According to the IUCN Red List Categories and Criteria (IUCN, 2012), species categorized as Extinct in the Wild are those that survive only under human care in ex situ facilities or naturalized outside their native range. Although ex situ facilities such as botanic gardens and seed banks limit plant species extinctions (Abeli et al., Reference Abeli, Dalrymple, Godefroid, Mondoni, Müller, Rossi and Orsenigo2020), there are several problems associated with long-term ex situ cultivation that make the reintroduction of Extinct in the Wild species difficult (i.e. low genetic diversity, inbreeding and adaptation to ex situ conditions; Ensslin & Godefroid, Reference Ensslin and Godefroid2020; Tremblay et al., Reference Tremblay, Alicea-Roman, Anaya-Reyes, Duclerc-Rodas and Medina-Tirado2022). As a result of these problems, and combined with the probable destruction or degradation of their original habitats, the rediscovery of species that are Extinct in the Wild has a significant impact on their conservation because it facilitates the coupling of ex situ and in situ conservation, including site protection, wild population reinforcement and revitalization of ex situ collections with new genetic material from the wild (Smith et al., Reference Smith, Abeli, Bruns, Dalrymple, Foster and Gilbert2023).

Nymphaea thermarum Eb. Fisch. (family Nymphaeaceae) is a small, iconic waterlily endemic to southern Rwanda that has become a model for the study of the evolution of early angiosperms (Povilus et al., Reference Povilus, DaCosta, Grassa, Satyaki, Moeglein and Jaenisch2020). The species was discovered and described in 1987 by Eberhard Fischer (Fischer, Reference Fischer1988; Fischer & Magdalena, Reference Fischer and Magdalena Rodriguez2011) near the Mashyuza hot spring in the area of Nyakabuye, Rusizi District. Nymphaea thermarum was last seen in its original site in 2008, and attempts to find it in the wild since then have been unsuccessful, leading to its categorization as Extinct in the Wild on the IUCN Red List in 2010 and 2019 (Juffe, Reference Juffe2010; Fischer et al., Reference Fischer, Ntore, Nshutiyayesu, Luke, Kayombo and Kalema2019). The species was initially cultivated and propagated at the Botanic Gardens of the University of Bonn, Germany, from several plants collected at the time of discovery, and it is currently cultivated in at least 13 botanical gardens (BGCI, 2023).

On 29 July 2023, authors TA and SMM, and Jean Marie Habiyakare, visited the area of the Mashyuza hot spring and found N. thermarum in a new site west of its original location, on private land (Plate 1). Given the interest of collectors and waterlily enthusiasts for this species (which is sold on the horticultural market for c. EUR 500/plant) the exact location of the discovered population is not provided here, and will not be disclosed until protection measures are established. On 29 August 2023, author SS visited the site and completed preliminary observations on the population status of N. thermarum.

Plate 1 The rediscovery of Nymphaea thermarum in Rwanda: (a) a wild flowering individual, (b) N. thermarum along ditches, (c) adults and seedlings in a muddy fishpond, and (d) part of the team that rediscovered the species. Photos: Thomas Abeli, 29 July 2023.

We found N. thermarum in small ditches and ponds in an area currently occupied by small plantations of banana, mango and avocado, and fishponds. We assessed the population to be healthy and to comprise hundreds of individuals, including reproductive adult plants, and seedlings (Plate 1). The area occupied by the species is c. 4,600 m².

There are serious and imminent threats to this wild N. thermarum population. Mining by a local cement factory (the property of which is c. 10 m from a group of N. thermarum) is the most significant threat, and the mining has already affected the population directly through excavations and road development and indirectly through impacts on the springs feeding the ditches and ponds where the species grows. Potentially, local or foreign collectors might overharvest the species given its high market value.

Although N. thermarum has not yet been reassessed for the IUCN Red List, it is likely to be categorized as Critically Endangered (IUCN, 2012) under Criterion B, considering its area of occupancy of < 4 km², extent of occurrence of < 100 km², the single location and the observed and projected continuing declines in extent of occurrence, area of occupancy, quality of habitat and number of mature individuals (IUCN, 2012).

Urgent conservation measures are required to ensure this population of N. thermarum is not extirpated. In the short term it will be important to: (1) protect and/or purchase land, especially in relation to the expansion of the mining site (investigations to identify the owners of the land where N. thermarum grows are in progress); (2) negotiate with the cement factory to ensure that sufficient water reaches the habitat of N. thermarum; and (3) reassess the species with the IUCN Red List Categories and Criteria to update its status from Extinct in the Wild to Critically Endangered, to prevent out-of-date information hampering prioritization of conservation measures for the species (Abeli et al., Reference Abeli, Sharrock and Albani Rocchetti2022). This reassessment is underway.

In the medium term it will be important to: (1) create a plant micro-reserve (sensu Laguna et al., Reference Laguna, Ballester, Deltoro, Kadis, Thanos and Laguna2013) or include the site of N. thermarum in the Mashyuza Natural Forest; (2) restore the areas already affected by mining in collaboration with the local cement factory and develop a ditch system for irrigation of the fishponds, providing benefits for both local farmers and the species; (3) reintroduce N. thermarum to its original site near the main Mashyuza hot spring pool; (4) propose to the Government of Rwanda the inclusion of N. thermarum in CITES Appendix II, which would guarantee the protection of the species and potentially allow local farmers to cultivate and trade it (this species is in demand as an ornamental plant because of its small overall size and large flowers, and it can be grown out of water, on wet soil); (5) genetically revitalize existing ex situ collections with seeds from the wild population; and (6) create a new ex situ collection of the species in Rwanda for conservation and awareness purposes.

In addition to these urgent conservation actions, research is needed, and is being planned, to clarify aspects of the biology and ecology of N. thermarum that could inform conservation decisions, including the total and effective population size, population structure, abiotic factors influencing occurrence, natural vegetation dynamics of the wild site, pollination biology and population genetics of the wild and ex situ populations. All stakeholders (i.e. Rwandan governmental authorities, local community members, landowners, private companies working in the area and conservation biologists) now need to collaborate to conserve the rediscovered wild population of this iconic species.