Comparative analysis of diversity and utilization of edible plants in arid and semi-arid areas in Benin

This study was conducted in two ecological regions of Benin (West Africa) namely the Sudanian and the Sudano-Guinean regions. Benin is located on the Atlantic coast, and borders Nigeria to the east, Togo to the west, and Burkina Faso to the northwest, and Niger to the north (Figure 1). The vegetation pattern shows a humidity gradient northward as a result of the joint effects of the climate and the soils [32].

The Sudanian region is a woodland and savanna region with ferruginous soils. The rainfall is unimodal with a mean annual for about 1000 mm (Table 1). The temperature ranges from 24 to 31°C [32, 33]. The main sociolinguistic groups are Bariba, Fulani and Otamari and related sociolinguistic groups [34]. Farming systems are mainly based on cotton and cereal cultivation and livestock breeding. In the Western part, farming systems are limited by both land degradation and availability leading to population migration into central part of the country [35].

The Sudano-Guinean region is a transitional zone between the Guinean forests in the south and the Sudanian woodlands and savannas in the north and is characterized by a vegetation mosaic of forest islands, gallery forests, and savannas. The rainfall is unimodal and lasts for about 200 days with an annual mean rainfall varying from 1100 to 1300 mm (Table 1). The temperature varies from 25 to 29°C [32, 33]. The main sociolinguistic groups are Fon, Yoruba-Nagot and related sociolinguistic groups [34]. Other ethnic groups found include Otamari, Yom-Lokpa and related sociolinguistic ethnics groups. These ethnic groups represent two sociolinguistic groups that form the principal actors of migratory dynamics in Benin [23, 36]. Members of these groups are motivated to leave their homes in the hilly and over-populated North-West part to the central part in search of the virgin and fertile lands [23, 35‐37]. Fulani herders are also found in the Sudano-Guinean region because of their nomadic pastoralist lifestyle. The yam-based cropping systems are dominant [35] while rice cultivation is also important. Cotton and cashew nut are the major cash crops in the Sudano-Guinean zone.

Based on biophysical and socio-economic gradients [32, 33, 38, 39], we selected Boukoumbé and Bassila municipalities for field investigations (Table 1, Figure 1). Boukoumbé (10°10'36.1"N and 01°06'22.0"E) is located in Atacora department (north-western Benin) and belongs to the arid Sudanian region. Main ethnic groups in Boukoumbé include Ditamari, M’Berme, Natimba and Berba, which form the Otamari socilolinguistic group [34]. Otammari ethnic groups compose 92.4% of the population of Boukoumbé [34]. Traditional social system of the Otammari is based on crop production [36, 40]. Limited access to arable land due to the Atakora mountain chain and land degradation lead Otammari people to leave their homes to the central part of the country [23, 35‐37]. The total population of Boukoumbé is 60568 with a population density of 58 habitants per km²[41]. The total land area is 1036 km². Eighty six percent of household depend primarily on agriculture for their livelihood [41]. About 72% of households in the department of Atacora (in which belongs Boukoumbé) are poor (the highest proportion at national level) and 51% of its population live in extreme poverty [38]. To reduce this severe food insecurity an emergency programme of the Benin government was implemented from 2009 to 2011 through various emergency projects [39]. Bassila (09°01'00.1"N and 01°40'00.1"E) is located in Donga department of (upper central Benin) and belongs to the semi-arid Sudano-Guinean region. Main ethnic groups in Bassila include in order of importance Nagot, Anii and Kotokoli ethnic groups [34]. Nagot people are considered as native dwellers of Bassila although they have known to be originated from Yoruba people of Nigeria with whom they share indeed many similarities [23]. Anii and Kotokoli originated from Togo. Other ethnic groups such Otamari, Lokpa and Fulani are found in Bassila due to population migration. Indeed, Bassila as well as other municipalities in the transitional Sudano-Guinean zone between the Sudanian zone in the north and the Guinean zone in the south, is a receptacle of strong dynamic migration [23, 36]. Otamari are from the Atakora mountain chain region while Lokpa people are from municipality of Ouaké (in northern Bassila) [34]. Apart Fulani who has a nomadic pastoralist lifestyle, other ethnic groups in Bassila are mainly tillers [34, 40]. The total population of Bassila is 71511 with a population density of 13 habitants per km²[41]. The total land area is 5661 km². Currently, Bassila is the second largest municipality of Benin and about half of its total land area is covered with forests. Eighty three percent of household depend primarily on agriculture for their livelihood [41]. About 40% of households in the department of Donga (in which belongs Bassila) are poor with 20% of its population living in extreme poverty [38]. Also, Bassila is in a moderate but steady food insecurity situation [39].

In each municipality, we identified main agricultural production zones with local extension service agents. Afterwards, villages were randomly selected in these zones and the number of villages by municipality is proportional (11 to 12%) to the total number of villages that each municipality holds. Bassila holds 31 villages whereas Boukoumbé holds 71 villages [41]. We surveyed four villages in Bassila (Adjiro, Aoro-Lokpa, Camp pionier and Mondogui) and eight villages in Boukoumbé (Dimatema, Dipokor 1, Ditchendia, Koukongou, Kounadogou, Koutchata, Okouaro and Tassayota). Each surveyed village exhibit a dominant sociolinguistic group. A sociolinguistic group is understood here as a group in which a member inherits a common language of communication and shares social attributes such as customs, history, and food habits as recognized by Achigan-Dako et al.[24]. We carried out a focus group discussion in each village with about 20 community members and with a balanced representation of men, women, and different age groups. The socially defined age classes (youth - an unmarried individual; an adult - an individual married, but not considered an elder; and elder [42]) were considered. Participants in focus group discussions are community’s members locally recognized as knowledgeable about edible plants. Those participants were invited by the chief of the village and their peers. We obtained a permission of the chief of each village before conducting a focus group discussion, and followed the ethical guidelines of the International Society of Ethnobiology [43]. Participants were asked to build a free and an agreed list of edible plants consumed in the villages and to indicate their status (e.g. cultivated, wild or under domestication), utilizations and plant parts used. For local people, a “cultivated” species is a crop plant that is only known to be cultivated in the village; a “wild” species referred to any other food plants ranging from truly wild (entirely wild and collected only when needed) to wild-protected (maintained and protected or preserved in fields and fallows or around habitats with a sort of ownership), and semi-domesticated plants (cultivated in home gardens or in selected parts of cultivated fields where farmers tend to conduct diverse experiments); an “under domestication” species referred to plants of “wild” category that are not considered fully cultivated yet, and are no longer considered truly wild. It includes wild-protected and semi-domesticated plants, and species that are cited as cultivated by some participants and as wild by other during focus group discussion. Cultivated plants that grow spontaneously in the bush or fallow were not considered as wild species. At the end of each focus group, a guided tour was organized in the village to collect vouchers of edible plants previously listed (by their local names) by participants. The vouchers were selected with the help of two informants who were consensually selected among the participants. Species were taxonomically identified following Akoègninou et al.[33]. We also used the illustrated reference book of traditional vegetable species in Benin [44] and an illustrated reference book of trees, shrubs and lianas of dry zones of West Africa [45] to identify plant species. Voucher specimens were sent to the National Herbarium of Benin at University of Abomey-Calavi for further taxonomic confirmation.

In addition to focus group discussions, we carried out semi-structured individual interviews with 90 farmers in each municipality using a questionnaire. Participants were randomly selected and included in the survey after obtaining their verbal prior informed consent. Participant included is an individual who has his farm and hold the decision-making of activities to be implemented on it. Farmers were asked to describe their farming practices starting from land clearing and preparation until post-harvest practices on field.

To estimate edible plant species richness in the two municipalities, we translated the utilization patterns of village communities into presence-absence data to generate species accumulation curve using EstimateS statistical device [46]. The species accumulation curve represents the number of edible plant species as a function of some measure of the sampling effort employed in the surveys. It yields the asymptotic richness of the assemblage and the computation of the asymmetrical confidence interval of the estimates [46, 47]. To assess the diversity of edible plant species in the two municipalities, we used the Shannon-Wiener index computed in EstimateS based on the species accumulation curve [46].

To compare the estimated edible plants species’ richness and diversity between the two areas and between cultivated and wild groups, we performed a t-test, a Welch’s test or a two-sample Wilcoxon test when appropriate. We used t-test when normality and homoscedasticity assumptions were met, Welch’s test when normality assumptions was met but not homoscedasticity and two-sample Wilcoxon test when normality and homoscedasticity assumptions were not met [48]. Shapiro-Wilk’s test for normality and Levene’s test for equality of error variances were used to test the normality and the homoscedasticity assumptions respectively. We also used a range of descriptive statistics to account for species diversity and use patterns.

To test if farming practices used by farmers were independent of their agroecological region, we used Fischer’s exact test or chi-square test when appropriate. Fischer’s exact test was used when some cells of the contingency table had frequencies less than five and these cells represent more than 20% of the total number of cell in the contingency table [48, 49].

To assess relationships among village communities in term of the utilization of edible plants species, we performed a hierarchical cluster analysis based solely on cultivated species, on wild species and then wild and cultivated together. Dendrograms were obtained using the complete linkage algorithm based on Jaccard coefficient of similarity [50] generated from the presence–absence data matrix in which villages were considered as sampling units and species as variables and scored, for each village, as 1 when present or 0 if not. All statistical analyses were performed using R version 3.0.2 [51].

The total edible plant species richness was estimated to vary between 110 and 122 with an absolute value of 115 species (Figure 2a). Species belong to 48 families and 92 genera. Families with the highest genera and species richness include Asteraceae, Poaceae, Anacardiaceae, and Cucurbitaceae. The first two had 6 genera, each one with 8 and 7 species respectively, whereas the last two had 5 genera each one with 7 and 6 species respectively (Table 2). About 48% of plant families had only one species while 54% of them had only one genus. A list of species, with their utilization and part consumed is presented in Table 3. About 50% of edible plants (57 species) have been cited in more than 50% of surveyed villages (see Table 3).

Estimation of cultivated species richness indicated 45 species with Confidence Interval (CI) of 42 to 48 species belonging to 23 families and 38 genera. They are dominated by vegetable (47%) species (e.g. Abelmoschus esculentus, Corchorus olitorius, Solanum macrocarpon, Vernonia amygdalina) and fruit (20%) species (e.g. Citrus sinensis, Mangifera indica, Psidium guajava). Cereals (e.g. Oryza sativa, Sorghum bicolor, Zea mays) and pulses (e.g. Arachis hypogea, Glycine max, Vigna unguiculata) accounted for 24% of cultivated species and roots and tubers for 13% (e.g. Colocasia esculenta, Dioscorea cayenensis–rotundata complex, Ipomoea batatas, Manihot esculenta).

The estimated wild edible species richness indicated 70 species (CI: 66–76) representing 61% of edible plants collected. They also represent 46% of the most cited species (species cited in more than 50% of surveyed villages) by communities (Table 3). About 25% of them are under domestication and had dual status depending on the village. Some species are entirely wild and collected only when needed (e.g., Ceratotheca sesamoides, Cissus populnea), others are maintained in agricultural environments (fields and fallow) or around habitats (e.g., Adansonia digitata, Vitellaria paradoxa, Parkia biglobosa, Blighia sapida) and others have dual status depending on the village (e.g., Sesamum radiatum, Hibiscus asper, Justicia tenella, Corchorus tridens, Talinum triangulare). Wild edible species belong to 38 families and 61 genera. Vegetable species represent about 57% while fruit species account for roughly 47% of wild edible plants collected. Several life forms were recorded, but dominated by trees (36%) and herbaceous species (33%). Many wild species (41%) have multiple edible parts, including edible leaves, fruits, flowers, pulps, kernel, and/or seeds (e.g. Adansonia digitata, Annona senegalensis, Blighia sapida, Cissus populnea). About 96% of herbs are used as vegetables whereas about 80% of tree species provide fresh fruits (Table 4). Shrubs provide fruits (56%) and also used as vegetable (39%). Other uses (about 50%) were recorded for shrubs and included purgative and laxative (e.g. Tamarindus indica), stimulation of milk production in lactating women (e.g. Zanthoxylum zanthoxyloides), toothbrush (e.g. Vernonia amygdalina). Comparison of estimated richness between wild and cultivated species indicated that wild plant richness is significantly higher than cultivated species richness (Figure 2a, p < 0.001).

The estimation of species richness per region indicated 99 species (CI: 94–104) with about 60% of wild species (59 species) in Bassila and 82 species (CI: 79–89) with about 54% of wild species (44 species) in Boukoumbé. Overall, edible species richness in Bassila was significantly higher than in Boukombé (Figure 3a, p < 0.05). The same trend was observed when taking into account only wild species (Figure 3b, p < 0.01). However, there is no significant difference between the two areas when considering only cultivated species (Figure 3c, p = 0.1334). Thirty five out of 45 cultivated species collected are shared by the two areas (e.g. Zea mays, Sorghum bicolour, Vigna unguiculata, Arachis hypogea, D. cayenensis–rotundata complex, Manihot esculenta, Abelmoschus esculentus, Corchorus olitorius, Mangifera indica, Psidium guajava). Main species only cultivated in Bassila include Ananas comosus and Elaeis guineensis. Digitaria exilis, D. iburua and Pennisetum glaucum are the three species only found in Boukoumbé.The Shannon-Wiener diversity index was high, up to 4.5 for all categories and followed an asymptotic function against the sample frequency (Figure 2b). Wild species diversity is significantly higher than cultivated species diversity (Figure 2b, p < 0.001). The Shannon-Wiener diversity index was equally high in the two regions. It was close to 4.5 and 4.2 in Bassila and Boukoumbé respectively. Overall, edible species diversity in the semi-arid Sudano-Guinean area was significantly higher than in the arid Sudanian area (Figure 4a, p < 0.001). The same trend is observed when taking into account only wild species (Figure 4b, p < 0.001) and also cultivated species only (Figure 4c, p < 0.001).Although about 83% of respondents indicated that they had livestock, livestock breeding differed significantly between the two areas. More farmers were involved in livestock breeding in arid area than in semi-arid one (Figure 5, p < 0.001). The majority of them explained that livestock resources are used for household needs (e.g., consumption, cultural ceremonies) and commercialized to earn additional financial resources. They argued that this supplementary resource is crucial not only during food shortage period but also during agricultural work period. Animal species raised were dominated by poultry and small ruminants (Figure 5).Although farmers in Bassila exploit an overall higher edible plant diversity compared to their counterpart in Boukoumbé, this is not necessary translated into resource-conservation practices. Land clearing and land preparation practices developed were region-dependent (Figure 6, p < 0.001). Farmers in the semi-arid area used fire to clear new land. On old fields seedling and sapling are pruned and burnt. They also used human traction and to some extent tractors. We noticed no tillage. In the drier area however, the majority of farmers indicated that they use herbicide to clear land. Animal traction is used for tillage and mulches are systematically incorporated during the tillage as soils are degraded. Soil fertility management practices were also region-dependent (Figure 7, p < 0.001). Chemical fertilizer, animal manure, mulch incorporation and mulching were more used by farmers in the arid area while farmers in the semi-arid area still relied on slash-and-burn cultivation, crop rotation, burning of crop residue and mulches, and mixed-cropping. Similarly, field management practices after harvest were related to region where farmers were living (Figure 8, p < 0.001). Farmers in the semi-arid area left fields for fallow and/or burnt crop residues. They explained that burning crop residues after harvest reduces weed pressure at beginning of the next cropping season. It also avoids attracting the nomadic Fulani people and their herds into their fields because cattle increase soil degradation and make tillage labour difficult at the next cropping season. However, farmers in the arid area left fields for fallow and/or grazed their own cattle and small ruminants on the fields. They explained that crop residues serve as fodder for their livestock and the manure serves as fertilizer to crops. Contrary to others farming practices, there is no significant difference between the two municipalities regarding pest management practices (Figure 9, p = 0.1974). Farmers widely used chemical insecticide in both regions, mainly on cotton and to some extent on cowpea.

To understand the impact of agroecological context and socio-cultural attributes on edible plants selection by a community, we performed a cluster analysis using villages as operational units (Figures 10, 11 and 12). Each village is dominated by only one sociolinguistic group. The dendrogram in Figure 10 presents data on all edible plants together. At 40% of similarity, it shows two groups that revealed the two phytogeographical zones. Cluster A1 is composed of all villages of Bassila and no clear grouping according to sociolinguistic membership was noted within the cluster. Cluster A2 is composed of all villages of Boukoumbé. There is no clear grouping according to sociolinguistic membership too. The dendrogram in Figure 11 is based solely on cultivated species, and shows at 40% of similarity two groups based on phytogeographical zones. Cluster B1 is composed of all villages of Sudano-Guinean municipality and Cluster B2 of all villages of Sudanian municipality. There is no clustering according to sociolinguistic membership. The data in Figure 12 take into account wild species only, the same trend was observed although this topology revealed three clusters: Cluster C1 composed of all villages of Sudanian region, Cluster C2 composed of one village (Modogui, a Nagot socio-linguistic community) of Sudano-Guinean region and Cluster C3 of the rest of villages (which are of immigrant community) of Sudano-Guinean region.