Mustangs in danger? Too many horses, too little land. (Population Ecology).

FORTY-FIVE THOUSAND untamed horses roam vast Western rangeland. These wild mustangs--descended from horses of explorers, Native Americans, and pioneers--are revered symbols of the Wild West. But mustangs don't roam the range alone. They share the U.S. government-owned land with ranch animals like cattle and sheep, and wild deer and elk. "You only have so much vegetation and water, and all the species need to share," says Maxine Shane of the U.S. Bureau of Land Management (BLM). The agency balances the needs of human ranching and recreation versus animal wildlife on public land. Of 260 million such Western acres, about 35 million are home to free-roaming wild horses. With few natural predators, horse herds easily swell to numbers the land can't sustain.

"You've got to make sure mustangs don't overgraze, or eat the plants down so much they can't regrow," Shane says. Population control is a must: BLM officials in helicopters swoop down and thunder behind bands of fleeing mustangs, herding them into temporary corrals. After roundup, surplus horses enter an owner-adoption program--a key to BLM's plan to reduce the mustang herd to a habitat-friendly 27,000 by 2005. "We feel the humane thing to do is remove excess horses and let them be adopted rather than leave them out there to starve to death."

Why thin mustang herds when around 4 million cows can graze on federal land? Opinions vary: "The political answer is ranchers don't want wild horses eating the same grass they want to give cows and sheep," says Jay Kirkpatrick, director of science and conservation biology at Zoo Montana in Billings. But ranchers pay the U.S. government for the right to graze cattle on public land, and "most beef producers don't have problems with wild horses on ranch land," says Jason Campbell, spokesperson for the National Cattlemen's Beef Association. "We think there's room for both. But if you've got areas with too many grazing animals, the ecosystem suffers."

Many ranchers and animal-welfare advocates agree that the adopt-a-horse program, which has placed at least 185,000 horses, is not a cure-all. "The real problem is too much reproduction," Kirkpatrick says. Wild horses reproduce at a rate of about 20 percent a year their population can double every three to four years. "Removing horses speeds up reproduction in the horses you leave behind," Kirkpatrick says. It's a natural phenomenon called compensatory reproduction--remaining animals increase reproduction in response to increased food and habitat availability.

Experts like Kirkpatrick control reproduction in some herds with vaccines that cause female horses to make antibodies (blood proteins that prevent infection) that stop sperm from fertilizing eggs. But wild horse roundups will continue, like it or not: "If you're out West and you see wild horses running through, it's very cool," Campbell says. "But it's not cool when you see a 50-horse herd that should have 13 animals, and the horses are nothing but skin and ribs and bones. It all goes back to how much range is out there for them."

WHAT DO YOU THINK?

Should wild horses be managed or left alone? To help you decide, check out www.wildhorseandburro.blm.gov

Did You Know?

* Wild horses evolved on the North American continent over 60 million years, but around 10,000 years ago they disappeared. The species was reintroduced in 1519, when Spanish explorers landed in Mexico, bringing horses with them by boat.

* Wild horses nurse their young for up to four years. This prolonged lactation slows mares' reproduction rate because lactating horses generally do not ovulate.

* The wild horse population on Assateague Island, off Maryland, is controlled entirely with immunocontraception. Females receive yearly injections of a birth-control vaccine made from pzp, a chemical found in pig eggs.

Cross-Curricular Connection

History: Research Wild Horse Annie and the events leading up to the passage of the 1971 Wild Free-Roaming Horses and Burros Act.

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National Science Education Standards 

Grades 5-8: structure and function in living systems * populations and ecosystems * population, resources, and environments

Grades 9-12: interdependence of organisms * population growth * natural resources * science and technology in local, national, and global challenges

Resources

"A Roundup of Wild Horses Stirs up a Fight in the West," by Evelyn Nieves, The New York Times, Feb. 25, 2002. Into the Wind: Wild Horses of North America by Jay Kirkpatrick, Northword Press, 1994 Visit rancher Dayton O. Hyde's Black Hills Wild Horse Sanctuary on-line at www.wildmustangs.com

CHECK FOR UNDERSTANDING

Directions: Answer the following questions in complete sentences.

1. What are mustangs and where do they live?

2. Give two reasons why measures are being taken to control mustang populations.

3. Name two herd-reduction methods.

ANSWERS

1. Mustangs are wild horses descended from the horses of explorers, Native Americans, and pioneers. They roam freely on government-owned Western rangeland.

2. (1) With few natural predators, horse herds swell at a rate of about 20 percent a year, doubling every three to four years. The land can't sustain such numbers. (2) Mustangs also share the land with ranch animals like cattle and sheep, and wild deer and elk. And overgrazing plants down to the ground makes them hard to regrow. Food supply dwindles.

3. (1) To control mustang population, helicopters swoop down and thunder behind bands of fleeing mustangs, herding them into temporary corrals. After roundup, surplus horses enter an owner-adoption program. (2) Experts are also trying to control mustang reproduction via vaccines that cause female horses to make antibodies (blood proteins that prevent infection) that stop sperm from fertilizing eggs.