Introduction

The significant effects of climate change on agriculture and livelihood in West Africa recently show the need to develop varieties of crops that can adapt to the rising temperatures, desertification, unpredictable rainfalls, floods and droughts and new diseases and pests, as well as meet the yield quality and quantity requirements of producers and consumers (Allen et al., 2019; Maxted et al., 2015; Mousavi-Derazmahalleh et al., 2018). Climate change has led to yield losses in different crops and will continue to adversely affect agriculture with considerable yield decline predicted in West Africa (Sultan, Defrance, & Iizumi, 2019). IPCC reported that the duration of the growing season in West Africa may be reduced by 20% in 2050, resulting in about 40% yield reduction in cereals (Zougmoré et al., 2016). The increasing population in the Western Africa region may further limit the ability of the region to meet the food and nutrient security needs of its growing population. CWR are wild plant taxa genetically related to domesticated crops and are widely recognised as a major reservoir of valuable diversity that can be used in plant breeding to sustain food and nutrient security in the future (Herden, Bonisch, & Friesen, 2020; Kioukis et al., 2020; Magos-Brehm et al., 2017; Maxted, Lloyd, Jury, Kell, & Scholten, 2006). Many CWR thrive in marginal environments (Jarvis, Fielder, Hopkins, Maxted, & Smart, 2015; Phillips et al., 2017; Vincent et al., 2019), making them better suited to withstand changing climate conditions. The extensive genetic diversity in CWR has been used globally in plant breeding programmes to produce crop cultivars with traits for high yield, drought tolerance, disease resistance, good handling quality, seed weight, early flowering time, cooking quality and better storage quality (Maxted & Kell, 2009; USDA, 2011).

CWR conservation and use contributes to the Sustainable Development Goal (SDG) of the United Nations (United Nations, 2015). Also, the United Nations’ Intergovernmental Science – Policy Platform on Biodiversity and Ecosystem Service (IPBES), described CWR as vital for future food and nutrient security, ameliorating ecosystems and adapting crops to marginal environments (IPBES, 2019).

There are about 45,000 plant species in Sub-Saharan Africa (Linder, 2014). In Nigeria alone, there are thought to be 7,895 different plant species (Federal Republic of Nigeria, 2010). However, the diversity of CWR is widely threatened by unsustainable use of natural resources, urbanization, deteriorating environmental conditions, the introduction of exotic species and climate change (Magos-Brehm et al., 2017; Maxted et al., 2009). Several CWR thrive on farmlands and are therefore threatened by agrochemical inputs and intensive agricultural systems (Capistrano-Gossmann et al., 2017; Jarvis et al., 2015; Vincent et al., 2019). Also, increasing population, poverty, habitat destruction, overgrazing, lack of land use planning and deforestation causes biodiversity loss in West Africa (Adejuwon, 2000). There is therefore the need for active in situ and ex situ conservation of CWR in West Africa, to ensure they continue to provide profitable genes to produce plant cultivars to meet the growing demand for ample food supply for the people of West Africa and beyond.

Developing a regional and national conservation plan is essential if poverty alleviation and food provision is to be maximised. This starts with making an inventory of CWR. Several countries already have CWR inventory, such as UK (Fielder et al., 2012), USA (Khoury et al., 2013), China (Kell et al., 2015) and Indonesia (Rahman, Magos-Brehm, & Maxted, 2019). A CWR checklist is a list of CWR taxa found in a defined geographical area. A CWR checklist may contain additional information on the priority CWR which are important for conservation planning including taxon distribution, reproduction and conservation status, turning the checklist into a CWR inventory. As reported by Magos-Brehm et al. (2017) the steps involved in the generation of a CWR inventory are: (i) compilation of a national flora, (ii) matching the national flora against an existing digitized list of crop genera to obtain a list of taxa of the same genera as the list in the national flora, thereby producing the CWR checklist, (iii) prioritization of the CWR checklist to generate a realistic and manageable number of priority CWR, and (iv) annotation of the priority list of CWR with additional information for active conservation action to produce a CWR inventory (Magos-Brehm et al., 2017; Maxted, Scholten, Codd, & Ford-Lloyd, 2007). Prioritization involves reducing the number of taxa in the CWR checklist into a number manageable for active conservation actions due to resource constraints and funding limitations. The prioritization criteria may include crop socio-economic value, CWR genetic closeness and ability to donate genes to the related crop, endemicity, occurrence, threat status and other related parameters (Magos-Brehm et al., 2017; Thormann, Kell, Magos-Brehm, & Dulloo, 2017). There is presently no complete CWR checklist or inventory for West Africa.

This paper aims at the generation of a regional CWR checklist for West Africa, prioritization of this CWR checklist and the compilation of a CWR inventory, using the method described by Maxted et al. (2007).

Materials and Methods

Results

The monographic CWR checklist for West Africa contains 1651 taxa from 379 genera. After the digitized list of crop genera was matched with the floristic list, a total of 392 CWR (and crops) resulted, belonging to 46 genera. Cultivated taxa without wild relatives in West Africa were removed, bringing the number to 379 taxa belonging to 33 genera. After applying the three criteria of the parallel method for prioritization (Kell, Ford-Lloyd, Magos-Brehm, Iriondo, & Maxted, 2017; Ng’uni et al., 2019), the CWR checklist was reduced to a total of 102 priority CWR from 18 genera with 24 sub-taxon (subspecies/varieties). The priority CWR are related to 15 crops or crop groups important for the West African region. The families with the highest number of CWR species are Poaceae (39), Papilionaceae (26), Dioscoreaceae (15) and Convolvulaceae (13). The genus with the highest number of CWR are Vigna (23), Dioscorea (15), Ipomoea (13), Oryza (6) and Cola (5) (Table 1).

Socio-economic Value of Related Crops

Yam (Dioscorea cayenensis subsp. rotundata (Poir) J. Miege) is the most economically valuable crop in West Africa, with the highest gross production value (Supplemental Table S3). It is followed by cassava (Manihot esculenta Crantz), rice (Oryza sativa L.), finger millet (Eleusine coracana (L.) Gaertn), sorghum (Sorghum bicolor (L.) Moench), cotton (Gossypium hirsutum L.) and cowpea (Vigna unguiculata (L.) Walp.) (Figure 1). Yam (Dioscorea cayenensis subsp. rotundata (Poir) J. Miège) also has the second largest number of CWR (15) after cowpea (23). Cassava (Manihot esculenta Crantz) and rice (Oryza sativa L.) which are the second and third in gross production value, have five and six CWR, respectively in the inventory (Table 1).

Threat Status of CWR

The threat status of 71 (69.6%) of the priority CWR has been determined under the IUCN threat assessment criteria (IUCN, 2012). All the priority CWR were globally assessed. Vigna desmodioides R. Wilczek is the only Endangered (EN) priority CWR. Two priority CWR are Near Threatened (NT): Dioscorea sensibarensis Pax and Gossypium anomalum Wawra. Two CWR are Data Deficient (DD): Gossypium herbaceum var acertifolium (Guill & Perr.) A. Chev. and Oryza brachyantha A. Chev. & Roehr. Sixty-six (64.7%) of the priority CWR are Least concern (LC) while 31 (30.4%) were Not Evaluated (NE) (Figure 2). Rice (Oryza sativa L.) was the only socio-economically valuable crop that had all its CWR assessed for threat status (Huchinson et al., 1958).

Discussion

Adejuwon (2000) reported that 20 species of plants in Nigeria were extinct, 431 were endangered species while 20 were vulnerable. Urbanization, soil degradation, natural calamities, deforestation, forest fires, overgrazing and other anthropogenic activities, particularly climate change, are reducing the availability of CWR for sustainable agricultural productivity (Maxted, Hawkes, Guarino, & Sawkins, 1997; Moore, Kell, Iriondo, Ford-Lloyd, & Maxted, 2008; Mounce, Rivers, Sharrock, Smith, & Brockington, 2017). For an effective and sustainable regional conservation strategy and its subsequent implementation, a priority CWR inventory is essential. A CWR inventory serves as a guide for a sustainable conservation action plan. The outcome of this research will form the blueprint for a systematic conservation and use strategy for West Africa. It will provide a starting point for a coordinated policy in the conservation and sustainable utilization of CWR diversity in the West African region. In this study, 379 taxa were identified as priority plant taxa, of which 122 were subsequently prioritized for urgent active conservation action. The remaining 257 plant species and their CWR could be considered for active conservation in the future as and when resources become available.

CWR of socio-economic valuable crops in West Africa have been reportedly used in crop improvement. For instance, Sood et al. (2015) reported the use of E. crusgalli (P. Beauv) and E. frumentacea (Link) to increase yield quality in Barnyard millet. D. abysinica (Hochst ex Kunth) and D. praehensilis have been reported to show resistance to yam mosaic virus and anthracnose , while D. bulbifera showed resistance to yam nematode and tolerance to drought (Lopez-Montes et al., 2012). Similarly, Kawuki et al. (2016) reported the use of M. esculenta subsp. peruviana Crantz, M. carthagenensis subsp. glaziovii (Müll. Arg.) Allem, M. dichotoma Ule and M. esculenta subsp. flabellifolia Crantz in breeding against cassava brown streak disease in cassava (Table 2). Traits for drought tolerance (Hajjar et al., 2007), yield improvement (Brar, 2004) and bacterial blight resistance (Brar, 2004; Jena, 2010) have been transferred from O. longistaminata A. Chev. & Roehr. to rice, while O. glaberrima Steud. has been reported to show drought tolerance (IRRI, 2006), iron tolerance (Brar & Khush, 2002), rapid leaf canopy establishment (Jones et al., 1997) and potential for tolerance to acid soil (Brar et al., 2002) and heat (Atwell et al., 2014). Also, S. bicolor subsp. verticillifolia (L.) Moench has reportedly shown resistance to leaf rust (Park et al., 2015), spot blotch (Yun et al., 2006), stem rust (Fetch-Jr et al., 2009) and tolerance to drought (Nevo et al., 2010). Resistance to white mold (Mkwaila et al., 2011) , bruchids (Osborn et al., 2003) , common bacterial blight and web blight (Beaver et al., 2012), and tolerance to drought have been documented in Phaseolus vulgaris var, aborigineus L. Chaudhary et al. (2014) reported stripe rust and powdery mildew resistance in T. turgidum L. (Table 2).

Maxted et al. (2015) and Kell et al. (2017) have opined that regional conservation is supplemental to national efforts as some CWR may be lacking in some countries in a region. West Africa, being a region dominated by agricultural nations, will find the implementation of the conservation plan from this inventory useful, as it will enhance the region’s global relevance in agricultural productivity. As reported by Maxted et al. (2008) and (Engels & Thormann, 2020), collaboration by neighboring nations could enhance the extensive and effective conservation of CWR genetic diversity. It is therefore the collective responsibilities of the neighboring nations where this CWR diversity is found to regionally conserve it (Maxted et al., 2008; Allen et al., 2019; Kell et al., 2017; Maxted et al., 2015).

Conclusion

This study shows that West Africa harbours CWR diversity that can contribute significantly to sustainable agricultural development in the region. Kell et al. (2015) noted that countries should widen their utilization of CWR across national boundaries and all nations are inter-independent in the quest for food security. Similar to an existing CWR inventory for the North African region (Lala, Amri, & Maxted, 2018), the CWR checklist, prioritization and prioritized inventory presented in the study will help in the development of a CWR conservation plan for West Africa. The conservation and utilization of CWR in this inventory for crop improvement has the potential to significantly reduce the over-dependence on synthetic agrochemicals and fertilizers in the region, which negatively impacts on its biodiversity and agricultural productivity. There is an urgent need to take a systematic and pragmatic approach in the conservation and sustainable utilization of CWR diversity in West Africa to ensure food security.

Conflict of interest statement

The authors declared that there is no conflict of interest.

Author contributions

Nigel Maxted and Joana Magos Brehm designed the study. All authors were involved in checklist compilation and prioritization. Michael Nduche wrote the manuscript while all authors discussed the results, commented on the manuscript, read and approved the final manuscript. 

Supplemental data

Supplemental Table 1: Related crop and concept level of priority CWR for West Africa.

Supplemental Table 2: Endemicity and International Union for Conservation of Nature and Natural Resources (IUCN) category of priority CWR.

Supplemental Table 3: Gross production value of socioeconomically valuable crops in West Africa.