Feral Horses in the Arid West

So, this has started out as an ethno-ornithology paper. Ultimately it is just about horses and how they affect the arid lands of the western United States. The paper only needed to be two pages and so I had to leave some stuff out, sadly. Maybe I'll add on to this post in the future.


Feral Horses in the Arid West

Modern horses appeared in the America’s in the late 16th century when the Spanish Conquistadors arrived. The progenitor of these horses originated in Europe, from species that are now extinct. The last time that the ecosystems of the Americas had included equids was about 10,000 years earlier. In the 10,000 years since the American species of horses went extinct, the “Western United States has become more arid and many of the horses natural predators, like the American lion and saber-toothed cat have disappeared” (Leopold 2010). Thus, the ecosystem modern horses were introduced to was far different from the one they came from, and from the one their distant relatives had left. In the approximately 400 years since horses began forming feral herds the rangelands of the west have been transformed. These new conditions were so ideal for modern equids that by the late 19th century there were 2-7 million horses roaming on arid lands (Beever 2003). The Taylor Grazing Act of 1934 facilitated the removal, domestication and extermination of feral horses, dramatically reducing their numbers (Beever 2003). Finally, legislation was drafted to protect feral horses and burros, via the Wild Free-Roaming Horse and Burro Act of 1971, sparking a population boom from 17,300 head in 1971 to 57,200 head in 1978. Today there are an estimated 38,815 head on 17.5 million ha of land managed by the Bureau of Land Management (BLM) and 1600 head on 1.1 million ha of land managed by the United States Forest Service (USFS).

Horses differ from the native grazing mammals in a number of ways. First, horses are cecal digesters, rather than ruminants. Second, horses have upper front incisors and a flexible upper lip, allowing them to graze plants lower than other grazing mammals. Horses use their ecosystems heterogeneously compared to other large grazing mammals. Additionally horses have single hard hooves, rather than the split hooves of native ungulates.

Cecal digestion occurs in a section of the digestive system that lies behind the stomach, called a cecum. Here, much like in the rumen, good bacteria aids in breaking down plant matter and freeing nutrients for further absorption by the intestines. Because cecal digestion happens at the end of the digestive tract horses don’t digest their food as thoroughly as ruminants. Thus, horse feces will have a greater amount of undigested seeds and poorly digested plant matter than ruminant feces. Ostermann-Kelm, et al found that horse feces increased native plant diversity along horse trails in their study quadrants (Ostermann-Kelm, et al 2009). The increased moisture, organic matter and nutrients, along with a high level of undigested seeds creates a ideal medium for growing plants. In areas where invasive grasses, such as cheat grass, proliferate it is likely that plant diversity will decrease as invasive seeds are dispersed through feral horse feces.

Arid ecosystems evolved with bison, deer and similar grazing mammals. The grasses in these systems are therefore physiologically designed to withstand heavy grazing pressure with periods of rest when the large herds of bison move on. Additionally, feral horses will graze native grasses so low that the recovery of the grass is delayed. This can directly cause die-off or indirectly cause erosion of the soils around the plant when it rains resulting in death of the plant.

Trampling, cause by horses’ single solid hooves and heterogeneous use of the landscape, creates a number of changes in arid ecosystems. Trampling directly causes erosion through hoof chiseling of soils or chipping of rocks. Indirectly, horses cause erosion by compacting soils and removing vegetative cover. Horse-grazed areas show a variability of soil strengths, with the highest strength concentrated in areas where horse trails, and thus trampling, occur. This results in a decrease in water infilitration and soil pore volume, inhibits or restricts plant root growth, decreases vegetation cover and increases soil temperature and soil erosion (Ostermann-Kelm, et al 2009; Beever 2003). In arid ecosystems nutrients tend to be concentrated in the surface of the soil. Therefore, eroding only the first few millimeters of soil can seriously alter the availability of nutrients in desert soils. A decrease in vegetative cover directly correlates to a decline in the predation on, and diversity and abundance of, birds, lizards and small mammals.

According to Beever and Herrick feral horse populations may increase by greater than 20% each year (Beever and Herrick 2005). Increasing herd size requires increased management action by both the BLM and USFS. In order to manage horses in balance with other public rangeland resources and uses more horses need to be removed from federal land. The increase in numbers of horses in captivity requires additional public expense. In 1996, 7369 feral horses were rounded-up by the BLM and put up for adoption. All but about 8% of those horses were adopted. With the decline in the economy adoption rates have plummeted. In 2009 only 53.6% of the 5961 horses up for adoption found homes. The cost of holding the excess feral horses, either in short-term holding pens or in long-term holding pastures is $36.9 million of the $63.9 million budget (BLM 2011). As rangelands are degraded and federal budgets decline there is a push to find a solution to the feral horse population problem.

There are a variety of organizations that advocate for the horses to be left alone and for minimal management. The arguments made by these organizations and individuals are generally emotion-based rather than fact-based. It is true that feral horses are magnificent to behold. It is also true that horses played an important part in the history of the United States, in expansion and in the realization of manifest destiny. Horses became an important part of many First Nations tribes, both historically and spiritually.

Native species of desert dwelling plants and animals live in careful balance with the land around them. Horses are not a natural part of North American desert environments and should not be treated as such. Increasing the number of horse “preserves” will only expand the area of land impacted by feral horses. Euthanasia, while a legal option, is highly unpopular and should therefore only be used with sick or injured horses. The best option seems to be to sterilization. Creating non-reproductively viable herds will allow this piece of U.S. history to remain and over time will decrease their destructive capacity.

Literature Referenced

Beever, E.A. 2003. Management implications of the ecology of free-roaming horses in semi-arid ecosystems of the western United States. Wildlife Society Bulletin. 31(3):887-895. Available at: http://fresc.usgs.gov/products/papers/1224_Beever.pdf. Accessed February 2, 2011.

Beever, E.A. and Herrick, J.E. 2006. Effects of feral horses in Great Basin landscapes on
soils and ants: Direct and indirect mechanisms. Journal of Arid Environments. 66, 96-112. Available at: http://ddr.nal.usda.gov/bitstream/10113/16128/1/IND43801323.pdf. Accessed February 4, 2011.

BLM National Wild Horse and Burro Program page. http://www.blm.gov/wo/st/en/prog/wild_horse_and_burro.html. Updated Febrary 2, 2011. Accessed February 7, 2011.

Leopold, B. Letter to the Wild Horse and Burro Program. Available at: http://joomla.wildlife.org/documents/policy/horse_comments_082010.pdf. Updated August 2010. Accessed February 2, 2011.

Ostermann-Kelm,S., et al. 2009. Impacts of Feral Horses on a Desert Environment. BMC Ecology. Available at: http://www.biomedcentral.com/content/pdf/1472-6785-9-22.pdf. Updated November 10, 2009. Accessed February 4, 2011.