The Wild Free-Roaming Horses and Burros Act of 1971 (P.L. 92-195), as amended by the Public Rangelands Improvement Act of 1978 (P.L. 95-514), requires the Bureau of Land Management (BLM) to “determine appropriate management levels for wild free-roaming horses and burros on [designated] public lands.” The legislation makes BLM responsible for deciding how these appropriate management levels (AMLs) of free-ranging horses and burros should be achieved within the agency’s multiple-use mandate, including consideration for wildlife, livestock, wilderness, and recreation. BLM is also directed to manage for a thriving natural ecological balance, to prevent deterioration of the range, and to use minimal management for free-ranging horses and burros.
An AML has been interpreted by BLM as being a population size with upper and lower bounds for each individual Herd Management Area (HMA). Options listed in the legislation for keeping horses and burros within set population levels include removal of animals from the range, destruction of animals,1 sterilization, and natural controls on population levels, although the legislation does not limit BLM to these actions or specify acceptable types of sterilization or natural controls. Much of the controversy surrounding the management of free-ranging horses and burros focuses on the appropriate limit, if any, for the numbers of these animals on the range and how to keep free-ranging equid populations within a prescribed limit. From submitted public comments and statements made by members of the public at information-gathering meetings, it was clear to the committee that stakeholders vary in their opinions about how AMLs are established and what constitutes an AML. Because AMLs are a focal point of controversy, how they are established, monitored, and adjusted should be transparent to stakeholders and supported by scientific information.
1 The destruction of healthy, unadoptable free-ranging horses and burros has been restricted by a moratorium instituted by the director of BLM since 1982 and by the annual congressional appropriations bill for the Department of the Interior since 1988.
The committee was asked to
- Evaluate BLM’s approach to establishing or adjusting AMLs as described in the Wild Horses and Burros Management Handbook (BLM, 2010).
- Determine, on the basis of scientific and technical considerations, whether there are other approaches to establishing or adjusting AMLs that BLM should consider.
- Suggest how BLM might improve its ability to validate AMLs.
To accomplish its assignment, the committee first investigated the basis of the Wild Horses and Burros Management Handbook approach to setting AMLs. The investigation included gaining an understanding of legislative definitions and interpretations that BLM has used to develop its AML policies. The committee then evaluated BLM’s approach to setting AMLs as described in the handbook. Finally, the committee explored alternative, improved approaches that BLM could consider in setting and validating AMLs.
Scientific methods can be used to assess the condition of rangeland and its ability to sustain foraging and browsing animals. However, decisions regarding what kinds of animals should occupy the land, how many species should be in an area, how the land should be used, and what the balance of different uses of the land should be are questions of policy, not science. The committee’s task in this chapter is to explore the science behind the establishment and adjustment of appropriate management levels.
The Wild Horses and Burros Management Handbook was written in response to a critique by the Government Accountability Office (GAO) stating that, as of 2008, BLM had not provided formal guidance to its field offices on how AMLs should be established and that there was a lack of consistency in setting AMLs in the agency (GAO, 2008). The following summarizes the legislative context for establishing and adjusting AMLs. It then draws conclusions about the challenges inherent in establishing and adjusting AMLs on the basis of the committee’s review of the legislation.
The Legislative Setting for Establishment of Appropriate Management Levels
The Public Rangelands Improvement Act of 1978 amended the 1971 act to state that information from rangeland inventory and monitoring, land-use planning, and court-ordered environmental impact statements should be used to determine whether horses are exceeding AMLs. The 1978 Code of Federal Regulations (CFR) asserted that BLM should ascertain the optimum number of free-ranging equids supported by an area and that enough forage should be allocated to horses and burros to maintain them at that number in healthy conditions while considering an area’s soil and watershed conditions, wildlife, environmental quality, and domestic livestock (43 CFR §4730.3 ). The concept of defining AMLs by the optimum number of horses that maintains a thriving natural ecological balance and avoids deterioration of the range was reaffirmed in Dahl v. Clark, 600 F. Supp 585, 592 (1984) and by the Department of the Interior’s Interior Board of Land Appeals (IBLA) (Animal Protection Institute of America, 109 IBLA 112, 119 ).
Under its enabling legislation, the 1976 Federal Land Policy and Management Act (P.L. 94-579), BLM is required to manage public lands under the principles of multiple use and sustained yield. The agency’s objectives are
1) to periodically and systematically inventory public lands and their resources and their present and future use projected through land-use planning processes; 2) to manage public lands on the basis of multiple use and sustained yield; 3) to manage public lands in a manner that will protect the quality of scientific, scenic, historical, ecological, environmental, air and atmospheric, water resource, and archaeological values; 4) where appropriate, to preserve and protect certain public lands in their natural condition; 5) to provide food and habitat for fish and wildlife and domestic animals; 6) to provide for outdoor recreation and human occupancy and use; and 7) to manage, maintain and improve the condition of the public rangelands so that they become as productive as feasible for all rangeland values in accordance with management objectives and the land use planning process. (BLM, 2001, p. I-1)
Those objectives originate with the Taylor Grazing Act of 1934 (P.L. 73-482), as amended and supplemented by the Federal Land Policy and Management Act of 1976, and the Public Rangelands Improvement Act of 1978. In addition, managers of free-ranging horses and burros must also be mindful of or necessarily follow (depending on the particular law) the guidance in the Wilderness Act of 1964 (P.L. 88-577), the National Historic Preservation Act of 1966 (P.L. 89-665), the Clean Water Act of 1972 (P.L. 92-500), the Endangered Species Act of 1973 (P.L. 93-205), the Forest and Rangeland Renewable Resources Planning Act of 1974 (P.L. 93-378), and others. Managers of free-ranging horses and burros must balance a litany of complex and even conflicting considerations when setting and maintaining AMLs in the context of those laws. A Senate conference report that accompanied the Wild Free-Roaming Horses and Burros Act states
The principal goal of this legislation is to provide for the protection of the animals from man and not the single use management of areas for the benefit of wild free-roaming horses and burros. It is the intent of the committee that the wild free-roaming horses and burros be specifically incorporated as a component of the multiple-use plans governing the use of the public lands. (U.S. Congress, 1971, p. 3)
Historically, BLM efforts to identify the appropriate number of free-ranging equids that should inhabit each HMA have been challenging and controversial, even after the term optimum was replaced in the CFR with the charge to “consider the appropriate management level for the herd, the habitat requirements of the animals, [and] the relationships with other uses of the public and adjacent private lands” while continuing to manage free-ranging horses and burros on designated HMAs (43 CFR §4710.3-1 ). Previous reviews of BLM’s setting of AMLs consistently reported that established AMLs were not based on thorough assessments of range conditions. The U.S. District Court for the District of Nevada, IBLA, and GAO all noted that AMLs of many HMAs in the 1970s and some in the 1980s were based on administrative decisions rather than information about the carrying capacity of the range (Dahl v. Clark, 1984; 109 IBLA 119; GAO, 1990). The agency acknowledged in its 2003 strategic plan (updated in 2005) that diverse methods had been used to establish AMLs (BLM, 2003, revised 2005). In general, more consistent data collection has also been recommended for grazing management (Veblen et al., 2011).
Even though AML determination has been harmonized to derive from an agency-wide land-use planning process, diversity is still an issue because each state office conducts habitat assessment in its own way (BLM, 2003, revised 2005). In addition to a critique that formal guidance on setting AMLs had not been given to field offices, the 2008 GAO report noted that, as late as 2002, AMLs had not been set for two-thirds of HMAs.
Major Challenges in Defining Appropriate Management Levels in Prescribed Legislation
The committee identified three overarching challenges that permeate any consideration of how to set or adjust AMLs. These challenges stem from the historical and legislative background of AMLs and the institutional and environmental context of BLM in considering the setting and adjustment of AMLs.
First, although biological and physical measurements are used to estimate the capacity of rangelands to support free-ranging horses and burros, the allocation of forage among multiple users is a policy decision.
Second, the legislation includes requirements that seem contradictory. As reviewed in Chapter 1, the 1971 act (as amended) calls for horses and burros to be managed “as an integral part of the natural system of the public lands” and that “all management activities shall be at the minimal feasible level” but also requires the protection of a thriving natural ecological balance, which encompasses other species—especially threatened, endangered, and sensitive species—and avoidance of range deterioration caused by overpopulation. As a result, horses and burros are limited to specified areas, populations are controlled, and herds are largely protected from starvation and drought. Thus, the stipulations for their management are different from those for wildlife, which can be hunted or left to self-regulate naturally, and for livestock, which can be removed from the range by their owners at BLM request.
Equids have been able to inhabit western rangelands for hundreds of years without human intervention despite weather, predation, and disease. On most HMAs, horse populations have demonstrated an ability to reproduce at a rate sufficient to sustain themselves and, in most cases, to increase in abundance. However, their reproductive success may cause them to migrate or disperse in search of more resources or to have undesirable effects on soils and vegetation, both of which can bring them into conflict with other land uses. Population processes involved in food limitation, climatically driven variations in food and water, fire, predation, or natural barriers that limit access to additional food can, in some circumstances, effectively operate to regulate populations without human intervention (see Chapter 3). However, allowing horses or burros to self-regulate by permitting them to starve or to suffer from disease outbreaks is unacceptable to a large portion of the public (see section “Consequences and Indicators of Self-Limitation” in Chapter 3) and herbivory-induced changes in soils and vegetation may be unacceptable to some. Restricting horses and burros to designated HMAs can interfere with processes involved in self-regulation when dispersal or migratory movements are disrupted, when key resource areas are made unavailable (see Chapter 3), or when natural predators are lacking (see section “Effects of Predation” in Chapter 3). Management interventions may become necessary as surrogates for self-regulation processes. Interventions likely involve removals because hunting, euthanasia, and sale for slaughter are not currently acceptable options.
Setting AMLs in light of conflicting mandates leads to expensive and controversial approaches to management of rangeland herbivores, including gathering and removing horses and burros, fertility control, manipulation of genetic attributes, adoption, and feeding or pasturing horses. Each of those actions takes management of free-ranging horses and burros further from the ideal of minimal management as envisioned in the original legislation, regardless of how they represent attempts to work within the institutional and legal framework that shapes and constrains the protections for free-ranging horses and burros.
Third, although the legislation calls for setting AMLs to maintain a thriving natural ecological balance and to prevent rangeland deterioration, these terms are uninformed by
science and open to multiple interpretations; precise definitions would improve the ability to use them as goals for management. For example, the concept of a thriving natural ecological balance does not provide guidance for determining how to allocate forage and other resources among multiple uses, which ecosystem components should be included and monitored in the “balance,” or when a system is considered to be out of balance. It brings up arguments over whether such a balance exists in nature or is even possible. Avoiding rangeland deterioration and setting of land health standards may be seen as a problem of developing specific ecological measurements and standards or as a matter of arriving at a consensus about how rangelands should be maintained. A standard, broadly agreed-on definition of rangeland deterioration and how to measure it has proved an elusive goal for decades.
The BLM Wild Horses and Burros Management Handbook was written to respond to GAO’s criticism that BLM had not provided guidance to its field offices on how AMLs should be established. To understand how AMLs were set without a specific protocol, the committee surveyed 40 HMAs (Box 7-1). Beever and Aldridge (2011) provided a comprehensive review of criteria used by BLM managers to establish AMLs.
The handbook seeks to rectify the lack of guidelines for setting AMLs by making recommendations for their establishment and adjustment in several sections. Most specifically, Appendix 3 of the handbook defines AMLs and provides guidelines for setting them.
AML decisions determine the number of WH&B [wild horses and burros] to be managed within an HMA or complex of HMAs. AML is expressed as a population range with an upper and lower limit. The AML upper limit is the number of WH&B which results in a TNEB [thriving natural ecological balance] and avoids a deterioration of the range. The AML lower limit is normally set at a number that allows the population to grow to the upper limit over a 4-5 year period, without any interim gathers to remove excess wild horses and burros. (BLM, 2010, p. 67)
Table E-1 in Appendix E shows the upper limit set for HMAs as of May 2012. The handbook states that an AML should be evaluated or re-evaluated “when review of resource monitoring and population inventory data indicates the AML may no longer be appropriate” (BLM, 2010, p. 18). Reasons that may warrant a re-evaluation include changes in the environment; newly federally protected threatened, endangered, or sensitive species; and other relevant data.
The handbook prescribes processes for the decision-making aspects of setting and adjusting AMLs. Chapter 2 of the handbook, on land-use planning, suggests that the process of setting and adjusting AMLs should take place as part of comprehensive planning, should be based on monitoring and evaluation, and should follow required decision- making procedures.
AML may be adjusted (either up or down) through the site-specific environmental analysis and decision process required by the National Environmental Policy Act of 1970 (NEPA) (P.L. 91-190). An analysis under NEPA is also required to establish a population range (upper and lower limit) for AMLs initially established as a single number. Development of a LUP [land-use plan] amendment or revision is not generally required. (BLM, 2010, p. 10)
The handbook states that an LUP should provide a process for adjusting AMLs once they are established. The process varies from one LUP area to another. If an LUP does not
Reasons Given by Managers for Setting and
Adjusting Appropriate Management Levels
The committee recognized that, by and large, AMLs for individual HMAs had been set before the publication of the handbook in June 2010 and that little time had passed for adjustments to be made between the publication and when the committee’s survey questions (Appendix D) were distributed to 40 HMAs in January 2012. The committee wanted to gain an understanding of how AMLs had been established and adjusted before publication of the handbook. The 40 HMAs in the survey were the same as those sampled for population-estimate and survey-method information (Appendix E, Table E-3).
Survey respondents reported considerable variation at the HMA level in the approaches used for assessment and monitoring on HMAs. Establishment of HMAs generally occurred through consultation with state departments of fish and game for habitat and wildlife assessment, as called for in the legislation; use of state or regional BLM standards for rangeland (or public land) health as the “Standards for Land Health” stipulated as a goal in the handbook (BLM, 2010, p. 59); and reliance on the number of horses and cattle on the range at the time of HMA establishment to determine a goal for population levels, and in some cases to establish a ratio of number of horses to number of cattle as a framework for adjusting numbers.
The committee asked BLM managers who had been surveyed how they allocated forage among horses, cattle, and wildlife. Only a few fully addressed the question, and their responses were diverse. Use at the time of AML establishment was the most common answer, along with use of the original numbers at the time of the establishment of the HMA, the number specified in accordance with a resource management plan, the outcome of a land-use planning process, or a combination of the three. For example, in one HMA, the allocation between free-ranging horses and livestock was based on the original AMLs in the resource management plan, maintaining the original ratio of forage use for livestock and horses so that livestock and horses were reduced at the same rate. In this HMA, forage allocations were not increased because all the areas were stocked at or above carrying capacity. In another state, managers reported that in consultation with their department of fish and wildlife, the biologists at BLM made forage allocations to the native and exotic ungulates. Often, the forage allocated for existing livestock grazing privileges in an HMA was subtracted from total forage availability to determine the amount available to wildlife and horses.
Participating districts reported that measures of range condition and trend, upland utilization (amount of forage grazed, also termed “actual use” away from water), noxious weeds, and other types of rangeland and vegetation monitoring were considered relevant to adjusting and setting AMLs. One district used “negatively impacted vegetation functionality” as part of the justification to adjust an AML. Such considerations were frequent among reasons listed by managers for resetting or reaffirming AMLs. No data were provided on the metrics used to make the decisions, although some managers referred to other reports and multiple-use directives that were used in arriving at decisions. Monitoring of range and animal conditions; threatened, endangered, and sensitive species; and habitat was also conducted as part of setting, maintaining, or adjusting AMLs according to the survey. Most respondents to the committee’s survey reported that rangeland-monitoring studies (upland utilization, upland and riparian trend, and noxious-weeds monitoring) were being used to assess and evaluate forage availability in HMAs.a
On one HMA in another state, the AML was set after an intensive 5-year monitoring program. Data that were used included actual use, range condition and trends, utilization, precipitation, range sites, observations, and frequency of concentration areas for free-ranging horses. To change the AML again, the district
provide a process for adjusting AMLs, it may need to be revised or amended so that AML adjustments can be made.
In the committee’s view, the setting of an AML within a NEPA planning process when allocating resources among uses is in concert with the recognition that tradeoffs and values are parts of management decisions. The NEPA process provides for public comment and review and increases public participation in environmental decisions although the relationship is consultative rather than collaborative, tends to be bureaucratic, and
reported that it would need to conduct a similar monitoring program that would include re-examining the entire HMA and potentially reallocating forage for all animals.
One district reported that in the 1975 HMA planning process, forage-production calculations from 1952 were used to estimate how many animals could be supported on BLM-managed land in the HMA. That carrying capacity was revised in 1975 because of rangeland seedings conducted in the HMA in 1974. BLM then identified the forage allocated to existing livestock grazing privileges in the HMA and subtracted that amount to calculate forage available to horses and wildlife. The state department of fish and wildlife was consulted to determine the forage required by wildlife. Forage allocations to livestock, wildlife, and free-ranging horses were made commensurate with the available forage within a reasonable distance from water and in consultation with the state wildlife agency.
Managers in one state reported limiting forage use to 55 percent of production. No details were provided as to how annual plant production was determined.
The committee received the most comprehensive response to the question of allowable use from managers who used forage production maps from 1958 to estimate total forage production and determined the forage available on the basis of 50-percent utilization rate. The biologists reported currently using monitoring studies to assess and evaluate forage allocations in the HMAs.
Because horses are on the range year-round but cattle are not, temporal separation has been used to distinguish horse and cattle effects on water holes and other features. Surveyed managers of districts in California, Oregon, and Wyoming cited effects on watersheds and riparian areas, riparian utilization, riparian trend, and insufficient or unreliable water as causes for adjustment. “Timing and duration of flow” was also provided as a reason for changing AMLs.
Managers of the 40 surveyed HMAs reported that AMLs often had been adjusted or reaffirmed since 1971. For example, on one HMA, AMLs were changed 13 times from 1979 to 2007. Reasons for the changes were related either to four essential habitat components (forage, water, cover, and space) or to the political process. Examples of reasons included emergency gathers after extensive wildland fire, free-ranging horse distribution data, absence or inadequacy of winter range available for horses, climate and weather, and change in space available to free-ranging equids (for example, because of land closures, land trades, land-use planning efforts, boundary discrepancies, or a “checkerboard” jurisdictional pattern adjoining HMAs). Responders also cited splitting current herds into smaller groups, adverse effects of horses on cultural resources, improving vegetation conditions, enhancing wildlife habitat, and updating management plans as reasons for adjusting AMLs.
a “All Bureau of Land Management grazing allotments are periodically evaluated to assess rangeland health and evaluate the trend in rangeland condition and the influence grazing management has on the multiple rangeland resources associated with these allotments. [As an example, one district] employs two methods of evaluating grazing allotments. The first strategy involves a one-time field assessment by an Interdisciplinary Team composed of various BLM resource specialists. This team completes an assessment based on observations of vegetation and soil conditions. The second, and most commonly used strategy, involves a formal allotment evaluation process. During this process, an interdisciplinary team composed of various resource specialists evaluates resource conditions and creates management recommendations for the allotment. The end product of this process is an allotment evaluation document which summarizes resource conditions and trend and makes recommendations for future grazing management and range improvements on the allotment. Typically allotment evaluations occur every five to 10 years depending on the resource concerns for a given allotment.” (Sharp, no date, p. 1)
does not foster deliberation (Hourdequin et al., 2012). In any case, the decision-making process should be clearly distinguished from the data-gathering and analysis that provide the information used in decision-making. The committee’s focus is on the scientific analysis that feeds into decisions that ultimately must reflect social values, compromise, and economic realities.
A multitiered analysis process is stipulated by the handbook for establishing and adjusting AMLs. Tier One instructs managers of free-ranging horses and burros to “determine
whether the four essential habitat components (forage, water, cover, and space) are present in sufficient amounts to sustain healthy [wild horse and burro] populations and healthy rangelands over the long-term” (BLM, 2010, p. 67). Assessing the amount of sustainable forage available for the animals’ use is required by Tier Two. Tier Three concerns the genetic health of populations. Tiers One and Two are germane to this chapter; issues pertaining to Tier Three are discussed in Chapter 5.
The Tier One evaluation as described in the handbook for four habitat factors—forage, water, cover, and space—determines whether the features necessary to support horse and burro basic needs are present. It considers water, forage, space, and cover as limiting factors and requires evaluation of whether they are sufficient. Because of the inherent climatic variability of typical rangelands, the handbook recommends evaluating rangelands under conditions when they are likely to be low in forage production. Tier Two considers forage availability and quantity in detail. This section first reviews the handbook’s approach to water, cover, and space and then discusses its approach to forage. Forage availability is described in greater detail because it must be measured and used as a primary method for determining the number of herbivores that the range will support in Tier Two of the handbook-prescribed analysis. The section concludes with a review of problems related to terms and consistency in the handbook.
In keeping with its approach of using limiting factors to evaluate habitat suitability for horses and burros, the handbook instructs managers that the amount of available water is to be calculated on the basis of the driest part of the year (BLM, 2010). However, the handbook does not expand beyond the limiting-factors concept and provides little information about the importance of water in sustaining populations or about specific protocols for water monitoring and assessment. Water quantity and availability are to be assessed, but the handbook does not discuss poor water quality (such as nutrient content, sediment load, and water temperature). One BLM district reported in the committee’s survey that in its 1975 HMA plan process, water was identified as a limiting factor for summer use in drought years; as a result, forage allocations to livestock, wildlife, and free-ranging horses were then made with specific attention to water supplies and carrying capacity. One concern of the committee would be the age of the data because water supplies, developments, and land use have often changed and are subject to further alterations because of climate change. Another concern would be the possibility of conflict arising from competition between BLM and state agencies with responsibilities for water management. For example, the Nevada Division of Environmental Protection is responsible for water-quality standards and monitoring in the state. To prevent overlapping or competitive efforts, cooperative interaction between that office and BLM would be valuable.
Although riparian condition has been used as one of a suite of criteria to justify removal of free-ranging equids, the handbook provides relatively little specificity on the criteria to use in such decisions. Areas near water should be considered foci of concentration for horses and burros and monitored accordingly. Analyses of habitat use by free-ranging horses in sagebrush (Artemisia spp.) communities reported that horses seek riparian habitats (Crane et al., 1997). Free-ranging horses typically range farther from water sources than domestic cattle but need more water than forage alone can provide in most seasons and locations. Free-ranging horses can travel to water every 3 days to twice a day, and numerous factors affect their drinking frequency, for example, ambient temperature, succulence of existing
vegetation, wind speeds, and activity levels (Pellegrini, 1971; Meeker, 1979; Greyling et al., 2007). Horses’ use of water can affect water sources that influence vegetation, soils, and other species, so amounts and effects of current use should also be considered in evaluating water as a habitat component (Greyling et al., 2007). Use of areas near streams can increase runoff (Dyring, 1990b; Rogers, 1994), break down streambanks (Dyring, 1990a), reduce water quality (Nimmo and Miller, 2007), cause vegetation trampling, alterations in stream flow, and downstream siltation (Rogers, 1991), and accelerate gully erosion (Berman et al., 1988). Boggy habitats also can be altered by free-ranging horses (Dyring, 1990b; Rogers, 1991; Clemann, 2002). Similarly, soils, vegetation, and small mammals in and adjacent to springs can be markedly affected by free-ranging equids even when livestock have been absent for extended periods (Beever and Brussard, 2000).
There is evidence of interaction between forage characteristics and riparian-area use; the characteristics of forage may be affected by concentrated animal use near water. In the Sheldon National Wildlife Refuge in Nevada, 3 years of exclusion of free-ranging horses from grazing in riparian zones led to a 40-percent increase in cover of plant litter compared to bare ground and a 30-percent decrease in extent of bare ground, whereas these metrics remained generally constant in the paired riparian plots that continued to be grazed by horses (Boyd et al., 2012). In the nonexclosed areas, estimates of use from September to October based on standing biomass varied from negligible to nearly 100 percent (Boyd et al. 2012). In contrast, Greyling et al. (2007), studying areas of heavy use around a waterhole in Namibia, reported that the “expected degradation gradient radiating out from the water troughs due to over-utilization by the horses was not found. Neither vegetation species composition, density, nor standing biomass measured at various distances from the troughs confirmed a degradation gradient.”
Methods of measuring riparian condition are available. Proper functioning condition is a monitoring tool developed by BLM to assess the physical functioning of riparian and wetland areas (BLM, 1998). It provides a consistent approach that takes into consideration hydrology, vegetation, and soil-landform attributes and encourages a team approach which includes wildlife, hydrology, and plant-science expertise. This method is qualitative by design and thus lacks rigorous quantitative analysis and statistical inference. However, it can provide a framework for identifying sites where water impairments have occurred and where improved management of water resources is required. Measures of water quality (such as temperature, salinity, nutrients, dissolved oxygen, and sediment) or hydrogeomorphology (such as groundwater discharge, active foodplain, sinuosity, and width and depth ratio) do not appear to be actively used by BLM and might serve as indicators for modifying management decisions related to free-ranging horses and burros (BLM, 1998). Soil conditions—such as storing moisture, allowing infiltration, stabilizing vegetation, and balanced release of water—and preventing rill or sheet erosion by water-caused or wind-caused dust are also possible indicators. A new synthesis of literature pertaining to riparian management practices (George et al., 2011) may provide insights on how to manage free-ranging horses in riparian areas. Further, a standard range-improvement action for mitigating damage to riparian areas involves fencing sensitive areas and providing troughs at locations away from natural waters. Given the extensive diversion, piping, and regulation of springs already in place across the western United States, additional use of troughs should be balanced against consideration for native fauna dependent on natural flows.
Cover and Space
Vegetation provides cover for free-ranging equids. For example, trees provide shade that allows equids to avoid direct insolation during the hottest times of the day, a rubbing surface that they can use to scratch topical irritations, visual concealment from predators, and forage (Pellegrini, 1971; Hanley, 1982). In the second paragraph of Chapter 3 of the handbook, an emphasis is placed on evaluating habitat suitability on the basis of access to “forage, water, or thermal or hiding cover.” The implication is that without access to those resources, horse removals may be necessary. Many models suggest that contemporary climate change may alter the distribution of trees and the balance of deciduous versus evergreen trees in parts of the domains of HMAs (Fuhlendorf et al., 1996, 2012; Tausch, 1999). The direct effects to horses of such changes are unknown. Before considering horse removals when cover and space are inadequate, where it does not cause conflict with other uses, managers may also consider increasing habitat availability by establishing greater connectivity between key habitats (through removing barriers and creating corridors for travel, habitat improvement, providing water at key points, land acquisition, or other methods).
It is not clear from the handbook (BLM, 2010) what is meant by space, and there does not seem to be a good definition or way of measuring it in the scientific literature. The analysis of adequate “space” in the handbook apparently is derived largely from whether the horses and burros will stay in the habitat. For example, the handbook states that the animals “require sufficient space to allow the herd to move freely between water and forage within seasonal habitats” (BLM, 2010, p. 13). The need to adjust AMLs because of changes in the area available to equids was cited several times by surveyed managers—such changes as land closures, land trades, LUP efforts, boundary discrepancies, or a “checkerboard” jurisdictional pattern adjoining or within HMAs.
To be more specific, the discussion in the handbook should emphasize the spatial movements of free-ranging horses and burros relative to water, cover, and forage. Other aspects that might be considered include the influence of sunshine, shade, the viewshed, predator escape routes, and slope position (e.g., leeward for shelter from weather and windy gaps for insect avoidance). There is a direct relationship between space and access to spatially heterogeneous resources (such as those listed) in landscapes where horses and burros may be (Coughenour, 1991, 2008). Those resources are often dispersed patchily. As a result, the four key habitat components (forage, water, cover, and space) and other resources are naturally heterogeneous in distribution and availability and should be evaluated on the basis of their spatial and temporal variability.
Because horses and burros, like most ungulates (Hobbs, 1996), use landscapes heterogeneously, assessment ideally would occur at multiple spatial resolutions. In particular, free-ranging equids will use some portions of the landscape often (especially when equid densities are high) and use other parts rarely or never (e.g., areas more than 15 km from water sources, slopes of more than 50 percent [Ganskopp and Vavra, 1987], and areas dominated by large boulders or monoliths). Multiple-resolution assessment could be especially valuable in situations in which dynamics at one spatial resolution can influence dynamics at other spatial resolutions (cross-scale dynamics; Allen, 2007).
In a case study in Appendix 3 of the handbook, the amount of forage available for sustainable use by herbivores, or the carrying capacity of an HMA, is the accessible, palatable
biomass that grows on the site annually, modified by an allowable use factor (AU). An AU is the percentage of annual production that can be grazed without causing plants to decline in production and growth. Typical AUs are between 25 and 60 percent, meaning that 25 to 60 percent of the annual forage growth can safely be allocated to grazing; that is, it is available forage. However, AUs are often adjusted for local conditions, as it is in the case study, and for season of grazing; AUs are higher in dormant than in growing seasons. AUs are based on data about the effects of specific percentages of “use” on plant species and communities that are rarely available. Studies of the response of specific species and plant communities to herbivory and how the species and communities are influenced by season of grazing, the amount and frequency of herbivory, and varied growing conditions have been numerous but by no means comprehensive (e.g., Hanley, 1982; Paige and Whitham, 1987; Paige, 1992; Belsky et al., 1993; Hawkes and Sullivan, 2001). In fact, it is usually difficult to determine exactly how even widely used AUs were derived.
The handbook’s case study details the use of at least 3 years of grazing utilization and use mapping data with annual population estimates of horses to determine weighted utilization data, potential carrying capacity, and a proposed carrying capacity. It is not clear where the AU for plant species is acquired. In the case study, it appears that all the available forage will be allocated to horses and that only horse data are used, although at the end it is shown that the results can be converted for other herbivores (BLM, 2010). The explanation of how to calculate the weighted average forage utilization is relatively clear, but it is not clear how annually adjusted population estimates of horses, expressed in animal unit months (AUMs), are acquired. An AUM is a standardized unit of forage consumed per “animal unit” each month. Knowledge of annual herd population sizes for at least 3 years is critical for the prescribed method in that they are the basis for establishing annual forage availability, the most common habitat factor used for establishing AMLs.
Use of utilization and use mapping data to infer forage production levels is a pragmatic approach that takes multiple factors into account, including “background” consumption by all users of forage, areas of concentration, and site-specific production limitations. Ideally, however, direct forage-production data should also be used to determine forage availability. Measuring how much forage is consumed by what species (horses and burros versus livestock versus wildlife) would be helpful in determining how many animals can be supported relative to forage supply, although the committee acknowledges that this can be difficult. The methods for assessing utilization are not described in the manual; however, examination of various BLM reports indicated that utilization was simply visually estimated. This method is prone to inaccuracy and is generally not well validated. More direct measures of utilization could be made through the use of grazing exclosures, particularly movable exclosures. Issues related to determining horse population size are detailed in Chapter 2.
Another complication is that a substantial part of the diet of horses may not be herbaceous plants, such as grass, and the case study includes only herbaceous growth to calculate forage availability. A fair amount of research on diets of free-ranging horses of the western United States that occurred in the 1970s and 1980s confirmed that horses are typically grazers (that is, most of the food that they consume is grasses and graminoids) but that the proportions of individual food items and even of plant life forms consumed vary markedly annually, among seasons, by location, and among individuals, including variation by age, sex, and reproductive state and history (Hansen, 1976; Hubbard and Hansen, 1976; Hansen et al., 1977; Olsen and Hansen, 1977; Krysl et al., 1984; McInnis and Vavra, 1987). That is due in part to the fact that the nutritive value of plant species can vary markedly among seasons and years (Miraglia et al., 2008). Utilization of browse should be identified and
incorporated into carrying-capacity calculations when it proves to be an important source of forage for horses.
The handbook guidelines recognize the high variability in forage production on arid rangelands, stipulating that forage-availability estimates should be based on 3-5 years of utilization and use-pattern mapping. In addition, the handbook states that to determine whether forage is sufficient for long-term sustainable equid grazing, production data, ecological site condition, trend, frequency, precipitation, and standards for land health may be used (BLM, 2010). It appears that each local office has a great deal of discretion in determining which methods to use. The handbook guidelines stipulate that years of above-average forage production are not to be used in calculations of forage availability—a conservative approach that aims to reduce the need for emergency gathers. Rangeland that is not commonly used is also not included. However, the committee considers 3 years of data to be inadequate typically for capturing variation in forage production on arid lands.
There are useful parallels between the setting of AMLs for free-ranging equids and the setting of sustainable stocking rates in managed livestock systems. Both endeavor to achieve ecological sustainability although management objectives and methods are quite different. Campbell et al. (2006) evaluated conditions that favor different ways of determining how to establish the number of livestock that can be supported on rangelands. They contrasted two types of strategies for setting a livestock stocking rate: conservative and tracking. A conservative strategy maintains a roughly constant stocking rate, which is set so that carrying capacity, the ability of the range to provide adequate forage, is unlikely to be exceeded even in dry years (Sandford, 1983, 2004; Tainton, 1999 in Campbell et al., 2006); this approach errs on the side of caution for dry years—in which overstocking can lead to livestock losses and vegetation deterioration—as does the handbook strategy. A strategy that tracks forage availability is less static and changes stocking rates to track variable forage supply; thus, more animals are on the range in years of high rainfall and fewer in dry years. Different conditions favor one strategy or the other (Table 7-1).
Campbell et al. (2006) summarized research that demonstrated that forage growth and distribution in semiarid rangelands are influenced by precipitation and are highly variable across time and space. Average annual rainfall is the key factor in temporal variation. Temporal variation increases as annual rainfall decreases (Ellis and Swift, 1988; Campbell et al., 2006; Briske et al., 2011). Because variability in rangelands also occurs on macroscales
|Environmental Conditions||Conservative Strategy— Setting numbers below average that can be supported over the long term is more likely to be optimal if:||Tracking Strategy— Managing animal numbers to follow changes in forage supply annually is more likely to be optimal if:|
|Predictability of environmental variability||Environmental variability is high and unpredictable.||Environmental variability is highly predictable.|
|State changes and thresholds||The system is prone to state changes and thresholds that limit reversibility through management.||The system has high resilience and changes are likely to be reversible with management.|
|SOURCE: Adapted from Campbell et al. (2006).|
(Campbell et al., 2006), even the largest HMA cannot buffer the variation in rainfall amount or distribution completely. As a result, variation in forage quality and quantity across space and time is high, and setting a static population level for herbivores runs counter to this complexity. The tracking strategy is argued to be more appropriate where environmental variability, such as in rainfall, is more predictable, allowing managers to anticipate need for adjustments in stocking, and a conservative strategy is more appropriate for locations with high variability and low predictability of environmental variability, such as in rainfall, because it reduces the number of years when drought would reduce forage production below levels adequate to support the animals (Campbell et al., 2006). As previously discussed, allowing free-ranging horses and burros to suffer from inadequate forage is precluded.
Extreme droughts will inevitably occur at unpredictable times. The location-specific effects of climate change are as yet largely uncertain. Studies suggest that temperature stress on ecosystems will be markedly higher (especially in summer) in the western United States, and there will probably be an increased frequency of extreme climatic conditions (Christensen et al., 2007; Mote and Redmond, 2012). It is clear that AMLs will need to be adaptable and periodically reassessed over the long run and subject to rapid adjustment in the short run. Gathers are the major means of adjusting the number of animals in response to drought. BLM may consider other options, which might include temporary supplemental forage or temporary movement or expansion by animals into unused range (if there were not conflicts with other resources). That might be accomplished through provision of water where there is no natural supply. However, unused range areas are quite possibly rare, and those options will only delay the need for a gather unless population growth is reduced.
In the case study in Appendix 3 of the handbook, despite the fact that allowable use was originally established to consider “year-round grazing” by horses, AUMs for horses are converted to their equivalents for other species, including livestock that are not on the range year-round (BLM, 2010). That highlights the difficulty of evaluating forage availability independently of allocation to various grazing animals. BLM considers a horse to be a single animal unit, consuming 1.0 AUM of forage per month. Horses consume more forage per unit of body weight than do ruminants (Hanley and Hanley, 1982), and the standard measure of an animal unit is a 1,000-lb cow and nursing calf. Several references to animal units for horses report that they consume more than 1.0 animal unit per month (1.2,2 1.3,3 or 1.0 for a 2-year-old horse and 1.5 for a horse 3 years old and older4). BLM should explain its choice of 1.0 animal unit for a horse.
As discussed in the section “Major Challenges in Defining Appropriate Management Levels in Prescribed Legislation,” vague definitions in the Wild Free-Roaming Horses and Burros Act and related legislation have created difficulty in implementing and assessing management strategies for free-ranging equids. The handbook does not provide any greater clarity. The committee reviewed two terms in detail to illustrate the problem: land health standards and thriving natural ecological balance.
2 Available online at http://www.gov.mb.ca/agriculture/crops/forages/bjb00s17.html/. Accessed October 8, 2012.
4 Available online at http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/faq6722/. Accessed October 8, 2012.
Land Health Standards
The handbook states that horses “should be managed in a manner that assures significant progress is made toward achieving the Land Health Standards for upland vegetation and riparian plant communities, watershed function, and habitat quality for animal populations, as well as other site-specific or landscape-level objectives, including those necessary to protect and manage threatened, endangered, and sensitive species” (BLM, 2010, p. 17). The basis for setting land health standards is not described in the handbook but is described elsewhere (BLM, 2001). However, land health standards are not specifically incorporated into the AML-setting process as outlined in the handbook, and this reflects a disconnect between AMLs and BLM land-health assessment procedures. If land health standards are to be at the crux of AMLs, a handbook should include procedures for their scientific determination or specific references to established procedures published elsewhere and recommendations for using such procedures to set AMLs.
The BLM land health standards policy has been in effect for over 15 years. BLM developed regulations for livestock grazing administration beginning in 1995-1997. One of the regulations was that each BLM state director would, in consultation with the Resource Advisory Council in that state, develop standards for public-land health. BLM posts a number of state-level land health guidelines developed accordingly.5 The purpose of the standards is to provide a measure to determine land health and methods or guidelines to improve the health of public rangelands (BLM, 2001). Rangeland health is defined as “the degree to which the integrity of the soil and ecological processes of rangeland ecosystems are sustained. Rangeland health exists when ecological processes are functioning properly to maintain the structure, organization and activity of the system over time” (BLM, 2001, p. I-7). That is significant in that it calls for assessments not only of states but of ecosystem processes. Processes of interest pertain to hydrology, nutrient cycling, primary production, and vegetation dynamics. The 2001 document outlines a set of general procedures that should be followed to assess and achieve rangeland health. Notably, they include not only a call to assess current land health but a determination of the causal factors that have led to the current state on the basis of the best data and resource information available. That would require the development of conceptual or quantitative models of ecosystem functioning.
“Deterioration,” like “health” or “condition,” is determined by some measure of the difference between the current state of the system and some reference state. The question is, What is the appropriate reference state of a minimally managed free-ranging equid system? The difficulty of defining that state is similar to the difficulty of defining what constitutes overgrazing. Overgrazing is a level of herbivory that leads to some level of rangeland deterioration. However, overgrazing in a livestock production system may be defined differently from overgrazing in a system that is being managed for natural processes. Coughenour and Singer (1991) reported that definitions of overgrazing also depend on differences in theories of how ecosystems that have abundant large herbivores function without human intervention. Indicators of deterioration in rangeland health may or may not constitute evidence of overgrazing, depending on management objectives and the theory or conceptual model that the management is based on. Differences in definitions used by livestock producers and wildlife managers are particularly relevant here. It is problematic to define overgrazing where there is a call for minimal management and “wildness.”
5 43 CFR §4180. Available online at http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=5d253348ae12c9c0d76a8114b7eec027&rgn=div5&view=text&node=43:220.127.116.11.92&idno=43#43:18.104.22.168.92.9/. Accessed October 8, 2012.
Thriving Natural Ecological Balance
The handbook does not provide guidance on how to assess a thriving natural ecological balance as called for in the legislation. It is also easily conflated with the allocation process, which is a policy-driven and sometimes court-adjudicated decision rather than something derived directly from currently available scientific information. The Wild Free-Roaming Horses and Burros Act is clear that habitat for wildlife and threatened, endangered, and sensitive species is not to be harmed by free-ranging horses and burros. Among the districts responding to the implementation survey, BLM consultation with state wildlife agencies (as instructed in the legislation) was fairly consistent. Wildlife considerations were mentioned in responses from several HMAs as reasons for adjusting AMLs, either explicitly in some HMAs or implicitly as reflected in allocation of forage. Concern for species listed as threatened, endangered, or sensitive under the U.S. Endangered Species Act was referred to by only one HMA complex in the survey sample to protect greater sage-grouse (Centrocercus urophasianus), an endangered species candidate, in Wyoming.
There are several possible interpretations of what constitutes a thriving natural ecological balance. It is not a scientific term.
Wildlife and Plant Diversity. BLM relies largely on state wildlife agencies to determine how to consider wildlife in the setting of AMLs. However, BLM has a responsibility to make sure that key indicators of free-ranging horse and burro effects are included in assessments of impacts to wildlife habitat.
Monitoring of wildlife and plant abundance and diversity is key to determining whether a thriving natural ecological balance is being preserved. Free-ranging horses and burros can affect species richness in a variety of habitats (Levin et al., 2002; Zalba and Cozzani, 2004). Manipulative experiments illustrate a dramatic array of indirect pathways by which free-ranging horses can affect components of marsh ecosystems (Levin et al., 2002). In addition to direct interference and other types of competition possibly experienced by large ungulates in areas that have free-ranging equids, wildlife sharing the range with free-ranging horses and burros can be subject to equids’ effects on the structure, composition, and function of vegetation. Vegetation provides perching habitat for raptors, nesting habitat for breeding birds (including cavity-nesting and stick-nest–creating birds), concealment cover for greater sage-grouse and other ground-nesting birds (Beever and Aldridge, 2011), shade and thermal refuge, refuge from predators, and nutrients and energy for diverse animals. Numerous species are affected by equid impacts to soil. Recovery of some species has been attributed to removal of free-ranging horses (Nano et al., 2003). Wildlife and plant abundance has been reported to be influenced by free-ranging horses’ presence (Coventry and Robertson, 1980; Mansergh, 1982; Gillespie et al., 1995; Beever and Brussard, 2000, 2004; Greyling, 2005). Reptile species richness was significantly lower at horse-used sites than at horse-removed sites studied in the western Great Basin (Beever and Brussard, 2004) and in the Austrian Alps (Coventry and Robertson, 1980; Mansergh, 1982), and reptiles are important as prey for numerous other species and as predators that influence biological integrity. Horse presence has been identified as potentially affecting ant mounds in the Great Basin (Beever and Herrick, 2006).
Interactions with Native Grazers. Native herbivores may have forage, space, water, and cover needs that overlap with those of free-ranging horses and burros. Therefore, monitoring of the status of native ungulates is crucial. In some cases, the presence of free-ranging horses has increased forage available to native species, as in the case of bitterbrush (Purshia tridentata)
in Utah (Reiner and Urness, 1982). Horses and burros can dominate other herbivores and exclude them from water sources, forcing a change in the habitat use of native grazers (Meeker, 1979; Berger, 1985; Ganskopp and Vavra, 1987; Coates and Schemnitz, 1994). The degree of overlap in diets and habitats determines the potential for competition. Elk and bison diets overlap with those of horses, but there have been few cases of concern about their interactions with horses. Elk inhabit low-elevation sagebrush-steppe habitats (Hobbs et al., 1996; Manier and Hobbs, 2007) and are found in some areas that have horses (Hansen et al., 1977), but their preferred habitats tend to be more mesic (moderately moist) and at higher elevations. Bison are primarily residents of the Great Plains and portions of the Rocky Mountains (Mack and Thompson, 1982); there is little sharing of range with horses.
In some areas, there has been concern about potential competition for forage between free-ranging equids and bighorn sheep. In the Pryor Mountains, Coates and Schemnitz (1994) found partial dietary overlap year-round, and Kissel (1996) found little overlap except in summer. Dietary overlap was minimized by the fact that a substantial fraction of bighorn sheep diets was shrubs, particularly the evergreen shrub mountain mahogany (Cercocarpus spp.), but shrubs were insignificant in horse diets. Horses and bighorn sheep also used markedly different habitats and overlapped little spatially. Modeling studies including consideration of diets and the extent of overlap in spatial ranges of the bighorns and horses also supported the idea that there was only a small degree of competition (Coughenour, 1999). Kissel (1996) concluded that there was little if any competition between horses and mule deer, inasmuch as the latter are primarily browsers and horses primarily grazers. Those two species had little spatial overlap because their habitat preferences are different. In contrast, competitive interactions between burros and bighorn sheep are important inasmuch as burros are mixed feeders and have substantial quantities of browse in their diets (Walters and Hansen, 1978; Seegmiller and Ohmart, 1981).
Interactions with Livestock. Free-ranging horses and livestock overlap in demands for forage and habitats. Cattle and horses are both primarily generalist grazers, consumers of palatable herbaceous vegetation. Horses and burros, however, are able to use lower-quality forage than cattle because of their cecal-digestive system (Hanley, 1982; Hanley and Hanley, 1982). Burros preferentially consume woody vegetation (shrubs, dwarf shrubs, stemmy forbs, and small trees). Horses and cattle use similar habitats, but they also diverge with respect to mobility and accessibility. Horses can travel great distances in a short time, they can travel further from water, and they can use rugged topography more readily than can cattle (Ganskopp and Vavra, 1987; Hampson et al., 2010).
Although it is often assumed that cattle, horses and burros, or wildlife always compete, recent research on zebras and cattle and on cattle and donkeys (donkeys served as surrogates for zebras in controlled experiments) showed that it is not always the case (Odadi et al., 2011a,b). When cattle were reared with donkeys, both grew faster than when each species was allowed to graze on its own. Facilitation occurred because the donkeys consumed tough, fibrous stems, allowing the cows to eat the more nutritious leaves, forbs, and regrowth; and the cattle helped to dilute the effects of ticks that plagued the donkeys. In semiarid habitat, the occurrence of light rains allowed grasses to continue growing after joint cropping by the two species changed the structure of the sward, thus improving forage quality. The extent to which horses and cattle can facilitate each other and improve rangeland in temperate grasslands requires further study and most certainly depends on the specifics of the ecosystem being considered. It is also critical to note that the Odadi et al. (2011a,b) research was carried out in African grasslands, which have a long, continuous coevolutionary history of herbivory by numerous ungulates and can have biomass and
graminoid diversity one to two magnitudes higher than some areas encompassed by HMAs of the western United States (Mack and Thompson, 1982). However, assuming that cattle and equids must compete because they share the same range is not necessarily warranted (du Toit, 2011).
Endangered Species. Of particular concern is the interaction of horses and greater sage-grouse. Possible interactions of free-ranging equids with greater sage-grouse were thoroughly outlined by Beever and Aldridge (2011). They described numerous mechanisms by which equids can influence their environment, and greater sage-grouse are known to be sensitive to those aspects of the environment (e.g., the height of herbaceous plants is important as concealment cover for nests), but no field research has directly addressed the relationships between equids and grouse. The authors outlined numerous research questions that might be addressed, given the continuing effort and concern related to greater sage-grouse and other sagebrush-associated species in western North America.
Possible alterations of the small-mammal community by free-ranging equids may be important because of the role of small mammals in aeration and bioturbation of soils, as prey for numerous terrestrial and aerial predators, in seed and nutrient redistribution, and as part of biotic integrity. Numerous other ecosystem processes and components are critically important for conserving the potential of BLM-administered landscapes to provide ecosystem services (e.g., clean water, noneroded soils, food, and fiber) and for allowing cost-effective maintenance of ecological function and biological diversity. All these are mandated by numerous laws, policies, and statutes related to rangeland health, water quality, endangered species, and other topics.
Challenges to Managing for a Thriving Natural Ecological Balance. Although allowing an equid ecosystem to self-regulate could be one approach to establishing a balance, it is also evident that this may not be a realistic objective in many cases, owing to human effects that are beyond the purview of BLM. Thus, as a result of human disruptions, a self-regulated system is not necessarily natural. Land use is a foremost human effect that constrains natural horse movements, dispersals, or migrations. In natural wildlife systems, herbivores are free to seek forage and avoid situations of depleted forage. Fragmentation of habitats due to land use or ownership that does not permit such movements is problematic for herbivore and vegetation sustainability (Coughenour, 2008). Another human intervention that disrupts natural ecological processes is the development of water sources that make otherwise unavailable areas of the landscape available for equid use. Water-scarce areas would naturally be refugia from horse use for a variety of plant and animal species that are less tolerant of horses’ presence. The recent incursion of invasive plant species, such as the Bromus species which includes cheatgrass (Bromus tectorum), is another example of human effects that alter the possibility of a natural balance, as is the extirpation of predators, such as wolves, in some environments where they would otherwise occur.
There are scientific approaches for assessing human effects on such ecosystems and the degree to which they impair free-ranging horse and burro numbers and management. They include scientifically based modeling studies of alternative scenarios of the presence or absence of human effects. Methods that meet the objective of minimal management could be identified, targeted, and justified to mitigate the adverse human effects. For example, if landscape fragmentation has altered the capacity of the habitat to support horses, model-based assessments would be able to quantify how this has occurred and therefore provide support for management interventions that mitigate it. Similar assessments could address lack of predation, invasive plants, and water development.
Managing horse and burro populations as free-ranging with the minimal management called for in the Wild Free-Roaming Horses and Burros Act thus entails conceptual challenges associated with defining what constitutes land deterioration or health. The handbook does not help in such definition. The handbook should address the challenge of defining terms used as management criteria, including appropriate, thriving, natural, in balance, healthy, and deteriorated. The approach would involve the development of a conceptual model for ecosystem functioning relative to management objectives and the development of indicators that can be used to measure the degree of departure from a scientifically informed conceptual model of an “appropriately” functioning free-ranging equid ecosystem.
Specificity of Methods and Their Consistency among Herd Management Areas
The handbook does not adequately respond to GAO’s request for guidance; the level of detail that the handbook supports is too limited. The handbook does provide for some degree of consistency in goals, forage allocation, and general habitat considerations and should help to improve consistency in how AMLs are set. However, it does not provide detail on monitoring and assessment methods. That is intended to allow BLM managers to decide what specific approaches fit local environmental conditions and administrative capacity, but it makes it difficult to review the program’s on-the-ground methods. A better approach would be to provide specific options. Similar issues were identified with respect to establishing AMLs, population inventory, use patterns, animal distribution, other site-specific and landscape-level management objectives, and forage allocation (BLM, 2010). For example, the handbook states that the amount of forage available to allocate to free-ranging horses and burros shall be determined through in-depth evaluation of resource-monitoring data after a site-specific environmental assessment and multiple-use decision process6 that includes public involvement. There is no explanation of any of the data-collection methods. The handbook would be more informative if it provided guidelines on how to conduct various kinds of assessments (even if there were a variety of appropriate methods available) or referenced appropriate sources, linking them to particular settings or situations. In general, the handbook lacks clear protocols for evaluating habitat components other than forage availability. That is critical because without clear protocols specific enough to ensure repeatability, the monitoring organization cannot determine whether observed change is due to changes in condition or to changes in methods. Protocols should also include
6 The multiple-use decision (MUD) process is generally used to establish livestock grazing, AMLs for free-ranging horses and burros, and recommendations for wildlife habitat management.
This process begins with an evaluation of range conditions; the evaluation assesses whether or not management and stocking levels for livestock, wild horses and/or burros, and wildlife are achieving rangeland objectives. If rangeland health objectives are not being met, changes in management or stocking levels are proposed. Proposed changes are analyzed in an environmental assessment and a proposed multiple-use decision is issued. Proposed decisions are subject to review and protest by parties affected by the proposal. BLM considers all protests filed and then issues a final multiple-use decision. BLM’s final decisions are subject to administrative review (appeal). (Appropriate Management Level. Available online at http://www.blm.gov/nv/st/en/prog/wh_b/appropriate_management.html. Accessed February 21, 2013.)
At the conclusion of the decision process the management actions are implemented and monitoring continues until the next evaluation. All decisions issued as a result of completion of an allotment evaluation are issued in the MUD format. The MUD format has four sections: Introduction; Livestock Grazing Management Decision; Wild Horse and Burro Management Decision; and Wildlife Decision. Each of these sections includes a rationale, citation of appropriate authority, and information about protests and appeal procedures. (Multiple Use Decision Process. Available online at http://www.blm.gov/nv/st/en/prog/grazing/multiple_use_decision.html. Accessed December 3, 2012.)
establishment of controls when the goal is to distinguish treatment or management effects from other causes of change.
The committee was asked to recommend methods for establishing and validating AMLs. The establishment of AMLs should be linked to consistent, scientifically supported models of range and herbivore interactions. Validating AMLs requires methods that draw on information on rangeland, equid, and wildlife dynamics for adaptive decision-making. Improved and more consistent monitoring is also needed. Processes for establishing and validating AMLs should be open and understood by stakeholders, and ultimately AMLs should be amenable to adaptation in light of new information and environmental and social change.
Understanding Ecosystem Dynamics
Numerous developments in ecological theory, in technologies and methods for assessing ecosystem status and trends at multiple resolutions, and in understanding arid rangelands dynamics and function have occurred since publication of the earlier National Research Council reports on free-ranging horses and burros (NRC, 1980, 1982). Developments in ecological research challenge the notion that a reliable minimum annual forage production that would allow the establishment of a static carrying capacity, or AML, over the long term can be determined. The research highlights the role of unpredictability on arid range-lands (Ellis and Swift, 1988; Westoby et al., 1989; Smith et al., 1995; Bestelmeyer et al., 2003; Briske et al., 2005; Vetter, 2005) and the importance of abiotic factors, such as weather events and fire, relative to biotic factors, such as competitive interactions among plants or grazing pressure, in determining vegetation expression. In short, the effects of a severe drought on forage availability often have more influence than herd population management.
It is important to distinguish between two foci for applying the concept of nonequilibrial rangeland dynamics. The first is a focus on plant-herbivore equilibria or nonequilibria. It was once theorized that plants and herbivores would come into a natural ecological balance or equilibrium if left undisturbed. Herbivore population growth would be slowed to zero at equilibrium because of density-dependent feedbacks arising from food limitation (see section “Density-Dependent Factors” in Chapter 3). However, it has been understood that population regulation also has density-independent terms, for example, weather variability (see section “Density-Independent Population Controls” in Chapter 3). Caughley (1987), who developed much of the theory of plant-herbivore dynamics, observed that, when abiotic variability is high, plant-herbivore systems exhibit nonequilibrial dynamics. That line of thought was further developed by others (DeAngelis and Waterhouse, 1987; Ellis and Swift, 1988; Behnke and Scoones, 1993; Illius and O’Connor, 1999). An important outcome of nonequilibrial plant-herbivore dynamics is that herbivore populations in such natural systems, where natural controls apply, cannot attain numbers high enough to degrade vegetation. Vegetation dynamics are driven largely by climate rather than herbivory. Vetter (2005) cited evidence from arid environments with annual rainfall coefficients of variation7 (CV) over 33 percent that suggests that these systems better fit the nonequilibrium plant-herbivore model (Ellis and Swift, 1988; Ward et al., 1998, 2000; Sullivan, 1998, cited in
7 The coefficient of variation is the ratio of the standard deviation to the mean.
Sullivan and Rohde, 2002; Fernandez-Gimenez and Allen-Diaz, 1999, cited in Vetter, 2005). In such areas, vegetation cover, composition, and productivity are influenced largely by rainfall and other abiotic factors, and grazing intensity has been reported to have much less influence on these three aspects of the vegetation (Vetter, 2005). In more mesic sites with lower annual rainfall variation or reliable soil moisture, grazing has been reported to cause such changes as brush encroachment (Desta and Coppock, 2002) and alteration of grassland species composition (Fernandez-Gimenez and Allen-Diaz, 1999). Those sites may occur in or be intermixed with arid rangelands. Caughley (1987) first observed that plant-herbivore equilibria diminish markedly in strength above an annual rainfall CV of 33 percent. Ellis and Swift (1988) extended the theoretical 33-percent CV threshold. On a regional scale, HMAs are most commonly in areas that have an annual rainfall CV exceeding 33 percent (Figure 7-1).
The second focus for the application of nonequilibrial rangeland dynamics has been more broadly on vegetation dynamics and the multiplicity of factors that drive them, including climate and disturbance. A wealth of evidence and observation, perhaps beginning with Westoby et al. (1989) and Laycock (1991) if not Gleason (1917, 1926, 1927), supports the idea that vegetation dynamics in arid climates do not necessarily follow the theory of linear successional dynamics first proposed by Clements (1916), which provided the basis for assessing rangeland condition in the United States for several decades (Dyksterhuis, 1949; Briske et al., 2003). The ecological dynamics of vegetation on arid rangelands are now commonly characterized by using state-and-transition models that posit that relatively stable configurations of vegetation, or “states,” exist and that they may “transition” to other such states as a result of the influence of biotic or abiotic factors, such as grazing, precipitation, species invasions, fire, and seed sources (Westoby et al., 1989; Bestelmeyer et al., 2003;
FIGURE 7-1 Coefficient of variation of annual rainfall in the contiguous United States.
SOURCE: Adapted from Maurer et al. (2002) and Lettenmaier et al. (2008).
Stringham et al., 2003; Briske et al., 2005, 2006). An inherent aspect of this concept is the acknowledgment that there may be thresholds between states and nonlinear dynamics: instead of a predictable, directional pattern of change, a state may transition to one of several alternative states, may not do so in any predictable timeframe, and may not transition back after a change (Belovsky, 1986; van de Koppel et al., 1997; Rietkerk et al., 2002; Peters et al., 2006; Bisigato et al., 2008). The state-and-transition framework does not exclude the occurrence of changes that follow linear successional trajectories (Bestelmeyer et al., 2003; Briske et al., 2005, 2006).
When an ecological threshold is crossed and an HMA or part of an HMA has entered an alternative state, the simple removal of horses or burros may not result in a return to the conditions of the previous state and AMLs may need adjustment. If recovery of biological structure and ecological processes that promote self-repair and facilitate long-term sustain-ability can be expected at all, such areas require additional resources and time. Areas that were suitable for horses or burros may become unsuitable, and areas that were unsuitable may become suitable.
In addition to the unpredictability and irregularity of rangeland dynamics, climate and social change add another level of uncertainty about future conditions. Setting of AMLs takes place in a context of ecological and social change (Bestelmeyer and Briske, 2012). Vegetation change, soil degradation, invasive species, and changing climate have already altered many rangelands, and such state changes are expected to occur more frequently (Williams and Jackson, 2007; Stafford Smith et al., 2007; Dai, 2011). Social values, economic conditions, and land use in HMAs as well as stakeholders, markets, and policies influencing ecosystem management are all undergoing change (Holmes, 2002; Sheridan, 2007; Brunson and Huntsinger, 2008). Ultimately, the challenges of these numerous sources of unpredictability demand that AMLs be adaptable.
State-and-transition models are synthetic, conceptual models that describe soil and vegetation dynamics (Bestelmeyer et al., 2003; Briske et al., 2005, 2006; Herrick et al., 2012). Models are refined as data become available, and they become increasingly data-driven rather than conceptual over time. Monitoring and site selection can be improved through identification of ecological sites with state-and-transition models (Herrick et al., 2012). The inventorying of ecological sites and linking of them to state-and-transition models are important efforts that BLM is already participating in with the U.S. Department of Agriculture’s Natural Resources Conservation Service (NRCS). Such models offer the opportunity to capture information about the influence of management and both linear and nonlinear dynamics of ecosystem change, and they are useful in modeling efforts. Good conceptual models implicitly or explicitly identify influences and short-term response indicators in the description of transitions and pathways (Herrick et al., 2012). Information about free-ranging horse and burro management should be linked to ecological sites whenever possible. Over time, the outcomes of adaptive management can be used to improve the state-and-transition models.
The NRCS effort to develop state-and-transition models to guide rangeland management throughout the West is a valuable opportunity to create a standardized basis for managing for desirable ecosystem states that will go a long way to maintaining a thriving natural ecological balance as mandated by the Wild Free-Roaming Horses and Burros Act as amended (see Box 7-2). The committee recognizes that the development of state-and-transition models is in its infancy, is difficult, and may be beyond the purview of BLM and instead be in the domain of NRCS or other natural-resources management agencies and the scientific community. Increased and better defined collaboration between such agencies, the scientific community, and BLM is needed.
The Pryor Mountain Wild Horse Range Example of the Natural Resources Conservation Service Approach
The Pryor Mountain Wild Horse Range is probably the most well-studied HMA in the nation (e.g., Garrott and Taylor, 1990; Coughenour, 1999; Fahnestock and Detling, 1999a,b; Singer and Schoenecker, 2000; Ricketts et al., 2004; Roelle et al., 2010). Widespread concern about the ability of the range to support wild ungulate populations prompted BLM to ask NRCS to perform a comprehensive inventory and assessment of the health of range in 2002-2003 (Ricketts et al., 2004). According to NRCS, its report was the most detailed assessment of any wild horse range to date (this presumably referred to all the HMAs under BLM purview). Although the Pryor Mountain Wild Horse Range supported 161 horses in 2003, the NRCS assessment determined carrying capacity should be 45-142 horses on the basis of the percent of rangeland in poor condition, the low similarities of vegetation to potential climax vegetation, a perceived downward trend in range condition, and evidence of severe erosion.
The approach taken by NRCS was different than that described in the BLM handbook. A more exhaustive methodology was used by NRCS, and this approach serves as an example of potential improvements to the BLM handbook approach. Nevertheless, it too has had notable limitations.
The approach used a systematic sampling of the entire landscape. The landscape was stratified into ecological sites on the basis of an earlier Soil Conservation Service soil survey. Transects were distributed among ecological sites within broader-scale inventory units by using stratified random sampling. Along each transect, 10 circular plots were sampled at 10- or 20-foot intervals. Vegetation biomass was determined by harvesting and weighing all plants, by double sampling with some being visually estimated, or by visual estimation only. Total forage availability was used to determine stocking rates. A “harvest efficiency” was applied in the same way as a proper use factor would be applied in the BLM approach. It was assumed, on the basis of an earlier literature review (Holechek, 1999), that 35- to 45-percent use is moderate for desert and semidesert environments. A value of 30 percent was used for preferred and desirable species and 10 percent for undesirables. Estimates were subjectively adjusted on the basis of judgments of whether plants had reached peak biomass and to account for grazing removals. Forage availability was further modified by distance from water and slope class. This approach is a more direct way of assessing forage biomass than the BLM handbook approach, but it is still subject to uncertainty in that visual estimates of biomass are used without clear evidence of calibration against actual measured weights; samples were taken in open,
An encouraging initiative in this direction is the collection of rangeland data on BLM lands by NRCS staff familiar with the National Resource Inventory data collection methods for rangelands that have been used since 2003 (L. Jolley, NRCS [retired], personal communication, February 2013). Those data are linked to development of state-and-transition models for specific ecological sites. Eventually, this will allow BLM to use the nation wide National Resource Inventory Resource Assessment database.8 This would be a valuable contribution to standardizing BLM methods and data nationwide.
Assessing Rangeland Deterioration
The handbook assumption appears to be that if forage and habitat components are adequate, range deterioration will not occur. Habitat structural characteristics and amount of forage available can be measured in a straightforward way, but what defines “range condition”—the “health” of the range—has been a subject of debate in scientific and management
8 Available online at http://www.nrcs.usda.gov/wps/portal/nrcs/detail/national/technical/nra/nri/?&cid=stelprdb1041620/. Accessed February 21, 2013.
grazed vegetation, and a subjective and unsubstantiated method of adjusting for grazing removal was used; sampling occurred only once in the growing season, but biomass is dynamic through the season; sampling occurred in only 1 year, but precipitation is highly variable among years (the BLM approach is superior in accounting for such variability); and a source based on pre-1976 range literature was cited for setting proper use levels. Proper use levels are based on available site-specific research, local experience, and trend data and should be adjusted through adaptive management (Swanson et al., 2006). Updates were not mentioned.
Rangeland condition was based on similarity in composition to that inside reference long-term exclosures. The underlying assumption was the traditional one: climax, ungrazed plant communities are in the best condition, as in the BLM method. However, plant communities grazed by herbivores cannot be expected to be like communities exclosed from grazing. They may be different, but they may be stable and productive. An attempt was made to assess trends in conditions relative to the presumed climax community, as evidenced by conditions in long-term exclosures. However, the assessment was based on judgments of condition at one time as determined by comparison to the presumed ungrazed climax condition rather than observations of changes over time. That underscores the need for long-term monitoring. It was not established that the vegetation community was changing, only that it was different from that in the ungrazed area. It is noteworthy that the grazed and ungrazed plots have not become more similar despite the fact that management removals have, for the most part, kept the horse population near the AMLs for many years. There is little evidence that all but elimination of the horses would result in such a convergence and no evidence that one “condition” is generally superior to the other.
Rangeland health was assessed by using a number of indicators “relative to soil and site stability, watershed and hydrologic function, and soil and plant community integrity” (Ricketts et al., 2004, p. 104). They included hydrological indicators—such as pedestaled plants, rills, gullies, and soil loss—and observations of plant mortality, bare ground, and litter (detritus). Although there was evidence of erosion due to overland flow and wind, it was not established how long it had been occurring, that it was not going on in the absence of grazing, or that changes in herbivore density would reduce erosion rates.
Finally, the assessment did not make substantial references to or comparisons with more detailed studies of vegetation and ecosystem functioning that used a greater number of grazing exclosures, measures of live and dead biomass dynamics over time, experimental design, statistics, and spatially explicit ecosystem modelling (Coughenour, 1999; Fahnestock and Detling, 1999a,b; Singer and Schoenecker, 2000).
communities for decades, and the handbook provides no additional clarity. As noted above, concepts underlying range-condition assessments in the 1970s, when the Wild Free-Roaming Horses and Burros Act was written, are now viewed as simplistic and not in line with current thinking regarding vegetation dynamics in arid lands. Although the handbook focuses primarily on the level of forage utilization as a determinant of AML, forage consumption is only one process associated with grazers’ use of the landscape. Horses and burros have mechanical effects on plants and soils through trampling and on shrubs through rubbing (Beever et al., 2008). Therefore, other factors sensitive to equid presence may indicate ecosystem change due to equid grazing, including insect activity, soil compaction, species richness, condition of woody vegetation, and cover of plants (Beever et al., 2003). Areas used by free-ranging horses have been reported to exhibit soil loss, compaction, and erosion (Dale and Weaver, 1974; Dyring, 1990b; Whinam et al., 1994; Nimmo and Miller, 2007); soil was the ecosystem component that differed most between horse-occupied and horse-removed sites in the western Great Basin study (Beever and Herrick, 2006).
Although invasive species are receiving increased management and conservation attention (both in and outside BLM), the committee observed that there was relatively little guidance in the handbook on the effects of invasive species on AMLs. Invasive species may
be brought in by free-ranging equids (Campbell and Gibson, 2001) and spread by them (Dyring, 1990b; Rogers, 1991; Weaver and Adams, 1996; Campbell and Gibson, 2001; Loydi and Zalba, 2009). Most seeds pass through the equine digestive tract in less than 2 days, but some can be carried and remain viable for much longer periods (Janzen, 1981) and so can be transferred long distances. Seeds are also carried on the animal body. It can be difficult in practice to ascribe causation of invasive-plant presence and density to free-ranging equids, especially without controlled experiments. It is likewise unclear whether alterations (either decreases or increases) in grazing intensity can be expected to reverse or halt the spread of invasive species. Although invasive-plant ecology is still an emerging field, the handbook should offer some guidance based on what is known. Given the importance of invasive plants in altered fire regimes in the domain of BLM HMAs, the topic of how free-ranging equids relate to distribution and abundance of invasive plants may deserve increased research attention.
Livestock grazing has been reported to alter soil properties via compaction, hoof action and consequent erosion, and redistribution of nutrients such as nitrogen (Archer and Smeins, 1991; van de Koppel et al., 1997). However, hoof action may break physical surface crusts that impede infiltration and seed germination. Soil surface horizons are involved in numerous biotic and abiotic pathways in communities (Thurow, 1991; Belsky and Blumenthal, 1997; Beever and Herrick, 2006). Given the importance of soil chemistry and physical attributes (e.g., related to compaction) for ecological function, the tight connection of soil measures to so many BLM mandates and arid-lands monitoring frameworks, and the relative dearth of information on equid-soil relationships, Beever and Aldridge (2011) reported that further research on these relationships would increase ecological understanding and identify the implications of the relationships for the management of equid influence on soil resources. For example, they asked provocative questions, including the following: To what depth below the soil surface does compaction extend? Under what conditions will treading by equids lead to favorable or adverse hydrological outcomes? What factors (such as soil texture or percentage of clays, concentration of calcium carbonate, and depth to water or an impervious layer) most strongly modify soil responses to horse and burro densities? Consideration of such factors and interactions would strengthen assessments used to set AMLs.
Beever et al. (2003) reported that 19 horse-grazed and horse-removed sites could not be clearly discriminated on the basis of the cover of key plant species consumed by horses (species measured by BLM specialists in horse-effects monitoring) or by using cover or frequency of all plant species. However, horse-occupied and horse-removed sites were clearly discriminated by using a diverse suite of variables that research had suggested were sensitive to grazing disturbance. The variables included density of ant mounds, soil-surface hardness, species richness, grass cover, forb cover, and shrub cover (Beever et al., 2003).
Monitoring and Assessing Forage Availability
Once AMLs are established it is essential to determine whether forage consumption is at the predicted level. To account for factors other than grazing (such as weather) that can affect rangeland condition, determining effects of equid foraging ultimately requires comparisons of vegetation in areas where foraging occurs and where it is prevented. Consumption levels can then be compared with rangeland productivity. Grazing and browsing effects are typically measured by comparing vegetation production and composition in areas where grazing occurs with those in areas where it is excluded.
One of the easiest and most effective ways to compare vegetation features in grazed and ungrazed areas is to establish pairs of exclosed and grazed plots. Plots are chosen at random
with one plot at each site as a control and the other enclosed by an exclosure device such as a 1-m3 cage covered in wire mesh. With grazing excluded, changes in vegetation height, biomass, and percentage cover in cages provide estimates of productivity. In the paired plots outside cages, grazing will reduce vegetation biomass. Thus, differences between vegetation biomass inside and outside cages provide estimates of consumption. Clipping-based estimates are more accurate than visual estimates. Cages are moved regularly (one to three times per year) to prevent the cages themselves from altering productivity. The temporary exclosures can be used to determine whether current herbivore densities, as set through management removals, are achieving anticipated or target levels of offtake. Larger, permanent exclosures can also be established to determine the extent to which grazing changes vegetation cover and composition over time. Such exclosures can also be used to test the hypothesis that removal of grazing results in vegetation recovery.
Temporary exclosures are easy to deploy and relatively inexpensive to implement, but many replicates in habitats are required because each cage and the control paired with it provides estimates of productivity and consumption over only a small area. Moreover, small cages generally exclude trees and shrubs that provide browse for burros. Care must be taken to avoid statistical pseudoreplication because samples in each permanent exclosure are likely to be spatially correlated to a greater extent than samples among exclosures, for example, in different vegetation types or patches of vegetation and soils in the larger-scale matrix of landscape heterogeneity.
Although small-scale and large-scale exclosures are similar in many respects, they differ in important ways. Fenced areas allow detection of long-term changes in vegetation where grazing is excluded, whereas 1-m3 cages enable frequent and easy movement for measurement of annual production. Long-term exclosures may foster the development of vegetation and soil conditions different from those in areas routinely grazed by large herbivores, whereas small, movable exclosures maintain conditions more similar to the conditions of grazed vegetation. The different approaches have two implications. First, the conditions inside long-term exclosures may enhance or suppress plant growth compared with grazed vegetation. Reduced growth could arise from self-shading, rainfall interception, and lower rates of nutrient cycling in the ungrazed than in the grazed vegetation. As a result, growth (primary production) of the vegetation that is grazed cannot be estimated from data collected in long-term exclosures. Comparisons of vegetation in and outside large permanent exclosures provide different estimates of production and consumption from those of temporary exclosures because vegetation that develops within long-term exclosures often becomes quite different from that outside the exclosure. The vegetation that develops in long-term exclosures should not be considered “natural” or “desirable” if the objective is to conserve free-ranging populations of large herbivores.
Beever and Brussard (2000) concluded that exclosures are nonetheless an excellent monitoring and experimental design tool that had been underused to quantify influences of free-ranging horses on vegetation and wildlife. That is particularly relevant for BLM managers of free-ranging equids because numerous exclosures have been in place for some time, and a strategically placed network of large exclosures could provide BLM with robust data for quantifying the effects of free-ranging equids among HMAs, seasons, and years of different weather.
Sampling vegetation in and outside either type of exclosure is labor-intensive. As a result, techniques that relay easily acquired, remotely sensed data have become popular (see Box 7-3), even though the data are relatively coarse and generally cannot be used to monitor exclosures. Images from satellites routinely measure many spectral bands of reflected light from vegetation and provide long-term time series for examining changes in vegetation.
Use of Remote Sensing
Remote sensing is an effective and universal tool adapted to a wide array of applications in natural-resources science and management (Gross et al., 2006; Kennedy et al., 2009). It has utility for landscape assessment ranging from site-specific habitat management to broad landscape-scale predictions. Remote sensing can be used effectively to characterize heterogeneous landscapes on the basis of detection of abrupt changes or gradual trends and patterns. It can provide consistent and reliable information on ecological effects and can be used to monitor landscape change and to extract unique or important features from complex ecosystems (Kennedy et al., 2009). It is an excellent tool for landscape-level applications, such as range assessment, ecological monitoring, weed-invasion detection, and woodland-encroachment assessment.
The spatial resolution of an image refers to the size of the smallest object that can be detected (resolved) on the ground. In raster-based information, the resolution of an image is limited by the smallest pixel size. High-resolution information is characterized by small pixel sizes and low resolution by large pixel sizes. When comparing images or datasets from different HMAs, it is critical that the resolution of the images be known and preferable that they be comparable. The accuracy and reliability of an analyzed (classified) remotely sensed image may depend on its resolution.
Diverse sensor types and remote-sensing platforms are available, each with specific-resolution and spatial-extent parameters. Which sensor is chosen depends on management objectives and expected outcomes. Several of the sensors provide specific advantages for management of free-ranging horses and burros (Table 7-2).
|Attribute||Image Type||Advantages and Opportunities|
|Patch-size detection||Fine grain: IKONOS,a Quickbird,b Aerial photographyc||High spatial resolution. Delineation of habitat heterogeneity. Characterization of primary horse grazing and drinking areas.|
|Gradual landscape change||Fine grain: IKONOS, Quickbird, Aerial photography||Fine-scale habitat structure and change in HMAs. Monitoring of effects and resource availability.|
|Moderate grain: Landsat, ASTERd||Regional disturbance assessment, forage detection availability, and landscape-change detection.|
The Normalized Difference Vegetation Index (NDVI) is widely used and compares infrared and near-infrared reflectance to measure vegetation abundance and quality. However, interpretation of those values in relation to actual rangeland quality requires “ground-truthing” based on actual measurements of vegetation. Moreover, sample ground-truthing in and outside large-scale exclosures remains essential for estimating consumption levels from remotely sensed spectral indexes and thus foraging effects on large areas. Care must
|Attribute||Image Type||Advantages and Opportunities|
|Coarse grain (MODIS,e AVHRRf)||Atmospheric influences and surface detection across broad spatial areas. Monitor regional shifts in vegetation structure.|
|Abrupt landscape change||Fine grain: IKONOS, Quickbird, Aerial photography||Inference of land use and land- use change by image analysis and interpretation. Annual or seasonal effect detection in HMAs.|
|Moderate grain: Landsat, ASTER, SPOT, hyperspectral, AVRIS||Detection of disturbance events in large areas.|
|Coarse grain (MODIS, AVHRR)||Large-scale disturbance and regional vegetation change such as drought. Cloud screening. NDVI of large areas for vegetation cover and predicted annual forage production.|
|aThe IKONOS sensor is a high-resolution satellite that captures 3.2-m multispectral images and 1-m panchromatic data. It has wide application in natural-resources assessment and mapping, agriculture, forestry, natural disasters, change detekction, and so on. It collects reflected wavelength bands that include panchromatic, blue, green, red, and near-infrared wavelengths; it can also be used to develop digital elevation models that represent the earth’s topographic surface (http://www.satimagingcorp.com/satellite-sensors).
bQuickbird is a high-resolution satellite sensor that collects 0.61-m resolution imagery. It is an excellent sensor for land-use and land-change detection, environmental analysis, and resource management. It has a short revisit time (93.5 minutes), making it effective in abrupt to gradual time-change analysis. Data come in panchromatic, red, blue, green, and near-infrared bands. It can be used to map and analyze fine-scale HMA features.
cSeveral types of aerial photography are available or can be produced, depending on the type of information needed. Since 2006, the National Agricultural Imaging Program (NAIP) has provided color, and for some states and dates, color- infrared imagery. The images have a 1-m resolution and can be used to identify and map landscape features. In contrast with most high-resolution satellite sensors, which can be expensive, NAIP imagery is free to the consumer.
dASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) is used to obtain information on surface temperatures, reflectance patterns, and elevation changes. It is used to predict variability and trends in climate, weather, and surface structure (http://asterweb.jpl.nasa.gov/index.asp).
eMODIS (Moderate Resolution Imaging Spectroradiometer) views the earth every 1-2 days, collecting data in 36 spectral bands. It is used to measure global dynamics and processes, including prediction of global climate change, to assist policy-makers in land protection. With a 250-m pixel size, the resolution can be considered relatively coarse-grained.
fAVHRR (Advanced Very High Resolution Radiometer) is a satellite sensor that collects the earth’s reflectance in five wide spectral bands (red, two near-infrared, and two thermal).
also be taken in interpreting the meaning of reflectance values because they are affected by the abundance of bare ground and the abundance of grasses and forbs relative to trees and shrubs. Once predictive statistical models are developed, remotely gathered data on large spatiotemporal scales can be used to measure changes in rangeland quality repeatedly. In that way, the effects of AMLs can be monitored from remote sensing and adjusted on a regular and timely basis.
Analytical and Modeling Approaches
Scientifically defensible approaches have been developed over the last 2 decades for assessing vegetation, herbivore, and ecosystem dynamics in spatially heterogeneous environments from landscape through regional and even to global scales. A wide variety of models have been developed that are capable of simulating vegetation, biogeochemistry and hydrology dynamics in response to soils, changing climate, and, to a lesser extent, herbivory A body of science in this field does exist focusing on vegetation and ecosystem responses to herbivory. Some models are capable of simulating interactive responses to herbivory, climate, and soils. Although computer modeling has been adopted by BLM to predict horse population responses to management and to assist in the setting of the lower bounds of AMLs (see Chapter 6), modeling has not been used to set the upper bounds of AMLs or to inform AML decisions. That would necessitate models of vegetation and ecosystem dynamics and the ability of such models to represent ecosystem dynamics in spatially heterogeneous environments and mobile herbivore populations. Assessments of AMLs could be made more robust and more informative by using the powerful analytical and modeling approaches.
A first step would be to use geographic information systems (GIS) and remote sensing to a greater extent in setting and evaluating AMLs. BLM uses GIS to some extent to quantify vegetation and forage production potentials in different range sites, as delineated by NRCS or older Soil Conservation Service soil surveys. Forage production estimates for each range site have been combined or scaled up by using GIS to derive forage production. However, there is a potential to do more with spatial data and to derive additional data from remote sensing, for example,
- Overlay spatial data on equid distributions, which are temporally variable, onto forage production estimates to predict percentage utilization across the landscape. Even if the equid distributions are coarse or estimated, they represent what is known.
- Use spatial modeling of equid habitat selection on a seasonal basis to provide estimates of equid distributions. Equid habitat-selection patterns will be influenced by distance to water, topography, forage quantity and quality, shrub and tree cover, barriers to movement, conflicting land uses, forage offtake by livestock, and other factors. All those can be represented as GIS data layers.
- Use remote-sensing data to cross-check and augment estimates of forage production.
- Use spatially explicit precipitation maps that account for patchy rainfall and topographic gradients to refine estimates of forage production.
- Estimate snowpack distributions by using remote-sensing products, SNOTEL data, snowpack modeling, and spatial interpolation to estimate areas that are available to horses in winter. The snowpack in turn affects the forage supply for the horses in winter.
As just noted, various vegetation and biophysical-ecosystem models have been developed over the last 3 decades. All have capabilities of simulating realistic scenarios of plant production and vegetation dynamics in response to soils and climate. However, few have focused on simulating vegetation or ecosystem responses to herbivory. Few have explicitly represented herbivores or their dynamic distributions on the landscape. However, one example that does is the application of the SAVANNA ecosystem model to the Pryor
Mountain Wild Horse Range (Coughenour, 1999) and to a variety of other large herbivore ecosystems in the western United States, East Africa, and elsewhere.
To be useful in informing and assessing AMLs, key capabilities of such an ecosystem model would include
- Prediction of plant-biomass dynamics and production responses to climatic variations and soils. Dynamics must be represented at least seasonally and ideally on a weekly or even daily basis. Dry, wet, and average years should be realistically simulated. Seasonal dynamics are important because forage biomass varies greatly owing to intraseasonal and interseasonal precipitation patterns and herbivore offtake on different parts of the landscape at different times throughout the year.
- Realistic simulation of plant-production responses to herbivory, including under-compensatory and overcompensatory responses.
- Simulation of changes in vegetation cover over multiyear periods.
- Differentiation of simulated plants into functional groups, including preferred and nonpreferred species for herbivores.
- Representation of spatially variable patterns of precipitation and temperature and their effects on vegetation. Spatial patterns of precipitation can be thought of as dynamic precipitation maps in the model.
- Simulation of dynamic snowpack distributions across the landscape because these affect forage availability and herbivore distributions.
- Simulation of dynamic herbivore habitat selection and resulting spatial distributions in response to water, forage, topography, cover, and barriers.
- Simulation of herbivore forage intake and resulting effects on herbivore body condition.
- Representation of key nutrient cycles, particularly nitrogen and soil-carbon dynamics.
- Representation of key hydrological responses, particularly runoff and infiltration responses to changes in vegetation cover, which may result from herbivory.
- Simulation of interactions with other species via competition for forage, water, and habitat and effects on other species resulting from equid-induced habitat alterations. Ecosystem modeling can represent forage competition, and effects on habitats could be represented by linkages to habitat models for other species.
With those modeling capabilities, it would be possible to predict the effects of different horse or burro densities and distributions on ecosystem dynamics and to assess whether horse or burro densities are sustainable in the long term. It would also be possible to infer or directly represent interactions with other species, including wildlife and livestock. Competition between livestock, wildlife, and horses or burros is affected by the degree of overlap in species forage preferences and spatial distributions. Modeling could also be used to assess the effects of restrictions on horse or burro movements that arise from fencing and other land uses. Such habitat fragmentation results in reduced opportunities for herbivores to access key grazing areas in times of food shortages on primary ranges. Restrictions of movement can also result in higher herbivore densities and grazing pressures than would occur if the animals could disperse or migrate. Vegetation or ecosystem models must be verified through comparisons with monitoring data described above. It is recognized that no single model is completely accurate; however, iterative adjustment of a model on the basis of data will improve it and make it more useful.
Environmental variability and change, changes in social values, and the discovery of new information require that AMLs be adaptable. Perhaps the most fundamental approach in this regard is adaptive management (Holling, 1978; Williams et al., 2009). Adaptive management can be used in a variety of social decision-making settings (see Chapter 8). Herrick et al. (2012) defined adaptive management as an iterative decision-making process that incorporates development of management objectives, actions to address the objectives, monitoring of results, and repeated adaptation of management until desired results are achieved. A key tenet of adaptive management that is relevant to managing free-ranging horses and burros is the treatment of management actions as testable hypotheses. In turn, maximizing long-term knowledge of the system and thereby improving management (balanced with achieving optimal short-term outcomes, given current knowledge; Stankey and Allan, 2009) hinges on several fundamental tenets of research and monitoring design. Those tenets include use of control plots (against which to evaluate the effects of a given management “treatment,” such as erecting exclosures, administering immunocontraception broadly in a population, or removing or transferring animals from a population); use of replication to increase confidence that results are generalizable rather than anomalous; and controlling for variability (such as that due to annual differences in precipitation and thus productivity), for example, through Before-After Control-Impact designs (Underwood, 1992, 1994). Also essential for adaptive management specifically and for applied ecology generally is the explicit incorporation of uncertainty (such as the use of 95-percent confidence intervals, standard errors or standard deviations, and probability density functions) into estimated measures (such as herd size, utilization rate in a site or HMA, and average penetration resistance in a landscape).
Several other approaches to analysis and interpretation of management actions and monitoring data can improve confidence in the results. First, if there is interest in understanding whether or how a particular factor (e.g., average site growing-season precipitation) affects the degree of ecosystem alteration caused by a given density of free-ranging horses and burros, ecosystem attributes mentioned above should be measured at numerous sites with comparable horse and burro density across a broad range of that factor (gradient analyses; Austin, 1985; Gosz, 1992). Such approaches provide quantitative information on the major driving variables, permit the generation of information for extrapolating between sites and across scales, and begin to address mechanistic explanations of phenomena relevant to management (Gosz, 1992). Although ideally other important factors would remain constant in all sites along the gradient, that is rarely the case; for example, soils may differ markedly along the gradient. In those situations, explicitly accounting for this key factor (e.g., soils) can be approached in a manner comparable with complete factorial or blocked designs (e.g., Underwood, 1994, 1997; Sokal and Rohlf, 2012). A related example might be the use of landscape-scale analyses to identify portions of the landscape most likely to be early-warning indicators of deterioration of landscape condition, such as areas of heavy use.
Numerous relatively recent advances in ecological monitoring that can further increase confidence in results are relevant and noteworthy for the Wild Horse and Burro Program. For example, if a particular question is being addressed in terms of testing of the null hypothesis and the null hypothesis fails to be rejected (that is, no effect of a management action or “treatment” was found), a post hoc power analysis can be performed to assess how likely the effort was to detect an existing effect (what power the effort had) given the sample sizes used for and the variability among replicates in the various groups. Over time, however, a priori power analyses have generally come to be regarded more favorably than post
hoc analyses. A priori analyses can tell managers and researchers what level of effect size (i.e., only if a 50-percent difference exists) can be detected for given levels of power, sample size, and variability within groups. BLM managers should note that the error structure (the partitioning of degrees of freedom) in these analyses reflects the design of their monitoring. In more complex designs, simulation analyses can be a more realistic alternative. Concepts related to power can improve setting and adjusting of AMLs by providing quantification of sensitivity of a monitoring system, that is, the ability to be an early-warning system of environmental change as opposed to confirming that a system has already been dramatically altered and perhaps crossed an ecological threshold.
The committee believes that the above principles could be more thoroughly integrated into the Wild Horse and Burro Program to increase the defensibility and scientific validity of management actions. Generally speaking, when the domain is as spatially vast and biotically heterogeneous as the area managed by BLM for free-ranging equids, a compromise approach can be taken. The compromise seeks to balance the incorporation of as much repeatability as possible (to permit analyses at numerous hierarchical spatial and temporal scales) with the ability to tailor management and monitoring efforts to local biota, interests, and priorities (to allow stakeholder involvement and investment and have relevance on both local and broader scales). That may mean, for example, that a core suite of field methods and monitoring indicators are used and that databases and analysis templates exist for all HMAs (Box 7-4). In contrast, individual HMAs or district offices may add to the core suite by creating standard monitoring approaches for monitoring locally important rare plants or animals or may add additional metrics for a given field method that are important to local interest groups.
Validity for Stakeholders
Because AMLs are a focal point for controversy, it is important to develop and maintain standards for transparency, quality, and equity in the establishment, adjustment, and monitoring of AMLs. Research suggests that transparency is an important contributor to the development of trust between agencies and stakeholders (Rowe and Frewer, 2000; Webler and Tuler, 2000). The public should be able to understand the methods used and how they are implemented and should be able to access the data used to make decisions. Transparency will also encourage adherence to a high level of quality in data acquisition and use. The data and methods used to inform decisions must be scientifically defensible. Allocation of resources to management of free-ranging horses and burros takes place in a context of contending uses for BLM lands, all of which have some standing in the agency’s charge for multiple-use management. The law makes clear that rangeland resources are to be protected from deterioration, but there is no known formula for creating a balance among such uses as cattle grazing, wildlife, hunting, mining, recreation, and free-ranging horses.
From submitted public comments and statements made by members of the public at information-gathering meetings, it is clear that stakeholders vary in their opinions about AMLs. Some believe that herd numbers should be higher and should take precedence over other rangeland uses administered and managed by BLM. Some believe that equid population size needs to be increased to protect genetic diversity or to ensure survival of the herds in an unpredictable environment. Some believe that herd levels are too high or that AMLs are not adequately adhered to and that free-ranging horses and burros are damaging habitat and taking resources away from other uses. Some argue that HMAs should be managed exclusively or primarily for horses and that other uses should be considered secondarily or excluded from allocation of forage and habitat resources. Different ideas about
Development of a Comprehensive Monitoring Strategy
BLM’s 2011 report Assessment, Inventory, and Monitoring Strategy for Integrated Renewable Resources Management (BLM, 2011), also known as the AIM strategy, is part of a laudable effort to standardize and improve monitoring and assessment agency-wide. The strategy will help considerably with the transparency of how AMLs are set and adjusted, and the committee strongly supports the effort. As the document states, “the AIM Strategy is intended to reach across programs, jurisdictions, stakeholders, and agencies to provide data and information valuable to decisionmakers.” The type of data to be acquired is described as follows:
To effectively manage renewable resources, the BLM needs information at multiple scales about resource extent, condition and trend, stressors, and the location and nature of authorized uses, disturbances, and projects. Acquiring and assessing this information will be accomplished through the integration of several fundamental processes, including the: (1) development and application of a consistent set of ecosystem indicators and methods for measuring them (i.e., core quantitative indicators and consistent methods for monitoring); (2) development and implementation of a statistically valid sampling framework; (3) application and integration of remote sensing technologies; and (4) implementation of related data acquisition and management plans (e.g., Geospatial Services Strategic Plan, Enterprise Geographic Information System architecture, and rapid eco-regional assessments). (BLM, 2011, p. 1)
The AIM strategy is based on the premise that a few carefully evaluated integrative indicators can be used to monitor complex ecological processes. Herrick et al. (2012) evaluated how to integrate such monitoring into a “holistic strategy for adaptive land management.” The report points out that monitoring cannot be separated from its objectives and that processes to be monitored include driving processes, short-term responses, and long-term responses. In the context of free-ranging horses and burros, short-term indicators of management effectiveness would include vegetation measurements to learn whether offtake levels are as predicted and to see whether the horse and burro populations are within the bounds of AMLs. Long-term indicators would include measures of vegetation composition and cover, soil fertility and hydrological properties, and riparian ecosystem functioning. Monitoring must always include climate; it is the foremost driving variable because it occurs outside the realm of management but affects system dynamics. The set of indicators used in the AIM strategy should be reviewed for their applicability to the objectives of the Wild Horse and Burro Program.
The committee recognizes and the AIM strategy report observes that BLM has limited staff and resources and that it is therefore difficult to make complete, distributed, and recurring assessments and evaluations. The report makes suggestions for setting priorities for assessment, data collection, and increased use of remote-sensing technologies (BLM, 2011). The AIM strategy argues that “remote-sensing indicators can complement and even replace ground-based indicators where spatially and temporally consistent relationships can be established” (Herrick et al., 2012).
what constitutes rangeland health and a thriving natural ecological balance pervade such debates. The multiple, and often conflicting, views regarding AMLs emphasize the need for robust data and transparent processes in the setting of AMLs. Data and transparency will of course not fully resolve differing public viewpoints about allocation. Chapter 8 discusses approaches to working with stakeholders.
Establishing and validating AMLs could involve six steps:
- Inventorying the landscape to assess the current states of the system quantitatively and qualitatively.
- Developing conceptual models and hypotheses for the processes that have led to the current states, particularly differentiating the relative roles of climate, horses and burros, livestock, wildlife, and other factors.
- Developing predictions of future changes based on conceptual and quantitative models, particularly of changes in response to alternative management practices that are hypothesized to lead to alternative desired states.
- Developing monitoring approaches to assess the success of the adopted management approach in bringing about a hypothesized, predicted change.
- Refining the models to improve accuracy and predictive power in setting AMLs.
- Providing transparent information about the data and decision-making process to stakeholders and obtaining their responses.
Essentially, this is an adaptive-management approach in that it calls for the development of a model or set of hypotheses, predictions of responses to management and environmental variables, learning from observed responses to management, and refinement of the model. It can fit a state-and-transition format.
To carry out this adaptive management process, BLM needs to solve five major challenges, which its handbook does not adequately address. Specifically, BLM should
- Increase the specificity and consistency of its protocols for establishing and adjusting AMLs.
- Develop a scientific approach to identifying objectively the constraints on equid populations and their explicit effects on the expression of natural processes under minimal management.
- Improve transparency of forage allocation.
- Manage for change and unpredictability in ecosystems and in social contexts.
- Improve the scientific validity of the concept of a thriving natural ecological balance.
Increased Specificity and Consistency
BLM should continue moving toward consistency in its protocols for setting and adjusting AMLs; repeatability is a hallmark of ecological monitoring. The BLM handbook should define terms explicitly and precisely, use them consistently, and include citations of research or methodological references in the text. An intermediate approach that achieves continuity and comparability among spatial resolutions for numerous ecological components and attributes (by using standardized methods) but allows for “stepping-down” or options in monitoring approaches to address issues or resources of local or regional concern may be an ideal compromise approach.
Direct forage-production measures should augment the inference of production from visual estimates of percentage utilization; back-calculations involving total offtake based, on equid counts, which may not be accurate; and assumed per-animal intake requirements. The use of small, temporary exclosures implemented in a spatially representative and statistically robust sampling design would provide more transparent and scientifically supported data. BLM should also develop approaches for quantitatively distinguishing horse or burro use from livestock and wildlife uses of forage, riparian areas, and other resources to verify utilization partitioning between livestock, horses, burros, and other herbivores. Table 7-2 describes various remote-sensing methods. BLM should use the ones that are applicable in monitoring and assessment for particular locations. GIS and spatial modeling
could be used to map and overlay total and percentage utilization by the different species in the landscape. The committee believes that BLM should continue to develop the AIM strategy and to participate in development of state-and-transition models for western rangelands with NRCS.
As is the case with all large herbivores, free-ranging horses and burros not only use the environment but change it, and these effects need to be considered in assessment of AMLs. Effects of trampling and concentrated use on soils, insects, small mammals, and plants should be monitored in addition to forage consumption. Given BLM’s multiple-use mandate, it may want to consider wider monitoring of one or more aspects of ecological condition and function that are not tied solely to equid health. Metrics related to such aspects should reflect the effects of processes that large-bodied herbivores impose on ecosystems—namely, patch creation, redistribution of nutrients via selective herbivory and later urination and defecation, compaction of upper soil horizons, and rubbing and trampling of vegetation. Although it can be challenging to measure ecological function directly, there are numerous methods and techniques for indexing ecological services, such as loss of soil by wind or water erosion; riparian-channel function; clean water; and physical structure of vegetation for perching, resting, or escape cover. Explicit attention to a reasonable subset of ecological services and ecosystem components is a good idea fiscally because conserving the potential of landscapes to remain resilient and to resist degradation may make expensive remediation, rehabilitation, or emergency recovery efforts unnecessary. Native threatened, endangered, and sensitive species require focused conservation attention. Such attention provides BLM with a mosaic of conservation elements that reflect diverse disturbance regimes, including parts of the landscape with no nonnative herbivores. Many disturbance-sensitive species seem likely to become increasingly rare, especially in the arid and semiarid landscapes of western North America that are being affected by invasive plants, climate change, and uncharacteristic fire regimes.
Water quality needs to be considered in addition to water supply in looking at availability for multiple species. Numerous methods have been developed to perform such monitoring, including ones that involve robust statistical designs, have been used specifically for grazing systems, and have been used by many local, state, and federal agencies that have diverse stakeholders (Beever and Pyke, 2004; Herrick et al., 2005a,b; Thoma et al., 2009). Consultation and collaboration with state and federal agencies charged with water quality responsibilities are necessary.
BLM should use a strategically placed network of large, long-term exclosures to quantify the long-term effects of free-ranging equids, livestock, and wildlife among HMAs, seasons, and years of different weather.
The Challenge of Minimal Management
The way that AMLs are established and adjusted ensures that population growth rate is maximized (see Chapters 2 and 3). The density-dependent and environmental constraints that would reduce population growth rate and keep a natural population in check are precluded by management removals to avoid range deterioration. In a self-regulating, food-limited system, a lack of adequate food eventually suppresses the population if predation does not (see Chapter 3), and this sometimes results in effects on vegetation, soils, and other species. Removals to prevent those effects also prevent self-regulation of the horse population and in fact may allow it to reach its maximum potential growth rate. Therefore, there is a need to predict and state explicitly the population-level outcomes of managing
for vegetation conditions that may be expected in a sustainable but differently functioning ecosystem that includes large herbivores.
A program of continuing, ad infinitum removals may not be economically sustainable or socially acceptable. However, letting horses become food-limited, having many horses in poor condition, and having horses die of starvation on the range are not acceptable to a sizable proportion of the public. The use of more benign methods to control population growth rate (such as contraception) may reduce (but perhaps not minimize) the level of management intervention while avoiding the unacceptable outcome of food limitation. Various fertility-control mechanisms are described in Chapter 4 with their consequences for population processes (see Chapters 3, 4, and 6) and genetic processes (see Chapter 5).
A scientific approach is needed to identify objectively the constraints on horse and burro populations and their effects on the expression of natural processes under minimal management. The ecosystem might look different and function differently in the presence of more minimally managed equid populations from how it does with no or markedly reduced populations, but it may nevertheless be sustainable over time. Such a scientific approach would provide a more solid justification of management interventions. For example, the anticipated effects of different equid densities on vegetation and rangeland ecosystem functioning should have a scientific basis. Likewise there should be a basis for assertions that barriers to dispersal or barriers to access to critical habitats preclude natural processes; and the assertions should be explicitly described and justified for a specific HMA on the basis of an understanding of how ecosystems would function with large herbivores and minimal management. Ideally, from a research standpoint, such questions would be addressed in a replicated spatial mosaic in which some herds or areas would be allowed to self-regulate and others would be managed as they are currently being managed.
Allocation versus Assessment
Transparent processes for allocation should be developed, such as participatory adaptive approaches. Participatory approaches are discussed in Chapter 8.
Managing for Unpredictability
The committee examined traditional pastoral systems adapted to arid ecosystems. BLM is charged with using “minimal” management for free-ranging horses and burros, so extensive pastoral systems adapted to arid rangelands that use little or no supplemental feeding, energy, and physical infrastructure might offer some insight into how to manage free-ranging equids. Traditional pastoral systems emphasize mobility, flexibility, and reserves (Oba et al., 2000). Mobility is the movement of animals from one area to another on scales from the local to across biomes; flexibility is being able to adjust boundaries, herd sizes and components, and timing and patterns of mobility. Reserves are areas that are grazed only during extreme events. The origins of those practices owe much to the natural movements and behaviors of free-ranging herds. Can BLM use this information in developing strategies for coping with the unpredictability of arid rangeland environments?
How much and within what kinds of bounds in nonequilibrium environments grazing influences vegetation trajectories is debatable; however, it is indisputable that there is great unpredictability in forage production and that grazing management cannot reduce it (Vetter, 2005). BLM’s system of calculating forage availability without including years of high production attempts to adjust for this unpredictability by removing high-productivity years from the calculation; however, there will always be extreme events in nonequilibrium
conditions. Even with a conservative approach, an important lesson from traditional pastoral systems is that the extreme events need to be planned for and that flexibility in numbers, timing, and boundaries is important. From the theoretical developments in rangeland ecological dynamics, it is also known that some sites will be permanently altered by unpredictable events. There will be a constant need for adaptation, so an adaptive-management process for setting and adjusting AMLs should be explored.
In addition to intensive monitoring of grazing utilization, rangeland ecological condition and trend, actual use and climate data, using NRCS ecological site descriptions and associated state-and-transition models for horse-occupied habitat would not only help to standardize ecological information agency-wide, but it would build on substantial previous work and facilitate use of the already existing National Resource Inventory database. That would provide value to the consistent investment by BLM that is needed at this time.
Ecological site descriptions are land-classification systems that identify and stratify lands on the basis of soil-, climate-, and herbivory-influenced ecological potential and ecosystem dynamics. State-and-transition models are included in individual ecological site descriptions that characterize thresholds, community phases within states, and irreversible transitions that degrade ecological processes and lead to alternative states (Stringham et al., 2003). In fact, BLM has already recognized the need to develop such models for BLM lands in its 2011 AIM monitoring strategy. Conceptual ecological models based on science and other expert input are being developed to provide a common language that addresses ecosystem sustainability, a means of identifying indicators of key ecosystem attributes, and a basis for resource decisions predicated on maintaining or restoring ecosystem capacities.
Managing for a Thriving Natural Ecological Balance and to Prevent Rangeland Deterioration
If maintaining a thriving natural ecological balance and preventing rangeland deterioration are to be used as scientific justifications for setting AMLs, these goals need a more scientific basis and clear definition. Recently developed concepts that might be of use in helping to set and adjust AMLs include those of ecological sustainability (Smith et al., 1995; Turner et al., 2003; Weltz and Dunn, 2003; Mitchell, 2010) and ecosystem resilience (Carpenter et al., 2001; Walker et al., 2006; Briske et al., 2008). As those concepts are developed and tested scientifically, adopting a sustainability or resilience framework would be a marked advancement, and it would be more likely that such a framework would have a credible scientific basis.
Allen, C. 2007. Interactions across spatial scales among forest dieback, fire and erosion in northern New Mexico landscapes. Ecosystems 10:797-808.
Archer, S. and F.E. Smeins. 1991. Ecosystem-level processes. Pp. 109-139 in Grazing Management: An Ecological Perspective, R.K. Heitschmidt and J.W. Stuth, eds. Portland, OR: Timber Press.
Austin, M.P. 1985. Models for analysis of species response to environmental gradients. Pp. 10-11 in Theory and Models in Vegetation Science: Abstracts, R. Leemans, I.C. Prentice, and E. Van Der Maarel, eds. Stockholm: Almqvist & Wiksell International.
Beever, E.A. and C.L. Aldridge. 2011. Influences of free-roaming equids on sagebrush ecosystems, with a focus on Greater Sage-Grouse. Pp. 273-290 in Greater Sage-Grouse: Ecology and Conservation of a Landscape Species and Its Habitats, S.T. Knick and J.W. Connelly, eds. Berkeley: University of California Press.
Beever, E.A. and P.F. Brussard. 2000. Examining ecological consequences of feral horse grazing using exclosures. Western North American Naturalist 60:236-254.
Beever, E.A. and P.F. Brussard. 2004. Community- and landscape-level responses of reptiles and small mammals to feral-horse grazing in the Great Basin. Journal of Arid Environments 59:271-297.
Beever, E.A. and J.E. Herrick. 2006. Effects of feral horses in Great Basin landscapes on soils and ants: Direct and indirect mechanisms. Journal of Arid Environments 66:96-112.
Beever, E.A. and D.A. Pyke. 2004. Integrated Monitoring of Hydrogeomorphic, Vegetative, and Edaphic Conditions in Riparian Ecosystems of Great Basin National Park, Nevada. U.S. Geological Survey, Scientific Investigations Report 2004-5185. Reston, VA: U.S. Geological Survey.
Beever, E.A., R.J. Tausch, and P.F. Brussard. 2003. Characterizing grazing disturbance in semiarid ecosystems across broad scales, using diverse indices. Ecological Applications 13:119-136.
Beever, E.A., R.J. Tausch, and W.E. Thogmartin. 2008. Multi-scale responses of vegetation to removal of horse grazing from Great Basin (USA) mountain ranges. Plant Ecology 196:163-184.
Behnke, R.H. and I. Scoones. 1993. Rethinking range ecology: Implications for rangeland management in Africa. Pp. 1-30 in Range Ecology at Disequilibrium: New Models of Natural Variability and Pastoral Adaptation in African Savannas, R.H. Behnke, I. Scoones, and C. Kerven, eds. London: Overseas Development Institute.
Belovsky, G.E. 1986. Generalist herbivore foraging and its role in competitive interactions. American Zoologist 26:51-69.
Belsky, A.J., W.P. Carson, C.L. Jensen, and G.A. Fox. 1993. Overcompensation by plants: Herbivore optimization or red herring. Evolutionary Ecology 7:109-121.
Belsky, A.J. and D.M. Blumenthal. 1997. Effects of livestock grazing on stand dynamics and soils in upland forests of the Interior West. Conservation Biology 11:315-327.
Berger, J. 1985. Interspecific interaction and dominance among wild Great Basin ungulates. Journal of Mammalogy 66:571-573.
Berman, D., P. Jarman, and A.J. Bowman. 1988. Feral Horses in the Northern Territory. Alice Springs, Australia: Conservation Commission of the Northern Territory.
Bestelmeyer, B.T., J.R. Brown, K.M. Havstad, R. Alexander, G. Chavez, and J.E. Herrick. 2003. Development and use of state and transition models for rangelands. Journal of Range Management 56:114-126.
Bestelmeyer, B. and D. Briske. 2012. Grand challenges for resilience-based management of rangelands. Rangeland Ecology & Management 65:654-663.
Bisigato, A.J., R.M.L. Laphitz, and A.L. Carrera. 2008. Non-linear relationships between grazing pressure and conservation of soil resources in Patagonian Monte shrublands. Journal of Arid Environments 72:1464-1475.
BLM (Bureau of Land Management). 1998. Riparian Area Management: Process for Assessing Proper Functioning Condition. Technical Reference 1737-9. Available online at ftp://ftp.blm.gov/pub/nstc/techrefs/Final%20TR%201737-9.pdf/. Accessed October 8, 2012.
BLM (Bureau of Land Management). 2001. Rangeland Health Standards, H-4180-1. Washington, DC: U.S. Department of the Interior.
BLM (Bureau of Land Management). 2003, revised 2005. Strategic Research Plan: Wild Horse and Burro Management. Fort Collins, CO: U.S. Department of the Interior.
BLM (Bureau of Land Management). 2010. Wild Horses and Burros Management Handbook, H-4700-1. Washington, DC: U.S. Department of the Interior.
BLM (Bureau of Land Management). 2011. Assessment, Inventory, and Monitoring Strategy for Integrated Renewable Resources Management. Washington, DC: U.S. Department of the Interior.
Boyd, C., K. Davies, G. Collins, and S. Petersen. 2012. Feral Horse Impacts to Riparian Areas and Adjacent Uplands. Presentation at the Symposium on Free-Roaming Wild and Feral Horses at the Society for Range Management 65th Annual Meeting, Spokane, WA, January 31.
Briske, D.D., S.D. Fuhlendorf, and F.E. Smeins. 2003. Vegetation dynamics on rangelands: A critique of the current paradigms. Journal of Applied Ecology 40:601-614.
Briske, D.D., S.D. Fuhlendorf, and F.E. Smeins. 2005. State-and-transition models, thresholds, and rangeland health: A synthesis of ecological concepts and perspectives. Rangeland Ecology & Management 58:1-10.
Briske, D.D., S.D. Fuhlendorf, and F.E. Smeins. 2006. A unified framework for assessment and application of ecological thresholds. Rangeland Ecology & Management 59:225-236.
Briske, D.D., B.T. Bestelmeyer, T.S. Stringham, and P.L. Shaver. 2008. Recommendations for development of resilience-based state and transition models. Rangeland Ecology & Management 61:359-367.
Briske, D.D., J.D. Derner, D.G. Milchunas, and K.W. Tate. 2011. An evidence-based assessment of prescribed grazing practices. Pp. 21-74 in Conservation Benefits of Rangeland Practices: Assessment, Recommendations, and Knowledge Gaps, D.D. Briske, ed. U.S. Department of Agriculture, Natural Resources Conservation Service.
Brunson, M. and L. Huntsinger. 2008. Ranching as a conservation strategy: Can old ranchers save the New West? Rangeland Ecology & Management 61:137-147.
Campbell, J.E. and D.J. Gibson. 2001. The effects of seeds of exotic species transported via horse dung on vegetation along trail corridors. Plant Ecology 157:23-35.
Campbell, B.M., I.J. Gordon, M.K. Luckert, L. Petheram, and S. Vetter. 2006. In search of optimal stocking regimes in semi-arid grazing lands: One size does not fit all. Ecological Economics 60:75-85.
Carpenter, S., B. Walker, J.M. Anderies, and N. Abel. 2001. From metaphor to measurement: Resilience of what to what? Ecosystems 4:765-781.
Caughley, G. 1987. Ecological relationships. Pp. 158-187 in Kangaroos: Their Ecology and Management in the Sheep Rangelands of Australia, G. Caughley, N. Shepherd, and J. Short, eds. Cambridge, UK: Cambridge University Press.
Christensen, J.H., B. Hewitson, A. Busuioc, A. Chen, X. Gao, I. Held, R. Jones, R.K. Kolli, W.-T. Kwon, R. Laprise, V. Magaña Rueda, L. Mearns, C.G. Menéndez, J. Räisänen, A. Rinke, A. Sarr, and P. Whetton. 2007. Regional climate projections. Pp. 847-940 in Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, S. Solomon, D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor, and H.L. Miller, eds. Cambridge, UK, and New York: Cambridge University Press.
Clemann, N. 2002. A herpetofauna survey of the Victorian alpine region, with a review of threats to these species. Victorian Naturalist 119:48-59.
Clements, F.E. 1916. Plant Succession: An Analysis of the Development of Vegetation. Washington, DC: Carnegie Institution of Washington.
Coates, K.P. and S.D. Schemnitz. 1994. Habitat use and behavior of male mountain sheep in foraging associations with wild horses. Great Basin Naturalist 54:86-90.
Coughenour, M.B. 1991. Spatial components of plant-herbivore interactions in pastoral, ranching, and native ungulate ecosystems. Journal of Range Management 44:530-542.
Coughenour, M.B. 1999. Ecosystem Modeling of the Pryor Mountain Wild Horse Range. Final Report to U.S.
Geological Survey Biological Resources Division, U.S. National Park Service, and U.S. Bureau of Land Management. Fort Collins, CO: Natural Resource Ecology Laboratory.
Coughenour, M.B. 2008. Causes and consequences of herbivore movement in landscape ecosystems. Pp. 45-91 in Fragmentation of Semi-Arid and Arid Landscapes: Consequences for Human and Natural Systems, K.A. Galvin, R.S. Reid, R.H. Behnke, Jr., and N.T. Hobbs, eds. Dordrecht, The Netherlands: Springer.
Coughenour, M.B. and F.J. Singer. 1991. The concept of overgrazing and its application to Yellowstone’s northern range. Pp. 209-230 in The Greater Yellowstone Ecosystem: Redefining America’s Wilderness Heritage, R.B. Keiter and M.S. Boyce, eds. New Haven, CT: Yale University Press.
Coventry, A.J. and P. Robertson. 1980. New records of scincid lizards from Victoria. Victorian Naturalist 97:190-193.
Crane, K.K., M.A. Smith, and D. Reynolds. 1997. Habitat selection patterns of feral horses in southcentral Wyoming. Journal of Range Management 50:374-380.
Dai, A. 2011. Drought under global warming: A review. Wiley Interdisciplinary Reviews: Climate Change 2:45-65.
Dale, D. and T. Weaver. 1974. Trampling effects on vegetation of trail corridors of north Rocky-Mountain forests. Journal of Applied Ecology 11:767-772.
DeAngelis, D.L. and J.C. Waterhouse. 1987. Equilibrium and nonequilibrium concepts in ecological models. Ecological Monographs 57:1-21.
Desta, S. and D.L. Coppock. 2002. Cattle population dynamics in the southern Ethiopian rangelands, 1980-97. Journal of Range Management 55:439-451.
du Toit, J.T. 2011. Coexisting with cattle. Science 333:1710-1711.
Dyksterhuis, E.J. 1949. Condition and management of rangeland based on quantitative ecology. Journal of Range Management 2:104-115.
Dyring, J. 1990a. Management Implications of the 1988-1990 Study: The Impact of Feral Horses (Equus caballus) on Sub-alpine and Montane Environments in Australia. Canberra, Australia: University of Canberra, Applied Ecology Research Group.
Dyring, J. 1990b. The Impact of Feral Horses (Equus caballus) on Sub-alpine and Montane Environments in Australia. M.A. thesis. University of Canberra, Australia.
Ellis, J.E. and D.M. Swift. 1988. Stability of African pastoral ecosystems: Alternate paradigms and implications for development. Journal of Range Management 41:450-459.
Fahnestock, J.T. and J.K. Detling. 1999a. Plant responses to defoliation and resource supplementation in the Pryor Mountains. Journal of Range Management 52: 263-270
Fahnestock, J.T. and J.K. Detling. 1999b. The influence of herbivory on plant cover and species composition in the Pryor Mountain Wild Horse Range, USA. Plant Ecology 144:145-157.
Fernandez-Gimenez, M.E. and B. Allen-Diaz. 1999. Testing a non-equilibrium model of rangeland vegetation dynamics in Mongolia. Journal of Applied Ecology 36:871-885.
Fuhlendorf, S.D., F.E. Smeins, and W.E. Grant. 1996. Simulation of a fire-sensitive ecological threshold: A case study of Ashe juniper on the Edwards Plateau of Texas, USA. Ecological Modelling 90:245-255.
Fuhlendorf, S.D., R.F. Limb, D.M. Engle, and R.F. Miller. 2012. Assessment of prescribed fire as a conservation practice. Pp. 75-104 in Conservation Benefits of Rangeland Practices: Assessment, Recommendations, and Knowledge Gaps, D.D. Briske, ed. U.S. Department of Agriculture, Natural Resources Conservation Service.
Ganskopp, D. and M. Vavra. 1987. Slope use by cattle, feral horses, deer, and bighorn sheep. Northwest Science 61:74-81.
GAO (U.S. General Accounting Office). 1990. Rangeland Management: Improvements Needed in Federal Wild Horse Program. Washington, DC: U.S. General Accounting Office.
GAO (U.S. Government Accountability Office). 2008. Effective Long-Term Options Needed to Manage Unadoptable Wild Horses. Washington, DC: U.S. Government Accountability Office.
Garrott, R.A. and L. Taylor. 1990. Dynamics of a feral horse population in Montana. Journal of Wildlife Management 54:603-612.
George, M.R., R.D. Jackson, C.S. Boyd, and K.W. Tate. 2011. A scientific assessment of the effectiveness of riparian management practices. Pp. 214-252 in Conservation Benefits of Rangeland Practices, Assessment, Recommendations, and Knowledge Gaps, D.D. Briske, ed. U.S. Department of Agriculture, Natural Resources Conservation Service.
Gillespie, G.R., W.S. Osborne, and N.A. McElhinney. 1995. The Conservation Status of Frogs in the Australian Alps: A Review. Canberra, Australia: Australian Alps Liaison Committee.
Gleason, H.A. 1917. The structure and development of the plant association. Bulletin of the Torrey Botanical Club 44:463-481.
Gleason, H.A. 1926. The individualistic concept of the plant association. Bulletin of the Torrey Botanical Club 53:7-26.
Gleason, H.A. 1927. Further views of the succession-concept. Ecology 8:299-326.
Gosz, J.R. 1992. Gradient analysis of ecological change in time and space: Implications for forest management. Ecological Applications 2:248-261.
Greyling, T. 2005. Factors Affecting Possible Management Strategies for the Namib Wild Horses. Ph.D. dissertation. North-West University, Potchesfstroom, South Africa.
Greyling, T., S.S. Cilliers, and H. Van Hamburg. 2007. Vegetation studies of feral horse habitat in the Namib Naukluft Park, Namibia. South African Journal of Botany 73:328.
Gross, J.E., R.R. Nemani, W. Turner, and F. Melton. 2006. Remote sensing for the national parks. Park Science 24:30-36.
Hampson, B.A., M.A. de Laat, P.C. Mills, and C.C. Pollitt. 2010. Distances travelled by feral horses in “outback” Australia. Equine Veterinarian Journal 42(Supplement 38):582-586.
Hanley, T.A. 1982. The nutritional basis for food selection by ungulates. Journal of Range Management 35:146-151.
Hanley, T.A. and K.A. Hanley. 1982. Food resource partitioning by sympatric ungulates on Great Basin rangeland. Journal of Range Management 35:152-158.
Hansen, R.M. 1976. Foods of free-roaming horses in southern New Mexico. Journal of Range Management 29:347.
Hansen, R.M., R.C. Clark, and W. Lawhorn. 1977. Foods of wild horses, deer and cattle in the Douglas Mountain area, Colorado. Journal of Range Management 30:116-118.
Hawkes, C.V. and J.J. Sullivan. 2001. The impact of herbivory on plants in different resource conditions: A meta-analysis. Ecology 82:2045-2058.
Herrick, J.E., J.W. Van Zee, K.M. Havstad, L.M. Burkett, and W.G. Whitford. 2005a. Riparian channel and gully profile. Pp. 74-78 in Monitoring Manual for Grassland, Shrubland, and Savannah Ecosystems. Vol 2: Design, Supplementary Methods and Interpretation. Las Cruces, NM: U.S. Department of Agriculture.
Herrick, J.E., J.W. Van Zee, K.M. Havstad, L.M. Burkett, and W.G. Whitford. 2005b. Riparian channel vegetation survey. Pp. 69-73 in Monitoring Manual for Grassland, Shrubland, and Savannah Ecosystems. Vol 2: Design, Supplementary Methods and Interpretation. Las Cruces, NM: U.S. Department of Agriculture.
Herrick, J.E, M.C. Duniway, D.A. Pyke, B.T. Bestelmeyer, S.A. Wills, J.R. Brown, J.W. Karl, and K.M. Havstad. 2012. A holistic strategy for adaptive land management. Journal of Soil and Water Conservation 67:105A-113A.
Hobbs, N.T. 1996. Modification of ecosystems by ungulates. Journal of Wildlife Management 60:695-713.
Hobbs, N.T., D.L. Baker, G.D. Bear, and D.C. Bowden. 1996. Ungulate grazing in sagebrush grassland: Effects of resource competition on secondary production. Ecological Applications 6:218-227.
Holechek, J.L., H. Gomez, M. Francisco, and D. Galt. 1999. Grazing studies: What we’ve learned. Rangelands 21:12-16.
Holling, C.S. 1978. Adaptive Environmental Assessment and Management. Laxenburg, Austria: International Institute for Applied Systems Analysis.
Holmes, J. 2002. Diversity and change in Australia’s rangelands: A post-productivist transition with a difference? Transactions: Institute of British Geographers 27:362-384.
Hourdequin, M., P. Landres, M.J. Hanson, and D.R. Craig. 2012. Ethical implications of democratic theory for U.S. public participation in environmental impact assessment. Environmental Impact Assessment Review 35:37-44.
Hubbard, R.E. and R.M. Hansen. 1976. Diets of wild horses, cattle, and mule deer in the Piceance Basin, Colorado. Journal of Range Management 29:389-392.
Illius, A.W. and T.G. O’Connor. 1999. On the relevance on nonequlibrium concepts to arid and semiarid grazing systems. Ecological Applications 9:798-813.
Janzen, D.H. 1981. Enterolobium cyclocarpum seed passage rate and survival in horses, Costa Rican Pleistocene seed dispersal agents. Ecology 62:593-601.
Kennedy, R.E., P.A. Townsend, J.E. Gross, W.B. Cohen, A.P. Bolstad, Y.Q. Wang, and P. Adams. 2009. Remote sensing change detection tools for natural resource managers: Understanding concepts and tradeoffs in the design of landscape monitoring projects. Remote Sensing of Environment 113:1382-1396.
Kissell, R.E. 1996. Population Dynamics, Food Habits, Seasonal Habitat Use, and Spatial Relationships of Bighorn Sheep, Mule Deer, and Feral Horses in the Pryor Mountains, Montana/Wyoming. M.S. thesis. Montana State University, Bozeman.
Krysl, L.F., M.E. Hubbert, B.F. Sowell, G.E. Plumb, T.K. Jewell, M.A. Smith, and J.W. Waggoner. 1984. Horse and cattle grazing in the Wyoming Red Desert, II. Dietary Quality. Journal of Range Management 37:252-256.
Laycock, W.A. 1991. Stable states and thresholds of range condition on North American rangelands: A viewpoint. Journal of Range Management 44:427-433.
Lettenmaier, D., D. Major, L. Poff, and S. Running. 2008. Water resources. Pp. 121-150 in The Effects of Climate Change on Agriculture, Land Resources, Water Resources, and Biodiversity in the United States, Synthesis and Assessment Product 4.3, P. Backlund, A. Janetos, and D. Schimel, lead authors. Washington, DC: U.S. Climate Change Science Program and the Subcommittee on Global Change Research.
Levin, P.S., J. Ellis, R. Petrik, and M.E. Hay. 2002. Indirect effects of feral horses on estuarine communities. Conservation Biology 16:1364-1371.
Loydi, A. and S.M. Zalba. 2009. Feral horses dung piles as potential invasion windows for alien plant species in natural grasslands. Plant Ecology 201:471-480.
Mack, R.N. and J.N. Thompson. 1982. Evolution in steppe with few large herbivores. American Naturalist 119:757-773.
Manier, D.J. and N.T. Hobbs. 2007. Large herbivores in sagebrush steppe ecosystems: Livestock and wild ungulates influence structure and function. Oecologia 152:739-750.
Mansergh, I. 1982. Notes on the range extension of the alpine water skink (Spenomorphus kosciuskoi) in Victoria. Victorian Naturalist 99:123-124.
Maurer, E.P., A.W. Wood, J.C. Adam, D.P. Lettenmaier, and B. Nijssen, 2002. A long-term hydrologically-based data set of land surface fluxes and states for the conterminous United States. Journal of Climate 15:3237-3251.
McInnis, M.L. and M. Vavra. 1987. Dietary relationships among feral horses, cattle, and pronghorn in southeastern Oregon. Journal of Range Management 40:60-66.
Meeker, J.O. 1979. Interactions Between Pronghorn Antelope and Feral Horses in Northwestern Nevada. M.S. thesis. University of Nevada, Reno.
Miraglia, N., M. Costantini, M. Polidori, G. Meineri, and P.G. Peiretti. 2008. Exploitation of a natural pasture by wild horses: Comparison between nutritive characteristics of the land and the nutrient requirements of the herds over a 2-year period. Animal 2:410-418.
Mitchell, J.E., ed. 2010. Criteria and Indicators of Sustainable Rangeland Management. Laramie, WY: University of Wyoming Extension.
Mote, P.W. and K.T. Redmond. 2012. Western climate change. Pp. 3-26 in Ecological Consequences of Climate Change: Mechanisms, Conservation, and Management, E.A. Beever and J.L. Belant, eds. Boca Raton, FL: CRC Press.
Nano, T.J., C.M. Smith, and E. Jefferys. 2003. Investigation into the diet of the central rock-rat (Zyzomys pedunculatus). Wildlife Research 30:513-518.
NRC (National Research Council). 1980. Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research. Phase I Final Report. Washington, DC: National Academy Press.
NRC (National Research Council). 1982. Wild and Free-Roaming Horses and Burros. Final Report. Washington, DC: National Academy Press.
Nimmo, D.G. and K.K. Miller. 2007. Ecological and human dimensions of management of feral horses in Australia: A review. Wildlife Research 34:408-417.
Oba, G., N.C. Stenseth, and W.J. Lusigi. 2000. New perspectives on sustainable grazing management in arid zones of sub-Saharan Africa. BioScience 50:35-51.
Odadi, W.O., M. Jain, S.E. Van Wieren, H.H.T. Prins, and D.I. Rubenstein. 2011a. Facilitation between bovids and equids on an African Savanna. Evolutionary Ecology Research 13:237-252.
Odadi, W.O., M.K. Karachi, S.A. Abdulrazak, and T.P. Young. 2011b. African wild ungulates compete with or facilitate cattle depending on season. Science 333:1753-1755.
Olsen, F.W. and R.M. Hansen. 1977. Food relations of wild free-roaming horses to livestock and big game, Red Desert, Wyoming. Journal of Range Management 30:17-20.
Paige, K.N. 1992. Overcompensation in response to mammalian herbivory: From mutualistic to antagonistic interactions. Ecology 73:2076-2085.
Paige, K.N. and T.G. Whitham. 1987. Overcompensation in response to mammalian herbivory: The advantage of being eaten. American Naturalist 129:407-416.
Pellegrini, S.W. 1971. Home Range, Territoriality and Movement Patterns of Wild Horses in the Wassuck Range of Western Nevada. M.S. thesis. University of Nevada, Reno.
Peters, D.P.C., B.T. Bestelmeyer, J.E. Herrick, E.L. Fredrickson, H.C. Monger, and K.M. Havstad. 2006. Disentangling complex landscapes: New insights into arid and semiarid system dynamics. BioScience 56:491-501.
Reiner, R.J. and P.J Urness. 1982. Effect of grazing horses managed as manipulators of big game winter range. Journal of Range Management 35:567-571.
Rietkerk, M.M., C. Boerlijst, F. van Langevelde, R. HilleRisLambers, J. van de Koppel, L. Kumar, H.H.T. Prins, and A.M. de Roos. 2002. Self-organization of vegetation in arid ecosystems. American Naturalist 160:524-530.
Ricketts, M.J., R.S. Noggles, and B. Landgraf-Gibbons. 2004. Pryor Mountain Wild Horse Range Survey and Assessment. Bozeman, MT: U.S. Department of Agriculture, Natural Resources Conservation Service.
Roelle, J.E., F.J. Singer, L.C. Zeigenfuss, J.I. Ransom, L. Coates-Markle, and K.A. Schoenecker. 2010. Demography of the Pryor Mountain Wild Horses, 1993-2007. U.S. Geological Survey Scientific Investigations Report 2010-5125. Reston, VA: U.S. Geological Survey.
Rogers, G.M. 1991. Kaimanawa feral horses and their environmental impacts. New Zealand Journal of Ecology 15:49-64.
Rogers, G.M. 1994. North Island seral tussock grasslands. 1. Origins and land-use history. New Zealand Journal of Botany 32:271-286.
Rowe, G. and L.J. Frewer. 2000. Public participation methods: A framework for evaluation. Science, Technology, and Human Values 25:3-31.
Sandford, S. 1983. Management of Pastoral Development in the Third World. New York: John Wiley & Sons.
Sandford, S. 2004. Factors affecting the economic assessment of opportunistic and conservative stocking strategies in African livestock systems. Pp. 56-67 in Rangelands at Equilibrium and Non-equilibrium: Recent Developments in the Debate Around Rangeland Ecology and Management, S. Vetter, ed. Durban, 26-27 July 2003. Bellville, South Africa: Programme for Land and Agrarian Studies.
Seegmiller, R.F. and R.D. Ohmart. 1981. Ecological relationships of feral burros and desert bighorn sheep. Wildlife Monographs 78:3-58.
Sharp, R. No date. Evaluating BLM grazing allotments. Burns BLM. Available online at http://extension.oregonstate.edu/harney/sites/default/files/Evaluating_BLM_Grazing_Allotments.pdf/. Accessed October 8, 2012.
Sheridan, T.E. 2007. Embattled ranchers, endangered species, and urban sprawl: The political ecology of the new American West. Annual Review of Anthropology 36:121-138.
Singer, F.J. and K.A. Schoenecker (compilers). 2000. Managers’ Summary—Ecological Studies of the Pryor Mountain Wild Horse Range, 1992-1997. Fort Collins, CO: U.S. Geological Survey.
Sokal, R.R. and F.J. Rohlf. 2011. Biometry: The Principles and Practices of Statistics in Biological Research, 4th edition. New York: W.H. Freeman.
Smith, E.L., P.S. Johnson, G. Ruyle, F. Smeins, D. Loper, D. Whetsell, D. Child, P. Sims, R. Smith, L. Volland, M. Hemstrom, E. Bainter, A. Mendenhall, K. Wadman, D. Franzen, M. Suthers, J. Willoughby, N. Habich, T. Gaven, and J. Haley. 1995. New concepts for assessment of rangeland condition. Journal of Range Management 48:271-282.
Stafford Smith, D.M., G.M. McKeon, I.W. Watson, B.K. Henry, G.S. Stone, W.B. Hall, and S.M. Howden. 2007. Learning from episodes of degradation and recovery in variable Australian rangelands. Proceedings of the National Academy of Sciences of the United States of America 104:20690-20695.
Stankey, G.H. and C. Allan. 2009. Introduction. Pp. 3-8 in Adaptive Environmental Management: A Practitioner’s Guide, C. Allan and G. Stankey, eds. Dordrecht, The Netherlands: Springer.
Stringham, T.K., W.C. Krueger, and P.L. Shaver. 2003. State and transition modeling: An ecological process approach. Journal of Range Management 56:106-113.
Swanson, S., B. Bruce, R. Cleary, B. Dragt, G. Brackley, G. Fults, J. Linebaugh, G. McCuin, V. Metscher, B. Perryman, P. Tueller, D. Weaver, and D. Wilson. 2006. Nevada Rangeland Monitoring Handbook, 2nd edition. Reno, NV: University of Nevada Cooperative Extension. Available online at http://www.unce.unr.edu/publications/files/ag/2006/eb0603.pdf/. Accessed October 8, 2012.
Sullivan, S. 1998. People, Plants and Practice in Drylands: Socio-political and Ecological Dimensions of Resource-use by Damara farmers in North-West Namibia. Ph.D. disseration. University College London.
Sullivan, S. and R. Rohde. 2002. On non-equilibrium in arid and semi-arid grazing systems. Journal of Biogeography 29:1595-1618.
Tainton, N.M. 1999. Veld Management in South Africa. Pietermaritzburg, South Africa: University of Natal Press.
Tausch, R.J. 1999. Transitions and thresholds: Influences and implications for management in pinyon and Utah juniper woodlands. Pp. 61-65 in Proceedings: Ecology and Management of Pinyon-Juniper Communities Within the Interior West, S.B. Monsen, R. Stevens, R.J. Tausch, R. Miller, and S. Goodrich, eds. General Technical Report, RMRS-P-9. USDA Forest Service, Rocky Mountain Research Station.
Thoma, D., D. Sharrow, K. Wynn, J. Brown, M. Beer, and H. Thomas. 2009. Water Quality Vital Signs Monitoring Protocol for Park Units in the Northern Colorado Plateau Network (ver. 1). U.S. Department of the Interior, National Park Service Inventory and Monitoring Program 2007.
Thurow, T.L. 1991. Hydrology and erosion. Pp. 141-159 in Grazing Management: An Ecological Perspective, R.K. Heitschmidt and J.W. Stuth, eds. Portland, OR: Timber Press.
Turner, B.L., R.E. Kasperson, P.A. Matson, J.J. McCarthy, R.W. Corell, L. Christensen, N. Eckley, J.X. Kasperson, A. Luerse, M.L. Martello, C. Polsky, A. Pulsipher, and A. Schiller. 2003. A framework for vulnerability analysis in sustainability science. Proceedings of the National Academy of Sciences of the United States of America 100:8074-8079.
Underwood, A.J. 1992. Beyond BACI: The detection of environmental impacts on populations in the real, but variable, world. Journal of Experimental Marine Biology and Ecology 161:145-178.
Underwood, A.J. 1994. On beyond BACI: Sampling designs that might reliably detect environmental disturbances. Ecological Applications 4:3-15.
Underwood, A.J. 1997. Experiments in Ecology: Their Logical Design and Interpretation Using Analysis of Variance. Cambridge, UK: Cambridge University Press.
U.S. Congress, Senate, Committee on Interior Insular Affairs, Subcommittee on Public Lands. 1971. Protection, Management, and Control of Wild Free-Roaming Horses and Burros on Public Lands: S.R. 92-242 to accompany S. 1116. Washington, DC: U.S. Government Printing Office.
van de Koppel, J., M. Rietkerk, and F.J. Weissing. 1997. Catastrophic vegetation shifts and soil degradation in terrestrial grazing systems. Trends in Ecology & Evolution 12:352-356.
Veblen, K.E., D.A. Pyke, C.L. Aldridge, M.L. Casazza, T.J. Assal, and M.A. Farinha. 2011. Range-Wide Assessment of Livestock Grazing Across the Sagebrush Biome. Reston, VA: U.S. Geological Survey.
Vetter, S. 2005. Rangelands at equilibrium and non-equilibrium: Recent developments in the debate. Journal of Arid Environments 62:321-341.
Walker, B., D. Salt, and W. Reid. 2006. Resilience Thinking: Sustaining Ecosystems and People in a Changing World. Washington, DC: Island Press.
Walters, J.E. and R.M. Hansen. 1978. Evidence of feral burro competition with desert bighorn sheep in Grand Canyon National Park. Desert Bighorn Council Transactions 22:10-16.
Ward, D., B.T. Ngairorue, J. Kathena, R. Samuels, and Y. Ofran. 1998. Land degradation is not a necessary outcome of communal pastoralism in arid Namibia. Journal of Arid Environments 40:357-371.
Ward, D., B.T. Ngairorue, J. Karamata, and I. Kapofi. 2000. The effects of communal and commercial pastoralism on vegetation and soils in an arid and a semi-arid region of Namibia. Pp. 344-347 in Vegetation Science in Retrospect and Perspective, P.S. White, L. Mucina, J. Leps, and E. van der Maarel, eds. Uppsala, Sweden: Opulus Press.
Weaver, V. and R. Adams. 1996. Horses as vectors in the dispersal of weeds into native vegetation. Pp. 383-387 in Proceedings of the 11th Australian Weeds Conference. Melbourne, Australia: Weed Science Society of Victoria.
Webler, T. and S. Tuler. 2000. Fairness and competence in citizen participation: Theoretical reflections from a case study. Administration and Society 32:566-595.
Weltz, M.A. and G. Dunn. 2003. Ecological sustainability of rangelands. Arid Land Research and Management 17:369-388.
Westoby, M., B. Walker, and I. Noy-Meir. 1989. Opportunistic management for rangelands not at equilibrium. Journal of Range Management 42:266-274.
Whinam, J., E.J. Cannell, J.B. Kirkpatrick, and M. Comfort. 1994. Studies on the potential impact of recreational horseriding on some alpine environments of the Central Plateau, Tasmania. Journal of Environmental Management 40:103-117.
Williams, J.W. and S.T. Jackson. 2007. Novel climates, no-analog communities, and ecological surprises. Frontiers in Ecology and the Environment 5:475-482.
Williams, B.K., R.C. Szaro, and C.D. Shapiro. 2009. Adaptive Management. U.S. Department of the Interior Technical Guide. Washington, DC: U.S. Department of the Interior.
Zalba, S.M. and N.C. Cozzani. 2004. The impact of feral horses on grassland bird communities in Argentina. Animal Conservation 7:35-44.