Feral horse activity reduces environmental quality in ecosystems globally

Feral horses occur on several continents, across a wide range of terrestrial biomes, and have had marked impacts on ecosystems worldwide. Despite their oft-reported negative impacts on plants and soils, a global synthesis of their effects has not been attempted. Here we present a meta-analysis of the global impacts of feral horses on ecosystem structure, function and composition using data from 78 studies across five continents. Two-thirds of these studies were from North America (47%) and Oceania (21%), and most studies examined feral horse effects on plants (58%) or soils (35%). Feral horse activity reduced environmental quality by 13% overall, and the magnitude of this decline increased with increases in the intensity of horse activity. Feral horse activity strongly reduced measures of ecosystem function by 19% on average, and had variable effects on composition, with measures of composition most strongly increased (by 21%) at arid sites. There were no overall effects of feral horse activity on ecosystem structure, with insufficient data to assess effects on plant height and cover. We found that feral horse activity had negative effects on soil, increasing erosion (by 31% on average) and reducing plant biomass and litter cover (by 25% and 31% on average respectively). Increases in soil physical properties generally intensified with increasing aridity. Given the generally negative effects of feral horses on ecosystems worldwide, resource managers and governments need to balance the needs of maintaining healthy functional ecosystems and their biota with social- and cultural-driven commitments to maintaining free-ranging herds of feral horses.