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.