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RESEARCH: The effects of wild horses on western rangelands


“Though feral horse effects likely vary by intensity and frequency of use as well as a host of other factors, our results suggest that feral horses have some ecological impacts on semiarid rangelands across a range of levels of utilization.”

BLM horses in holding.

A study published last month in the Ecological Society of America journal, ECOSPHERE, has determined, mostly, what many have assumed all along: Because of their feral and unmanaged nature, wild horses can have negative impacts on rangelands.

Well, it’s more nuanced than that.

The study – “Effects of feral free-roaming horses on semi-arid rangeland ecosystems: an example from the sagebrush steppe” – is isolated and full of qualifiers. But it doesn’t detract from what complicates the management of horses on rangelands.

“The concern with feral horses is that unmanaged and poorly managed non-native grazers can have substantial impacts on ecosystem integrity as seen with poorly managed livestock in the western United States…” write the study’s authors, personnel from the Eastern Oregon Agriculture Research Center (USDA ARS) and Sheldon-Hart Mountain National Wildlife Refuge Complex (USDI Fish and Wildlife Service).

The issue, in short, is management. Cattle are managed via fencing, rotation in grazing regimes, dietary supplements and other human interventions. Wild horses mostly don’t receive those same treatments.

Left to roam, the estimated 40,000, or more, wild horses and burros occupying federally managed land are usually only gathered when their populations grow beyond what ecosystem estimates are believed to sustain. Too many animals beyond what the resources can sustain usually means the ecosystem suffers.

Livestock are subject to management whims, oft debated and litigated. The horses are capped at certain numbers based on their respective areas. Once over Herd Management Area (HMA) estimates, then the federal government swoops in for gathers of wild horses and burros, seeking accordance with the federal 1971 Wild Horse and Burro Act by keeping population numbers at estimated HMA levels. (Though, the BLM has been on record saying that gathers will be reduced because of funding.)

Absent human management, like the livestock interventions mentioned above—and no viable predation—horses roam unfettered until resources go sour and their numbers rise.

The point of this study was to determine what impact, as precisely as possible, the wild horses may have on one of their western ecosystems. The study occurred over five years – 2008 to 2013 – at the Sheldon National Wildlife Refuge in northern Nevada.

The study’s particulars are academically interesting (read the complete article here). The uniqueness of the research, the authors determine, is that “our results are the first study that empirically measured effects of free-roaming feral horses on the sagebrush steppe.”

Their study was site specific and with limitations. Nevertheless, grazing by wild horses appeared to have an impact on plant biodiversity and soil characteristics. Their conclusion, in full:

“Feral horse exclusion increased sagebrush density and plant species diversity and promoted recovery of important soil surface characteristics. Collectively, these results suggests that feral horse grazing at the utilization levels occurring in this study can affect the ecological function of semi-arid rangelands and may degrade the habitat value of these communities for associated wildlife. Feral horse grazing elevates the risk of soil erosion by increasing soil penetration resistance and decreasing soil aggregate stability. However, our study demonstrated that relatively short-term feral horse exclusion can initiate recovery of some variables. Other variables (e.g., perennial grass density and forb cover and density) may increase with extended horse exclusion, but long term studies are needed for verification. Though feral horse effects likely vary by intensity and frequency of use as well as a host of other factors, our results suggest that feral horses have some ecological impacts on semiarid rangelands across a range of levels of utilization. Our results agree with Beever and Aldridge (2011) assertion that feral horsesvalue to society must be weighed against their ecological costs. The collective results from our study suggest that the effects of feral horses should be considered when developing conservation plans for sagebrush steppe rangelands and other semi-arid and arid ecosystems.”

Note the absence of specific policy recommendations. Critics of feral horse management quickly adopt a poise of policy expertise while cherry picking data and sources. Peer-reviewed science often outlines research results and leaves it at that.

It’s important to note this research is not about whether horses should be on the range, or how we should view horses. It simply determines that unmanaged herbivores can have negative ecosystem impacts.

Related: Scientists rebuke livestock grazing critics.

Bob Conrad
Bob Conradhttp://thisisreno.com
Bob Conrad is publisher, editor and co-founder of This Is Reno. He has served in communications positions for various state agencies and earned a doctorate in educational leadership from the University of Nevada, Reno in 2011. He is also a part time instructor at UNR and sits on the boards of the Nevada Press Association and Nevada Open Government Coalition.