written by Phil Rasmussen

Introduction

Ensuring the sustainability of medicinal plants and thus their availability to humans to treat illness and disease in the future, is imperative. With the global market for herbs and botanicals continuing to grow, we need to understand where and how they are sourced, before ending up as an ingredient in a ‘dietary supplement’, natural health product or practitioner-prescribed formulation, waiting for us to ingest or apply them.

As most medicinal herbs are currently collected from their natural habitat in the wild (‘wildcrafted’) rather than from cultivated sources, understanding more about what is happening in these natural habitats, and how the health of each species is tracking, is important.  Apart from harvesting practices used for personal use or the medicinal plant trade, factors such as urban sprawl or conversion of natural landscapes to agriculture or forestry, and climate change, can impact on the abundance of plant species.

While various studies have considered each of these factors in isolation in relation to particular species, few have taken a widespread lens and attempted to quantify the contribution of each, in trying to understand and project the impact of humans on the health and population levels of medicinal plants over an extended period of time. 

Natal lily

Clivia miniata, known as Natal lily or bush lily, is a popular plant in its native South Africa and Swaziland, being widely used as a medicine especially by indigenous communities. Clivia is also a highly sought after ornamental plant with attractive and long-lasting flowers, and there are many different cultivars now available through nurseries in many countries.

One of its most popular traditional uses in southern Africa, is as an aid to induce or augment childbirth through an apparent oxytocin-like effect. Effects on uterine contractions have also been shown in animal studies^{(1, 2)}. Other traditional uses include to treat fevers, snake bites, infertility, and urinary tract infections. Phytochemicals with potential anti-diabetic activity have been characterized⁽³⁾, and anticholinesterase effects (implicating potential anti-dementia properties) have been reported for some of its alkaloids⁽⁴⁾. Crude extracts of the roots and leaves also exhibit antiviral activity against poliomyelitis, Coxsackie, Semliki forest, herpes and measles viruses^{(5, 6)}, with an alkaloid lycorine, being contributory. Potential activity against HIV, has also been suggested ⁽⁷⁾.

Under threat

Natal Lily is a highly traded plant in medicine markets in South Africa, and in 2008 it was assessed by the South African National Biodiversity Institute (SANBI) as a “vulnerable” species, after data revealed its population had declined by an estimated 40% over the previous 90 years⁽⁸⁾. The Red List of SANBI in 2016, lists Clivia miniata as in danger of extinction and now rarely occurring in its ecological niches⁽⁹⁾. Furthermore, high volumes in trade, plant scarcities and shortages have been reported by traders in several regional medicinal plant markets.

In order to better understand the response of Clivia miniata to individual and multiple pressures on its survival, a multidisciplinary team of scientists from South Africa, the UK and USA, recently simulated its future range and abundance by modelling the impact of different scenarios of climate change, changes in land cover, and harvesting practices⁽⁷⁾. All pressures were considered in isolation and in combination, to predict future population trends.

Study methods

The effects of climate change, were modelled based upon two scenarios from the Intergovernmental Panel on Climate Change (IPCC). One assumed the increase in global annual greenhouse gas emissions peaked between 2010–2020 and is now declining substantially, resulting in a projected global mean temperature rise of 0.4° to 1.7°C by the end of the century relative to 1850. The other scenario assumed that emissions continue to rise throughout the 21st century and the global mean temperature rises by 2.6° to 4.8°C.

For the influence of changes in land use, they used two different scenarios. One assumed a halt in the expansion of agricultural land and urban areas that has encroached upon Clivia miniata’s habitats, and that farming intensifies in existing agricultural areas only in order to meet future food demand. The other scenario extrapolated from recent trends in land cover change in which cropland and urban land cover gradually replace suitable habitat in proximity to existing locations.

Each of these scenarios was incorporated into a species distribution model and subsequently a metapopulation model, to assess and predict future population densities and extinction risks for the plant over the next 30 years. Habitat suitability was projected for each year between 2015 and 2055, and for each climate scenario used, habitat suitability maps were produced.

Two different harvesting scenarios were used, one based upon the harvest of juvenile plants only (representing the preference of traders for these, which have a lower water content than older plants), and the other which assumed that traders do not discriminate and demand plant material from all life stages equally.

An assumption was also made that harvesting only took place where the plant population size was at least 50 individuals. This relied upon continuation of traditional harvesting practices, in which only a small proportion of the available plant biomass from each location was harvested each time (five plants every second year) to allow time for recovery, and that a minimum population size of 50 plants was set to make harvest viable. This also assumed that smaller populations were more difficult to locate and were therefore visited and harvested less frequently..

Outcomes:

All of the different scenarios used, pointed to continuing declines in suitable habitat and abundance of Clivia miniata by the 2050’s.

Somewhat surprisingly, harvesting in isolation had the least impact, although it is important to note that each scenario was based upon limitations on the number of plants that could be harvested from each population per year. However, harvesting of plants from all stages resulted in a faster decline in abundance than extracting only juvenile plants.  

Each climate change scenario reduced the mean suitable habitat area by around 14%, driven by increasing temperatures and decreasing precipitation. However, not all scenarios caused a consistent decline, with some scenarios leading to an increase in suitable habitat area before a reduction of around 20% (relative to the start of the simulation) by 2050.

Land use change however, caused a substantially higher loss of suitable habitat area with more than 61% relative to the initial conditions. When combined with climate change scenarios, the suitable habitat area declined by 71 to 73%.

While the researchers tried to ascertain whether the interactions between these different pressures on the species were synergistic, additive, or antagonistic, no clear conclusions could be drawn.

Considering pressures independently, the future loss of suitable habitat was mainly driven by land cover change. In many countries including South Africa, conversion from a natural environment to farming for food production or forestry is a significant contributor; in others it is urban drift and increased construction of houses, towns and cities. Once land cover has changed, land is usually permanently lost to the species. This is in line with previous studies that established land cover change as a major threat to biodiversity over the next decades^{(10, 11)} .

Summary

This systematic study by a team of experienced and renowned researchers from South Africa and the UK, found that ongoing inadequate management of populations of Clivia miniata in the wild will have negative consequences on the wellbeing of people relying on it for medicine, and the many others for whom harvesting and trading in it, is a source of income.

While traditional and measured harvesting practices had minimal impact on future populations of the plant, the researchers modeled this on relatively modest and respectful harvest yields. It should be noted that for many at risk species now, harvesting practices are sometimes poorly undertaken and poorly regulated, such as taking plants at all stages of growth, in the case of Clivia miniata. Increasing pressure from land use change, is also likely to further contribute to declines in medicinal plant populations.  

Also, this study focused on a single medicinal plant, known to be relatively hardy and relatively resilient to climate change, but how wild populations of the thousands of other medicinal plants will fare in the face of global warming and increasing human encroachment on their natural environments, remains largely unknown. Much more research, is clearly needed.

A key message from this study is that greater efforts to introduce more cultivation of medicinal plants, are urgently needed.  However, a key comment I noted when reviewing this study, was a statement by the authors that efforts to cultivate had failed to date due to lack of commercial or government institutional support. Without commenting on the relative wealth or funding availability for such agronomy research in South Africa, I suspect that this hurdle is probably a factor in many other countries, particularly those with a relatively low GDP. To me it reiterates the importance of ensuring adequate attention including funding for cultivation trials over several years, in order to achieve the step change we probably need to move from an over-dependence on wildcrafted plants.  Such a change will need a collaborative combination of support and planning by governments or local regional development institutions and communities. Adequate funding and support from both the industry and other stakeholders is required over several years, to research and develop, viable and sustainable cultivation methods.

Finally, while considering the sustainability of an individual species in its native or original habitat is really important, in reviewing this study I realized that I have a couple of plants of Clivia miniata that have flourished in a semi-shady area of my garden for more than 20 years, despite receiving virtually no human attention. This reminded me yet again, that plants that may be increasingly at risk in the natural environment of one country or where they originally evolved, may be much less at risk and potentially even noxious or become ‘weedy’, in others.

References:

1. Veale DJ, Oliver DW, Arangies NS, Furman KI. Preliminary isolated organ studies using an aqueous extract of Clivia miniata leaves. J Ethnopharmacol. 1989;27(3):341-346.
2. Veale DJ, Oliver DW, Havlik I. The effects of herbal oxytocics on the isolated “stripped” myometrium model. Life Sci. 2000;67(11):1381-1388.
3. Pereira ASP, den Haan H, Peña-García J, Moreno MM, Pérez-Sánchez H, Apostolides Z. Exploring African Medicinal Plants for Potential Anti-Diabetic Compounds with the DIA-DB Inverse Virtual Screening Web Server. Molecules. 2019;24(10):2002.
4. Hirasawa Y, Tanaka T, Hirasawa S, et al. Cliniatines A-C, new Amaryllidaceae alkaloids from Clivia miniata, inhibiting Acetylcholinesterase. J Nat Med. 2022;76(1):171-177. 
5. Ieven M, et al. Planta Med 1979; 36, 311.
6. Ieven M, Vlietinck AJ, Vanden Berghe DA, et al. Plant antiviral agents. III. Isolation of alkaloids from Clivia miniata Regel (Amaryllidaceae). J Nat Prod. 1982;45(5):564-573.
7. Groner VP, Nicholas O, Mabhaudhi T, et al. Climate change, land cover change, and overharvesting threaten a widely used medicinal plant in South Africa. Ecol Appl. 2022;32(4):e2545. doi:10.1002/eap.2545.
8. http://redlist.sanbi.org/species.php?species-2081-5
9. Redlist of South African Plants. 2016. http://redlist.sanbi.org/stats.php
10. Jewitt, D , Goodman P. S, Erasmus B. F. N, O’Connor T. G, and Witkowski E. T. F.. 2015. “Systematic Land‐Cover Change in KwaZulu‐Natal, South Africa: Implications for Biodiversity.” South African Journal of Science 111(9–10): 1–9. 
11. Pereira, H. M. , Leadley P. W., Proença V., Alkemade R., Scharlemann J. P. W., Fernandez‐Manjarrés J. F., Araújo M. B., et al. 2010. “Scenarios for Global Biodiversity in the 21st Century.” Science 330(6010): 1496–501.