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