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Effects of feral free-roaming horses on semi-arid rangeland ecosystems: an example from the sagebrush steppe

Kirk W. Davies, G. Collins, C. S. Boyd
Ecosphere 2014 v.5 no.10 pp. 1-14
Artemisia, soil penetration resistance, grazing, plant density, ecological function, soil aggregates, risk, ecosystem management, ecosystems, issues and policy, plant growth, cattle, wildlife management, species diversity, Centrocercus urophasianus, steppes, wildlife, rangelands, feral animals, semiarid soils, soil erosion, aggregate stability, horses, Australia, Nevada, South America
Feral horses (Equus caballus) are viewed as a symbol of freedom and power; however, they are also a largely unmanaged, non-native grazer in North America, South America, and Australia. Information on their influence on vegetation and soil characteristics in the sagebrush (Artemisia L.) steppe has been limited by confounding effects of cattle grazing and a lack of empirical manipulative studies. We compared vegetation and soil surface characteristics in feral horse grazed areas and ungrazed exclosures at five sites in northern Nevada. Horse grazed areas had lower sagebrush density and plant diversity, greater soil penetration resistance, and lower soil aggregate stability than ungrazed areas. Though herbaceous cover and density generally did not differ between treatments, we speculate that vegetation will differ with longer horse exclusion and as soils recover. The cumulative effect of feral horse grazing on soil characteristics suggests that they can negatively affect the ecological function of semi-arid rangelands by increasing the risk of soil erosion and potentially decreasing the availability of water for plant growth. The two-fold increase in sagebrush density with horse exclusion suggest that feral horses limit sagebrush recruitment and thereby may negatively impact Greater sage-grouse (Centrocercus urophasianus) and other sagebrush associated wildlife. These results suggest that the effects of feral horses on sagebrush ecosystems should be considered when developing conservation plans for sagebrush steppe communities and associated wildlife. The results of this study also suggest that the policy of minimal management of feral horses is not scientific sound ecosystem management for arid and semi-arid rangelands.