DZATO, CHINA—It's a sight to behold on mornings in May and June: Hardy nomads and enterprising villagers from Nepal to western China spread out over the Tibetan Plateau and the Himalayas, pickaxes in tow, and ascend unforgiving peaks in the thin mountain air. Anyone who can make the journey goes—children, yak herders, pregnant women. By midday, the plateau is dotted with crouched forms combing the grass on their hands and knees.

The trekkers are searching for caterpillar fungus (Cordyceps sinensis), an elusive fungus that grows on the caterpillar of the Thitarodes ghost moth, which lives at altitudes over 10,000 feet (3,000 meters). A prized medicine believed to boost immunity and increase stamina, caterpillar fungus is a popular cure for everything from cancer to erectile dysfunction among Han Chinese in the nation's east. (It had been for the Tibetans, too, but now they can't afford it.) Collected and traded east as early as the Tang dynasty (A.D. 618 to 906), the medicine has long been central to the Tibetan economy. But rising incomes in eastern China are now pushing demand for the fungus, sparking a frenzied collection that exacerbates environmental degradation, sometimes erupts into violence, and threatens the fungus's very existence.

As China's economy booms, per capita disposable incomes are rising by as much as 10 percent yearly in the affluent east. Many middle class Chinese are spending some of their extra cash on traditional medicine. Tonics such as caterpillar fungus, believed to ensure good health when taken in soup or steeped in hot water, are in particular demand. Last year, prices for caterpillar fungus doubled in just months. By December, 1.8 ounces (50 grams, or roughly a handful) of prime caterpillar fungus retailed for 25,000 yuan ($3,376). That same month, Chinese headlines exclaimed that 2.2 pounds (one kilogram), enough to last a few months, cost more than a small Mercedes—500,000 yuan ($67,522).

High demand now appears to be endangering supply. "I started picking when I was as young as this boy," said 48-year-old nomad Buchang from his yak hair tent in this mountain encampment last June, gesturing to his small grandson. "Then, every person could pick 200 pieces per day. Now we find 15." Indeed, in January a report on medicinal plants from London-based conservation organization Botanic Gardens Conservation International highlighted Cordyceps sinensis as a threatened species.

The application of Western science to Chinese medicine—an ambitious, 15-year project the Chinese government terms "modernization"—hasn't helped. Since the 1990s, Chinese scientists have led dozens of studies on caterpillar fungus, documenting its effects on hyperglycemia, the respiratory system and testosterone levels. Such studies are accompanied by high-profile testimonials. When two women shattered world records in three running events at China's 1993 National Games in Beijing, for example, track coach Ma Junren attributed their success to a diet of caterpillar fungus. (That was likely a bluff; in the 2000 Olympics China dismissed four of Ma's athletes ahead of the games for testing positive for the hormone erythropoietin, which increases the blood's oxygen-carrying capacity, hence an athlete's endurance.)

The fungus is one of hundreds of Chinese medicinal species threatened by unsustainable collection. At a 2004 seminar, Chen Shilin, deputy director of China's Institute of Medicinal Plant Development, said that up to 70 percent of the country's 3,000 threatened plants are used in traditional medicine, endangering their survival. Among others at risk: wild ginseng, orchids and the crocodile newt. But the story of this particular medicine is instructive.

Caterpillar fungus forms during the winter, when fungal spores attach to a caterpillar and begin to consume its body. As it dies, the caterpillar burrows underground and the fruiting fungus overtakes it, sprouting a small "tail" that breaks through the soil, like a thick stalk of grass. By late May, the tail disperses spores that ensure a crop of fungus for next season. (This process is contained in the fungus's Tibetan name, yartsa gunbu, which means "summer grass winter worm".)

To unearth the fungus, a picker brings a pickax down just adjacent of the tail, plucking the L-shaped specimen from the upturned clump of dirt. For hundreds of years, interviewed nomads said, locals collected in the areas around their tents, selecting only mature fungus that had dispersed spores. These days, Tibetan townspeople, Muslim Hui traders and Han migrants flock to the plateau during fungus season, lured by the prospect of profit. That increased competition compels pickers to take to the hills in the spring, shortly after the fungus sprouts through the soil.

Early harvesting is a serious problem, says Chinese ecologist Yang Darong, who has studied caterpillar fungus for decades from Xishuangbanna Tropical Botanical Garden in southwestern China. Surveying select areas near his base last year, he found that species totals in western China had fallen to between 3.5 and 10 percent of their totals of 25 years ago. "The fungus doesn't get the opportunity to disperse spores," he says. That diminishes the species's chance for regeneration.

Indian zoologists Chandra Negi, Prithvi Raj Koranga, and Hira Singh Ghinga recently conducted a similar study in parts of Himalayas, documenting a 30 to 50 percent decline of the fungus over a two-year period. In findings published in the International Journal of Sustainable Development and World Ecology in 2006, they agreed with Yang's assessment. The zoologists found some villagers starting the harvest as early as April—a change, they wrote, that had "grave repercussions."

With the stakes so high, fungus collection frequently turns to a rush that pits nomads against their neighbors. Last July in Dabba County, an ethnically Tibetan part of Sichuan province, a dispute over picking rights sparked an armed clash between neighboring encampments that left six people dead and 110 injured. In 2005 tension between locals and outsiders here in Dzato prompted a two-week orgy of looting and fighting in which at least one man died.

The rush also exacerbates erosion on the stressed plateau, Yang says. Many profiteers no longer replace the soil they upturn with the fungus, leaving clumps of dirt dotting the hills. Impromptu picking camps spring up and disband in a matter of weeks, leaving garbage pits and barren land in their wake. Irresponsible collection "is destroying the habitat," agrees Xu Hongfa, China director for TRAFFIC International, a Cambridge, England–based wildlife trade monitoring organization overseen by the World Wildlife Fund and the International Union for Conservation of Nature.

Ecologists say the only way to raise species counts is to restrict access to sensitive and overpicked areas. Local governments have established checkpoints restricting access to the highlands, and the national Chinese government has set up over 50 cordyceps research centers throughout the plateau. But Xu says so far regulation has had little effect. "It's hard to reach the area," he says, "so enforcement is very difficult."

Toward the end of fungus season, the pickers in Dzato sell their yield to traders—last year's going price was 20 to 50 yuan per fungus ($2.70 to $6.75)—who broker it east, doubling or tripling the price along the way. In pharmacies in Shanghai, Beijing and other wealthy cities along China's coast, the shriveled caterpillar shells are prominently displayed in glass cases, laid out on red velvet. Now a status symbol, the fungus is also incorporated into products that might cancel out prospective health benefits, like cigarettes and hard alcohol.

Will enthusiastic consumption by the growing number of middle class Chinese spell the medicine's demise? After surveying part of the plateau last year, Yang says the species' prospects are grim. He hasn't, however, given up hope for its survival: "There are some places that people can't get to. There is still caterpillar fungus there."