Beever et al. examined disturbance created by feral horses (Equus caballus), in 9 mountain ranges of the western Great Basin, USA, in Nevada, using plants, mammals, ants, and soil compaction, as indicators. In both reciprocal averaging (RA) and TWINSPAN analyses, abiotic factors, plant cover, and key plant species cover characterized sites more strongly by mountain range than by presence of horses. Abiotic factors can strongly influence the results of monitoring, and need to be accounted for in any monitoring schemes performed over broad areas. In contrast to other data sets, disturbance related variables separated all but one or two sites according to the presence of horses in analyses of both 1997 and 1998 data. Percent cover of all plant species was the best data set to distinguish horse-occupied from horse-removed sites in both years. The authors recommend development of an expanded monitoring strategy based on established vegetation measurements, investigating forage consumption, and including disturbance-sensitive variables that more completely reflect the suite of effects that a large-bodied grazer may impose on mountain ecosystems, independent of vegetation differences.
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