A self-described "old-timer," Skip Wissinger has spent 32 years traversing the park's 300 square miles and identifying its natural treasures. But now many of the park's most prized resources -- its American ginseng plants, black bears and unusual butterflies -- are disappearing.

Looking over a vast hollow filled with wildflowers and vibrant grasses last month, Wissinger tried in vain to spot the small ginseng plants that used to grow there in profusion but are now being stolen to make tea, health supplements and aphrodisiacs.

"When I look down in that hollow and see no viable ginseng population, to me that is a very serious matter," said Wissinger, a National Park Service special agent. "In my view they are an integral part in the portfolio of the nation's natural resources."

That portfolio is now showing heavy losses. While the National Park Service does not keep comprehensive statistics on how much poaching occurs in its nearly 400 parks, its 2006 budget request reported that thefts have helped spur the decline of at least 29 wildlife species. "The poaching of wildlife from national parks has been steadily increasing each year for the past several years," the document said.

Some of these resources are scarce to begin with, and the toll that poaching takes on the national parks is rising.

"If there's something with a dollar amount attached to it in a park, somebody is trying to make a profit off it," said Dennis Barnett, law enforcement administrator for the Park Service.

Because national parks enjoy special protections -- hunting is prohibited, and activities such as mining and logging are restricted -- they are home to plants and animals that are scarce elsewhere. That has attracted thieves who often go to extraordinary lengths to spirit away flora and fauna and then sell to the highest bidder.

In Shenandoah National Park, the ginseng and the black bears that thrive along the edge of the Blue Ridge Mountains are the biggest draw for poachers. Wild ginseng sells for $400 a pound on the open market, 10 times the price of cultivated ginseng. And a black bear's dried gallbladder sells for $1,000 in Asia, making it worth more per ounce than cocaine.

Living things are not the only targets. Potsherds from national parks end up being sold on the Internet, sometimes by the pound. On occasion, poachers have transported their ill-gotten goods by water to evade authorities by cutting down ancient trees in California's Redwood National Park and floating them downstream.

Park visitors also have taken more obscure items with less commercial value, such as owl pellets, the fur-coated bones that owls regurgitate after eating small prey.

"Who would even think to look for owl pellets?" asked special agent Todd Swain, who works in more than half a dozen national parks in California. "All of it goes back to people being pack rats by nature. There aren't any resources, other than air, I've come across that people don't collect."

Park officials are most concerned, however, that poaching is depleting nonrenewable resources, such as remnants of ancient civilizations and very rare species. Thieves take at least one archaeological artifact from a park every day, they say, and a 1988 federal survey found poachers had taken 105 wildlife species from 153 parks the year before. That illegal take included 12 threatened and endangered species, including the desert tortoise, Steller sea lion and Schaus swallowtail butterfly.

"The national parks are the best sanctuaries for these plants and animals," said Peter Dratch, who runs the Park Service's endangered species program. "That's why we get concerned when these genetic resources get hammered."

These estimates may be low, officials add, because they lack the resources to properly track what is missing. When agents at Joshua Tree National Park in California got special funding to monitor thefts in the mid-1990s, for example, the reported number of stolen archaeological artifacts mushroomed.

Park officials said the biggest problem they face is a lack of money for enforcement. The Interior Department has only 51 special agents for 388 national parks, which means that each agent is responsible for patrolling more than 1.5 million acres.

"It means a lot of stuff is going to go on that they don't detect," Swain said. "They have to be in the right place at the right time."

Just this month, the FBI began investigating the theft of a ceremonial tomahawk taken from a display case during visiting hours at the Whitman Mission National Historic Site in Washington state. Experts believe the tomahawk was used to kill Marcus Whitman, a 19th-century missionary who helped settle the Northwest.

Barnett said the government might want to reassess its funding for parks law enforcement, given the threats parks face. "Staffing would be an issue we need to look at," he said.

While the government has provided more money for enforcement in recent years, officials could not say whether it was aimed at poaching or other activities. Blake Selzer, legislative representative at the National Parks Conservation Association, said the lack of enforcement "is yet another consequence of the funding shortfall facing our national parks."

Rangers and special agents have become more creative, however, devising new tools to identify stolen federal property. In Shenandoah, for example, officials are coating ginseng with an orange dye that glows under black light, essentially creating a bar code that can prove that roots that end up in a marketplace originated in the park.

Out West, park officials have begun embedding devices in cactuses that mark them as federal plants, and they have set up remote cameras and sensors to monitor areas where poachers may lurk.

State and federal sting operations have nabbed dozens of park thieves in recent years. Operation VIPER, a joint effort of the Park Service and Virginia's Department of Game and Inland Fisheries, entailed setting up a fake storefront in Elkton, Va., where officers sold black-market goods to customers. Prosecutors ultimately indicted dozens of defendants on more than 100 charges. The government has already obtained 31 convictions and collected $150,000 in fines and restitution for those crimes.

"You have to be especially sneaky to get into position to see what they're doing, but it's possible," Swain said.