Weeds In Disturbed Areas May Be Source Of More Medically Important Compounds Than Plants In Tropical Rainforests

The study, by John R. Stepp at UGA and Daniel E. Moerman of the University of Michigan-Dearborn, appears to turn some theories of medicinal flora on their heads.

"I was really amazed by what we found," said Stepp. "The study is based on my field work with the Highland Maya in Chiapas, Mexico. We also used an exhaustive database of over 2,500 medicinal plants used by Native North Americans."

The research was published today in the Journal of Ethnopharmacology. The database used in the study was compiled by Moerman.

The idea that tropical rainforests may hold the key to new medicines that can solve everything from AIDS to cancer has been around for some time. Indeed, one study found that of the 95 plant species now used for prescription drugs, 39 originate in and around tropical forests.

Stepp, however, began to ask a simple question during his doctoral field work in the Mexican state of Chiapas and research with North American tribes: Why would indigenous people walk miles to find medicinal plants if the plants were available on a roadside a few houses down? Working with the Maya in Chiapas, Stepp found that, in fact, nearly all the medically important plants being used grow as weeds in disturbed areas not far from their houses or villages.

"What we found is that people use what they have nearby, except on rare occasions," said Stepp.

While considerable scientific attention has been given to the potential value of medicinal plants deep in rainforests, very little research has been done on the potential medicinal value of common weeds. (Weeds are usually considered as plants that thrive in sunlight and disturbed areas, establish their presence quickly and grow where other plants can’t.)

The Highland Maya–with a population of around 800,000—have inhabited the same region in Mexico for millennia. Earlier research by Brent and Elois Ann Berlin from the University of Georgia established that there is widespread medicinal knowledge for about 600 plant species. Since the Maya don’t cultivate medicinal plants but gather them fresh when needed, Stepp worked in the rugged mountains of Chiapas to discover where the plants are located.

He conducted research in six communities, tracking plant-gathering activities of 208 individuals over a period of seven months through weekly interviews. The Maya used 103 plant species during this period to treat a number of conditions, though more than 80 percent were respiratory or gastrointestinal disorders.

"The Highland Maya of Chiapas rely almost exclusively on disturbed areas for medicinal plants," said Stepp, "even in communities that are adjacent to stands of primary forest."

The scientists analyzed the discoveries by comparing the collected medicinal plants to a known database of some 9,000 plants found in Chiapas. Of these 9,000, 1178 are considered weeds. If weeds were randomly distributed in the medicinal flora studied, there should have been about 13 weed species present. Instead, there were an astonishing 35.

An analysis, done by Daniel Moerman, of medicinal plants used by Native North Americans reveals an equally striking use of weeds as medicinal plants. While 9.6 percent of the plants in North America are considered weeds, 25.8 percent of the taxa used by Native Americans for medicinal purposes are weeds.

There are a number of reasons why the Maya use as medicinal plants weeds found in nearby areas. First, there is some evidence that the plants lose their effectiveness over time and must be used when freshly picked. That makes long treks into the rainforest illogical. Second, the Maya have lived around disturbed areas for centuries, and so they have found plants that are effective right where they live.

There is also good biochemical evidence that supports the hypothesis that plants in disturbed areas are likely to have more chemicals in them for defense, said Stepp.

The possible placebo effects from the medicinal plants is unclear, but Stepp said that 30 percent of the effects of all medicines, including pharmaceuticals, have been found to be placebo effects in some studies. What is clear is that most of the plants used by the Maya do have biologically active compounds that have been shown by generations to be effective against specific disorders. "The rainforests should be conserved for many intrinsic and economic reasons," said Stepp. "But the reason that we should save them primarily to use the medicinal plants found there is not a very good one."

The idea of medicinal plants and self-treatment is somewhat alien to many Western cultures where going to the doctor and being given a prescription is the standard. The vast majority of people in the world, however, take care of themselves and use healing plants that have been used for hundreds of generations.

The discovery that easy-to-find weeds may be more important as medicinal plants than exotic species "hidden" deep in the rainforests may catch some people by surprise, Stepp said. In a class he taught at the University of Georgia, Stepp took students to the side of a nearby railroad track and collected all the plants in the area. Of the 60species collected, an astonishing 38 are already listed as having medicinal properties.

"We think the evidence supports a closer examination of the relationship between weeds and medicinal floras," said Stepp. "We may find that the next plant-derived pharmaceutical is in the abandoned lot down the street."