The Wild Free-Roaming Horse and Burro Act recognizes the wild horse as an "integral component of the natural system." It stipulates that horses can only be removed from public lands if it is proven that they are overpopulating or are causing habitat destruction. It further mandates that the government "maintain specific ranges on public lands as sanctuaries for their protection and preservation."
In order to remove wild horses from public lands, the Bureau of Land Management (BLM) has claimed that horses are destroying critical habitat, competing for grazing lands, and overpopulating. But reports by the General Accounting Office and the National Academy of Sciences dispute such claims: BLM has never presented any evidence that horses destroy habitat, nor that their population levels are what it claims they are. In fact, reducing horse populations in a given area has a negligible effect on range conditions: after massive wild horse roundups, herd areas show little or no improvement, especially in instances when cattle numbers remain the same (or increase).
In stark contrast with BLM’s assertions, scientific studies have shown that horses actually benefit their environment in numerous ways; vegetation seems to thrive in some areas inhabited by horses, which may be one reason the Great Plains were once a "sea of grass." Generally, range conditions in steep hilly areas favored by horses are much better than in lower areas frequented by cattle.
Cows have no upper front teeth, only a thick pad: they graze by wrapping their long tongues around grass and pulling on it. If the ground is wet, they will pull out the grass by the roots, preventing it from growing back. Horses have both upper and lower incisors and graze by "clipping the grass," similar to a lawn mower, allowing the grass to easily grow back.
In addition, the horse’s digestive system does not thoroughly degrade the vegetation it eats. As a result, it tends to “replant” its own forage with the diverse seeds that pass through its system undegraded. This unique digestive system greatly aids in the building up of the absorptive, nutrient-rich humus component of soils. This, in turn, helps the soil absorb and retain water upon which many diverse plants and animals depend. In this way, the wild horse is also of great value in reducing dry inflammable vegetation in fire-prone areas. Back in the 1950s, it was primarily out of concern over brush fires that Storey County, Nevada, passed the first wild horse protection law in the nation.
The fact that horses wander much farther from water sources than many ruminant grazers adds to their efficacy as fire preventers. This tendency to range widely throughout both steep, hilly terrain and lower, more level areas, while cattle concentrate on lower elevations, also explains why horses have a lesser impact on their environment than livestock: when one looks at a boundary fence where horses range on one side and cattle range on the other, the horses’ side typically reveals about 30% more native grasses. Their nomadic grazing habits cause horses to nibble and then move to the next bunch of grass. This is why horse range is seldom denuded unless the horses' natural grazing patterns are disrupted by human interference, mostly in the form of fencing.
A team of Russian scientists, part of a cooperative venture with the United States, came in 2001 to study the effects of grazing animals on riparian areas in Nevada. They tested streams for nutrients and examined the desert and Sierra to learn techniques to improve the environment of their homeland. The scientists found that cows, which tend to camp around water sources, cause more damage to the stream banks than wild horses, which tend to drink and move on: "When we saw horses drinking from creeks, we didn't see much impact except for hoof prints. The water looked clean, had good overhanging branches and there was no sign of erosion on the banks. There was an abundance of insects and animals, including frogs and dragonflies and water-striders." Areas extensively used by cattle had fewer nutrients in the water and showed signs of bank erosion and other damage, concluded the study.
Horses have proven useful to other species they share the range with: in winter months, they have the instinct to break through even deep crusted snow where the grass cannot be seen. They also open up frozen springs and ponds with their powerful hooves, making it possible for smaller animals to drink. During the historic blizzard of 1886, hundreds of thousands of cattle were lost on the Plains. Those that survived followed herds of mustangs and grazed in the areas they opened up. Another positive effect of wild horses on biodiversity was documented in the case of the Coyote Canyon horses in the Anza Borrega National Park (California). After wild horses were all removed from the Park to increase big horn sheep population, bighorn sheep mortality actuality skyrocketed: mountain lions, wild horse predators, compensated the loss of one of their prey species by increasing their predation on other species.
Wild horses should not be used as scapegoats for range degradation that is in fact primarily caused by private livestock: for instance, environmentalists have determined that in Nevada, home of the vast majority of America's remaining wild horses, the herds have little impact on the ecosystem compared with the hundreds of thousands of cattle that also roam the Nevada range. The Western Watersheds Project acknowledges that "the main cause of degradation of public lands in the arid west is livestock use and not wild horses."