Rupert Cornwell: Time catches up with wild symbol of the Old West

Out of America: Government measures to control Nevada's herds of feral horses have stirred growing protests

They are among the most potent symbols of that great fantasy land that is the American West. Their name was borrowed by an iconic American car, as well as by one of the world's better known brothels. But last week, none of this could spare Nevada's wild mustangs from the indignity of a round-up.

It began on Monday, as a pair of helicopters took to the low skies over the rugged scrub country north-east of Reno. After four days of combing the area, they had chased 140 mustangs into corrals, from where the animals were to be taken by truck east, to a new home out on the Great Plains. This latest "gather", as it is called, is set to run for two months. When all is done, the Bureau of Land Management, the federal agency in charge of the operation, will have removed more than three-quarters of the 3,000-plus wild horses roaming an area of some 1,000 square miles – unless, of course, a mounting public ruckus stops the project in its tracks.

Celebrities, including the actors Viggo Mortensen and Kevin Nealon, and the singer Willie Nelson have pleaded with the White House to impose a halt, and protesters demonstrated outside the San Francisco offices of Dianne Feinstein, the California senator who heads the committee that oversees funding for the Interior Department and the BLM, to demand congressional action.

For animal rights activists, the round-ups are the result of a dastardly campaign by Nevada ranchers to secure more grazing land for their cattle, sneakily started by the bureau during the Christmas/New Year break, when it was least likely to be noticed. Round-ups, they maintain, are cruel and inhumane, terrifying and often injuring the animals, and separating foals from their mothers. But to no avail.

The government replies that such fears are overblown; no evidence exists of animals being traumatised. More important, it argues, unless the mustang population is heavily thinned, a fragile ecosystem that has already been weakened by drought, wildfires and shortage of forage will be facing collapse. According to BLM experts, the land on which the new round-up is taking place can support 600, at most 700 animals. But left to its own devices, the local mustang population – which by law cannot be hunted and which has no natural predators – would double to 6,000.

However, science's cold reasonings count for little against the ingrained beliefs of the old West. The ancestors of the modern horse did indeed originate in North America, experts say, but disappeared some 12,000 years ago (perhaps hunted to death by our own ancestors). The species was then reintroduced from Asia and Europe, by the first Spanish explorers, among others. However, contrary to legend, today's wild herds are not the direct descendants of handsome caballos that might have escaped during expeditions by the early conquistadores. The more prosaic likelihood is that native Indians came into possession of horses after the 1680 Pueblo revolt in what is now New Mexico, when the indigenous rebel leaders rose against the Spanish colonists.

Later, as is well known, American Indians fell upon bad times – but their wild horses flourished. Today, an estimated 37,000 of them live on 51,000 square miles of land, an area slightly larger than England, spread across 10 western and south-western states. Since 1971, when Congress passed the "Wild Free-Roaming Horse and Burro Act" the mustang population is believed to have doubled.

And in some circles at least, the magic of the mustang is wearing thin. Land management specialists complain that while a herd of wild horses can stir the heart, it can also do great damage to waterholes and the grasses and shrubs on which other species on the sierras and high plains depend. Adding insult to injury, an article in Audubon magazine, that bible for nature enthusiasts, had the gall in 2006 to refer to mustangs as "feral equids".

In reality of course, the mustang controversy is merely a variant of a debate that rages on every continent about species control, be it elephants in Africa, seals in Canada or hedgehogs in Scotland's Western Isles: how many is too many? The difference is that, unlike the elephants, seals and hedgehogs, the mustang cull does not involve killing them.

Instead, they are given veterinary check-ups and sent to government-run boarding centres in Kansas or Oklahoma. The problem is that these facilities are bursting at the seams, holding almost as many animals as roam the wild. As a result, the US taxpayer is now shelling out close to $30m (£18.5m) a year to maintain 30,000 wild horses at pasture.

To get some of the money back, the BLM has for years been running an Adopt-a-Horse programme, whereby members of the public can buy a mustang to take home and look after themselves. For enthusiasts a wild horse, once broken in, has qualities his domestic counterpart lacks. In the words of one, they are "wise, sure-footed and they know the difference between a mountain lion and a piece of debris in the wind".

But as anyone who keeps horses also knows, it's an expensive business – and not surprisingly, rising feed prices and now recession have seen adoptions tumble. A year or two ago, there was even talk of killing some of the captive population, so that numbers on the open range could be further reduced. That idea, predictably, did not get far. For the animal rights lobby, the spectacle of these emblems of freedom being herded into pens by clattering helicopters has been bad enough. And for any right-thinking American, whatever his view of "feral equids", the only thing worse than mustangs being put to death is the thought of slaughterhouses sending the carcasses to foreign markets for horse meat.