Given the demand for a continuous and uniform supply of medicinal plants and the accelerating depletion of forest resources, increasing the number of medicinal plants species in cultivation would appear to be an important strategy for meeting a growing demand (Uniyalet al.2000).
But why are so few species cultivated and why are some species cultivated and so many others not?
One explanation may be found in the observation that cultivated plants are sometimes considered qualitatively inferior when compared with wild gathered specimens. For instance, wild ginseng roots are 5-10 times more valuable than roots produced by artificial propagation. The reason is primarily cultural, as the Chinese community, which is the largest consumer group of wild ginseng, believes that the similarity in appearance of gnarled wild roots to the human body symbolizes the vitality and potency of the root. Cultivated roots lack the characteristic shape of wild roots and are therefore not as highly coveted by consumers (Robbins 1998). In Botswana, traditional medicinal practioners said that cultivated material was unacceptable, as cultivated plants did not have the power of material collected from the wild (Cunningham 1994).
Scientific studies partly support this. Medicinal properties in plants are mainly due to the presence of secondary metabolites which the plants need in their natural environments under particular conditions of stress and competition and which perhaps would not be expressed under mono-culture conditions. Active ingredient levels can be much lower in fast growing cultivated stocks, whereas wild populations can be older due to slow growth rates and can have higher levels of active ingredients. While it can be presumed that cultivated plants are likely to be somewhat different in their properties from those gathered from their natural habitats it is also clear that certain values in plants can be deliberately enhanced under controlled conditions of cultivation (Palevitch 1991; Uniyalet al.2000).
In general, in all countries, the trend is towards a greater proportion of cultivated material. The majority of companies, the mass-market, over-the-counter pharmaceutical companies as well as the larger herb companies, prefer cultivated material, particularly since cultivated material can be certified biodynamic or organic (Laird and Pierce 2002).
From the perspective of the market, domestication and cultivation provide a number of advantages over wild harvest for production of plant-based medicines: (i) While wild collection often offers material adulterated with unwanted, sometimes harmful other plant species, cultivation provides reliable botanical identification. (ii) Wild harvest volumes are dependent on many factors that cannot be controlled and the irregularity of supply is a common feature. Cultivation guarantees a steady source of raw material. (iii) Wholesalers and pharmaceutical companies can agree on volumes and prices over time with the grower. (iv) The selection and development of genotypes with commercially desirable traits from the wild or managed populations may offer opportunities for the economic development of the medicinal plant species as a crop. (v) Cultivation allows controlled post-harvest handling and therefore (vi) quality controls can be assured and (vii) product standards can be adjusted to regulations and consumer preferences. (viii) Cultivated material can be easily certified organic or biodynamic although certifiers are also presently developing wildcrafting standards (Leaman 2002; Palevitch 1991; Pierceet al.2002).
However, domestication of the resource through farming is not always technically possible. Many species are difficult to cultivate because of certain biological features or ecological requirements (slow growth rate, special soil requirements, low germination rates, susceptibility to pests, etc.).
Economical feasibility is the main rationale for a decision to bring a species in cultivation but it also is a substantial limitation as long as sufficient volumes of material can still be obtained at a lower price from wild harvest. Cultivated material will be competing with material harvested from the wild that is supplied onto the market by commercial gatherers who have incurred no input costs for cultivation. Low prices, whether for local use or the international pharmaceutical trade, ensure that few species can be marketed at a high enough price to make cultivation profitable (Cunningham 1994). Domestication of a previously wild collected species does not only require substantial investment of capital (up to 200 000 US$; Plescher in litt.) but also requires several years of investigations (e.g. 12 years for Alchemilla alpina; Schneider et al.1999).
On a time scale of sometimes many decades, the transition from wild harvesting to possible cultivation goes through various phases (Figure 1):
(i) Discovery Phase: At this point the demand can be met by wild harvest. Extractivism is done for local use or for barter with others.
(ii) Expansion Phase: It is clear that the product is potentially useful and that demand is likely to increase. Harvest is done for local or regional sale and eventually for international markets. In general, species with naturally low densities are unlikely to become important sources of commercially large quantities.
(iii) Stabilization Phase: The species is unlikely to be attractive to growers unless prices are high enough and wild-harvested resources are scarce enough. However, desirable species may be grown on farm land and planted around settlements.
(iv) Decline Phase: Prices increase with scarcity due to transport costs, search time and the long-distance trade. Wild populations will have to decline further before cultivation is a viable option. The trade is characterized by fluctuations in supplies, often to the extent of disrupting the trade balance. For slow growing species, if controls on collection are not strictly enforced, wild populations will be more seriously eroded before cultivated material is available (Cunningham 1994; Iqbal 1993).
(v) Cultivation Phase: Now, formal cultivation systems are developed and instituted. The plants are domesticated and incorporated in agroforestry systems sometimes for the benefit of small-scale farmers. If international market opportunities exist, commercial plantations are created with substantial investment and genetic selection, cloning, breeding and biotechnology may be applied. More resilient species may recover in their wild populations (Figure 1).
Figure 1: Transition phases from wild harvesting to cultivation: after wild resources decline with over-harvesting, raw material prices increase and cultivation becomes economically feasible; more resilient species can recover (after Homma 1992 and Cunningham2001).