Cultivation of medicinal plants is widely viewed not only as a means for meeting current and future demands for large volume production of plant-based drugs and herbal remedies, but also as a means for relieving harvest pressure on wild populations (FAO 1995; Lambert et al. 1997; Palevitch 1991; de Silva 1997; WHO, IUCN and WWF 1993). In this chapter we want to assess the benefits and risks associated with such recommendations.
Booming markets with rapidly rising demands often have devastating effects on wild collected species. A closer look reveals that not all species are affected in the same way by harvesting pressures. The seven forms of rarity described by Rabinovitz (1981) make clear that a species which (i) has a narrow geographic distribution, (ii) is habitat specific, and (iii) has small population sizes everywhere, is more easily over-harvested than species of any other pattern (Table 4).
Table 4: Seven forms of rarity (after Rabinowitz 1981) | |||
Geographic distribution |
|||
Habitat specificity |
|||
Local population size |
|||
wide |
broad |
somewhere large |
least concern |
everywhere small |
|||
restricted |
somewhere large |
||
everywhere small |
|||
narrow |
broad |
somewhere large |
|
everywhere small |
|||
restricted |
somewhere large |
||
everywhere small |
highly susceptible |
Secondly, the susceptibility or resilience to collection pressure varies among species owing to biological characters such as different growth rates (slow growing vs. fast growing), reproductive systems (vegetative or generative propagation; germination rates; dormance; apomixis) and life forms (annual; perennial; tree).
Species can be distinguished quite well in their susceptibility to over-collection if their life form and the plant parts collected are viewed together (Table 5). Harvesting fruits from a long-lived tree presents a far lower threat to the long-term survival of the species than does collecting seeds from an annual plant. In the latter case, if the seed is gone the plant is gone. In some cases the harvest impacts are more complex, e.g. with slow growing trees which reproduce from seed but only produce few, large fruits (example: Araucaria araucana, monkey puzzle tree). This will increase their susceptibility to over-harvest from low to medium or even high. A thorough summary of predictors of resilience or vulnerability to harvesting wild populations is presented by Cunningham (2001).
In summary we can state that species most susceptible to over-harvest are habitat specific, slow growing and destructively harvested for their bark, roots or the whole plant. These species suffer most from harvesting and many of them have been seriously depleted, for example Prunus africana in West Africa, Warburgia salutaris in southern Africa and Saussurea costus in the Himalayas.
For threatened medicinal plant species cultivation is a conservation option because the constant drain of material from their populations is much higher than the annual sustained yield. If the demand for these species can be met from cultivated sources the pressure on the wild populations will be relieved. In these cases, the need for strict conservation of remaining populations, improved security of germplasm ex-situ and investment in selection and improvement programmes is extremely urgent as the example of Jaborandi (Pilocarpus jaborandi) in Brazil shows (Pinheiro 1997).
However, among the species that can be marketed at a high enough price to make cultivation profitable, only few are in the highest threat categories. Examples for threatened but cultivated species are Garcinia afzelii, Panax quinquefolius, Saussurea costus and Warburgia salutaris (Cunningham 1994). With respect to economic viability many highly endangered MAP do not qualify for cultivation. This group of plants will enter cultivation only with the help of public domestication programmes.
Table 5: Susceptibility of species to overcollection as a function of Life Form and Plant Parts Used | ||||||
Wood |
Bark |
Root |
Leave |
Flower |
Fruit / Seed | |
Annual |
--- |
--- |
high |
medium |
medium |
high |
Biannual |
--- |
--- |
high |
medium |
medium |
high |
Perennial |
--- |
medium |
high |
low |
low |
low |
Shrub |
medium |
medium ? |
medium ? |
low |
low |
low |
Tree |
medium |
medium ? |
medium ? |
low |
low |
low |
For all other harvested MAP species the priority conservation option is sustainable harvest from wild populations, for a variety of reasons.
Let's imagine that a valuable medicinal plant is exploited by local collectors. A pharmaceutical company has domesticated and begun to cultivate the plant on a commercial scale. When the company no longer needs the wild-harvested material, local harvesters have to abandon the harvest and any incentive the local collectors might have had to protect the wild populations is gone. The domestication of MAP species has an environmental implication in the sense that it reduces the economic incentives for forest dependent people to conserve the ecosystems in which the MAP species occur (Leaman et al. 1997; Vantomme in Anon. 2002a).
If collectors and collecting communities can be involved in the development of propagation and management methods, the likelihood of their having an interest in protecting the wild populations from over-exploitation, particularly if these are understood to be the genetic resource "bank" for the domestic enterprises, will be greater.
Another aspect to consider is the genetic diversity of the species which is in demand. Long before non-sustainable harvest practices lead to extermination of a whole species, selection of favoured growth forms and concentration on certain harvesting areas which may hold certain ecotypes will lead to a degradation of genetic diversity of the wild populations. The same is true under domestication: Industry requirements for standardization encourage a narrow genetic range of material in cultivation. Domestication will not achieve conservation of genetic diversity because a narrow group of high yielding individuals will be selected for planting.
As a summary of chapters 3-5, table 7 in Appendix 1 indicates the advantages and disadvantages for the three aspects distinguished: "species/ecosytems", "market" and "people".