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Feral horses (Equus caballus) are a non-native grazer in North America, South America, and Australia that are relatively unmanaged.� This has led to concern about their potential to damage natural resources.� Information about their influence on vegetation and soil characteristics in semi-arid rangelands and riparian areas has been limited by confounding effects of grazing by livestock and a lack of empirical manipulative studies.� We compared plant community and soil surface characteristics in feral horse grazed areas and ungrazed exclosures at five sagebrush (Artemisia) steppe and riparian communities in northern Nevada.� In the sagebrush steppe, horse grazed areas had lower sagebrush density and plant diversity, greater soil surface compaction, and lower soil aggregate stability compared to ungrazed areas.� In the riparian communities, feral horse grazed areas had 7-fold greater bare ground, reduced litter and visual obstruction, and increased rush density compared to ungrazed areas.� The cumulative effect of feral horses on soil characteristics in uplands suggests that they may negatively affect ecological function by increasing the risk of soil erosion and potentially decreasing availability of water for plant growth.� The two-fold increase in sagebrush density with horse exclusion suggests that feral horses may limit sagebrush recruitment and thereby negatively impact greater sage-grouse (Centrocercus urophasianus) and other sagebrush-associated wildlife.� The greater bare ground in horse grazed riparian areas increases the risk of erosion, and subsequently stream channel incising, and susceptibility to exotic plant invasion. �The effects of feral horses on native ecosystems should be considered when developing conservation plans for these ecosystems and associated wildlife.