occurred.

Functional-group diversity typically

reduced (often greatly). Site potential

is fundamentally altered due to

diminished resource availability, site

productivity, and capacity to support

characteristic functional groups. 

Denuded sites (perhaps with sparse

& fluctuating populations of annuals)

represent extreme cases. 

B. INVADED STATE/DYNAMIC

Major functional groups,

soil-resource conditions, and

ecological processes reasonably

intact, but system dynamics altered

by presence of functionally

significant plant invader. Resistance

& resilience of system to

disturbances/stressors (e.g.,

climatic fluctuations, fire, soil

disturbances) are somewhat

diminished relative to reference state.

C. WOODY-DOMINATED

STATE/DYNAMIC

Persistent increase in woody plants

and decrease / suppression of

herbaceous plants relative to reference

state -- representing a major shift in

functional-group structure. System

structure, functioning, & dynamics

(including disturbance regime)

significantly altered. System resistance

& resilience to disturbances/stressors

are diminished relative to reference state.

D. ANNUALIZED STATE/DYNAMIC

Persistent dominance by weedy annual

plants (typically exotic grasses),

representing major shift in

functional-group structure. System

structure, functioning, & dynamics

(including disturbance regime) significantly

altered. System resistance & resilience

to disturbances/stressors are diminished

relative to reference state.

1

4

7

5

2

6

2

3

3

1 Exotic plant invasion

2 Woody encroachment / thickening (fig. 10)

3 Soil erosion / redistribution (figs. 11 & 12)

4 Soil erosion / redistribution (figs. 14 & 15)

5 Exotic conversion (fig. 16)

6 Exotic conversion (fig. 17)

7 Soil erosion / redistribution (figs. 18 & 19)

TRANSITION DEGRADATIONAL PROCESSES

Figure 9. Conceptual model depicting major types of degraded alternative states (boxes B through E) that characterize dryland 

ecosystems of the region. Numbered arrows reflect general ecological processes responsible for transitions among states (see 

accompanying figures for detailed process models). Although not shown here, each state is characterized by a typical pattern of 

dynamic behavior. For state A, that pattern of behavior encompasses the “natural range of variability.” Dashed boxes associated with 

state A reflect the likelihood that global-change processes (for example, altered atmospheric chemistry, temperatures and precipitation) 

will cause future patterns of variability to drift away from historic patterns.

38  Structure and Functioning of Dryland Ecosystems
natural disturbances and anthropogenic stressors. However, 

as a consequence of on-going global-change processes (for 

example, altered atmospheric chemistry, temperatures, and 

precipitation patterns), managers should recognize that 

future ecosystem characteristics are likely to drift away from 

historic patterns of variability even in the absence of local 

land use effects. Accordingly, the concept of the “reference 

state” should be sufficiently flexible to account for uncertain 

trajectories of global atmospheric change (for example, Currie, 2001; Shafer and others, 2001; Hannah and others, 2002; 

Walther and others, 2002) as well as factors such as altered 

landscape configurations and the introduction of invasive 

exotic species to regional and local species pools. 

Despite uncertainties associated with future trajectories 

of change, four commonly observed types of degraded dryland 

states remain pertinent for purposes of ecosystem management and monitoring (fig. 9, boxes B through E). These are 

described here as follows: 

The invaded state (state B)—characterized by the presence of a functionally significant invasive exotic plant 

(for example, Bromus tectorum). System structure and 

functioning are somewhat altered relative to the reference state, but major functional groups and associated 

ecosystem processes generally remain intact. Presence 

of the invader may affect ecosystem resistance or resilience to natural disturbances or anthropogenic stressors.

The woody-dominated state (state C)—characterized 

by a persistent increase in the ratio of woody plant 

biomass to herbaceous plant biomass relative to the 

reference state. Associated with this major structural 

change, ecosystem functioning (for example, disturbance regimes, rates of geomorphic and biogeochemical processes) is also significantly altered relative to 

the reference state. 

The annualized state (state D)—characterized by 

persistent ecosystem dominance by weedy annual 

plants—often including invasive exotics species 

such as Bromus tectorum. Associated with this major 

structural change, ecosystem functioning (for example, 

disturbance regimes, rates of geomorphic and biogeochemical processes) also is significantly altered relative 

to the reference state.

The severely eroded state (state E)—characterized by 

severe loss or redistribution of soil resources. Site conditions have been fundamentally altered due to diminished (or persistently redistributed) resource availability, site productivity, and site capacity for supporting 

biotic functional groups characteristic of state A. 

Depending on site-specific properties of particular ecosystems, there may be numerous expressions and intergradations of these four basic types of degraded states. In addition, 

there is tremendous variability among particular sites in 

their susceptibility to various transitions depicted in figure 9. 

Transition susceptibility varies as a function of (1) ecosystem 

•

•

•

•

exposure to driving processes, and (2) ecosystem resistance 

and resilience to driving processes. These factors are discussed 

briefly at the end of this section. 

The following are conceptual models describing degradational processes associated with transitions between states 

depicted in figure 9. As with figure 9, these process-based 

models are necessarily generalized and will require modification for site-specific applications. The relative significance 

of processes depicted in the models can be expected to vary 

widely among sites. The models are intended to assist managers in the consideration of ecosystem attributes to be included 

in long-term monitoring programs designed to support goals 

of sustaining the health or integrity of dryland ecosystems. 

The models are not meant to imply that ecosystem degradation 

is the certain outcome of particular factors or processes. 

Transition 1—Exotic Plant Invasion

The transition from the desired state to the invaded state 

is caused by the establishment and spread of a functionally 

significant plant invader. Propagule dispersal clearly is an 

important factor in this transition, but resources also must 

be available if the introduced population is to reach a size 

sufficient for it to affect ecosystem functioning. Davis and 

others (2000) recently proposed a simple conceptual model 

illustrating their hypothesis that an ecosystem becomes more 

susceptible to invasion when there is an increase in the amount 

of resources that otherwise limit invasion. According to this 

model, factors that cause a pulse in resource supply (for 

example, precipitation events or fire) or a reduction in resource 

uptake (for example, episodic mortality of community dominants) may enable the rapid population expansion of responsive invaders that previously existed in the ecosystem at low 

levels. Other workers also have emphasized the importance of 

temporal and spatial patterns of resource availability as factors 

affecting ecosystem susceptibility to invasion (Johnstone, 

1986; With, 2002). Consistent with the resource-based hypothesis of Davis and others (2000), there is increasing evidence 

that ecosystems or microsites that are rich in soil resources are 

particularly susceptible to invasion by exotic species (Stohlgren and others, 1999; Levine and D’Antonio, 1999; Búrquez–

Montijo and others, 2002). 

For example, the pulsed population expansion of Bromus 

tectorum in a relatively undisturbed grassland in Canyonlands 

NP was attributed to enhanced soil-moisture conditions resulting from an El Niño episode (Belnap and Phillips, 2001). Subsequent to this population expansion, community dynamics 

in relation to interannual climatic fluctuations were substantially different than prior to El Niño. In experimental studies, 

Smith and others (2000) found that the presence of the exotic 

annual grass red brome (Bromus madritensis ssp. rubens) in 

an otherwise intact Mojave Desert ecosystem affected ecosystem response to elevated concentrations of atmospheric CO2

. 

Similarly, the presence of invasive exotic plants can significantly degrade the resilience of dryland ecosystems to wildfire 

(Brooks and others, 2004), thereby increasing the likelihood 

Ecosystem Dynamics  39
of a subsequent transition to a state dominated by an invader 

which promotes recurring fire (for example, transition 5 and 

state D in fig. 9). 

Transition 2—Woody Plant

Encroachment/Thickening

A persistent increase in the relative dominance of 

woody vegetation is perhaps the most widely documented 

transition in dryland ecosystems around the world (Archer, 

1994; Archer and Stokes, 2000; see http://cnrit.tamu.edu/

rlem/faculty/archer/bibliography.html). Factors proposed 

most commonly as explanatory mechanisms include excessive grazing and associated trampling by domestic livestock, 

active fire-suppression efforts, and climate (fig. 10). Elevated 

atmospheric CO2 also has been suggested as a factor (Polley 

and others, 1996, 1997), although Archer and others (1995) 

argued that CO2 enrichment is an insufficient explanation 

for observed patterns of vegetation dynamics. Climate plays 

an important role due to effects on population dynamics and 

competitive relations of herbaceous versus woody plants, 

but persistent overgrazing by domestic livestock generally 

has been implicated as the most important driver of transitions involving increasing dominance of unpalatable woody 

plants (Archer and others, 1995; Van Auken, 2000). Selective herbivory can affect the competitive relationships of 

plants, favoring the establishment and growth of unpalatable 

plants over those of palatable plants (Briske and Richards, 

1994). The reduction of aboveground herbaceous biomass 

and litter by grazing also can reduce the availability of fine 

fuels required to support a regime of frequent surface fires. 

Where such a fire regime is important for constraining the 

dominance of fire-intolerant woody vegetation, the removal 

of fine fuels by grazing may be more important than reduced 

herbaceous competition or active fire-suppression efforts as a 

driver of ecosystem change (Archer, 1994; Archer and others, 

1995; Van Auken, 2000). 

In this region, examples of persistent woody transitions outside the range of natural variability include juniper 

encroachment in grasslands of northern Arizona (Johnsen, 

1962; Jameson, 1962) and thickening of juniper and piñon 

in grasslands and savannas of north-central New Mexico (for 

example, Bandelier NM; Allen, 1989). These examples correspond with the piñon-juniper grass savanna ecosystem (sensu

Romme, Floyd–Hanna, and Hanna, 2003; table 3), which is 

characterized by a natural disturbance regime of frequent, 

low-severity surface fires. This model also may apply to 

former sagebrush steppe ecosystems and piñon–juniper shrub 

woodlands (Romme, Floyd–Hanna, and Hanna, 2003; table 

3) where land use activities have altered ecosystem structure, 

lengthened fire-return intervals, and increased the range-wide 

proportion of systems that are shrub- or tree-dominated (Connelly and others, 2004). In some circumstances, transition to 

this woody-dominated state may increase the likelihood of 

subsequent transitions to eroded or annualized states (that is, 

transitions 4 and 6 to states E and D in fig. 9). It is important 

to recognize that this model does not apply to piñon–juniper 

forest ecosystems found at Mesa Verde NP (Romme, FloydHanna, and Hannah, 2003) and possibly elsewhere. Nor does 

it apply to sparse desert shrublands or semidesert grasslands 

referred to as galleta—three-awn shrub steppe. 

Frequency of

surface fire

FIRE

SUPPRESSION

EXCESSIVE GRAZING CLIMATE Cover / vigor

of perennial grasses

Fine surface

fuels

WOODY PLANT

DOMINANCE

Figure 10. Conceptual model illustrating the processes by which excessive grazing, fire suppression, and 

climate interactively can lead to increasing dominance by woody plants.

40 Structure and Functioning of Dryland Ecosystems
Piñon-Juniper Grass Savanna Piñon-Juniper Shrub Woodland Piñon-Juniper Forest

Pre-1900 fire 

regime

Frequent, low-severity, surface fires carried by grassesModerately frequent, high-severity, 

crown fires carried by shrubs & trees

Very infrequent, very high-severity, 

crown fires carried by tree crowns

Pre-1900 stand 

structure

Sparse trees, few shrubs, dense grass, 

and other herbs

Sparse to moderately dense trees, sparse 

to very dense shrubs, moderately dense 

to sparse herbs, all depending on time 

since last fire

Dense trees, sparse to moderately dense 

shrubs, sparse herbs

Pre-1900 stand 

dynamics

Low tree density and high herbaceous 

biomass maintained in part by recurrent 

fire

Seral trend from herb to shrub to tree 

dominance, interrupted periodically by 

fire which returns a stand to early seral 

herb dominance

Stable/stationary tree age structure and 

little change in shrub or herbaceous layers during the long intervals without fire; 

very slow recovery after fire

Post-1900 changes 

in disturbance 

regime

Reduced fire frequency, great increase in 

fire severity

Reduced fire frequency, small increase in 

fire severity

Little change in fire frequency or fire 

severity

Post-1900 changes 

in structure

Increasing tree density, decreasing herbaceous biomassIncreasing tree density, decreasing 

shrubs and herbs

Little change in tree density or in shrubs 

or herbs

Overall current 

status

Outside HRV for disturbance regime, 

structure, & composition

Outside HRV for disturbance regime, 

structure, & composition

Still within HRV for disturbance regime, 

structure, & composition

Implications for 

restoration Urgent need for active restoration Urgent need for active restoration No need for restoration, protect instead

Current stand age 

structure

Very old trees (> 300 years) present, but 

not numerous; young trees (< 150 years) 

dominate stands

Very old trees (> 300 years) absent or 

rare; young trees (< 150 years) dominate 

stands

Very old trees (> 300 years) numerous; 

stands with all-aged structure, including 

old & young trees

Distribution: soil 

characteristics Deep, fine-textured soils Deep, fine-textured soils Shallow, rocky, or coarse-textured soils

Distribution: precipitation regime Summer peak in precipitation Winter peak in precipitation Variable

Distribution: topographic characteristics

Gentle plains and broad valley bottoms, 

with few barriers to fire spread

Gentle plains and broad valley bottoms, 

with few barriers to fire spread

Rugged slopes, canyons, and mesa tops, 

with many barriers to fire spread

Distribution: 

adjacent vegetation 

types

Grasslands, ponderosa pine, or other 

types that burn frequently

Grasslands, big sagebrush, or other types 

that burn frequently

Desert scrub, “slickrock,” or other types 

with sparse herbaceous vegetation that 

rarely burn

Geographic distribution

Most common in northern Mexico, 

southern New Mexico & Arizona, 

northern New Mexico, and possibly 

southeastern Colorado

Most common in the northern and 

central Great Basin and the Colorado 

Plateau

Scattered throughout the Colorado 

Plateau, Great Basin, central Oregon, 

southern Rocky Mountains, and southern 

California mountains

Examples

Jameson, 1962; Dwyer and Pieper, 1967; 

Allen, 1989; Segura and Snook, 1992; 

Dick-Peddie, 1993; Miller, 1999

Tausch and others, 1981; Koniak, 1985; 

Tausch and West, 1988; Miller and others, 1995; Miller and Tausch, 2001

Tausch and others, 1981; Tress and 

Klopatek, 1987; Kruse and Perry, 1995; 

Wangler and Minnich, 1996; Miller and 

others, 1999; Tausch and Nowak, 1999; 

Floyd and others, 2000; Waichler and 

others, 2001; Floyd and others, 2004

Table 3. Structure, disturbance dynamics, distribution patterns, current status, and restoration needs of three contrasting types of 

piñon–juniper ecosystems in western North America: A synthesis and set of hypotheses for further research. HRV = historic range of 

variability (from Romme, Floyd-Hanna, and Hanna, 2003, p. 346).

Ecosystem Dynamics  41
Coppice dunes formed by wind-driven movement of unstable soil where most perennial grasses and shrubs 

have been removed. Dunes formed where persistent shrubs and exotic annual plants such as tumbleweed 

(Salsola sp.) captured sediment. Navajo County, Arizona. Photograph by Mark Miller, USGS.

Although human land use activities frequently are implicated as primary drivers of woody transitions, it is important 

to recognize that increases in the relative dominance of woody 

vegetation also can occur in the absence of such activities. 

Between 1958 and 1996, juniper and piñon increased in 

sagebrush shrublands on a remote mesa in Grand Canyon NP 

that was unimpacted by human land use activities (Rowlands 

and Brian, 2001). Similarly, Harris and others (2003) found a 

significant increase in the dominance of woody vegetation on 

an ungrazed mesa in southern Utah between 1948 and 1993. 

In both cases, increases in the dominance of woody vegetation appear to be driven by natural successional processes. 

As indicated previously, area-specific research is necessary to 

evaluate natural disturbance regimes and processes responsible 

for observed dynamics. 

Transition 3—Soil Erosion/Redistribution

Transition 3 (fig. 9) is associated with accelerated 

erosional processes that result in a significant and persistent 

decline in soil-resource availability, site productivity, and site 

capacity for supporting biotic functional groups characteristic of the reference state. Soil resources can be eroded and 

redistributed by aeolian processes (fig. 11), fluvial processes 

(fig. 12), or both. Whether wind or water is the driving force, 

factors leading to accelerated rates of soil erosion and redistribution are similar. Trampling and other soil-surface disturbances can diminish soil stability by damaging biological soil 

crusts that protect and retain soils against erosive forces of 

wind, rain, and runoff (Williams, 1995b; Belnap and Gillette, 

1998; Okin and others, 2001). Soil-surface disturbances also 

can disrupt stable soil aggregates that enhance soil stability 

and soil infiltration capacity (Thurow, 1991). By reducing herbaceous cover and organic-matter inputs from litter and roots, 

excessive grazing can diminish soil protection and soil aggregate stability (Thurow, 1991). Reductions in ground cover 

can result in accelerated erosion due to diminished capacity 

to obstruct wind and overland flow of water (Davenport and 

others, 1998; Reid and others, 1999; Ludwig and Tongway, 

2000). Depending on site characteristics, such as soil texture, 

soil structure, and the vertical and horizontal structure of vegetation, reductions in ground cover may result in threshold-like 

increases in runoff and erosion (Davenport and others, 1998). 

Because of the complex and poorly understood effects of biological soil crusts on hydrologic processes, in figure 12 some 

uncertainty is indicated regarding effects of biological soil 

crusts on water and sediment capture and retention. Off-site 

conditions (that is, the functional status of adjacent landscape 

units; fig. 8) can contribute to accelerated rates of erosion. 

Examples are saltating soil particles that produce aeolian soil 

42  Structure and Functioning of Dryland Ecosystems
Saltating

particles

OFF-SITE

SOIL DISTURBANCES

EROSION/REDISTRIBUTION

OF SOIL (including organic matter

and nutrients) BY WIND

Herbaceous

ground cover

Biological

soil crust cover

Sediment

capture & retention

Soil protection

& stability

CLIMATE--DROUGHT &

AND EROSIVE WIND EVENTS Resource

availability

TRAMPLING &

OTHER SOIL-SURFACE

DISTURBANCES

EXCESSIVE GRAZING

Soil

quality

Soil detachment

& transport

Figure 11. Conceptual model illustrating the processes by which excessive grazing, soil-surface disturbances, off-site soil-surface 

disturbances, and climate interactively can lead to accelerated erosion/redistribution of soil resources by wind. 

Herbaceous

ground cover

Biological

soil crust cover

Water & sediment

capture & retention

Runoff

EROSION/REDISTRIBUTION

OF SOIL (including organic matter

& nutrients) BY WATER

OFF-SITE

HYDROLOGIC

FUNCTION

CLIMATE--DROUGHT &

EROSIVE PRECIP. EVENTS

TRAMPLING &

OTHER SOIL-SURFACE

DISTURBANCES

Resource

availability

Soil

quality

EXCESSIVE GRAZING Soil protection

& stability

Soil detachment

& transport

Infiltration &

soil-water storage

?

Figure 12. Conceptual model illustrating the processes by which excessive grazing, soil-surface disturbances, off-site hydrologic 

function, and climate interactively can lead to accelerated erosion/redistribution of soil resources by water.

Ecosystem Dynamics  43
Figure 13. Example of site where accelerated erosion has led to the significant loss of soil 

resources and persistent alteration of ecosystem structure and functioning (BLM photos). 

In A, the area above the red line retains a sandy-loam surface horizon that, though unstable 

and eroding, facilitates water infiltration and provides conditions suitable for vascular-plant 

establishment. Where this thin surface horizon has been lost below the red line, soil physical 

and chemical properties inhibit plant establishment and facilitate water-driven erosion. Panel 

B illustrates extreme results of erosion at this site. Photographs are of a site in Headquarters 

Valley, Grand Staircase-Escalante National Monument, Utah. Soil is transitional between an 

Elias series (Natriargid) and a Barx series (Calciargid) (Kent Sutcliffe, pers. commun., 2004).

Surface horizon

gone, exposing

sodium-affected

argillic horizon

Destabilized, eroding

surface horizon

(sandy loam)

A

44  Structure and Functioning of Dryland Ecosystems
detachment (fig. 11; Toy and others, 2002) and runoff that 

causes fluvial soil detachment and transport (fig. 12; Thurow, 

1991). Climate plays a role in this transition due to its effects 

on vegetative ground cover and the frequency of erosive wind 

and precipitation events (for example, fig. 5). As soil resources 

are lost due to erosional processes, declining resource availability generates a positive feedback facilitating further 

declines in vegetative ground cover and further increases in 

erosion (figs. 11 and 12; for example, Tongway and others, 

2003; Friedel and others, 2003; Sparrow and others, 2003). 

In the Colorado Plateau region, good examples of this 

transition can be found on stream-terrace soils that have been 

impacted by soil-surface disturbances (fig. 13). In this landscape setting, a typical intact soil profile consists of a thin (0–5 

cm) surface horizon of fine sandy-loam texture that overlies a 

thick subsurface argillic (clay-rich) horizon with high sodium 

content caused by capillary rise of alluvial groundwater (for 

example, the Elias series—for the official series description see http://ortho.ftw.nrcs.usda.gov/cgi-bin/osd/osdname.

cgi). Where unimpacted by land use activities, such sites are 

generally characterized by well-developed biological soil 

crust communities, productive stands of the C4 bunchgrass 

Sporobolus airoides (alkali sacaton), and the shrub Sarcobatus 

vermiculatus (greasewood). Destabilization and loss of the 

surface horizon expose a horizon with physical and chemical 

properties that tend to inhibit establishment of vascular plants 

and biological soil crusts, as well as enhance runoff and waterdriven erosional processes. In all cases where this occurs, 

ecosystem structure and functioning are persistently altered; 

in extreme cases, erosion leads to the development of badland 

topography. Examples similar to those depicted in figure 13 

can be found in Capitol Reef NP.

Transition 4—Soil Erosion/Redistribution

Erosional processes associated with transition 4 (figs. 14 

and 15) are similar to those associated with transition 3 (figs. 

11 and 12), but in the former, processes are modified due to 

an increase in the dominance of woody vegetation compared 

with the reference state. Relative to transition 3, within-site 

soil redistribution is more important in Transition 4 (figs. 14 

and 15). Soil resources transported from interspaces may be 

captured and retained within the subcanopy environments of 

woody plants because of interactions of shrubs and trees with 

wind- and water-driven geomorphic processes. The redistribution of soil resources from intercanopy spaces to subcanopy 

environments contributes to a positive feedback favoring 

continued increases in the ratio of woody to herbaceous 

biomass. Regional examples of the positive-feedback linkage 

Saltating

particles

OFF-SITE

SOIL DISTURBANCES

EROSION/REDISTRIBUTION

OF SOIL (including organic matter

and nutrients) BY WIND

Herbaceous

ground cover

Biological

soil crust cover

Sediment

capture & retention

Soil protection

& stability

CLIMATE--DROUGHT &

AND EROSIVE WIND EVENTS

TRAMPLING &

OTHER SOIL-SURFACE

DISTURBANCES

EXCESSIVE GRAZING

Soil

quality

Soil detachment

& transport

Resource

availability

Resource

availability

WOODY PLANT

DOMINANCE

Figure 14. Conceptual model illustrating the processes by which increasing woody-plant dominance, excessive grazing, soil-surface 

disturbances, off-site soil disturbances, and climate interactively can lead to accelerated erosion/redistribution of soil resources by 

wind.

Ecosystem Dynamics  45
Herbaceous

ground cover

Biological

soil crust cover

Water & sediment

capture & retention

Runoff

EROSION/REDISTRIBUTION

OF SOIL (including organic matter

& nutrients) BY WATER

OFF-SITE

HYDROLOGIC

FUNCTION

CLIMATE--DROUGHT &

EROSIVE PRECIP. EVENTS

TRAMPLING &

OTHER SOIL-SURFACE

DISTURBANCES

Soil

quality

EXCESSIVE GRAZING Soil protection

& stability

Soil detachment

& transport

Infiltration &

soil-water storage

Resource

availability

Resource

availability

WOODY PLANT

DOMINANCE

Figure 15. Conceptual model illustrating the processes by which increasing woody-plant dominance, excessive grazing, soil-surface 

disturbances, off-site hydrologic function, and climate interactively can lead to accelerated erosion/redistribution of soil resources by 

water.

between soil redistribution and increasing dominance of 

woody vegetation include coppiced shrublands of Monument 

Valley (pers. obs.) and piñon–juniper rocklands of Bandelier 

NM where severe erosion has been facilitated by past grazing 

and fire-regime alterations that led to increased tree cover and 

diminished herbaceous cover (Allen, 1989; Wilcox and others, 

2003; Davenport and others, 1998; Jacobs and Gatewood, 

1999). This positive-feedback model has been proposed as a 

common pathway of dryland degradation or “desertification” 

(Schlesinger and others, 1990; Havstad and others, 2000). 

Transition 5—Exotic Conversion

The persistent conversion of dryland ecosystems to dominance by exotic annuals is a widespread pathway of degradation (for example, Billings, 1990; Connelly and others, 2004). 

Many weedy annuals may be involved, but exotic annual 

grasses are the most common dominants. Several factors may 

contribute to processes associated with this transition to an 

“annualized” state (fig. 16), but all of them invariably affect 

the availability of resources that otherwise limit population 

growth in the exotic species (Davis and others, 2000). In this 

context, resources may include safe sites for germination and 

establishment (Harper, 1977; Grubb, 1977), as well as water, 

mineral nutrients, and light. Soil-surface disturbances may 

facilitate establishment of ruderal exotic species if availability 

of safe-site opportunities is a limiting factor (Crawley, 1987). 

However, if soil-surface disturbances do not ameliorate 

resource limitations, then soil-surface disturbance alone will 

not facilitate a transition to exotic dominance (Hobbs, 1991). 

Excessive grazing can reduce the competitive abilities of 

native perennial grasses relative to exotic annuals if the former 

are preferentially grazed, thereby favoring population expansion of lesser grazed exotic annuals. Resource enrichment 

associated with climatic episodes, fire, resource transfers from 

adjacent sites, excreta from localized livestock concentrations, or atmospheric N deposition may facilitate population 

expansion and persistent conversion to dominance by exotic 

annuals—particularly if associated factors (for example, selective herbivory by livestock) simultaneously have an adverse 

effect on native competitors. Increasing dominance by exotic 

annuals can lead to a dramatic shift in fire regime where annuals drive increases in the abundance and continuity of fine 

fuels and in the frequency of surface fires (Brooks and others, 

46  Structure and Functioning of Dryland Ecosystems
2004). Although a shift in fire regime does not always accompany ecosystem annualization, the positive-feedback linkage 

between exotic grasses and wildfire is sufficiently widespread 

to have been proposed as an important element of global 

change (D’Antonio and Vitousek, 1992). Fenn, Baron, and 

others (2003) suggested that, in some locations, atmospheric N 

deposition could trigger fire/exotic grass cycles by increasing 

resource availability for exotics. Experimental results from the 

Mojave Desert suggest that increasing levels of atmospheric 

CO2 may have a similar triggering effect (Smith and others, 

2000). Romme, Oliva, and Floyd (2003) expressed concern 

that invasive exotic species could respond to natural fires in 

piñon-juniper ecosystems of Mesa Verde NP and subsequently 

initiate a fire/exotic species cycle. Examples of annualized 

ecosystems are in several NPS units in the Colorado Plateau 

region, including Canyonlands NP, Arches NP, Golden Spike 

NHS, Zion NP, and Capitol Reef NP. In some circumstances, 

conversion to annuals may accelerate erosional processes that 

lead to further site degradation (for example, transition 7 to 

state E in fig. 9).

Transition 6—Exotic Conversion

Similar to the concerns of Romme, Oliva, and Floyd 

(2003) regarding the potential for exotic conversion following 

wildfire in Mesa Verde NP ecosystems, West (1999) suggested that persistent increases in the dominance of woody 

vegetation could facilitate exotic conversion by increasing the 

likelihood of high-intensity fires (fig. 17). Due to increased 

levels of competition, unnaturally dense populations of 

woody plants also may be more susceptible to drought, insect 

infestations, or other disturbances/stressors. Whether mortality of woody dominants is triggered by fire or some other 

factor, the sudden pulse of resource availability and release 

from competitive suppression could enable rapid expansion of existing exotic-species populations. If the population 

expansion is sufficient to cause a persistent increase in the 

abundance and continuity of surface fuels, a self-maintaining 

fire–exotic species cycle could be initiated. This type of transition may be a concern in parks such as Bandelier NM where 

the early 21st century drought has triggered near-complete 

mortality in unnaturally dense piñon stands. 

Soil

stability

Resource

availability/enrichment

ATMOSPHERIC

NITROGEN DEPOSITION

SOIL-RESOURCE TRANSFERS

FROM ADJACENT SITES

(soil, nutrients, water)

Safe-site

availability

for ruderals

DOMINANCE BY

EXOTIC ANNUAL GRASSES

Biological

soil crusts

Exotic annual

grass recruitment,

growth & reproduction

Fine surface

fuels

Frequency of

surface fire

CLIMATE--OCCURRENCE

OF DRY & WET PERIODS

LIVESTOCK

MANAGEMENT- CONCENTRATIONS

PRESCRIBED FIRE

AND MECHANICAL

THINNING

WILDFIRE

TRAMPLING &

OTHER SOIL-SURFACE

DISTURBANCES

EXCESSIVE GRAZING

Soil

quality

Cover/vigor

of perennial grasses

Figure 16. Conceptual model illustrating the processes by which excessive grazing, soil-surface disturbances, managementprescribed fire and mechanical thinning, wildfire, atmospheric nitrogen deposition, concentrated livestock activites, landscape-level 

soil-resource transfers, and climate interactively can lead to increasing dominance (and site conversion) by exotic annual grasses.

Ecosystem Dynamics  47
MANAGEMENTPRESCRIBED FIRE

WILDFIRE

Resistance/resilience

to drought, insects, &

other disturburbance/stress

Resource

availability/enrichment

Fine surface

fuels

Frequency of

surface fire

INSECT PESTS

(e.g., bark beetles)

DOMINANCE BY

INVASIVE EXOTIC WEEDS

Probability of

high-intensity fire

CLIMATE--DROUGHT

and FIRE-DRIVING WIND

High-intensity

fire

OFF-SITE

FUEL CONDITIONS

& FIRE MANAGEMENT

TRAMPLING &

OTHER SOIL-SURFACE

DISTURBANCES

Ignition

WOODY PLANT

DOMINANCE

Exotic annual

grass recruitment,

growth & reproduction

Woody plant

mortality

Figure 17. Conceptual model illustrating processes by which increasing woody-plant dominance, soil-surface disturbances, insect 

pests, climate, off-site fuel/fire conditions, wildfire, and management-prescribed fire interactively can lead to increasing dominance 

(and site conversion) by exotic annual grasses.

Transition 7—Soil Erosion/Redistribution 

Severe erosion (transition to state E in fig. 9) may be 

more likely in ecosystems dominated by exotic annual plants 

than in ecosystems dominated by perennial species because 

of greater fluctuations in annual ground cover in relation to 

climate (figs. 18 and 19). Greater soil exposure to erosive 

forces during unfavorable climatic periods could trigger spiraling processes of degradation as soil losses cause decreased 

ground cover and even greater soil exposure to erosive forces. 

Examples of soil erosion in annualized ecosystems can be 

found in Arches NP (fig. 20) and Canyonlands NP. 

Ecosystem Susceptibility to Change

As discussed previously, there is much variability 

among dryland ecosystems in their susceptibility to transitions depicted in figure 9. Transition susceptibility varies as a 

function of (1) ecosystem exposure to driving processes, and 

(2) ecosystem resistance and resilience to driving processes. 

Depending on the goals and objectives of a long-term monitoring program, factors affecting ecosystem exposure, resistance, and resilience may play a role in spatial and temporal 

aspects of the monitoring design. 

Ecosystem Exposure

Ecosystem exposure to drivers of change—particularly 

those considered to be anthropogenic stressors—is an important factor influencing ecosystem susceptibility to change 

beyond the range of conditions represented by the reference 

state. For example, sites exposed to frequent soil-surface disturbances or excessive herbivory by large mammals will be more 

susceptible to transitions affected by those processes than sites 

that are not so exposed, all else being equal. Landscape configuration also must be considered when evaluating ecosystem 

exposure where stressors may be associated with ecological 

48  Structure and Functioning of Dryland Ecosystems
Saltating

particles

OFF-SITE

SOIL DISTURBANCES

EROSION/REDISTRIBUTION

OF SOIL (including organic matter

and nutrients) BY WIND

Herbaceous

ground cover

Biological

soil crust cover

Sediment

capture & retention

Soil protection

& stability

CLIMATE--DROUGHT &

AND EROSIVE WIND EVENTS Resource

availability

TRAMPLING &

OTHER SOIL-SURFACE

DISTURBANCES

EXCESSIVE GRAZING

Soil

quality

Soil detachment

& transport

Fluctuations in

herbaceous production &

ground cover due to climate

DOMINANCE BY

EXOTIC ANNUAL

GRASSES

Figure 18. Conceptual model illustrating processes by which increasing site dominance by exotic annual grasses, excessive grazing, 

soil-surface disturbances, off-site soil-surface disturbances, and climate interactively can lead to accelerated erosion/redistribution of 

soil resources by wind.

flows from connected landscape units (fig. 8). At a regional 

scale, there is variation in ecosystem exposure to climatic factors (for example, erosive, high-intensity precipitation events) 

that affect the probability of transitions caused by erosional 

processes (fig. 6). Ecosystem exposure is not a static property. 

It can change with changes in climate and atmospheric conditions, landscape configuration, management, magnitude of 

existing stressors, or emergence of novel stressors. 

Ecosystem Resistance and Resilience

Persistent transitions to degraded states occur when 

ecosystem properties that confer resistance and resilience are 

damaged or otherwise exceeded (Archer and Stokes, 2000; 

Tongway and Hindley, 2000; Stringham and others, 2003). 

Ecosystem resistance and resilience are not static and can 

change in relation to varying climatic conditions and dynamic 

ecosystem properties affected by climate, land use, and 

management (Scheffer and others, 2001; Scheffer and Carpenter, 2003). Some aspects of resistance and resilience also are 

strongly controlled by ecosystem properties that are relatively 

stable through time. Examples of these inherent ecosystem 

properties include landscape position and soil characteristics 

such as rock content, texture, mineralogy, profile development, 

and depth (for example, fig. 13). 

A generalized description of factors affecting the resistance and resilience of dryland ecosystems is presented in 

appendix table C. Although generalizations can be useful, 

detailed considerations of resistance, resilience, and other 

aspects of system stability require that the variable of interest, 

benchmark conditions, spatial scale, temporal scale, and disturbance characteristics be clearly identified on a site-specific 

basis (Grimm and Wissel, 1997). Details matter—for example, 

the effects of soil properties on plant resistance and resilience 

to drought may vary with drought characteristics and among 

plant species in relation to particular ecophysiological traits. 

Implications for Ecosystem Classification

Given the importance of ecosystem resistance and resilience in affecting probabilities of ecosystem change beyond 

the range of natural variability, an ideal ecosystem classification system to support ecosystem management and monitoring 

would be based on stable properties that influence ecosystem 

resistance and resilience. From the preceding discussion and 

appendix table C, general factors that most strongly affect 

resistance and resilience are climatic conditions, soil structure, 

Ecosystem Dynamics  49
Figure 20. Example of an annualized ecosystem at Arches National Park where 

climate-driven fluctuations in ground cover and persistent soil instability likely 

contribute to accelerated rates of wind erosion. Photograph by Mark Miller, USGS. 

Figure 19. Conceptual model illustrating processes by which increasing site dominance by exotic annual grasses, grazing, soil-surface 

disturbances, off-site hydrologic function, and climate interactively can lead to accelerated erosion/redistribution of soil resources by 

water.

Herbaceous

ground cover

Biological

soil crust cover

Water & sediment

capture & retention

Runoff

EROSION/REDISTRIBUTION

OF SOIL (including organic matter

& nutrients) BY WATER

OFF-SITE

HYDROLOGIC

FUNCTION

CLIMATE--DROUGHT &

EROSIVE PRECIP. EVENTS

TRAMPLING &

OTHER SOIL-SURFACE

DISTURBANCES

Resource

availability

Soil

quality

EXCESSIVE GRAZING Soil protection

& stability

Soil detachment

& transport

Infiltration &

soil-water storage

Fluctuations in

herbaceous production &

ground cover due to climate

DOMINANCE BY

EXOTIC ANNUAL

GRASSES

50 Structure and Functioning of Dryland Ecosystems
vegetation structure, and inherent ecosystem properties 

described by substrate characteristics and landscape setting. Although climatic conditions themselves are not stable, 

latitude, elevation, and aspect are three invariant factors that 

are certain to affect climatic conditions irrespective of future 

global climate change. To support ecosystem management 

and monitoring, it is therefore recommended that ecosystems 

be classified in relation to invariant climatic zones, substrate 

characteristics, and landscape setting rather than by the current 

composition of biotic communities. 

For dryland ecosystems, the most widely used classification scheme based on general climatic zones and inherent site properties is the ecological site system of the U.S. 

Department of Agriculture Natural Resources Conservation 

Service (USDA NRCS). In this classification system, 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, p. 279; Creque and others, 1999). Soil surveys 

produced by NRCS match soil series with ecological sites, 

and many ecological site descriptions are available on-line.12

These descriptions provide basic information on dynamics of 

soils and plant communities, and NRCS staff and cooperators 

currently are developing site-specific conceptual models that 

incorporate hypotheses regarding ecosystem resistance, resilience, and non-linear (threshold) responses in relation to land 

use and climate (USDA Natural Resources Conservation Service, 2003; Bestelmeyer and others, 2003, 2004). Challenges 

in the application of this scheme to planning, implementation, and interpretation of long-term monitoring may include 

lack of recent soil/ecological site mapping (older data may 

be less complete or accurate than recent data), complex map 

units (more than one soil series and ecological site per map 

unit), and inconsistent ecological site classifications across 

political boundaries. Alternative approaches to the classification of ecological land units for purposes of assessment and 

monitoring are provided by Bourgeron and others (2001a, b). 

Conclusion

This report presents a literature review and a series of 

ecological conceptual models intended to inform policymakers and researchers in the selection of environmental 

attributes for long-term monitoring of dryland ecosystems of 

the Colorado Plateau. Climate and atmospheric conditions, 

soil-resource characteristics, major biotic functional groups, 

and typical natural disturbance regimes are reviewed because 

of their significance for ecosystem structure, functioning, 

and sustainability (Chapin and others, 1996). A generalized 

conceptual model describes (1) functional relationships among 

12

 See the Electronic Field Office Technical Guide at http://www.

nrcs.usda.gov/technical/efotg/.

key structural components of dryland ecosystems; (2) effects 

of climate, natural disturbances, and anthropogenic stressors 

on the functioning of selected ecosystem processes; and (3) 

how these drivers of ecosystem change can interact to produce 

particular patterns of ecosystem dynamics. Because a common goal of long-term monitoring is to provide managers with 

information needed to help sustain the health or integrity of 

ecosystems, additional models emphasize common processes 

and pathways of dryland degradation. Although developed 

to support the identification of long-term monitoring needs 

in NPS units, information in this report is broadly applicable 

well beyond NPS management boundaries and should be of 

use to other organizations or persons involved in management, 

conservation, and monitoring of dryland ecosystems in the 

Colorado Plateau or bordering ecoregions. 

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Wright, H.A., 1980, The role and use of fire in the semidesert 

grass–shrub type: Ogden, UT, U.S. Department of Agriculture, Forest Service, Intermountain Forest and Range Experiment Station, General Technical Report INT–85, 24 p.

64  Structure and Functioning of Dryland Ecosystems

Published in the Central Region, Denver, Colorado

Manuscript approved for publication August 30, 2005

Page layout and cover design by Mari L. Kauffmann

(Contractor, ATA Services)
Appendix
Appendix Table A.  Primary functions and attributes/measures associated with components of dryland ecosystems. Components are organized in re

interactive controls of ecosystem structure and function.

INTERACTIVE CONTROLS* COMPONENTS/PROCESSES PRIMARY ECOSYSTEM FUNCTIONS ATTRIBUTES/MEASURES Precipitation Water inputs; drives water-limited ecosystem and population processes (for example, nutrient cycling, C and N fixation, seed germination); erosive force for detachment, entrainment, and overland redistribution and export of soil, litter, and propagules; drives fire disturbances due to effects of interannual variability on fuel production and flammability; strongly affects ecosystem resistance and resilience to natural disturbances and anthropogenic stressors. Seasonality, quantity, intensity 

per unit time), duration, tempovariability (among seasons, wyears), spatial variability, form Frequency, intensity, and duraevents. Wind Drives the redistribution and export (transfer among patches and among systems) of soil, litter, and propagules; affects ecosystem- atmosphere gas-exchange (CO2 intake and evapotranspiration); energy-balance modification (transfer of sensible and latent heat). Average sustained and peak v

(and frequency-magnitude disseason), seasonal and diurnavariability. Frequency, intensiterosion-driving events. Radiant energy Energy inputs for photosynthesis and sensible heat; effects on ecosystem-atmosphere gas-exchange (CO2 uptake and evapotranspiration) and energy balance; interacts with precipitation to affect resource availability and drive biotic processes through 

effects on timing and rate of metabolic activity. 

Maximum, minimum, and ave(heat), spectral characteristicsvariability (seasonal and diurn(horizontally and vertically). 

N2 and CO2 Nitrogen and carbon inputs (gaseous). Atmospheric concentration.

Regional climatic and atmospheric conditions 

Dust & other airborne constituents Mineral nutrient inputs. Quantity, chemical compositio

(seasonality), spatial distributi

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; and (2) by affecting ecosystem resistance/resilience to other natural disturbances (for example, extreme precipitation events, wind storms, insect outbreaks, and/or fire) and anthropogenic stressors. 

Seasonality, intensity, duratiotiming in relation to extreme pevents.

Extreme precipitation 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, duratiorelation to drought, and the ambiotic and biotically structured and functions to recover from 

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, duratiorelation to drought, and the ambiotic and biotically structured and functions to recover from 

Disturbance regime

Episodicclimatic events

Extreme temperature events

Drives change in ecosystem structure and function by inducing selective mortality, potentially resulting in long-term shifts in dominants.

Intensity, duration, frequency, to other disturbances. 
Appendix Table A.—Continued 

INTERACTIVE CONTROLS* COMPONENTS/PROCESSES PRIMARY ECOSYSTEM FUNCTIONS ATTRIBUTES/MEASURES 

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; and (3) increasing runoff and soil erosion. Over longer time scales, fire may decrease erosion and runoff due to increases in herbaceous ground cover. 

Intensity, spatial extent and pain relation to other disturbanceprecipitation and wind events. 

Insect/disease outbreaks 

Drives change in ecosystem structure and function by inducing selective and often widespread mortality, potentially resulting in long-term shifts among dominant functional groups. May reduce ecosystem resistance/resilience to other disturbances such as fire. 

Intensity, duration, frequency, to other disturbances. 

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. Excretion further alters the spatiotemporal distribution of resources. 

Intensity, selectivity, spatial exfrequency, timing in relation to as drought, and the amount oand biotically structured ecosyfunctions to recover from drou

Digging/burrowing 

Alters soil structure and function (creation of macropores potentially increases 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 extenttiming in relation to other distuextreme wind and precipitatio

Disturbance regime(continued) 

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 directly due to trampling of vegetation and indirectly due to effects on soil characteristics. 

Intensity, spatial extent and pain relation to other disturbanceand precipitation events, droutime required for biotic and bioecosystem components and fdrought.

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, populadynamics; physiological condi

Herbivores

See Herbivory, above. May also impact ecosystem structure and function by digging/burrowing (see above). [Potential agents of disturbance – see herbivory, above.] 

Composition, quantity, populadynamics; physiological condi

Biotic functional groups 

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, populadynamics; physiological condi
Appendix Table A.—Continued 

INTERACTIVE CONTROLS* COMPONENTS/PROCESSES PRIMARY ECOSYSTEM FUNCTIONS ATTRIBUTES/MEASURES VEGETATION: 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 waterborne 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 an

structure and dynamics, verticadistribution/heterogeneity, photlongevity, litter quantity and quaflammability, productivity, physcondition, resistance/resilience dominant natural disturbances 

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 and soil temperature); soil-temperature increases (decreased albedo); habitat creation (due 

to long-term soil stabilization). 

Composition, quantity (cover andistribution and contiguity, micrheterogeneity/surface roughnescondition, productivity, resistanfunction to dominant natural disstressors. 

The Soil Surface 

Roots

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

and organic acids). 

Morphology, density, horizontaspatial and temporal patterns oproductivity. 

Bioticfunctional groups (continued) 

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 (biomassvertically and horizontally, temptemporal patterns of physiologiresistance/resilience of structurnatural disturbances and anthro

The Soil Continuum 

Soil mineral matrix Soil organic matterSoil 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 in

Mineralogy and texture by deptthese properties, depth. 

Dynamic properties (subject to 

and bulk density (structure), orgquality (for example, C:N ratio)inherent property, but subject to scales), erosion rate, infiltration crusting (biotic or physicochemspatial heterogeneity of these presistance/resilience of structurnatural disturbances and anthro

Soil resources 

Soil temperature Regulates physiological activity of autotrophic and heterotrophic soil biota, including roots. Maximum, minimum, and avera

temporal variability (seasonal a(horizontally and vertically). 

PPrimary sources: Whitford (2002), Herrick and others (2002), Belnap and Lange (2003), Ehleringer and others (2000), Seybold and others (1999), Whisenant (1999), and Ludwig and others (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; C
Appendix Table B.  Potential effects of major anthropogenic stressors on structure and functioning of dryland ecosystems. Potential impacts of cha

atmospheric chemistry, and atmospheric circulation patterns (including precipitation) are excluded.

Stressors Stress mechanisms Potential effects on ecosystem components and functions (emphasizing effects on energy flow, nutr

and water capture and retention)

Trampling of soil and vegetation 

Damaged biological soil crusts and vascular plants; decreased N fixation by biological crusts; decreased soil-surface recruitment opportunities for exotic plants favored by soil destabilization; altered vegetation structure; decreased soicrusts; decreased soil aggregate stability; decreased soil stability; decreased resistance of soil to erosion by wind andensity; decreased infiltration; increased overland flow of water; decreased soil-water availability for plant growth, socycling; decreased root growth and soil-organic-matter inputs; decreased plant growth; decreased resistance and reincreased redistribution and export of soil, litter, nutrients, and water; decreased capacity of ecosystem to capture amultiple cascading effects due to feedbacks among soil functions, soil-resource retention, resource heterogeneity, a

Park use by terrestrial recreationists 

Introduction of invasive exotic plants See exotics, below. Trampling of soil and vegetation See trampling, above. Excessive herbivory 

Altered competitive relations of plants; reduced plant canopy cover; reduced fine fuels and frequency of low-intensitvegetation structure (for example, dominance shift from palatable plants to unpalatable plants or from herbaceous pplants eliminated by fire); reduced plant-canopy protection of soil; reduced vegetative obstruction of wind and overlacapture and retention of soil resources; reduced litter deposition and litter-protection of soil; reduced soil-organic-maaggregate stability; decreased resistance and resilience of soil to trampling; decreased root growth; decreased resis

plants to drought; multiple cascading effects due to feedbacks among vegetation structure, soil-resource retention, rsoil functions. 

Excretion Nutrient losses (N volatilization); nutrient immobilization in dung pats; increased spatial and temporal heterogeneity 

alteration of vegetation structure; facilitation of exotic-plant invasion where nutrients are locally enriched (see exotics

Livestock & other large herbivores 

Introduction of invasive exotic plants See exotics, below. 

Fire suppression Altered fire regime 

Decreased frequency and extent of fire; altered vegetation composition/structure (for example, dominance shift from by fire to woody plants eliminated by fire); increased accumulation of coarse woody fuels; increased likelihood of higdecreased ecosystem resistance/resilience to impacts from drought and insect outbreaks. 

Competition with native plants Altered vegetation composition/structure; eventual alteration of soil-resource dynamics and/or heterogeneity due to Altered soil-resource dynamics 

Altered nutrient dynamics (for example, exotic characterized by different tissue chemistry and/or by different spatioteuptake and litter deposition than native plants); altered soil-water dynamics (for example, exotic characterized by difof water use than native plants); eventual alteration of vegetation composition/structure due to soil-vegetation feedb

Invasive exotic plants 

Altered disturbance regime Increased frequency and extent of fire (facilitated by increased quantity, flammability, and/or spatial continuity of fue

vegetation composition/structure and soil-resource availability due to strong feedbacks among fire, vegetation struct

Trespass livestock See livestock, above. Introduction of exotic plants See exotics, above. Accelerated transfers of sediment, nutrients, and water 

Soil-resource enrichment; eventual alteration of vegetation composition/structure; facilitation of exotic-plant invasion enriched (see exotics, above); increased overland water flow (increased erosive energy of water); increased redistrinutrients. 

Fire from adjacent lands Increased susceptibility to high-intensity fire and associated ecosystem changes. 

Adjacent land use activities 

Altered landscape structure Altered habitat structure, connectivity and habitat-use patterns of wildlife. Nitrogen deposition Soil-resource enrichment; potential alteration of vegetation composition and structure--including facilitation of exotic

above). Air pollutants 

Ozone Altered competitive relations between ozone-sensitive and ozone-tolerant plants; altered vegetation composition & s
Appendix Table C.  Climatic conditions, inherent ecosystem properties, and dynamic ecosystem properties hypothesized to confer resistance and re

ecosystem attributes in relation to particular disturbances or stressors. (BSC = biological soil crust; -- = not applicable)

1Inherent ecosystem properties are relatively insensitive to change in relation to land use, management, and climate (for example, soil texture, mineralogy, horizonation, and depth; landscape/topograph2Dynamic ecosystem properties are sensitive to change in relation to land use, management, and climate (for example, soil structure, biotic activity, and vegetation composition and structure). Dynamic by biotic processes. 

Climatic conditions Inherent ecosystem properties1 Dynamic ecosyste

Disturbance or stressor

Ecosystem attribute Resistance Resilience Resistance Resilience Resistance Native grass populationstructure 

Deep sandy soil; run-on landscape position

Soil and vegetation structure that enhance site capacity to capture, retain, and cycle water and mineral nutrients; physiological condition of native grass populations unimpacted by other disturbances or stressors (for example, excessive herbivory) 

Drought

Shrub/tree populationstructure 

Coincidenttemperatures and winds that do not exacerbate evaporationaldemands

Antecedent moisture & temperature conditions favorable for seed production by native plants Subsequent moisture & temperature conditions favorable for plant growth & soil biotic activity; precipitationcharacterized by timing, intensity, and event size that maximize likelihood of infiltration and soilwater recharge 

Rocky soil (including rock outcrops with joints and fractures); run-on landscape position

Natural stand structure (that is, not overstocked); soil and vegetation structure that enhance site capacity to capture, retain, and cycle water and mineral nutrients; physiological condition of native shrub-tree populations unimpacted by other disturbances or stressors 

Insect outbreak (Here assumed to be triggered by drought stress)

Treepopulationstructure 

Antecedent and coincident moisture & temperature conditionsfavorable for tree growth 

Antecedent moisture & temperature conditions favorable for seed production by native plants Subsequent moisture & temperature conditions favorable for tree growth 

Rocky soil (including rock outcrops with joints and fractures); run-on landscape position

Natural stand structure (that is, not overstocked); soil and vegetation structure that enhance site capacity to capture, retain, and cycle water and mineral nutrients; physiological condition of native shrub-tree populations unimpacted by other disturbances or stressors 
Appendix Table C.—Continued

Climatic conditions Inherent ecosystem properties Dynamic ecosy

Disturbance or stressor

Ecosystem attribute Resistance Resilience Resistance Resilience Resistance 

Surface fire (As natural disturbance) 

Nativevegetationcomposition& structure 

Antecedent moisture & temperature conditions that diminish fuel production and/or diminish fuel flammability; calm wind conditions 

Subsequent moisture & temperature conditions favorable for plant growth & soil biotic activity 

Natural landscape fragmentation (that is, sites potentially capable of supporting surface fire are discontinuously distributed in the landscape, separated by natural fuel breaks)

--

Abundance and spatial continuity of fine surface fuels inadequate to carry fire

Stand-replacing canopy fire (As natural disturbance) 

Nativevegetationcomposition& structure 

Antecedent moisture & temperature conditions that diminish fuel production and/or diminish fuel flammability; calm wind conditions 

Antecedent moisture & temperature conditions favorable for seed production by native plants Subsequent moisture & temperature conditions favorable for vegetative reproduction (for sprouters), plant establishment, plant growth, & soil biotic activity 

Natural landscape fragmentation (that is, sites potentially capable of supporting high-intensity standreplacing fire are discontinuously distributed in the landscape, separated by natural fuel breaks)

--

Abundance and spatial continuity of ladder and canopy fuels inadequate to carry fire 
Appendix Table C.—Continued

Climatic conditions Inherent ecosystem properties Dynamic ecosys

Disturbance or stressor Ecosystem attribute Resistance Resilience Resistance Resilience Resistance Erosive wind events Soil retention (Includes fine soil particles, litter, organic matter, nutrients) Antecedent and coincident moisture & temperature conditions favorable for plant growth, BSC growth, soil biotic activity, maintenance of moist soil conditions, formation of stable soil aggregates, and/or formation of physical crust Antecedent moisture & temperature conditions favorable for seed production by native plants Subsequent moisture & temperature conditions favorable for plant growth, BSC growth, soil biotic activity, maintenance of moist soil conditions, formation of stable soil aggregates, and/or formation of physical crust Landscape setting that provides protection from erosive winds due to aspect, absence of wind-funneling landforms, minimal fetch exposure, etc.; soils with low erodibility by wind (for example, fine- textured soil not exposed to bombardment from saltating sand particles; soil surface covered by rock fragments) Soil physical and chemical properties and landscape setting (for example, north- facing aspect) that favor plant growth, BSC growth, soil biotic activity, and/or reformation of physical crust Soil aggregate stability and ground cover of BSCs, stable litter, and/or other soil-stabilizing features at maximum levels relative to potential (that is, undiminished by other disturbances or stressors); vegetation structure that obstructs wind and reduces near-surface wind velocity (for example, tall, closely spaced vegetation) Erosiveprecipitation/flood events 

Soil & water retention

(Includes fine soil particles, litter, organic matter, nutrients)

Antecedent moisture & temperature conditions favorable for plant growth, BSC growth, soil biotic activity, and formation of stable soil aggregates

Antecedent moisture & temperature conditions favorable for seed production by native plantsSubsequent moisture & temperature conditions favorable for plant growth, BSC growth, soil biotic activity, and formation of stable soil aggregates

Deep sandy soil (high infiltration and storage capacity); soil surface covered by rock fragments; low slope gradient; short slope length; landscape position that does not receive runoff from upslope positions Soil physical and chemical properties and landscape setting (for example, northfacing aspect) that favor plant growth, BSC growth, and soil biotic activity 

Soil aggregate stability and ground cover of BSCs, stable litter, and/or other soil-stabilizing features at maximum level relative to potential (that is, undiminished by other disturbances or stressors); vegetation structure that obstructs over-land water flow, increases flow tortuosity, and reduces flow velocity 

Trampling & other soilsurfacedisturbances

Soil stability 

(Susceptibility to wind erosion) 

Antecedent moisture & temperature conditions favorable for plant growth, BSC growth, soil biotic activity, and formation of stable soil aggregatesCoincident moisture & temperature conditions favorable for maintaining soils in moist or frozen state (frozen or dry in case of clay soils) 

Antecedent moisture & temperature conditions favorable for seed production by native plantsSubsequent moisture & temperature conditions favorable for plant growth, BSC growth, soil biotic activity, formation of stable soil aggregates, and/or formation of physical crust 

Soil surface covered by rock or rock fragmentsSoil physical and chemical properties and landscape setting (for example, northfacing aspect) that favor plant growth, BSC growth, soil biotic activity, or reformation of physical crust 

Maximum degree of soil stabilization provided by biotic features relatively unimpacted by trampling oother soil-surface disturbances, such as stable litter, continuous herbaceous ground cover, and continuous canopy cover. Soil stability in relation to wind erosion not dependent on integrity of sensitive biological or physical crusts. 
Appendix Table C.—Continued

Climatic conditions Inherent ecosystem properties Dynamic ecosyste

Disturbance or stressor 

Ecosystem attribute Resistance Resilience Resistance Resilience Resistance 

Trampling & other soilsurfacedisturbances

Soil stability andinfiltration capacity 

(Susceptibility to water erosion)

Antecedentmoisture & temperature conditions favorable for plant growth, BSC growth, soil biotic activity, and formation of stable soil aggregates Coincidentconditions: in presence of BSCs, moist or frozen soils; in absence of BSCs, frozen or dry soils

Antecedent moisture & temperature conditions favorable for seed production by native plants Subsequent moisture & temperature conditions favorable for plant growth, BSC growth, soil biotic activity, and formation of stable soil aggregates Occurrence of wet-dry or freeze-thaw cycles required to enhance recovery of compacted soils

Sandy soil or soil surface covered by rock or rock fragments Soil physical and chemical properties and landscape setting (for example, north-facing aspect) that favor plant growth, BSC growth, and soil biotic activity 

Maximum degree of soil stabilization provided by biotic features relatively unimpacted by trampling or other soil-surface disturbances, such as stable litter, continuous herbaceous ground cover, and continuous canopy cover; site hydrologic functioning and soil stability in relation to water erosion not dependent on integrity of sensitive BSCs 

Excessive herbivory 

Nativevegetationcomposition& structure 

Antecedent and coincident moisture & temperature conditions favorable for plant growth and soil biotic activity 

Antecedent moisture & temperature conditions favorable for seed production by preferred forage species Subsequent moisture & temperature conditions favorable for soil biotic activity and regrowth, seed production, and germination/establishment of defoliated plants 

Soil physical and chemicalproperties and landscape settings that DO NOT favor growth of plants that are palatable to predominant large-bodiedherbivoresSoil physical properties and landscape settings that DO NOT facilitate access or mobility of predominant largebodied herbivores 

Soil physical and chemicalproperties and landscape setting (for example, rich alluvial soils, riparian corridors) that favor vigorous plant growth and soil biotic activity 

Plant communities with no plant species palatable to predominant large-bodied herbivoresORPlant communities in which all species are equally preferred by predominant large-bodied herbivores and exhibit similar physiological/morphological responses to defoliation (that is, preferential herbivory does not favor one species over another) 

Invasiveexotic plants 

Nativevegetationcomposition& structure 

Conditions that favor native plant communities relative to invaders (will vary among communities and invaders); in general, conditions that constrain resource availability 

Soil physical and chemicalproperties and landscape settings that present harsh resourceconstraints for vascular plants 

--

Growth, competitive ability, and reproduction of native plant populations undiminished by other disturbances or stressors; soil-surface conditions that restrict safe-site opportunities for invasives