Free-roaming horses have been described as a widespread conservation challenge by two American rangeland scientists, who say their effects would likely be greatly reduced if they were better managed.
Kirk Davies and Chad Boyd, writing in the journal BioSciences, say the grazing-related impacts of free-roaming horses remain largely unmanaged, leading to concerns about their effects on native plant communities and ecosystems.
The pair, with the Eastern Oregon Agricultural Research Center, part of the US Department of Agriculture’s Agricultural Research Service, examined the scientific literature to determine the ecological effects of free-roaming horses across North America’s rangelands.
The effects are extensive, they said, ranging from soil compaction and domination of scarce water resources to the over-grazing of native plant species.
Free-roaming horses, they conclude, can alter plant community composition, diversity, and structure and can increase bare ground and erosion potential. Free-roaming horses have also been linked to negative impacts on native fauna, they said.
“Horses have repeatedly been shown to limit and even exclude native wildlife’s use of water sources.
“These effects would likely be greatly reduced if the horse populations were better managed, but sociopolitical factors often preclude improved management.
“Using rigorous ecological research to educate politicians and the general public may facilitate the development of science-based management of free-roaming horses; however, ecological effects may have to become more severe before such changes can be realized.”
The pair acknowledge that the management of free-roaming horses is a contentious topic, with pressure from multiple special interest groups, such as wildlife enthusiasts, hunters, animal-rights groups, ranchers, conservationists, environmental groups, horse advocates and enthusiasts.
These groups have what the pair describe as divergent demands.
“A large part of the challenge is that humans often have a strong emotional connection to horses, because horses have been ingrained in human cultures for centuries.
“Free-roaming horses are viewed as a symbol of freedom and strength, as well as an icon of the American West, but the modern free-roaming horse is also an exotic species in North America that may influence ecosystem function and integrity if the population is left unmanaged.”
The pair say an additional concern is that most of the free-roaming horses on public lands in the United States occur in the driest state, Nevada, with plant communities and ecological sites sensitive to disturbance.
In their paper, they catalog many effects from horses on local landscapes. Unrestricted free-roaming-horses affect vegetation in uplands and riparian areas, they say. In general, horses alter the structural characteristics and, at times, the abundance of native vegetation.
They pointed to a study that found that “areas from which horses had been excluded compared with horse-occupied areas in Great Basin uplands had two to three times greater native grass cover and frequency.”
As a result, the authors caution, “free-roaming-horse use over time could permanently affect the productivity and function of some areas.”
One of the more concerning impacts may be their effects on soils and erosion potential, they say.
“Unrestricted free-roaming-horse use can result in high levels of bare ground, particularly in areas they repeatedly select. Similar impacts were observed with other large herbivores when they were allowed to repeatedly use preferred areas.”
Free-roaming-horses has also been linked to negative impacts on insects, small mammals, birds, and estuarine fauna.
Davies and Boyd point out that free-roaming horses and burros occupy 31.6 million acres of federal lands, with a population of 81,951 animals, exceeding the Bureau of Land Management’s established “appropriate management level” by more than 55,000 animals.
Further complicating matters is the uncertainty that beleaguers attempts to understand their effects.
The pair note that “domestic livestock grazing often confounds the ecological effects of free-roaming-horse grazing, giving rise to considerable uncertainty regarding the full extent and degree of impact of horses on rangeland ecological processes.”
They draw a clear distinction between continuous year-round grazing by free-roaming horses and planned grazing by livestock, which is comparatively limited in duration, takes place during a specific season, and allows for periodic rest.
But the uncertainty, they say, makes managing horse population challenging, especially in light of free-roaming horses’ prominent place in the public’s imagination.
They note that public resistance to horse removal has thus far stymied efforts to address rising numbers.
The solution the authors propose is rigorous ecological research with an eye toward promoting sociopolitical change.
“Limiting the ecological effects of free-roaming-horse use will require the successful navigation of a complex and emotionally charged sociopolitical environment,” they write.
“We currently have sufficient ecological literature to inform management decisions regarding free-roaming horses. However, science-based ecosystem conservation is confounded by human psychology and the politics and sociology of horse advocacy groups.
“We would like to believe that change in the sociopolitical arena around free-roaming horses can be stimulated by rigorous ecological research and using that research to inform scientifically sound management of free-roaming horses, as well as educating politicians and the public.
“However, we recognized that science is often ignored and, in some cases, blatantly discounted because of people’s emotional connection to horses.
“People’s emotional connections to other animals cause similar issues around the world.”
In Dubai, for example, camel use is unrestricted because of a cultural reverence for them, even though they pose a threat to inland deserts. Feral pigs in Hawaii are not eliminated because of a cultural tradition of pig hunting.
“The ecological impacts of free-roaming horses will likely have to become more severe before the sociopolitical environment surrounding this issue changes sufficiently to alter management.
“That said, the status quo of largely not managing free-roaming-horse populations is neither ecologically tenable nor compatible with the conservation of North American rangelands and their native fauna.”
BioScience is the journal of the American Institute of Biological Sciences.
Kirk W Davies, Chad S Boyd, Ecological Effects of Free-Roaming Horses in North American Rangelands, BioScience, biz060, https://doi.org/10.1093/biosci/biz060
The review can be read here.