West Africa is rich with taxa diversity, endemism and biodiversity heritage, while CWR diversity and flora distribution of the region have been reported in various studies (Huchinson and Dalziel, 1958; Oates et al., 2004; Bergl et al., 2007; Idohou et al., 2013; Hounsou-Dindin et al., 2022). However, as a purpose of this, further study is needed to determine the gaps in in situ and ex situ conservation action in the region, as this will complement and consolidate the national efforts of the individual countries. According to recent CWR ecogeographic diversity analysis, West Africa has been identified as a region of global importance with high CWR diversity for food security (Castañeda-Álvarez et al., 2016; Vincent et al., 2019). The highest CWR diversity identified in the provinces of Benin, is because of the recent Flora of Benin (Akoègninou et al., 2006), and the high number of occurrence records found in Benin, relative to other countries in the region (Figs. 1 and S1). High CWR diversity was also identified at Accra and Volta regions of Ghana, North – Central and South- Western zone of Nigeria. Other areas include Lacs district of Cote d’ Ivoire and Nzerekore region of Guinea (Fig. 2). These areas correspond to some areas of predicted distribution such as Nzerekore region of Guinea where Mont Nimba, Diecke, Pic de Fon, Pic de Tibe Classified Forests are located (Fig. 4). Similarly, these areas of species richness are in congruence with the Guinean forest, categorized as one of the 36 biodiversity hotspots in the world (Maxted and Vincent, 2021; Vincent et al., 2022) and the highest conservation value in Africa (Luiselli et al., 2019). The Guinean Forest covers an area of 621,705 km2, extending from Guinea, Sierra Leone, Liberia, Cote d’ Ivoire, Ghana, Togo, Benin to Nigeria. However, Guinean Forest is one of the most exploited biodiversity hotspots in the world, though 15% of the original forest is still unexploited (Conservation International, 2007).
The network of PAs in West Africa conserves a substantial number of the priority CWR with 61% (63) of the taxa found in PAs (Table S2). However, further field survey should be carried out to ascertain the presence of the priority CWR in those PAs where they were identified. For the 28 taxa (27.4%) that were found in less than five PAs, field survey should be done in areas of predicted distribution to determine their locations and to identify more taxa populations in network of PAs, to ensure they meet or surpass the required minimum number for active in situ conservation (Dulloo et al., 2008). Also, effective management and monitoring should be put in place to ensure active in situ conservation of the priority CWR in their respective PAs (Maxted et al., 2008b). Relevant institutions, stakeholders, non – governmental organizations (NGOs), and protected area managers should synergistically, ensure the maintenance of the PAs for optimal and active conservation action. Pendjari National Park in Benin with an area of 2,765 km2 and Comoe National Park in Cote d’ Ivoire occupying an area of 11,500km2 are the PAs with the highest number of CWR (Table S1). The presence of more CWR in Pendjari National Park may have resulted from the fact that the site was better surveyed than other PAs, as shown in the number of occurrence data recorded in Benin as compared to other countries (Fig.s 1 and S1). UNESCO (2019) reported that 620 plant species are found in Comoe National Park, which agrees with the high number of CWR present in Comoe National Park. The site contains great diversity of plants, endemic species and diverse ecological habitats ranging from savannah, forest to grasslands. (UNESCO, 2019). Large PAs such as Sahel (30,693 km2), Comoe National Park (11,500 km2), W National Park Benin (10,000 km2), Niokolo – Koba National Park (9,130 km2), were design to conserve diverse ecogeographical populations, which CWR is a subset. However, they contain only small of CWR population per unit area. The designation and development of the 18 reserve site found outside PAs, as other effective based conservation measures (OECM) will augment the preservation of CWR population outside PA network (Iriondo et al., 2021).
Identifying priority sites for the in situ conservation of CWR, based on species richness may be misleading since the approach relays only on taxa richness sites neglecting those taxa that require urgent protection (Brooks et al., 2006). However, to overcome this challenge, complementarity analysis through reserve site selection is used (Fielder et al., 2015; Contreras-Toledo et al., 2019; Mponya et al., 2020). Complementarity analysis have been used to identity priority site in regions such as Southern African Development Commission (SADC) (Magos Brehm et al., 2022) and Middle East (Zair et al., 2021). Twenty-nine reserve sites were identified in this study, with 11 in PA and 18 spotted outside PAs. The 11 reserve sites located in PAs will require minimal cost to establish and manage, being in existing PAs. It will augment and complement the protective function offered by the existing PAs and provide benefits to the local communities (Maxted et al., 2008b; Maxted and Kell, 2009). The remaining 18 reserve sites not located in PAs also present an opportunity for those countries with low number of PAs where taxa were found such as Guinea – Bissau (3), Mali (4), Niger (5), Sierra Leone (7) and Senegal (9) (Fig. 3). The outcome of the complementary analysis showed that the location of some reserve sites corresponds with some CWR hotspots in West Africa. These areas are Atakora, Alibori, Donga and Bongou provinces in Benin; Accra region of Ghana; North – Central and South – West zones of Nigeria and Lacs district of Cote d’ Ivoire (Figs. 2 and 3)
The areas of predicted distribution were highest in Woroba and Montangnes districts of Cote d’Ivoire where the reserve site; Mont Nimba is located and some protected areas such as Mont Tia, Mont Sangbe, Pic de Fon, Pic de Tibe, Mt Yonon are found. The area of high predicted distribution also extended to the Nzerekore, Faranah, Kindia, and Boke regions of Guinea, where the reserve site; Mont Nimba and Diecke Classified Forest are located (Fig. 4). Mont Nimba is strategic because it is located between Guinea and Cote d’ Ivoire. It occupies a total land area of 175.4 km2, with 125.4 km2 in Guinea and 50 km2 in Cote d’ Ivoire. UNESCO (2019) reported diverse flora and endemic plant species in the site, including epiphytes and over 2000 vascular plant species. Similarly, Diecke Classified Forest is one of the largest undisturbed areas of the Guinee Forestiere with diverse plant species including several threatened tree species. The presence of Cola attiensis Aubrev. Pellegr. in the site (Table S3) was also reported by (Couch and Haba, 2021). The area of predicted distribution appears to be larger than the area of observed distribution, which shows that the region is under surveyed. Efforts should be made for ex situ collection of taxa in predicted areas outside PAs, as they may be under threat by urbanization, change in land use and habitat destruction (Mponya et al., 2020).
For active in situ CWR conservation, effective ex situ conservation is needed to complement it. Ex situ conservation methods include seed bank, genebank, DNA bank, cryopreservation, botanical garden and in- vitro conservation (Maxted et al., 1997c; Maxted, 2013). 55% (56) of the priority taxa were represented in genebanks, however more accessions need to be collected for ex situ conservation. Further collection actions should be undertaken for the 20 taxa with occurrence records but not present in genebanks, the 26 taxa without occurrence data and the 44 taxa underrepresented in genebanks to reflect the recommendation by (Brown and Marshall, 1995 ) and (Guerrant et al., 2004) of 50 taxa population for effective representation in genebank. Additionally, taxa already present in PAs should be conserved ex situ in genebanks as a back – up to the in situ conservation to protect them in the event of natural disaster, war or fire outbreak (Ford-Lloyd and Maxted, 1993). Genebank accessions should be duplicated regionally and internationally to ensure effective and long term ex situ conservation (FAO, 2014; Magos Brehm et al., 2022).
The crop genepools with the highest number of taxa not represented in genebanks are yam (3), potato (3), sorghum (3), cowpea (3) and cola (3). Among the taxa that are not present in genebanks are Dioscorea abyssinica Hochst. ex. Kunth, used to improve yam for resistance against yam mosaic virus and anthranose (Lopez- Montes et al., 2012), Manihot carthagenesis (Jacq.) Mull. Arg, M. dichotoma Ule, M. esculenta subsp. peruviana Crantz and M. esculenta subsp. flabellifolia Crantz, used to improve for resistance against cassava brown streak disease (Kawuki et al., 2016). Echinichloa frumentacea Link and Eleusine Africana Kenn – O’Byne are used to breed Barnyard millet (Sood et al., 2015) and finger millet (Dida and Devos, 2006), respectively for high yield. Other taxa that are not present in genebanks include Phaseolus vulgaris var. aborigineus (Burkart) Baude, used for the improvement of common bean against bruchid (Osborn et al., 2003), white mould (Mkwaila et al., 2011), web and bacterial blight (Beaver et al., 2012) and for high yield (Wright and Kelly, 2011). Sorghum purpureosericeum (Hochst ex. A. Rich) Schweinf & Asch. has confirmed used in the improvement of sorghum for resistance against sorghum shoot fly (Nwanze et al., 1990), while Sorghum bicolor subsp. verticiliforum (L.) Moench is used in breeding sorghum for resistance against stem and leaf rust (Fetch Jr et al., 2009; Park et al., 2015b), increase in seed size and weight (Pillen et al., 2004). Hordeum bulbosum L. is used in breeding barley for resistance against barley mild mosaic virus (Ruge et al., 2003; Wendler et al., 2015), barley yellow virus (Wendler et al., 2015), powdery mildew (Pickering and Johnston, 2005; Johnston et al., 2009), stem and leaf rust (Fetch Jr et al., 2009; Johnston et al., 2013; Park et al., 2015b) and leaf scald (Pickering et al., 2006). Ex situ conservation will be a safety net for some CWR that have their adaptive scenario outside PA. For instance, some herbs and shrubs thrive on lawns, waste lands, swamps and agricultural lands (Maxted and Kell, 2009).
A major objective of in situ conservation is to confirm and preserve diverse CWR genes in a defined location for optimal used in crop improvement to ensure food and nutrient security. Ecogeographical diversity can work as proxy for genetic diversity (Korona, 1996; Parra-Quijano et al., 2012a ). The frequency of ecogeographical diversity outside PAs is higher, compared to that in PA. Therefore, ex situ collection of priority CWR outside PAs will capture taxa in ELC zones not represented or underrepresented in network of PA. The ELC map shows all resilient environmental conditions present within the geographical location of the target taxa population. ELC zone 2 had more accessions in genebanks and the highest frequency of occurrence in PA compared to other ELC categories (Table S4 and Fig. 6). However, taxa found in rare ELC zones present unique genes (Contreras-Toledo et al., 2019; Parra - Quijano et al., 2021) and should be prioritized in ex situ collection and conservation for use in crop improvement of their related crops. Complementarity analysis showed that 11 ELC categories were present in the reserve sites within PA, compared to 15 ELC categories represented in all PAs. This shows a high degree of complementarity in capturing the ecogeographical categories diversity of the priority CWR. On the average, the diversity of ELC categories per taxa was higher (26. 3%) compared to that for all PA network (23.4%) (Table S4). For ELC zones 12,14,16,17,19,22,23,24 where taxa were not represented in genebanks and ELC zones 4,7,20,21 with low genebank representation, based of frequency of occurrence (Table S9), further collection action should be carried out to ensure their representation. Similarly, ex situ collection should be done to represent all the ELC zones and ensure the preservation of novel and vital genes (Rubio-Teso et al., 2013 ; Parra - Quijano et al., 2021). The presence of these taxa in different ELC zones helps to identify those that thrives in adverse and marginal environments, as they may possess profitable genes for adapting their related crops to erratic climatic conditions (Garcia et al., 2017).