ASSATEAGUE ISLAND - It's a bright, dazzlingly blue afternoon on this arid claw of land stretching beside the Atlantic Ocean, and Allison Turner has been slogging for hours through salt marshes and loblolly pines, a rifle slung over her shoulder.
Suddenly, she aims at a pony with a shaggy reddish coat munching grass near a dune. A pop sounds as she fires a 4-inch orange dart that pierces the horse's haunch.
For the past decade, the National Park Service has been injecting contraceptives into the mares among this herd of 160 wild horses on the Assateague Island National Seashore in an attempt to control the population. But now, officials have concluded they might be forced to act more aggressively to reduce the herd by about one-third.
Although the ponies are adored by millions of young visitors, they're an invasive species that is destroying the grass and dunes on this fragile barrier island.
"The horses are hurting the ecosystem," said Carl Zimmerman, a resource management specialist at the park. "The plants on the island haven't evolved for large grazing animals, and the damage is pretty apparent. We can't wait a long period of time to deal with this problem."
As a result, officials might try to reduce the population on the Maryland side of the island by 35 to 60 ponies, bringing the total to 100 to 125, Zimmerman said.
Killing horses to thin the herd is not being considered, he said. But after seeking public input this spring, the Park Service might sell or donate excess ponies after a careful screening to make sure their new homes are safe.
"These Assateague horses have a special place in the nation's heart," Zimmerman said. "We are not going to be sending any horses to the slaughterhouse."
Park officials know they're dealing with a sensitive subject, because the wild horses are rare and have millions of passionate young supporters, many of whom have read Marguerite Henry's book Misty of Chincoteague.
Over the past three centuries, the ponies have adapted to this 37-mile-long strip of sand and scraggly pines, developing short legs and barrel chests. They have a mythical past as the descendants of survivors from a Spanish shipwreck, although park officials say their origins are more likely in horses hidden on the island by nearby landowners seeking to avoid livestock taxes.
Any sale would be a first for the horses on the Maryland side of the island, although the more domesticated ponies on the Virginia side have their numbers reduced every July by the famous "Pony Penning" auction in Chincoteague.
The Virginia horses are separated from the Maryland herd by a barbed wire fence that cuts across the island. While the northern ponies are owned by the National Park Service, the southern horses are owned by the Chincoteague Volunteer Fire Department, which follows a different management philosophy, Zimmerman said.
The Virginia horses are rounded up several times a year, given veterinary care (but not contraception), fed, branded, bred with horses imported from California and elsewhere, and kept away from tourists, dunes and sensitive environmental areas by fences and moats.
By contrast, the Maryland horses are allowed to roam as wild animals. But their freedom has caused problems: They sometimes steal treats from picnickers, rip open the tents of campers and bite folks who mistake them for "Misty" and try to pet them, park officials say.
More importantly, the lack of fences protecting the dunes and marshes on the Maryland side has meant that the ponies have ripped up grasses, wetlands and the habitats of other animals, said Jack Kumer, natural resource specialist for the Park Service.
The ponies' overgrazing is contributing to severe erosion. The lack of grass prevents dunes from forming to replace sand lost to wind, storms and rising sea levels caused by global warming, Kumer said.
The strain on the ecosystem is compounded by the grazing of 350 miniature Japanese elk, also known as sika deer. They have multiplied since the Boy Scouts introduced them to the island as part of an ill-conceived project in the 1920s, Kumer said.
"The ecosystem here is way out of whack, not in balance, primarily because of these two exotic grazers," Kumer said. "Instead of a healthy, tall salt marsh, we have a short, unkempt lawn and packed mud. And that hurts our nursery for fiddler crabs, marsh birds, rails, shellfish, fish, snails and many other creatures."
As Turner prowled the island with her contraceptive darts on a recent afternoon, sometimes on foot and other times in a truck, she described how the Maryland herd's population ballooned from 28 in 1968 to 175 in 2001.
She said the birth control program started in 1994. It uses a vaccine gleaned from pig eggs that induces an immune response in female horses that is about 95 percent effective in preventing fertilization.