Natural Quality

USFS Wilderness Character Monitoring Technical Guide

2 Natural Quality

The objective of monitoring the Natural Quality is to assess the effects of modern civilization on the integrity of wilderness ecosystems, with a focus on plants, animals, air and water, and ecological processes. The Wilderness Act defines wilderness as an area that "is protected and managed so as to preserve its natural conditions" and that these areas should be free from the effects of "an increasing population, accompanied by expanding settlement and growing mechanization" (sections 2(c) and 2(a), respectively). Human-caused changes to wilderness ecological systems can be intentional or unintentional. While managers may have control over some impacts to natural ecosystems in wilderness, many threats come from external sources outside of their jurisdiction (e.g., air pollutants and nonindigenous species). In contrast to the Untrammeled Quality, which monitors actions that manipulate or control ecological systems, the Natural Quality monitors the effects on wilderness ecosystems from actions as well as external forces. While this quality encompasses all the naturally occurring species, physical resources, and ecological functions and processes in wilderness, practical limitations require that a relatively small but significant subset of possible measures are monitored.

For the Natural Quality, a single monitoring question provides the broad context and four indicators provide the structure for this monitoring (as summarized in table 1.3.1).

Table 1.3.1—Monitoring question, indicators, measures, and measure types for the Natural Quality.
Natural Quality
Monitoring question: What are the trends in the natural environment from human-caused change?
Indicator	Measure	Measure type
Plants	Acres of nonindigenous plants	Required
Animals	Index of nonindigenous terrestrial animal species	Required to select at least one
Animals	Index of nonindigenous aquatic animal species	Required to select at least one
Air and water	Concentration of ambient ozone	Required to select at least one
	Deposition of nitrogen
	Deposition of sulfur
	Amount of haze
	Index of sensitive lichen species
	Extent of waterbodies with impaired water quality	Required
Ecological processes	Watershed condition class	Required to select at least one
Ecological processes	Number of animal unit months of commercial livestock use	Required to select at least one

3.1 Monitoring Question

A single monitoring question is used to monitor the Natural Quality: What are the trends in the natural environment from human-caused change?

This monitoring question assesses the trends in natural wilderness ecosystems that result from human-caused threats occurring since designation of the area as wilderness. Importantly, this monitoring question seeks to distinguish between natural variability, which is integral to all ecosystems and does not degrade wilderness character, and human-caused change. In wilderness, the primary goal is to allow ecosystems to function and change without impacts or interference from modern civilization; therefore, the Natural Quality should not be used to set a target to maintain a particular ecological state or condition. In addition, this monitoring question does not include actions taken to restore ecological systems in wilderness. There are several reasons for not including these actions, including: (1) actions are tracked in the Untrammeled Quality, not the Natural Quality that tracks effects; (2) restoration actions are highly site-dependent and no single national protocol to measure such actions and their effects has been developed; (3) restoration actions typically assume static or historical ecological conditions contrary to wilderness as a place where human-determined states are not appropriate; and (4) the effects of restoration actions should eventually show, with monitoring, as an improving trend in the Natural Quality.

Four indicators assess a range of ecosystem components, structures, and functions in wilderness: (1) plants, (2) animals, (3) air and water, and (4) ecological processes. Practical and conceptual constraints mean that not everything important to wilderness ecosystems can be included in this monitoring. Likewise, not all ecological data currently collected by scientists are relevant or necessary to include in WCM. The measures under each indicator are not all encompassing; rather, the measures are selected because they are known human-caused threats to the indicators. Part 2, section 3.6, provides a detailed discussion of the criteria and process used for selecting measures under the Natural Quality; this section should guide local units considering the use of locally developed measures under this quality.

3.2 Indicator: Plants

This indicator focuses on threats to indigenous plant species and communities. Indigenous plant species (also referred to as native plant species) and plant communities are an essential biological component of natural wilderness ecosystems. Indigenous plant species and plant communities are those that evolved in an area and therefore have intrinsic value within a wilderness. In addition, they are critically important to the entire ecosystem by providing food and habitat to indigenous animals, preventing soil erosion, adding soil nutrients, and maintaining the local environmental conditions and biodiversity.

3.2.1 Measure: Acres of Nonindigenous Plant Species

This measure assesses the total number of acres, or the estimated percentage of acres, occupied by selected nonindigenous plant species in wilderness. The introduction and spread of nonindigenous species (also referred to as non-native, alien, or exotic species) is the second leading cause of plant and animal species endangerment and extinction worldwide (Lowe et al. 2000). Although many nonindigenous species are present throughout the United States, invasive nonindigenous species (i.e., those species that increase quickly in abundance and distribution) are a particular threat to wilderness character and are therefore the focus of this measure.

This measure was selected because nonindigenous plants may directly and indirectly alter the composition, structure, and function of natural communities in significant ways by degrading or eliminating habitat for native plant and animal species, and causing multiple cascading effects throughout the entire ecosystem. The adverse impact of these species on the Natural Quality of wilderness character is significant. Because of established concerns about nonindigenous species, this measure is relatively simple and cost effective to monitor.

This measure is required for all Forest Service wildernesses. A 5-percent or greater change in the number of measured or estimated acres, or any change in defined "percentage occupied" categories, will result in a change in trend for this measure. An increase in the acreage occupied by nonindigenous species corresponds with a degrading trend.

Refer to part 2, section 3.2.1, for detailed guidance on data sources and compilation protocols, analysis, data adequacy, and interpreting the threshold for meaningful change.

3.3 Indicator: Animals

This indicator focuses on threats to indigenous animal species and communities. Indigenous animal species (also referred to as native animal species) and animal communities are an essential biological component of natural wilderness ecosystems. Indigenous animal species and communities are those that evolved in the area and therefore have intrinsic value within a wilderness. Additionally, they are critically important to the entire ecosystem by providing food and habitat to other animals, digesting plant material and thereby making nutrients available in the soil for plants to use, scavenging carcasses of dead animals, and contributing to a wilderness ecosystem in many other ways.

3.3.1 Measure: Index of Nonindigenous Terrestrial Animal Species

This measure is an index that assesses the geographic distribution and estimated impact of selected nonindigenous terrestrial animal species. Nonindigenous animal species generally occur inside a wilderness because of human influence, such as intentional and unintentional introductions and transplants. Once nonindigenous species become established outside a wilderness, they may spread naturally or disperse into that wilderness. Nonindigenous animals include livestock that intentionally graze in wilderness, as well as feral domesticated animals, such as feral livestock, horses, goats, and pigs. Examples of nonindigenous terrestrial insects include: Asian longhorned beetle, emerald ash borer, gypsy moth, and hemlock woolly adelgid. Terrestrial pathogens and diseases are included in this measure because even though they are not animals, they are not considered plants either and creating a separate measure for them is not warranted. Examples of terrestrial pathogens and diseases that would be included in this measure are sudden oak death, chronic wasting disease, and whitenose syndrome.

This measure was selected because nonindigenous terrestrial animals, insects, and pathogens and diseases may significantly alter the composition, structure, and function of natural communities by degrading or eliminating habitat for indigenous species, and causing multiple cascading effects throughout the entire ecosystem. The adverse impact of these species on the Natural Quality of wilderness character is significant.

Units are required to select either this measure or the following measure, Index of Nonindigenous Aquatic Animal Species, or may select both measures if relevant to the individual wilderness. A 5-percent or greater change in the measure value will result in a change in trend for this measure. Once there are five measure values, the threshold for meaningful change will switch to regression analysis, and statistical significance will determine the trend in the measure. An increase in the measure value corresponds with a degrading trend.

Refer to part 2, section 3.3.1, for detailed guidance on data sources and compilation protocols, analysis, data adequacy, and interpreting the threshold for meaningful change.

3.3.2 Measure: Index of Nonindigenous Aquatic Animal Species

This measure is an index that assesses the geographic distribution and estimated impact of selected nonindigenous aquatic species (NAS), including amphibians, fish, crustaceans, mollusks, gastropods, aquatic insects, and aquatic pathogens and diseases. NAS are typically introduced into a given wilderness by anthropogenic vectors, although species introductions may also have originated outside of a wilderness and the species subsequently moved into the wilderness by upstream or downstream movement. Aquatic pathogens and diseases are included in this measure because even though they are not animals, they are not considered plants either and creating a separate measure for them is not warranted. Examples of an aquatic pathogens and diseases that would be included in this measure are: whirling disease, iridoviruses, and chytrid fungus.

This measure was selected because nonindigenous aquatic animal species may alter the composition, structure, and function of natural aquatic communities, and adversely impact indigenous species, reduce biodiversity, and degrade natural aquatic ecosystems.

Local units are required to select either this measure or the preceding measure, Index of Nonindigenous Terrestrial Animal Species, or may select both measures if relevant to the individual wilderness. A 5-percent or greater change in the measure value results in a change in trend for this measure. Once there are five measure values, the threshold for meaningful change will switch to regression analysis, and statistical significance will determine the trend in the measure. An increase in the measure value corresponds with a degrading trend.

Refer to part 2, section 3.3.2, for detailed guidance on data sources and compilation protocols, analysis, data adequacy, and interpreting the threshold for meaningful change.

3.4 Indicator: Air and Water

This indicator focuses on threats to air and water quality. Air and water are fundamental physical resources of wilderness ecosystems, and both are essential to maintain properly functioning natural systems inside wilderness. Both air and water resources are vulnerable to degradation by pollutants produced outside of wilderness as a result of human development and industrial activity.

Units are required to select at least one of the five air quality measures included under this indicator (see sections 3.4.1 through 3.4.5), or may optionally select more than one air quality measure. The Clean Air Act of 1977 mandates special protections for values related to air quality in both Class I and Class II areas, many of which are also designated wildernesses. The presence of airborne pollutants in soil and water within wilderness can have direct adverse effects on sensitive plant and animal species and can directly impact essential ecosystem functions, such as nutrient cycling. Certain air pollutants also can reduce visibility. The effects of air pollution on plants, animals, soil, and water are important in all wildernesses, regardless of whether a wilderness is designated as Class I or Class II according to the Clean Air Act.

In addition to air pollutants, water quality and water flows also are vulnerable to the effects of physical manipulations inside and outside of wilderness. For example, dams outside a wilderness can markedly affect water quantity and quality, as well as stream morphology, inside a wilderness. Most existing NFS wildernesses include relatively undeveloped headwater watersheds with few water quality impacts. More recent additions to NFS wildernesses may include areas that are impacted by upstream watershed activities, such as by agriculture, mining, and land development.

3.4.1 Measure: Concentration of Ambient Ozone

This measure assesses the 3-year rolling average of ozone concentration (fourth highest daily maximum 8-hour concentration) based on the Forest Service Air Resource Management Program's annual analyses of national ozone monitoring data. Ozone is a pollutant formed when emissions of nitrogen oxides (NOX) and volatile organic compounds react in the presence of sunlight. Human activities such as the burning of fossil fuels and industrial processes produce these pollutants, which can then travel long distances resulting in elevated ozone levels in wildernesses. In most places in the United States, reductions in human-generated NOX will cause a reduction in ground-level ozone. Ozone is one of the most toxic air pollutants to plants and its effects include visible injury to leaves and needles, premature leaf loss, reduced photosynthesis, and reduced growth in sensitive plant species. Continued exposure of vegetation to ozone over time may also result in increased susceptibility to disease and damage from insects, as well as changes in species diversity and community structure.

This measure of air pollution was selected based on the potential impact of ozone on wilderness vegetation and the availability of ozone measurements. Considering all of the potential negative effects on wilderness vegetation, increasing ozone levels in or near a wilderness are a direct human-caused threat to the Natural Quality of wilderness character. A network of long-term air quality monitors measure ambient ground-level ozone concentrations across the United States. The monitors are primarily intended to track whether NAAQS, established to protect human health and natural resources, are being met. Data from this network receive rigorous QA and QC review before being entered into the EPA's Air Quality System (AQS) database available at https://www.epa.gov/aqs. Using these data, staff in the Forest Service Air Resource Management Program calculate a suite of ozone statistics for all monitoring sites in the United States each year.

If most relevant, local units may select just this measure of the five air quality measures included under this indicator (see sections 3.4.1 through 3.4.5), or may optionally select more than one air quality measure. A finding of statistical significance results in a change in trend for this measure. An increase in the average ozone concentration corresponds with a degrading trend.

Refer to part 2, section 3.4.1, for detailed guidance on data sources and compilation protocols, analysis, data adequacy, and interpreting the threshold for meaningful change.

3.4.2 Measure: Deposition of Nitrogen

This measure assesses the amount of nitrogen deposition in a wilderness by using either the average total deposition (based on nationally modeled or measured spatial data) or the trend in wet deposition (based on the Forest Service Air Program's annual analyses of spatially interpolated data). Nitrogen oxides (NOX) are one of the major pollutants emitted into the atmosphere during the burning of fossil fuels. Agricultural activities, especially livestock management and fertilizer application to soils, are the primary source of ammonia (NH3) released to the atmosphere. These pollutants return to terrestrial and aquatic environments as atmospheric deposition of nitric acids and ammonium. In sensitive ecosystems, these compounds can acidify soil and surface waters, which affects nutrient cycling, impacts the growth of vegetation, and causes the decline or death of aquatic insects and fish. Even in ecosystems that can buffer incoming acid compounds, excess nitrogen deposition can lead to chemical and biological changes that affect plant growth, species composition, and aquatic food webs. Descriptions of the effects of nitrogen deposition on natural resources are available on the Forest Service Air Quality Portal website available at https://www.srs.fs.usda.gov/airqualityportal/critical_loads/atmospheric_deposition.php.

Nitrogen deposition was selected as a measure based on potential and observed negative impacts on wilderness ecosystems and the availability of deposition estimates across most wildernesses. While a few wildernesses may have direct nitrogen deposition measurements available, most will rely on estimates created through modeling based on data derived from long-term air quality monitoring stations that record nitrogen deposition across the United States.

If most relevant, local units may select just this measure of the five air quality measures included under this indicator (see sections 3.4.1 through 3.4.5), or may optionally select more than one air quality measure. A finding of statistical significance, or any change in defined categories, results in a change in trend for this measure. An increase in the amount of nitrogen deposition corresponds with a degrading trend.

Refer to part 2, section 3.4.2, for detailed guidance on data sources and compilation protocols, analysis, data adequacy, and interpreting the threshold for meaningful change.

3.4.3 Measure: Deposition of Sulfur

This measure assesses the amount of sulfur deposition in a wilderness by using either the trend in wet deposition (based on the Forest Service Air Resource Management Program's annual analyses of spatially interpolated data) or the average total deposition (based on nationally modeled spatial data). Sulfur dioxide (SO2) is emitted during the burning of fossil fuels, especially coal, and can be transported long distances through the atmosphere before being deposited in the form of sulfuric acid. In sensitive ecosystems, sulfuric acid can contribute to acidification of soil and surface waters, affect nutrient cycling and impact the growth of vegetation, as well as lead to the decline and death of aquatic insects and fish. These effects have been more prevalent in the eastern United States due to historically high sulfur deposition levels.

Although sulfur deposition has been declining and fish kills from acidification are now infrequent, sulfur bound and held in the soil continues to affect soil chemistry, soil buffering capacity, and the nutrient status of soils. Detailed descriptions of the effects of sulfur deposition on natural resources are available on the Forest Service Air Quality Portal website available at https://www.srs.fs.usda.gov/airqualityportal/critical_loads/atmospheric_deposition.php.

Sulfur deposition was selected as a measure based on observed negative impacts on wilderness ecosystems and the availability of deposition estimates across most wildernesses. While a few wildernesses may have direct sulfur deposition measurements available, most will rely on estimates created through modeling based on data derived from networks of long-term air quality monitoring stations that record sulfur deposition across the United States. Eastern national forests are likely to be more interested in using the sulfur deposition measure over the nitrogen measure because sulfur continues to exert a stronger influence on many ecosystems in the Eastern United States.

If most relevant, local units may select just this measure of the five air quality measures included under this indicator (see sections 3.4.1 through 3.4.5), or may optionally select more than one air quality measure. A finding of statistical significance, or any change in defined categories, results in a change in trend for this measure. An increase in the amount of sulfur deposition corresponds with a degrading trend.

Refer to part 2, section 3.4.3, for detailed guidance on data sources and compilation protocols, analysis, data adequacy, and interpreting the threshold for meaningful change.

3.4.4 Measure: Amount of Haze

This measure assesses the trend in average deciview for the 20 percent most impaired days, based on the Forest Service Air Resource Management Program's annual analyses of national visibility monitoring data. Although air quality managers often refer to visibility (or the lack thereof) in terms of its impacts on human perception, visibility is a general indicator of air quality monitored for its inherent value, just as one would monitor the biophysical condition of water quality.

This measure was selected because visual air quality (visibility) measurements provide a direct link between the concentration of pollutants in the atmosphere and degradation of the natural and physical condition of clean air in wilderness. Reduced visibility can affect local climate and photosynthetic activity. Additionally, visibility directly affects many wildlife and insect species that depend on clear, clean air (e.g., foraging raptors, pollinators).

Particles suspended in the atmosphere that absorb and scatter light cause regional haze. Impairment is operationally defined as the portion of haze which results from human activity. Fine particles (particles less than 2.5 μm in diameter) are routinely split into six distinct categories: (1) sulfates, (2) nitrates, (3) organics, (4) elemental carbon, (5) sea salt, and (6) soil.

A simple algorithm is used to identify the 20 percent of sample days each calendar year that are likely to be most affected by anthropogenic pollutants. The visibility conditions on these 20 percent "most impaired" days are converted to deciview and averaged annually.

If most relevant, local units may select just this measure of the five air quality measures included under this indicator (see sections 3.4.1 through 3.4.5), or may optionally select more than one air quality measure. Any change in defined categories results in a change in trend for this measure. An increase in the amount of haze corresponds with a degrading trend.

Refer to part 2, section 3.4.4, for detailed guidance on data sources and compilation protocols, analysis, data adequacy, and interpreting the threshold for meaningful change.

3.4.5 Measure: Index of Sensitive Lichen Species

This measure assesses the trend in air pollution scores for nitrogen and sulfur derived from the presence and abundance of sensitive lichen species, based on the Forest Service Air Resource Management Program's analyses of local biomonitoring data. Air pollution scores are calculated for each wilderness biomonitoring plot by surveying epiphytic lichen species (i.e., those growing on trees) with varying sensitivities to nitrogen and sulfur air pollution. Lichens are important contributors to critical ecosystem processes, such as nutrient cycling, and they provide food and nesting material for birds and other animals. The composition of an epiphytic lichen community is a well-known biological indicator of air pollution in forested ecosystems because epiphytic lichens rely completely on atmospheric sources of nutrition.

The lack of a waxy cuticle on the lichen surface permits absorption and leaching of nutrients in very similar proportion to what is present in the atmosphere. Lichen species that are sensitive to nitrogen and sulfur deposition eventually die or diminish from the forest if pollution levels are elevated. Epiphytic lichen communities that retain the species most sensitive to air pollution indicate good air quality. Nitrogen and sulfur air pollutants can cause measurable lichen community changes within a 5-year monitoring period depending on the spatial and temporal extent of deposition.

This measure was selected because the presence or absence of sensitive lichens over time indicates improving or degrading air quality (Matos et al. 2017). Many Forest Service regions routinely collect data on epiphytic lichen communities; this measure of air pollution may be especially useful for wildernesses that are not near other air pollution monitors, such as in Alaska.

If most relevant, local units may select just this measure of the five air quality measures included under this indicator (see sections 3.4.1 through 3.4.5), or may optionally select more than one air quality measure. Any change in defined categories results in a change in trend for this measure. A change in the trend category indicating an increase in air pollution corresponds with a degrading trend.

Refer to part 2, section 3.4.5, for detailed guidance on data sources and compilation protocols, analysis, data adequacy, and interpreting the threshold for meaningful change.

3.4.6 Measure: Extent of Waterbodies With Impaired Water Quality

This measure assesses the miles of streams or number of lakes inside wilderness with impaired water quality, based on national or state 303(d) lists of impaired water bodies or local monitoring data. Water quality is influenced by a wide range of biological and physical variables from both inside and outside a wilderness. This measure focuses on human-caused threats to wilderness water quality and not on natural variation in water quality. Despite the general importance of water and a myriad of national water monitoring programs, water monitoring in wilderness is generally conducted only for site-specific threats. For example, impacts from grazing (sediment, manure), mining (sediment, heavy metals, and other toxins), air pollutants (nitrogen, sulfur), and recreation (sediment, fecal coliform bacteria) vary tremendously from wilderness to wilderness and from one site to another within that wilderness.

This measure was selected because of the fundamental importance of water quality to the Natural Quality of wilderness character. Water quality directly influences the health of plant and animal communities. While many headwater wilderness watersheds have good water quality, degradation from historical activities such as mining or from upstream developments outside a wilderness may impact water quality in wilderness.

Measures related to different aspects of water are included in other indicators under the Natural Quality. For example, changes to biological aspects of water are monitored under the plants or animals indicators. The measure Watershed Condition Class (see section 3.5.1) uses the Forest Service WCF, which includes water quality as one of 12 indicators that determine watershed condition. While WCF assesses the overall watershed condition of the entire 6th code Hydrologic Unit Code (HUC), this measure provides a more specific focus on water quality within a wilderness.

This measure is required for all Forest Service wildernesses. A 5-percent or greater change in the miles of impaired streams or number of lakes will result in a change in trend for this measure. An increase in the extent of impaired waterbodies corresponds with a degrading trend.

Refer to part 2, section 3.4.6, for detailed guidance on data sources and compilation protocols, analysis, data adequacy, and interpreting the threshold for meaningful change.

3.5 Indicator: Ecological Processes

This indicator focuses on threats to ecological processes that affect biotic and abiotic components of wilderness ecological systems. Ecological processes are the interactions among the biotic and abiotic components of ecosystems and include disturbance events (e.g., fire and wind storms, insect and pathogen outbreaks), predation, competition, decomposition, symbioses, and nutrient cycling. Ecological processes involve multiple components of wilderness ecosystems and are critical to all aspects of ecosystem composition, structure, and function, resulting in long-term and cascading effects on the natural community in wilderness.

The integrity of ecological processes within wilderness is crucial to maintaining the Natural Quality of wilderness character. Ecological processes are complex and difficult to quantify. Of the vast number of threats to ecological processes that could be used for WCM, this technical guide includes only those that take advantage of existing datasets and provide an overall synthesis of the condition of an ecological process within wilderness. This indicator does not include measures on the effects of climate change on ecological processes in wilderness because of the difficulty in separating the localized effects of natural change from climate change, combined with the general lack of wilderness-specific data on the natural variability of ecological processes (see section 3.6 in part 2, and Appendix 2).

3.5.1 Measure: Watershed Condition Class

This measure assesses the average wilderness watershed condition class, based on Forest Service Watershed Condition Classification (WCC) data. The WCF is a nationally consistent, reconnaissance-level approach for classifying NFS watershed conditions that uses a comprehensive set of 12 indicators to represent the underlying ecological, hydrological, and geomorphic functions and processes that affect watershed condition (USDA Forest Service 2011b,c). WCC maps generated from the WCF characterize the health and condition of NFS lands in more than 15,000 watersheds across the country. These maps, instituted in 2011, established watershed baseline conditions along with information on ecological, social, and economic factors, as well as partnership opportunities to establish watershed restoration priorities.

This measure was selected because it reflects the integrity and ecological importance of watersheds, including biotic integrity, resiliency, connectivity, and important ecosystem services such as high-quality water, the recharge of streams and aquifers, maintenance of riparian communities, and the moderation of climate variability and change. Updating the WCC ratings for each watershed is planned at five-year intervals with the next update initiated in 2016.

There is some redundancy between this measure and the Extent of Waterbodies With Impaired Water Quality measure. While this measure uses all 12 WCF indicators, including the indicator for water quality, the Extent of Waterbodies With Impaired Water Quality measure relies heavily on EPA and individual state 303(d) lists of streams or lakes with impaired water quality. The Extent of Waterbodies With Impaired Water Quality is appropriate as a measure under the Air and Water indicator as it provides a site-specific assessment of water quality in wilderness. Using all 12 WCF indicators in this measure provides a more complete overall assessment of watershed condition because it includes additional aquatic and terrestrial physical and biological information. This measure is therefore located under the Ecological Processes indicator rather than the Air and Water indicator.

Local units are required to select either this measure or the following measure, Number of Animal Unit Months of Commercial Livestock Use, or may select both measures if relevant to the individual wilderness. Any change in the average wilderness watershed condition class results in a change in trend for this measure. An increase in the watershed condition class corresponds with a degrading trend.

Refer to part 2, section 3.5.1, for more detailed guidance on data sources and compilation protocols, analysis, data adequacy, and interpreting the threshold for meaningful change.

3.5.2 Measure: Number of Animal Unit Months of Commercial Livestock Use

This measure assesses the 3-year rolling average of commercial livestock use, based on an annual count of wilderness animal unit months (AUMs) within a wilderness. The Wilderness Act states that, "The grazing of livestock, where established prior to the effective date of this act, shall be permitted to continue subject to reasonable regulations as are deemed necessary by the Secretary of Agriculture" (Section 4(d) (4)(2)). Subsequent wilderness legislation and the Congressional Grazing Guidelines (House Reports 96–617 and 96–1126 that are included in the Colorado Wilderness Act of 1980) uphold this mandate from the Wilderness Act. In practice, this means that livestock grazing cannot be reduced or phased out simply because an area is designated as wilderness—any adjustments in livestock grazing must be made through revisions in the normal rangeland management and land management planning and policy-setting processes. These processes consider legal mandates, range condition, and protection of the range resource from deterioration.

This measure was selected because the presence of livestock, even though allowed under the Wilderness Act, represents a nonindigenous, domestic animal that impacts many aspects of the Natural Quality of wilderness character (Belsky et al. 1999; Beschta et al. 2014). Livestock grazing may impact indigenous plant and animal communities, soil, and watershed conditions within a wilderness. This measure does not directly monitor the ecological impacts of livestock grazing; rather it is based on the assumption that a declining number of AUMs results in an improving trend in ecological processes within wilderness, even though the adverse ecological effects of livestock may persist (Nussle et al. 2017).

The amount of annual livestock use is based on the AUMs of livestock grazing authorized by a grazing permit for allotments located wholly or partially within a wilderness. AUMs are the preferred unit of measurement instead of head months and should be used if available.

Local units are required to select either this measure or the preceding measure, Watershed Condition Class, or may select both measures if relevant to the individual wilderness. A 5-percent change in the 3-year rolling average amount of commercial livestock use will result in a change in trend for this measure. Once there are five measure values, the threshold for meaningful change will switch to regression analysis, and statistical significance will determine the trend in the measure. An increase in the average amount of commercial livestock use corresponds with a degrading trend.

Refer to part 2, section 3.5.2, for more detailed guidance on data sources and compilation protocols, analysis, data adequacy, and interpreting the threshold for meaningful change.