Tubers in Trouble

Serene Hargreaves and Paul Wilkin report on the risk of extinction in Dioscorea, revealing how threatened they are, and why it matters.

Serene Hargreaves
Paul Wilkin

By Dr Serene Hargreaves and Dr Paul Wilkin

Grassland n Central Madagascar, Bat Vorontsova / RBG Kew, 2014

The risk of extinction

Extinction by its very nature is irreversible. Once a species is extinct, it’s too late for conservation practitioners to act. So, for us working on the front line of plant conservation, instead of just questioning whether a species is extinct, we need to look at the risk of extinction. Using assessments of extinction risk, we can uncover trends that help us to target the conservation resources we have available towards those species that need it most. The IUCN Red List of Threatened Species is one of the most well recognised lists that documents the extinction risk of species.

This week, in its latest publication, the IUCN Red List will publish Kew’s assessments of 41 species of Dioscorea, also known as yams, principally those in the genus that are only found in Madagascar and southern Africa.

It’s thought that around 20 per cent of the world’s plants are threatened with extinction. While this level is shocking, our assessments of yams show that this group has around twice that level of risk, with over 40 per cent of the species we studied at risk of extinction.

Dug to death

Yams are an important food in Madagascar. They are usually eaten boiled and provide an important source of carbohydrates, fibre, potassium and a range of micronutrients. While cultivated varieties are available, much of Madagascar’s rural community opt for eating wild yams. Rural families will harvest their own, while more urban communities may buy locally-sourced wild yams. They’re an important food especially in the late dry season “hungry gap”, when other foods like rice have been used up.

However, these yams are disappearing, with people reporting that they need to walk further and further to find yams to harvest. Over 30 yam species in Madagascar are only found there and, based on our assessments, many of these are likely to continue to decline if no action is taken.

    The useful and unusual at risk

    While the wild yams of southern Africa aren’t often used as agricultural food crops, they do include some of the most unusual species in the genus and most of them form a distinct evolutionary lineage. The lineage contains unique diversity, including species with enormous above-ground tubers as well as non-twining species that can be found growing in grasslands. The risk of extinction in southern Africa is even greater than that in Madagascar; 44 per cent are at risk of extinction according to our assessments, mostly due to habitat decline – from overgrazing, agricultural changes and other land development.

    The most distinctive and well-known species are those related to the elephant’s foot yam, D. elephantipes. Their extraordinary form makes them agriculturally important, and D. sylvatica in particular (which is assessed as Vulnerable) has been used as a source of steroids for the contraceptive pill and other medicines. This group also contains the fascinating D. strydomiana, harvested for its supposed cancer-curing properties, which is one of the most endangered yams anywhere in the world. There may be other life-saving properties, or opportunities to breed traits into cultivated varieties, from some of the species at risk. Our first priority though is to conserve them.

      The future

      Now we know the risks to both southern African and Malagasy yams, there is important work to do to ensure that we conserve these plants for future generations. Through funding received from the Darwin Initiative and the April Trust, Kew is working to reverse Malagasy yam declines. We are supporting 85 separate Malagasy rural communities to cultivate edible and threatened wild yam species as crops alongside conserving them in living collections and seed banks. Currently we have 23 species in germplasm collections, 17 banked as seed and 12 in cultivation with communities. In the future, these collections could be used for future crop breeding programmes, pharmaceutical drug discovery or even to reintroduce plants to areas from where they have disappeared.

      In South Africa, there are also numerous conservation initiatives involving yam species, in particular those of SANBI (South African National Biodiversity Institute) and other organisations.

      Meanwhile at Kew, we are redoubling our efforts to produce extinction risk assessments of thousands more plant species, co-ordinated through Kew’s Plant Assessment Unit. This work benefits from Toyota funding, which is being channelled to Kew through Toyota Motor Corporation's five year partnership with the International Union for the Conservation of Nature to develop the Red List of Threatened Species(TM). 


        • Toyota Motor Corporation
        • Kew Madagascar Conservation Centre (KMCC) particularly Mamy Tiana Rajaonah
        • Members of the IUCN SSC Madagascar Plant Specialist Group, especially Prof. Vololonoiana Jeannoda
        • Members of the IUCN SSC Crop Wild Relative Specialist Group especially Shelagh Kell and Joana Magos Brehm
        • Members of the IUCN SSC South African Plant Specialist Group, especially Lize von Staden
        • Darwin Initiative for funding project 22-005
        • April Trust for supporting the project Yam Cultivation for Conservation at Bongolava and Menabe
        • James Kennerley, Ryan Hills, Steven Bachman, Odile Weber & Emma Williams


        IUCN (2017). The IUCN Red List of Threatened Species. Version 2017-2. <>. Downloaded on 14 September 2017IUCN Red List

        Maurin, O., Muasya, A.M., Catalan, P., Shongwe, E.Z., Viruel, J., Wilkin, P. and van der Bank, M. (2016). Diversification into novel habitats in the Africa clade of Dioscorea (Dioscoreaceae): erect habit and elephant’s foot tubers. BMC evolutionary biology, 16(1), p.238.

        Viruel, J., Segarra‐Moragues, J.G., Raz, L., Forest, F., Wilkin, P., Sanmartín, I. and Catalán, P. (2016). Late Cretaceous–Early Eocene origin of yams (Dioscorea, Dioscoreaceae) in the Laurasian Palaearctic and their subsequent Oligocene–Miocene diversification. Journal of Biogeography, 43(4), pp.750–762

        Wilkin, P., Burrows, J., Burrows, S., Muasya A.M. & van Wyk, E. (2010). A critically endangered new species of yam (Dioscorea strydomiana Wilkin, Dioscoreaceae) from Mpumalanga, South Africa. Kew Bulletin 65(3), pp 421–433.

        Wilkin, P., Kennerley, J.A., Rajaonah, M.T., Huckël, G.M., Rakotoarison, F., Randriamboavonjy, T. and Cable, S. (2017). A new species of critically endangered edible yam endemic to northern Madagascar, Dioscorea irodensis (Dioscoreaceae) and its conservation. Kew Bulletin, 72(1), p.15.

        World Bank, (2016). Addressing Chronic Malnutrition in Madagascar. Available at <> Accessed 23 November 2017.

          Read & watch