Wild horses (Equus ferus) roam as unspoken architects of the land, shaping environmental interactions and nurturing ecological balance through their untamed presence. Often found in landscapes that support domestic livestock, such as in the midwestern United States, wild horses are frequently regarded as an invasive species (Lönker, Fechner, and Abd El Wahed 2020). With complex inter and intraspecies relationships, wild horses are essential to maintaining ecosystem dynamics and promoting biodiversity.
Introduced to the Americas by Spanish colonists in the 1500s, horses became fundamental to Western settlement (Schoenecker, Esmaeili, and King 2023). Post-industrialization, many domestic horses were released or escaped to the wild, becoming feral. Today, wild horses inhabit public lands divided into herd management areas (HMAs) by the Bureau of Land Management. HMAs are intended for various uses including recreation, wildlife, and rangelands; horses that roam outside their designated HMA may be considered excess animals subject to removal (Schoenecker, Esmaeili, and King 2023). Understanding how horses navigate their habitat is key to judiciously managing HMAs.
Wild horses’ spatial distribution and movement patterns are influenced by their anatomy and behaviours. Their cecal digestive system is less efficient than sheep and other herbivores in terms of water use, making them more vulnerable to drought (Mutillod et al. 2024). Their grazing habits also play a key role in promoting plant biodiversity. A study comparing the grazing impacts of wild horses and domestic sheep found that wild horse grazing was associated with higher species richness, evenness, and heterogeneity (Figure 1) (Mutillod et al. 2024). As mono-grastic grazers with a single-chambered stomach, wild horses are more selective foragers than sheep and seek sources of higher nutritional value. They forage for longer, travel farther between feeding stations, and exhibit higher chewing efficiency, which helps them access a wider variety of plant species and prevent localized overgrazing (Shingu, Kondo, and Hata 2010). These behaviours contribute to a more heterogeneous landscape, supporting the intermediate disturbance hypothesis, which postulates that moderate disturbances, like grazing, enhance biodiversity by preventing the dominance of a single species. Nevertheless, the presence of wild horses does come with ecological trade-offs.
Figure 1: Boxplot of plant species richness per square metre between sheep (blue), domestic horses (orange), and wild horses (green). A total of 171 plant species were recorded. Black diamonds represent mean values and black lines represent median values per groups of plots. Plant species richness is significantly higher in wild horse plots (28.83+-5.36) than in sheep plots (22.14+-4.42), with a p-value of less than 0.001 indicating high statistical significance. Species richness in wild horse plots is also higher than in domestic horse plots (Mutillod et al. 2024).
Like many large herbivores, wild horses impact resource competition and the terrain. For example, indirect competition over shared water resources rises between horses, desert bighorn sheep, and elk (Lönker, Fechner, and Abd El Wahed 2020). Horse trampling causes soil compaction, reducing water infiltration and nutrient cycling, which negatively affects vegetation and favours conditions for non-native species. Wild horses can also express superior competitive abilities due to traits selected during domestication, including reproductive output which can increase population growth (Boyce and McLoughlin 2021). The complex social structure of wild horses further enables them to outcompete native herbivores in competitive interactions.
Wild horses are an integral force within the heartbeat of ecosystems. They promote plant diversity by preventing overgrazing, but their impact on resources and terrain must be carefully managed. The unique socio-political context of wild horse management highlights the need for a balanced approach to wildlife conservation and livestock interests. It is essential to adopt practices that prioritize the natural behaviours of wild horses and consider diverse perspectives. We must boost awareness and engage with policymakers to ensure sustainable practices that allow wild horses to thrive while protecting the land they help shape.
References
Boyce, Paul N., and Philip D. McLoughlin. 2021. “Ecological Interactions Involving Feral Horses and Predators: Review with Implications for Biodiversity Conservation.” The Journal of Wildlife Management 85 (6): 1091–1103. https://doi.org/10.1002/jwmg.21995.
Lönker, Nelly Sophie, Kim Fechner, and Ahmed Abd El Wahed. 2020. “Horses as a Crucial Part of One Health.” Veterinary Sciences 7 (1): 28. https://doi.org/10.3390/vetsci7010028.
Mutillod, Clémentine, Elise Buisson, Laurent Tatin, Gregory Mahy, Marc Dufrêne, François Mesléard, and Thierry Dutoit. 2024. “Managed as Wild, Horses Influence Grassland Vegetation Differently than Domestic Herds.” Biological Conservation 290 (February):110469. https://doi.org/10.1016/j.biocon.2024.110469.
Schoenecker, Kathryn A., Saeideh Esmaeili, and Sarah R. B. King. 2023. “Seasonal Resource Selection and Movement Ecology of Free-Ranging Horses in the Western United States.” The Journal of Wildlife Management 87 (2): e22341. https://doi.org/10.1002/jwmg.22341.
Shingu, Yuko, Seiji Kondo, and Hiroshi Hata. 2010. “Differences in Grazing Behavior of Horses and Cattle at the Feeding Station Scale on Woodland Pasture.” Animal Science Journal 81 (3): 384–92. https://doi.org/10.1111/j.1740-0929.2010.00748.x