What does the scientific community think about Feral Horses?
"THE WILDLIFE SOCIETY
5410 Grosvenor Lane • Bethesda, MD 20814-2144
Tel: (301) 897-9770 • Fax: (301) 530-2471
E-mail:
tws@wildlife.org
Final Position Statement
Feral Horses and Burros in North America
Feral horses and burros are invasive species in North America. Exotic, non-native species are
among the most widespread and serious threats to the integrity of native wildlife populations
because they invade and degrade native ecosystems. When invasive species are perceived as a
natural component of the environment, the general public may regard them as “natural,” not
understanding the damages they inflict on native systems. These misperceptions create special
challenges for wildlife managers. As a result, some groups advocate conservation and
management of exotic species that promote their continued presence in landscapes where they
are not native. Because these species are exotic, few policies and laws deal directly with their
control. Feral horses (Equus caballus) and burros (E. asinus) that roam freely across western
North America and along the Atlantic coast are examples of such species: they are iconic and
beloved by some, but damage wildlife habitat and require improved and sustainable management
practices. The numbers and impact of feral horses and burros can be difficult to control,
amplifying their effects on native habitat and wildlife. In some cases, management of feral horses
and burros and their effects divert resources (human and financial) from management of native
species and habitat.
Feral horses and burros in North America are descendants of domestic horses and burros that
either escaped from or were intentionally released by early European explorers and later settlers.
Although many horse lineages evolved in North America, they went extinct in North America
approximately 11,400 years ago during the Pleistocene, along with many other mammals. All
horses and burros now present in North America are descendants of those domesticated in
Eurasia and Africa (respectively) and were subjected to many generations of selective breeding
(artificial selection) before they were introduced to North America by settlers. Since native North
American horses went extinct, the western United States has become more arid and many of the
horses’ natural predators, such as the American lion and saber-toothed cat, have also gone
extinct, notably changing the ecosystem and ecological roles horses and burros play.
Herds of feral horses and burros can damage the habitat they occupy. Estimates suggest that
these herds range across more than 45 million acres in 10 American states and 2 Canadian
provinces in western North America. Feral horses are also found in eastern North America on
barrier islands off the coasts of Maryland, Virginia, Georgia, North Carolina, and Nova Scotia.
Large herbivores (both native and non-native) disturb landscapes by trampling soils and
vegetation, selectively grazing palatable plants, and altering the distribution of nutrients in the
ecosystem. Research in the Great Basin has reported that areas inhabited by feral horses have
fewer plant species and less grass, shrub, and overall plant cover than areas without horses, and
more invasive plant species and weeds such as cheatgrass, an invasive species that degrades
wildlife habitat. Riparian and wetland areas may also be impacted by feral horses and burros
through soil compaction and increased erosion. The overall impact feral horses and burros have
on any type of ecosystem depends on intensity and duration of use, timing, and the health and
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resilience of the area. Where feral horse and burro density is high, lands are degraded, water
resources are limited, and native species are already stressed, impacts can be substantial.
When feral horses and burros are introduced to an ecosystem, much of the native habitat is used
by these non-native grazers. Free-ranging horses typically use higher elevations and steeper
slopes than cattle, often moving to higher elevations for grazing, defense, and temperature
control. Because of horses’ flexible lips and long incisors, they are able to crop vegetation close
to the soil surface, which can delay re-growth of grazed plants. The digestive systems of burros
and horses dictate that they must ingest more forage per unit of body mass than any other largebodied
grazer in western North America. Feral horses are also dominant among native Great
Basin ungulates in social interactions, notably at watering areas. There may not be aggressive
behavior among horses, deer, and bighorn sheep (Ovis canadensis), but the presence of horses
can affect the distribution of native species and their use of the habitat.
The diet of feral burros overlaps a great deal with that of bighorn sheep and uncontrolled burro
populations have been predicted to lead to greater competition for forage and a decline in the
populations of bighorn sheep and other native animals. Burros have one of the most-inclusive
diets of large mammals. Given the climates that their ancestors inhabited, extant burros typically
live in the hotter, drier ecosystems of North America. In those systems, rainfall is so scant that
annual productivity is very low, and recovery from disturbance has been reported to require
decades to centuries, depending on the type, intensity, and duration of the disturbance.
The small reptiles and mammals in the western North American ecoregion that depend on
burrows and brush cover to survive and breed are lower in species diversity and less abundant in
horse- and burro-occupied sites. These reptiles and mammals are an important component of the
ecology of desert systems because they are a link in the food web, and perform numerous critical
ecosystem functions (e.g. prey base, nutrient cycling, seed dispersal, insect control).
A variety of management practices have been in use since Congress passed the Wild Free-
Roaming Horses and Burros Act in 1971, which guides management of feral horses and burros
on Bureau of Land Management (BLM) and U.S. Forest Service lands in the western U.S.
Existing management practices include: periodic population counts and rapid assessments of
ecosystem status to determine where overpopulation exists; roundups to capture and transport
animals; use of contraception to reduce productivity; adoption of animals to private owners; and
the humane euthanasia of old, ailing, or unadoptable animals. However, management involving
euthanasia, and sometimes roundups, is severely restricted by public opinion. While the public
and interest groups express concern for the affected horses and burros, they often fail to consider
the conservation of native plants and animals in the ecosystem, and the likelihood that horses and
burros will die from starvation, thirst, and exposure when their numbers exceed the carrying
capacity of the region.
Due to public opinion, animals passed over for adoption are not euthanized; instead, they are
placed into short- or long-term holding facilities. The number of animals adopted annually has
declined in recent years, necessitating additional holding facilities. In turn, program costs are
rising to unsustainable levels and diverting funding that could be used to manage and sustain
habitats for native wildlife. Sound, scientifically-based feral horse and burro management
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practices should be employed to conserve the highly sensitive arid and semiarid ecosystems of
the West and keep taxpayer costs to an acceptable level.
The policy of The Wildlife Society regarding feral horses and burros is to:
1. Encourage the BLM and U.S. Forest Service to place primary emphasis upon the habitat
needs of native wildlife and plants when developing, revising, and implementing herd
management plans and to include wildlife biologists with differing areas of expertise on
planning teams.
2. Encourage the U.S. Fish and Wildlife Service and National Park Service to remove feral
horses and burros from all refuges and parks to protect wildlife and their habitat, historic
and archaeological resources, and other trust values.
3. Encourage the BLM to eliminate feral horse and burro populations in Herd Areas that
have been determined to have insufficient habitat resources necessary to sustain healthy
horse populations.
4. Recommend that BLM and other responsible agencies direct adequate attention and
resources toward accurately and precisely identifying the impacts of feral horses and
burros on wildlife populations, habitats, and other natural resources managed for public
benefit by 1) developing and implementing appropriate survey and removal methodology
2) conducting surveys and removals in a timely manner to minimize impacts on natural
resources that can result from the overpopulation of feral horses and burros and 3)
identifying and mitigating impacts on perennial and ephemeral riparian and wetland
habitats, upland habitats, and threatened, endangered, and special status species of
wildlife. Inventories should be performed using scientifically-based abundance
estimation techniques that quantify population size and associated estimate error.
5. Support the use of roundups to remove feral horses and burros from rangeland while
simultaneously seeking opportunities to improve the knowledge and use of the best and
most humane capturing and handling methods.
6. Recognize that adoption programs are a socially acceptable method for removal and
relocation of feral horses and burros, but that the pool of possible adopters is declining
and adoption is not a viable long-term solution to overpopulation.
7. Support euthanasia as a humane method for removal of old, ailing, or unadoptable feral
horses and burros and as a possible method to control population size.
8. Recognize that no feral horse or burro management plan should depend solely on fertility
control given the uncertainty, logistical difficulty, and great expense that still exist
regarding these methods.
9. Support increased funding for scientifically-defensible assessments of ecosystem
conditions and interactions between feral horses and burros and native wildlife used to
make decisions related to feral horses or burro management. Such assessments should
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consider the welfare of the feral horses and burros, and the ability of the system to
conserve native plant and animal populations and provide ecosystem services such as
clean air, clean water, and carbon sequestration.
10. Support the management of feral horses and burros at or below Acceptable Management
Levels using a statistically valid sampling methodology. Underestimated populations can
hinder management plans and lead to increased levels of resource damage.
11. Discourage the conversion of currently viable, ungrazed native or converted grasslands to
pasture lands to house unadoptable horses and burros, privately or publically owned.
12. Cooperate with the conservation and animal-welfare communities to educate the public
and key decision makers about the evolutionary history and ecological role of feral horses
and burros and the negative impact they have on native vegetation and wildlife, including
mammals, birds, reptiles, amphibians, and endangered species.
Approved by Council July 2011. Expires July 2016."