GLOBAL DIMENSIONS AND CAUSES OF BIODIVERSITY LOSS

The work of cataloguing the world’s flora is still far from complete. So far, scientists have named and described more than 250,000 species of “higher” plants—a group that includes flowering plants, conifers, ferns, and horsetails. Yet many species remain unknown to science, particularly those in the richly diverse tropical regions. We can only make an educated guess, but recent estimates of the number of flowering plant species that exist worldwide range from 270,000 to 422,000 (Bramwell, 2002; Govaerts, 2001).

Loss of medicinal plant biodiversity reflects the general decline in plant diversity that is occurring globally. The current rate of plant and animal extinction is estimated to be 100 to 1000 times greater than previous average levels (Tuxill, 1999). Many researchers of biodiversity agree that we are entering a phase of mass extinction that has been unprecedented since the meteorite that struck off the coast of the Yucatan ended the age of the dinosaurs 65 million years ago (Wilson, 2002). In North America, the Heritage Program considers approximately one third of the continent’s flowering plant species to be at risk (Stein, 2000).

Human cultures in every region of the world have identified and used native species of plants for their medicinal value. The diversity of these regional pharmacopoeias is remarkable. Estimates for the number of species used medicinally worldwide include 35,000 to 70,000 (Farnsworth, 1991) and 53,000 (Schippmann, 2002). The great majority of medicinal plant species are used only in traditional or folk medicine. Schippmann and colleagues (2002) estimate that the total number of medicinal and aromatic plant species in international trade is about 2500. Only a fraction of these—probably no more than a few hundred—are in formal cultivation for commercial purposes.

No reliable estimate is available for the number of medicinal plants that are globally threatened, but the number has been variously calculated as 4160 or 10,000 (respectively, Schippmann, 2002, and Vorhies, 2000). Medicinal plants known to be globally extinct are very few (Hamilton, 2004).

The principal causes of the general decrease in plant diversity are well known. They include habitat destruction, competition from alien species, mortality from introduced diseases, pollution, and overexploitation. Natural populations of medicinal plants are subject to all of these pressures, but the fact that they are selectively targeted and collected poses special problems for those working to conserve them. For example, one strategy that is frequently included in species recovery plans for federally protected plants in the United States is the carrying out of “reintroductions.” This involves propagating plants off-site, then planting them out to establish new populations of a species in areas of suitable habitat within its historical range. This approach can work well for little-known species that are not likely to attract much attention from the public. However, in cases of native medicinal species with commercial value (e.g., ginseng, goldenseal), the creation of artificial populations amounts to little more than stocking the shelves for future wild-harvesters.

An example of this dilemma was witnessed during a visit to Karoo National Botanical Garden in South Africa in 1996. The Karoo Garden is located 120 km from Cape Town near Worcester, at the foot of the Brandwacht Mountains. It concentrates on plants from the semidesert areas of South Africa, which are some of the most unusual succulent species in the world. Many of these species overcome dry conditions by storing water in their leaves and stems, or in underground roots and bulbs. Some species are quite large, such as the colorful tree aloes; others, such as tiny stone plants, are nearly indistinguishable from the gravelly soils in which they grow. In addition to horticultural collections, the garden includes a 144-hectare nature reserve. When one of the authors (JA) visited the Karoo Garden, a staff member pointed to a row of large, coarsely textured bulbs that were lying on the ground. These were the underground storage structures of the Candelabra Lily—Brunsvigia josephinae (Amaryllidaceae)—which is a striking plant with the biggest bulb (8-inch diameter) and the greatest inflorescence of any of the South African geophytes. These plants had been taken from poachers who were discovered in the nearby nature reserve. A total of 49 plants were recovered. The papery tunic of the bulbs is used medicinally as a dressing. Among some native South Africans, a young man’s passage to manhood is marked by a circumcision ritual. (In his autobiography, Nelson Mandela [1994] gives an account of his rite of passage ceremony.) The traditional bandage that is applied following circumcision is the cottony tissue of the Candelabra Lily, so the plant has great ritual significance and value. Staff members at the Karoo Garden were hesitant to return the bulbs they had seized to their original location in the reserve because they would surely be stolen again. They were looking for a safer place within the garden boundaries to reestablish them. This dilemma is encountered whenever reintroduced species are valuable and easily recognizable.

Although many medicinal plant species are under stress, others have become more common and more widely distributed, thanks to human activities. Some medicinal species have ecologic traits that preadapt them for easy dispersal and survival in disturbed habitats. Many, such as dandelion (Taraxacum officinale), various plantains (Plantago spp), Saint John’s Wort (Hypericum perforatum), and Artemesia annua—the source of the antimalarial drug artemesinin—have become cosmopolitan weeds. Recent analyses of herbal use in Mexico and North America indicate that plants with weedy traits are overrepresented in these traditional pharmacopoeias (Stepp, 2001). The distribution and abundance of medicinal plants reveal the same two trends that currently dominate the plant kingdom as a whole: (1) a mixing or homogenization of the global flora, as aggressive species become transported and established around the globe, and (2) a reduction in biodiversity and abundance of many species, as habitats are destroyed and exploitation pressures increase.

PROBLEMS ASSOCIATED WITH BIODIVERSITY LOSS AND OVEREXPLOITATION

From a commercial standpoint, the most obvious consequence of the disappearance of medicinal plant populations is a dwindling supply of wild-harvested plants, but this is only the tip of the iceberg. Unwise patterns of harvest and usage have led to numerous biological and environmental problems; have negatively affected product quality and availability; and have increased socioeconomic tensions within local communities that are involved in the collection and use of native species of medicinal plants.

The harvest pressure on wild populations of medicinal plants appears to be increasing (Hamilton, 2004; Sheldon, 1997), and many species have suffered reductions in the number and size of populations, as well as in overall geographic range. This trend has had serious biological and evolutionary consequences for individual species. In small populations, genetic factors such as changing demographic structure, inbreeding, genetic isolation, and genetic drift have led to smaller effective population sizes (Ellstrand, 1993), which, in turn, can increase the risks of local extirpation and extinction. Genetic diversity is often directly related to population size (Frankham, 2002), and reduced diversity can reduce individual fitness and decrease the likelihood of population persistence. Small populations tend to become increasingly unstable.

Threats to medicinal plant populations may affect ecologically related species. Removal of a plant species can affect the pollinators, herbivores, soil fauna, microorganisms, and other plants and animals that depend on that species and the physical environment it creates. Saw palmetto (Serenoa repens) is not a threatened species, but the manner in which it is used by wildlife illustrates the important role that a medicinal plant species can play within an ecosystem. Until recently, much of the scientific literature on saw palmetto focused on methods of eradicating the plant (Carrington, 2000). The fruits of this scrubby palm, which is native to the coastal plain of the southeastern United States, are valued in the treatment of benign prostatic hyperplasia. The plant grows densely in open fields and pine understories, and many landowners consider it a rangeland weed that transforms good pasture into an obstacle course for cattle. The fruits of saw palmetto have been likened in taste to “rotten cheese steeped in tobacco,” but they were an important source of starch for the Seminole Indians, and they are consumed by many species of wildlife, including raccoons, foxes, gopher tortoises, whitetail deer, fish, and waterfowl. They are also a major food source for the state-threatened Florida black bear (Ursus americanus subsp floridanus), and they provide cover for this species and for the endangered Florida panther (Bennett, 1998). Black bears often give birth under the cover provided by saw palmetto thickets. Although saw palmettos are common, the Florida black bear has experienced a significant population reduction. It prefers areas with dense understory vegetation, including pine flatwoods and other areas dominated by saw palmetto. Researchers in Florida are concerned that indiscriminate harvesting of saw palmetto fruits could adversely affect the black bear and other wildlife species, especially in low–fruit production years, when prices for saw palmetto berries may be high. Bears have reportedly shifted their ranges in search of fruits during the fall. Researchers plan to identify the extent of fruit harvesting in areas used by black bears to determine whether management practices should take the harvesting impact into account (University of Florida Web site: http://wfrec.ifas.ufl.edu/range/sawpalm/research.html).

Uncontrolled harvest of wild plants can also lead to erosion and other physical damage to the environment. This pattern was observed in an arid region of Argentina, where wild-harvest of medicinal plants is an important component of the local economy. Córdoba, a province in north central Argentina, is well known for its diversity of native herb species, many of which are collected from the wild for commercial use in herbal teas and medicines. In the mountainous western districts of the province, known as the Sierras de Córdoba, as many as 80% of families collect herbs as a primary source of income. Natural populations of many commercially harvested species in the region are declining in geographic distribution and abundance. This is a result of both indiscriminate collection and widespread habitat destruction. The arrival of railroads in the region during the 1930s resulted in large-scale deforestation. Mountain valleys that once possessed extensive hardwood forests now contain only small residual stands. Removal of the forest cover and burning of vegetation to control weeds and insects have hastened erosion of soils by wind and summer rains. Plant collectors in this region often harvest woody species by ripping up entire plants by the roots. Many species grow on the slopes of mountains and hillsides, and the erosional gullies that result are a frequent sight in such habitats (Lagrotteria, 1999).

Loss of medicinal plant species affects practitioners and consumers. Adulteration becomes more likely as wild-harvested plants become scarce. Substitution with different (and sometimes harmful) species may occur, but more often, less effective plant parts may be used (e.g., stems instead of leaves, leaves instead of roots). In addition, increasing scarcity makes desirable species less available, which drives prices up. Finally, as a species becomes rare, various chemotypes are lost, thus reducing the genetic variation and hence the potential range of phytochemicals that provide medicinal activity.

Loss of medicinal plant diversity has socioeconomic implications as well. Ethnobotanists have noted that as desired plant species disappear, local people have difficulty finding plants to satisfy their own health needs (Balick, 1996; Tuxill, 1999). The livelihoods of local wildcrafters may be threatened, and poaching and trespassing may increase (Hamilton, 2004). The negative impact of overcollection is often confined to the local level until such times as regional or global scarcity becomes critical. When the need to initiate horticultural production is finally recognized, tension may develop between local wildcrafters and “outsiders” who appear on the scene to encourage commercial production of plants.