Diversity of Useful Plants in Cabo Verde Islands: A Biogeographic and Conservation Perspective

Cabo Verde’s biodiversity is threatened by activities that meet human needs. To counteract this, an integration of scientific and indigenous knowledge is required, but no comprehensive list of the useful local plants is available. Thus, in this work, we assess (1) their diversity and phytogeography; (2) the role of geophysical, historical, and socio-economic factors on species distribution and uses; and (3) potentially relevant species for sustainable development. Data were obtained from flora, scientific publications, historical documents, herbarium specimens and field work. Many species were introduced since the 15th century to support settlement and commercial interests. We identified 518 useful taxa, of which 145 are native, 38 endemic and 44 endangered. The number of useful taxa is correlated with altitude and agricultural area, as well as with rural population indicators, but not with total population or socio-economic indicators such as gross domestic product. Native taxa are mostly used for fuelwood, forage and utilitarian purposes. Agrobiodiversity and traditional practices seem crucial to cope with recurrent droughts and ensure food security. Most of the introduced species do not present conservation problems, contrasting with the overuse of some native taxa. The safeguarding of native populations will ensure the sustainable exploitation of these resources and benefit the local economy.


Introduction
A sixth mass extinction of life on Earth is under way, and habitat loss is among the most important anthropogenic threats, followed by over-exploitation, introduced species and climate change, leading to a loss of species and ecosystems [1]. Although island and mainland regions have undergone equivalent past habitat loss, projections indicate (Psidium guajava) and lemon as well as sugarcane, tomato (Solanum lycopersicum), carrot (Daucus carota), papaya and mango (Mangifera indica) [29] growing together in small fields.

Taxonomic Diversity
The complete lists of plants used in Cabo Verde, with common names and respective uses, are presented in Table 1 (endemic and non-endemic native taxa) and in Table 2 (introduced taxa); species for which there is information on their historical use (until the end of the 18th century) are also indicated. Table 1. Native useful taxa occurring in Cabo Verde, including information on family, common names (mostly in creole), main uses (Fd, food; Fr, forage; Ml, melliferous; P, poison; S, social uses; Fu, fuelwood; T, timber; U, utilitarian uses; Mt, materials; O, ornamental; E, environmental use; for details, see Materials and Methods), and historical use (Hist.). Taxonomic authorities according to Plants of the World Online [30].

Taxa
Common Names Uses Hist.
Most of the useful plants found in Cabo Verde were introduced (373 taxa, representing 72%) (Supplementary Materials Table S1); among them, about 86% are cultivated in the country. Except for the Poaceae, most of the better-represented families are mainly composed of exotic species ( Figure 1A). Exotics also predominate in the best-represented genera (e.g., Acacia, Euphorbia, Ficus or Amaranthus), with the exceptions of Cyperus (Cyperaceae), Echium (Boraginaceae), Launaea (Asteraceae) and Eragrostis and Setaria (Poaceae) ( Figure 1B).
Considering the distribution of the useful species in the nine Cabo Verde islands (Santa Luzia, an uninhabited island, is not included in this analysis), there are very high positive correlations of "total useful taxa" with "altitude" (r = 0.900), area occupied by agriculture (r = 0.933), as well as with "total taxa number" (useful or not) present in each island (r = 0.933) (Figure 2). The "rural population" and the "total number of farms" display high positive correlations as well as all the farm categories: "rainfed", "irrigated", and "livestock farming". Less relevant (moderately positive) is the correlation with "forest holdings".  Table S1); among them, about 86% are cultivated in the country. Except for the Poaceae, most of the better-represented families are mainly composed of exotic species ( Figure 1A). Exotics also predominate in the best-represented genera (e.g., Acacia, Euphorbia, Ficus or Amaranthus), with the exceptions of Cyperus (Cyperaceae), Echium (Boraginaceae), Launaea (Asteraceae) and Eragrostis and Setaria (Poaceae) ( Figure 1B).
Considering the distribution of the useful species in the nine Cabo Verde islands (Santa Luzia, an uninhabited island, is not included in this analysis), there are very high positive correlations of "total useful taxa" with "altitude" (r = 0.900), area occupied by agriculture (r = 0.933), as well as with "total taxa number" (useful or not) present in each island (r = 0.933) (Figure 2). The "rural population" and the "total number of farms" display high positive correlations as well as all the farm categories: "rainfed", "irrigated", and "livestock farming". Less relevant (moderately positive) is the correlation with "forest holdings". No significant correlations were found between the "total useful taxa" and the variables "total population", and indicators of other economic activities such as "tourists" or "gross domestic product" (GDP).

Main Uses of Cabo Verdean Flora
Among the 11 considered classes of use, the most frequent are ornamental, with 183 taxa (corresponding to 35.3%); forage and pasture, with 171 taxa (33%); food, with 158 taxa (30.5%); environmental, with 72 taxa (13.9%); and melliferous with 71 (13.7%). The other classes represent less than 10% each (Supplementary Materials Table S1 and Figure 3). No significant correlations were found between the "total useful taxa" and t ables "total population", and indicators of other economic activities such as "tour "gross domestic product" (GDP).

Main Uses of Cabo Verdean Flora
Among the 11 considered classes of use, the most frequent are ornamental, with (corresponding to 35.3%); forage and pasture, with 171 taxa (33%); food, with 158 taxa environmental, with 72 taxa (13.9%); and melliferous with 71 (13.7%). The other classe sent less than 10% each (Supplementary Materials Table S1 and Figure 3).  No significant correlations were found be ables "total population", and indicators of oth "gross domestic product" (GDP). Some taxa are used for multiple purposes is used for ornamental purposes and for forag objectives, in addition to being a melliferous p Some taxa are used for multiple purposes, for instance, Vachellia nilotica subsp. indica is used for ornamental purposes and for forage, fuelwood, materials, and environmental objectives, in addition to being a melliferous plant. Moringa oleifera is a very useful plant for alimentary, pasture, ornamental and materials purposes. In addition, the native Ziziphus mauritiana is used for food, forage, melliferous, fuelwood and timber. However, most taxa (350 taxa, corresponding to 67.6%) are reported for only one use, the top three being ornamental (111), forage (89) and food (88).

Main Uses of Cabo Verdean Flora
Native plants (including endemic taxa) represent most of the taxa used as fuelwood (57.1%), as forage (63.2%) and for utilitarian applications (53.3%) (Figure 3). Most of the endemic taxa (26) are reported as forage, exceeding the other categories by far. Exotic species are mainly present in the other categories and are particularly well-represented as ornamentals (172 taxa, 94%).

Growth form Diversity and Uses
The useful plants of Cabo Verde present a high diversity of growing habits and life cycles. About 28.4% are trees or palms, 25.3% are shrubs or subshrubs, 23.2% are annual or biennial herbs, 16.8% are perennial herbs and 6.3% are climbers, including vines and lianas ( Figure 4). Trees are the most used for environmental purposes and, as expected, timber; annuals and biennials are most commonly used for forage and human food; ornamentals are mostly trees and shrubs.
Native plants (including endemic taxa) represent most of the taxa used (57.1%), as forage (63.2%) and for utilitarian applications (53.3%) (Figure 3) endemic taxa (26) are reported as forage, exceeding the other categories by fa cies are mainly present in the other categories and are particularly well-re ornamentals (172 taxa, 94%).

Growth form Diversity and Uses
The useful plants of Cabo Verde present a high diversity of growing h cycles. About 28.4% are trees or palms, 25.3% are shrubs or subshrubs, 23.2 or biennial herbs, 16.8% are perennial herbs and 6.3% are climbers, includi lianas ( Figure 4). Trees are the most used for environmental purposes and, timber; annuals and biennials are most commonly used for forage and huma mentals are mostly trees and shrubs. The use categories encompassing the widest variety of species are fo food and ornamental, including all growth forms.

Distribution across Cabo Verde Archipelago
Santiago and Santo Antão are the islands with more useful taxa, at 388 respectively (Supplementary Materials Table S1, Figure 5B). Fogo, Brava and have 297, 243 and 234 taxa, respectively. The remaining islands have betw Vicente) and 123 taxa (Sal). The use categories encompassing the widest variety of species are forage, human food and ornamental, including all growth forms.

Distribution across Cabo Verde Archipelago
Santiago and Santo Antão are the islands with more useful taxa, at 388 and 372 taxa, respectively (Supplementary Materials Table S1, Figure 5B). Fogo, Brava and São Nicolau have 297, 243 and 234 taxa, respectively. The remaining islands have between 203 (São Vicente) and 123 taxa (Sal). The islands where agricultural activities prevail (i.e., with higher "number o or larger "agriculture area"-Supplementary Materials Table S2), are also thos higher numbers of useful taxa are reported (e.g., Santo Antão, Santiago and Fogo Based on the number of species per use category (see heatmap in Figure  UPGMA analysis reveals two main groups of islands: 1) Santo Antão, Santiago, F Nicolau and Brava, the islands with the highest values for most of the use categor Santiago presenting the highest values for ten categories; and 2) a group inclu remaining islands, with the lowest values in most of the use categories.
Correlation coefficients of "total number of uses" and individual uses roughl the same pattern as the "total useful taxa" (Figure 2).

Uses vs. Biogeographic Origin
The huge biogeographic diversity of exotic species among the useful flora Verde is remarkable.
Taxa of Afrotropical origin prevail as forage ( Figure 6). That is the case for g cies such as Andropogon gayanus and Urochloa xantholeuca, reported as excellent fo the leguminous species from the genera Crotalaria, Desmodium, Grona, Macrotylom chosia, Sesbania, Tephrosia and Vigna, besides several endemic species of Lotus. The islands where agricultural activities prevail (i.e., with higher "number of farms", or larger "agriculture area"-Supplementary Materials Table S2), are also those where higher numbers of useful taxa are reported (e.g., Santo Antão, Santiago and Fogo).
Based on the number of species per use category (see heatmap in Figure 5A), the UPGMA analysis reveals two main groups of islands: (1) Santo Antão, Santiago, Fogo, São Nicolau and Brava, the islands with the highest values for most of the use categories, with Santiago presenting the highest values for ten categories; and (2) a group including the remaining islands, with the lowest values in most of the use categories.
Correlation coefficients of "total number of uses" and individual uses roughly follow the same pattern as the "total useful taxa" (Figure 2).

Uses vs. Biogeographic Origin
The huge biogeographic diversity of exotic species among the useful flora of Cabo Verde is remarkable.
Taxa of Afrotropical origin prevail as forage ( Figure 6). That is the case for grass species such as Andropogon gayanus and Urochloa xantholeuca, reported as excellent forage, or the leguminous species from the genera Crotalaria, Desmodium, Grona, Macrotyloma, Rhynchosia, Sesbania, Tephrosia and Vigna, besides several endemic species of Lotus. The taxa used for ornamental purposes are mainly of Neotropical origin, namely Apocynaceae (e.g., Asclepias curassavica, Cascabela thevetia, Plumeria rubra), Fabaceae (e.g. Caesalpinia pulcherrima), and Lamiaceae (e.g., Salvia spp.), only to mention a few examples Of the same origin are most taxa used as food (e.g., species of the genera Amaranthus Annona, Cucurbita, Capsicum, and Solanum). The Neotropical region is also the main origin of melliferous plants and of the taxa used to obtain materials and timber, however with considerably lower importance.
The Austral origin prevails in the environmental purposes category, with the wellrepresented genera Acacia and Eucalyptus.
Most taxa (364 taxa) occur in only one biogeographic region. Overall, 432 taxa, corresponding to 83.4% of all the useful taxa, occur as native in either Afrotropical, Neotropical Australotropical or Oriental regions (in single or mixed classes). Of the 86 taxa that do not occur in at least one of these regions, more than half (54) have an exclusively Palearctic distribution.

Native Species Conservation
Only 202 (37.6%) of the studied taxa were assessed by the International Union for Conservation of Nature (IUCN) Red List of Threatened Species [36] and Romeiras et al [37]. Most of them (144) are classified as Least Concern (LC), 7 as Near Threatened (NT) 9 as Vulnerable (VU), 23 as Endangered (EN), and 5 as Critically Endangered (CR). Fourteen taxa are classified as Data Deficient (DD).
Forage is the use category that includes most threatened Cabo Verdean species The taxa used for ornamental purposes are mainly of Neotropical origin, namely Apocynaceae (e.g., Asclepias curassavica, Cascabela thevetia, Plumeria rubra), Fabaceae (e.g., Caesalpinia pulcherrima), and Lamiaceae (e.g., Salvia spp.), only to mention a few examples. Of the same origin are most taxa used as food (e.g., species of the genera Amaranthus, Annona, Cucurbita, Capsicum, and Solanum). The Neotropical region is also the main origin of melliferous plants and of the taxa used to obtain materials and timber, however with considerably lower importance.
The Austral origin prevails in the environmental purposes category, with the wellrepresented genera Acacia and Eucalyptus.
Most taxa (364 taxa) occur in only one biogeographic region. Overall, 432 taxa, corresponding to 83.4% of all the useful taxa, occur as native in either Afrotropical, Neotropical, Australotropical or Oriental regions (in single or mixed classes). Of the 86 taxa that do not occur in at least one of these regions, more than half (54) have an exclusively Palearctic distribution.

Native Species Conservation
Only 202 (37.6%) of the studied taxa were assessed by the International Union for Conservation of Nature (IUCN) Red List of Threatened Species [36] and Romeiras et al. [37]. Most of them (144) are classified as Least Concern (LC), 7 as Near Threatened (NT), 9 as Vulnerable (VU), 23 as Endangered (EN), and 5 as Critically Endangered (CR). Fourteen taxa are classified as Data Deficient (DD).
Two Endangered endemic species are used for their edible fruits: Phoenix atlantica and Sideroxylon marginatum; and Urochloa caboverdiana, classified as Vulnerable, is used in times of food shortage (seeds).
The populations of several endemics, such as Echium vulcanorum, E. hypertropicum (EN), Euphorbia tuckeyana (NT) and Sideroxylon marginatum, have been depleted for firewood or charcoal. Also threatened are species once widely used for leather tanning, such as Periploca chevalieri (leaves), and Euphorbia tuckeyana (sap). Asteriscus daltonii subsp. vogelii (NT) and Pulicaria diffusa (EN) are reported as used in fumigations, and Dracaena caboverdeana (CR), produces the famous dragon's blood, a red resin used as varnish, besides being a valuable ornamental species. Other relevant endemic ornamentals are the palm Phoenix atlantica and the Crassulaceae Aeonium gorgoneum (EN).
The use of some non-endemic native taxa is also of concern. This is the case with the use as fuelwood of Arthrocaulon franzii, used in lime kilns, Tamarix senegalensis, and Tetraena gaetula subsp. waterlotii.
Some of the introduced species are also classified as threatened in their native areas of distribution (e.g., Jacaranda mimosifolia, Kalanchoe daigremontiana, or Khaya senegalensis). However, in most cases, the unknown origin of the introduced plants in Cabo Verde (wild populations or plant nurseries) hampers a correct evaluation of their relevance for species conservation.

Agrobiodiversity and Traditional Knowledge
The plants cultivated and preserved by rural communities for a long time and, as such, extremely well adapted to the diversity of bioclimatic conditions of the archipelago constitute a valuable reservoir of plant genetic resources. The cultivation of this agrobiodiversity, together with the traditional knowledge on cultural practices (selection, propagation, and conservation), is crucial to face the drought cycles that are common in Cabo Verde and to ensure food security. However, in drought years, many of these genetic resources are lost, as farmers lose their seeds when crops fail to grow due to lack of rain.
Beans are perhaps the crop with most varieties. For example, in Santo Antão, the "feijão-caqui" (a variety of Lablab purpureus subsp. purpureus), highly resistant to dryness, keeps the pods closed at the end of maturation, thus avoiding the need to collect grains from the ground [29]. Regarding corn, the selection is made by choosing the best ears (those fully filled with grains and with more "rows"), which are not threshed until the time of sowing and from which only the largest and best-formed grains are used [29].
An example of a traditional technique for propagule conservation is, in Santo Antão, the storage of potatoes (Solanum tuberosum) in cool places such as caves, dug out of pozzolanic rocks, which are very common in the region due to their volcanic origin [29].
In Santiago, it is documented that sweet potato (Ipomoea batatas) seedlings or cuttings are sold/offered to farmers in highlands, where the cooler climate allows for their conservation; during the planting season, these same plants are again sold/offered to farmers in lower areas. This is a secular practice that is used in this and also in other islands (e.g., Fogo) and also with other crops such as cassava.
To prevent the emergence of pests during storage of seeds and the serious damage they cause, namely in maize and beans, plant species that are rich in essential oils and act as biocides are traditionally used. This is the case of pink pepper leaves (Schinus molle), "losna" (Artemisia gorgonum), neem (Azadirachta indica), laurel (Laurus nobilis), leaves and fruits of eucalyptus (Eucalyptus globulus) and fruits of chilli pepper (Capsicum frutescens). The latter seem to allow the conservation of seeds and maintain their germination ability for many years [29].

Discussion
In Cabo Verde, as well as worldwide, the use of plants for a variety of purposes is a common practice. The data provided in this paper improve our knowledge of the flora used by local populations in Cabo Verde and underline the high dependence of the populations on the use of plants for multiple purposes.
Our comprehensive inventory, including historical works, collected previously ignored information on particular uses of species and also drew attention to some species that are no longer used, contributing to the general knowledge of useful species, as has been done in other tropical regions, such as South America (e.g., Cámara-Leret et al. [38]), Asia (e.g., Vu and Nguyen [39]) or Africa (e.g., Nortje and van Wyk [40]; Welcome and Van Wyk [41]), where this knowledge is still insufficient.

Taxonomic Diversity
Useful plant species make a considerable portion of total Cabo Verdean flora. With 518 taxa, they are mainly represented by introduced species (72%).
Species not previously recorded for the archipelago, to the best of our knowledge, are here reported, e.g., Tabebuia rosea (a Bignoniaceae with several centuries-old specimens and recently used as ornamental street tree) and the Fabaceae Gliricidia sepium, both in Santiago Island.
Following a common worldwide pattern [42], the three most diverse families of useful plants are the Fabaceae, the Poaceae and the Asteraceae, with a high number of crops of global significance. In addition, these families are among the largest plant families, corresponding, respectively, to the first (Asteraceae), third (Fabaceae), and fifth (Poaceae) best-represented families [43].
The best represented families are mostly composed of exotic species, with Poaceae being an exception (see Figure 1A). The high dispersal ability as well as its extraordinary adaptability to dryness [44] place this family among the most successful in the archipelago.
Species diversity is closely related with altitude, with the highest islands (Santiago, Santo Antão and Fogo) presenting the highest numbers of useful species. Furthermore, the two groups highlighted on the heat map (see Figure 5A) show that altitude is responsible for a similar pattern in the distribution of the types of uses among islands. This is certainly related with the greater suitability of high-altitude islands, with better climatic conditions, for agricultural activities (consequently housing a larger rural population) and also to the presence of a richer flora [13,45]. Moreover, reinforcing the relevance, in this respect, of more traditional economic sectors, such as agriculture, forestry and livestock farming, the diversity of useful plants is neither related with touristic activities nor with the gross domestic product-two indicators that are highly correlated (r = 0.800; data not shown).

History behind Diversity
The archipelago was uninhabited until it was discovered by the Portuguese in 1456, and most of Cabo Verde's inhabitants are of mixed Portuguese and African ancestry [46]. With a five century long settlement history, the combined influences of both cultures are evident in the use of plants, with many commonly used species (e.g., fruits and vegetables from Europe, cereals from West Africa). This knowledge was further enriched with the introduction of useful plants from other regions, such as the American continent (namely Brazil and Western Indies), resulting from the overseas trade of the Portuguese since as early as the 16th century [47].
In the early times, the introduced species were essential, if not decisive, for human survival. At the same time, they supported the dominant commercial interests linked to the slave trade and supply of merchant ships.
It is interesting to reference the example of the attempt of the first European settlers to maintain their eating habits in Cabo Verde. According to Torrão [48], seeds and other propagules accompanied the Portuguese colonizers, hoping that their Mediterranean crops would provide both food and a sociocultural link to their homeland. Early chroniclers such as Valentim Fernandes by 1506 (in Monod et al. [18]) reported that, in Santiago Island, some valleys were cultivated with fruit trees used in Europe (e.g., pears, apples, oranges, lemons, figs, grapes, etc.), reflecting the wish of European settlers to maintain their own traditions. However, the climatic constraints of the tropical climate seldom allowed the successful development of some of them (e.g., the cultivation of some cereals such as wheat and barley).
In addition, with the slaves from the West African coasts came the crops included in their dietary habits, namely rice (Oryza glaberrima), and "milho", referring to Sorghum and/or Pennisetum, two common crops in Guinean coasts (not Zea mays, the maize from South America, not yet introduced in Africa, and later also named "milho" by the Portuguese) [48]. The emotional memory attached to food was certainly an important promoter of the plant diversity that is still found everywhere in these islands.
The high number of species whose use dates back to the beginning of colonisation is remarkable. Historical documents (until the end of the 18th century) report almost 70 species, not including vegetables, which are rarely mentioned (see Tables 1 and 2).
Even considering the overall climatic constraints, the orographic diversity of the archipelago accounts for a wide range of habitat types, allowing the presence of species from temperate zones, such as Northern Europe or New Zealand, as well as of those with a tropical or subtropical distribution, e.g., from Central and South America or from India. In fact, introduced species have been a constant presence in Cabo Verde since the early times of colonization.
Due to geographical and historical circumstances, this archipelago in the middle of the North Atlantic became a pivotal region between Europe, Africa and America. This excellent location fostered the introduction of a wide variety of economically valuable plants, for acclimatization and further dissemination to other regions of the world. In fact, Cidade Velha (in Santiago), the former capital of the archipelago and the first town built in the tropics by Europeans (in the late 15th century), was an essential Atlantic port of call and rapidly became a commercial hub between Europe, Africa and the New World [49,50] promoting the introduction and later diffusion of many useful plants.
Overall, species were introduced to meet the needs of local populations, and the shifts over time reflect the changing in socio-economic requirements, from the most basic needs-food, fuelwood or timber-to higher-level demands, e.g., aesthetics.

Plant Uses and Sustainability
Ornamental followed by forage/pasture and food purposes are the primary uses of plants in Cabo Verde. The common use of ornamental plants in this archipelago is not surprising. The same occurs worldwide, with estimates pointing to 28,000 plant species of ornamentals (including gardening and landscaping plants), while cultivated crops correspond to about 7000 species [51]. However, it is worth mentioning that more than half of the species reported as ornamentals in Cabo Verde are also used for other purposes, namely for medicinal uses (data not included in the present analysis), food or for environmental projects.
Species used as forage or pasture, the second most reported use, are mainly from the families Poaceae and Fabaceae and are very common in grasslands and savannas. Several species are reported as high-quality forages (e.g., Desmodium tortuosum and Teramnus labialis), revealing their extraordinary value to improve natural pastures for cattle raising, a main economic sector in Cabo Verde [52].
Concerning edible species, it is interesting to note the presence of several commonly overlooked fruit-trees such as the introduced Syzygium jambos or Spondias mombin, and the reference to some native species whose fruits are used for human consumption such as Momordica charantia, Grewia villosa, Ficus sur, F. sycomorus, Solanum scabrum (leaves also used as vegetable) and the endemics Phoenix atlantica and Sideroxylon marginatum. The flour made from dry fruits of Ziziphus mauritiana is consumed by the populations living in dry areas, and, formerly, the fruits of Tamarindus indica were cooked together with meat [53]. Other native species used for food (vegetables) are Launaea intybacea, an ingredient of the most emblematic dish in Cabo Verde ("cachupa"), Portulaca oleracea, to make soups, Celosia trigyna, and the edible tubers of Cyperus esculentus and C. rotundus. Senna occidentalis was used, until recently, as a coffee substitute.
Particularly interesting are several native species reported as being used in times of food shortages (e.g., seeds used to make flour): the Malvaceae Melhania ovata and the Poaceae Dactyloctenium aegyptium, Setaria barbata, Urochloa caboverdiana and U. ramosa.
Note that several grass species occurring in Cabo Verde are considered as millets, a group of cereal crops with small grains used for human consumption. That is the case of the Guinea millet Urochloa deflexa, or the browntop millet Urochloa ramosa. Although abandoned many years ago, the millets are now being considered valuable functional foods for their good nutritional properties [7], and their use could be relevant to improve food security in arid regions owing to their ability to withstand adverse agroecological conditions [54].
Guinea millets, together with tubers, e.g., cassava, were the food base of island populations until the introduction of maize by the end of the 15th century/early 16th century [55]. These small grain crops are now uncommon, with maize (Zea mays) and beans (Cajanus cajan, Lablab purpureus, Phaseolus lunatus, Ph. vulgaris and Vigna unguiculata) being the prime food species in Cabo Verde [6].
Currently, maize and bean are the ingredients of traditional dishes: the "xerém" and "couscous", prepared in different ways with maize, and "cachupa", prepared with maize, several species of beans, cabbages, cassava and sweet potato.
Besides millets, other valuable small grain crops-Amaranthus caudatus and A. cruentus-are also present in Cabo Verde. These minor crops are presently underused but are becoming increasingly relevant as alternative crops in dry and semi-dry areas where major crops do not develop well [56], representing a promising resource to support food security.
In the early centuries of the archipelago's colonization, several plants played crucial roles in the local economic activities. That was the case of native tanning plants, such as Periploca chevalieri and Euphorbia tuckeyana (tanned leather was one of the most reputed exports) and the orseille, Roccella spp. or Ramalina spp. (lichens, a taxonomic group not included in the present work), widely used to dye textiles ("panos da terra") [55] and exploited in Cabo Verde since 1469 [57]. Other relevant productions were indigo (obtained from Indigofera tinctoria), extracted and used around the 16th century [58], cotton (Gossypium spp.), sugarcane (Saccharum officinarum) and the American physic nut (Jatropha curcas), whose seed oil was extracted to make soap or candles, all main exports during the 19th century [55]. Today, and except for sugarcane-used to produce "grogue" (an alcoholic spirit similar to rum), one of the main exports, produced in Santo Antão, São Nicolau and Santiago-and coffee (Coffea spp.)-in Fogo and São Nicolau islands (where it was introduced in 1778, [59]-these activities are almost completely abandoned. Species used for utilitarian purposes include Sida rhombifolia, to make brooms; Urena lobata and Calotropis procera, to obtain fiber, with the latter used to fill mattresses and pillows (as reported by Roberts and Defoe [22]) as well as for firewood; Phoenix atlantica leaves, for basketry; Cyperus alternifolius subsp. flabelliformis, for mat weaving; and Dichanthium annulatum and Imperata cylindrica, for roofing.
For centuries, the continuous need for wood as a fuel for cooking and as a building material led to the overexploitation of the few native woody species and, therefore, to a severe reduction of their populations. There are interesting references in the early 18th century [22] to the over-exploitation of wild fig trees (probably Ficus sycomorus and F. sur) to build canoes, and of the dragon tree (Dracaena caboverdeana) to build houses in São Nicolau. Other widely used species were Sideroxylon marginatum, for timber, and Arthrocaulon franzii, Calotropis procera, Dichrostachys cinerea, Echium vulcanorum, Launaea arborescens, Tamarix senegalensis and Tetraena gaetula subsp. waterlotii for firewood.
The social and cultural role of plants is also relevant in Cabo Verde, with particular species being of great symbolic value for ceremonial festivities. A few examples are the use of leaves of Phoenix spp. for religious ceremonies, such as at Easter time, or to decorate the streets to welcome governors in colonial times, or the use, in São Nicolau, of the endemic Asteriscus smithii in bonfires on Saint John's eve.
To meet population needs, as well as to restore degraded land and improve soil characteristics, several woody species were introduced through afforestation programs, especially by the mid-20th century. In the highlands of Santo Antão, Santiago and Fogo, species of the genera Eucalyptus, Hesperocyparis and Pinus, among others, were and still are widely planted, while the lowlands are extensively afforested with the Southern American Prosopis juliflora, species of the genera Acacia and Vachellia, and Ziziphus mauritiana. To halt the erosion of slopes, several exotic species such as Aloe vera, Lantana camara, and Furcraea foetida were formerly used. Native grasses, such as Heteropogon contortus and Bothriochloa bladhii, are also reported as important species for this purpose, and their use may be a good alternative to consider.
With the increasing valuation of native species (e.g., Bozzano et al. [60]), the use of autochthonous resources in reforestation/afforestation programs is now being promoted by the Cabo Verdean authorities in charge of forestry-e.g., Direção Geral da Agricultura Silvicultura e Pecuária (DGASP-MAAP) and international agencies (such as the World Bank, the United States Agency for International Development, or the Global Climate Change Alliance). These species are better adapted to local conditions and more likely to enhance biodiversity and improve ecosystem services while providing traditional products (e.g., fruits, wood) to local communities.
Most of the useful species in Cabo Verde are introduced and/or distributed worldwide and, as such, they do not represent serious conservation issues, except for the environmental impacts resulting from the invasive behaviour of some of them. This is the case with Lantana camara, Furcraea foetida, Prosopis juliflora or Eucalyptus spp., which are currently seriously damaging native species and ecosystems.
More worrying is the overuse of some native species (e.g., the non-endemic Tamarix senegalensis or Ficus spp.) and, in particular, of the 38 endemic species listed in the present work, most of them endangered. Reported for all use types, they are relevant in the livelihood of Cabo Verdean population.
The end of some commercial activities (e.g., tanning, dyeing) and the switch from firewood to cooking gas, especially in rural households, have reduced anthropogenic pressure, but the sustainability of some native plant populations (e.g., Sideroxylon marginatum, Dracaena caboverdeana) is far from certain.
Finally, most ornamental species are introduced and do not raise conservation concerns. However, some native or even endemic species (e.g., Echium spp., Nauplius spp., Phoenix atlantica or Aeonium gorgoneum) are become increasingly attractive to local population as ornamentals and for other purposes, which may threaten their populations. This situation can be avoided, provided that cultivation is promoted and no pressure is imposed on wild populations. On this issue, it is interesting to note that schemes are being proposed (e.g., Krigas et al. [61]) to assess the potential of neglected or underused local endemic plants. Ensuring the safeguarding of native populations, which in Cabo Verde involves their prior recovery, will allow for the sustainable exploitation of these resources and benefit the local economy. In this respect, it is important to mention that several local initiatives, promoted by government agencies and non-governmental organizations (for example, the "Associação Projecto Vitó" on Fogo Island), have played a relevant role in informing and involving local populations in the safeguarding of threatened taxa.

The Study Area
Cabo Verde is a volcanic archipelago in the Atlantic Ocean, with 10 islands and several islets, about 600 km off the West African coast. The topography is generally very rugged, with high massifs and deep valleys. The island of Fogo reaches the highest elevation at 2829 m, followed by Santo Antão (1979 m), Santiago (1392 m), and São Nicolau (1304 m) [13].
A dry tropical climate with two well-marked seasons (humid and dry) and a limited and irregular rainfall (mean annual value about 300 mm) constrains the distribution of flora and vegetation. However, the topography contributes to significant spatial variations according to altitude and exposure to prevailing winds, leading to contrasting weather conditions [13,45]. Cabo Verde's biodiversity is poor when compared to the other archipelagos of Macaronesia [62]. Native flora comprises about 400 taxa [63], of which 92 are endemic [32]. Plant communities are diversified and include open woodlands, scrubs, savannas and grasslands [33].
Among the main economic activities are livestock rearing (cattle, goats, poultry, pigs, rabbits, donkeys, and horses) and agriculture, with the latter limited to areas of adequate topographic and edaphoclimatic conditions [4,52].
To mitigate the effects of erosion, afforestation programmes were initiated in the 19th century and intensified during the 20th century, especially in mountainous areas of higher altitude islands (namely, Santo Antão, Santiago and Fogo) and in the more humid windward-facing slopes; the forestation of arid lowlands is more recent, dating from the second half of the 20th century [64,65].

Listing Useful Plants
A comprehensive review of the literature, including the Flora of Cabo Verde [66][67][68] and scientific publications (e.g., [11,15,16,25,29,31,63,), was undertaken to compile the list of useful plants in Cabo Verde and respective common names. We also examined several historical documents published between 1506 and the late 19th century, namely Valentim Fernandes 1506-1510 (in Monod et al. [18]), Gaspar Frutuoso, 1522-1591 (in Frutuoso [20]), Roberts and Defoe [22], Feijó (in Feijó [24], and Gardère et al. [25]), Forster [23], Chelmicki & Varnhagen [26,27], and Ficalho [58], which allowed us to identify the species used since the first settlements. These historical references help to understand how and when species were introduced and to provide a historical perspective on this subject. Information collected from herbarium specimens, especially LISC Herbarium (University of Lisbon), which hosts one of the most complete collections of Cabo Verdean plant species, was also used, as well as data collected in all the islands during field surveys performed by the authors (especially M.C.D., I.G., S.G., A.R., and M.M.R.) during the last decades.
Taxa nomenclature primarily follows Plants of the World Online [30]. Other databases, such as World Flora Online [34], were occasionally used. Note that there is still some uncertainty about the classification of some endemic taxa, which has led to frequent nomenclatural changes; the most relevant cases (for example, in the Apiaceae) are duly noted.
Information about growth form (habit) was obtained from taxa descriptions in the Flora of Cabo Verde and in World Flora Online [34]. Seven categories were considered: annual/biennial herbs, perennial herbs, shrubs/subshrubs (inc. rosette shrubs), trees (including palms and tree-like species), annual herbaceous climbers (annual vines), perennial herbaceous climbers (perennial vines) and woody climbers (lianas).
Eleven categories of uses (adapted from Cook [97]) were considered: food for humans (including beverages, food additives, spices, condiments, colorants, etc.); forage for livestock (including plants for pasture); materials (including plants producing gums, resins, oils, latex, waxes, tannins, dyes, etc.); timber; poison (e.g., for hunting and fishing, or used as biocides); melliferous; social uses (including stimulant, smoking materials, and plants used in ceremonial or ritual practices); fuelwood (firewood, charcoal); utilitarian (including plants used to make domestic utensils or tools, and sources of fibres); ornamental (garden plants, street trees, hedge plants, etc.); and environmental use (for revegetation, forestation and erosion control, as windbreaks, etc.). Medicinal species and gene source plants were not included in the present analysis.
Species distribution in Cabo Verde and worldwide was mainly based on the Flora of Cabo Verde [66][67][68], Sánchez-Pinto et al. [63] and Plants of the World Online [30]. Biogeographic distribution was established for each taxon using the regions established by Morrone [98] (Nearctic, Palaearctic, Neotropical, Afrotropical, Oriental, Australotropical, Andean, Afrotemperate, Antarctic, Neoguinean, Australotemperate and Neozelandic). When the species occurred in two or more regions, classification was based on the main distribution area(s). To avoid a large number of classes with a very low representation in Cabo Verde, some regions were merged (for details see the legend of Figure 6).
The conservation status was obtained from Romeiras et al. [37] and the IUCN Red List of Threatened Species [36].

Geographic and Socio-Economic Data
Fourteen indicators, summarized in Supplementary Materials Table S2, were selected to study the relationships between the diversity and distribution of useful flora in Cabo Verde with geographic, demographic, and economic factors. Data were obtained from official sources, produced by public authorities, namely Instituto Nacional de Estatística [14, 99,100], Ministério da Agricultura e Ambiente [101] and Ministério do Desenvolvimento Rural [102].

Data Analysis
The relationships between the diversity and distribution of useful species and the geographical, demographic and economic indicators were analyzed using Spearman's rank correlation coefficients. Correlation values were calculated using the "cor" function and the "Spearman" method. A probability value equal to or less than 0.05 was used to determine statistical significance. The visualization of the correlation matrix was produced with the packages "ggplot2" and "ggcorrplot" and the function "ggcorrplot". All these statistical analyses were performed in R v. 4.0.5 [103].
The chord diagram showing the relation between the uses and the habit of the taxa was performed with the package "circlize" v.0.4.14 and the function "chordDiagram".
The heatmap was produced based on the number of taxa with different use categories, for each island. This analysis was performed with the function "heatmap", and the resulting chart was normalized by columns.
The treemap chart, providing a hierarchical view of the data, was created in Microsoft Excel v.2202.

Conclusions
This study shows that the biodiversity of Cabo Verde is threatened by human activities that meet the basic needs of local populations in the particularly harsh environment of this Macaronesian archipelago. This calls for the integration of scientific information with local indigenous knowledge, but comprehensive knowledge of the plants traditionally used is unavailable. Therefore, we used an integrative approach (i.e., (1) the taxonomic diversity and phytogeographic distribution of useful plants in Cabo Verde; (2) the role of geophysical, demographic, historical, economic, and socio-cultural factors on species distribution and uses; and (3) the potentially relevant species, particularly native ones) for the sustainable development of the archipelago. Information about the plant species and their uses was obtained for 11 categories of uses (e.g., food, forage/pasture, materials, timber, poison, melliferous, social, fuelwood, utilitarian, ornamental and environmental), and species biogeographic distributions and conservation status, as well as 14 geographic and socio-economic indicators, were also taken into account.
Cabo Verde was uninhabited before the mid-15th century, and many exotic plant species were introduced, first to ensure human survival and later also to support commercial interests. In this study, we identified 518 useful taxa (88 plant families), of which only 145 are native and 38 are endemic. Taxa of Afrotropical origin prevail as forage, whereas ornamentals are mainly Neotropical. Only less than 38% of the recorded taxa were assessed by the IUCN Red List, and they include 44 species classified with some level of threat. The number of useful taxa is correlated with altitude and agricultural area, as well as with the size of the rural population, number and type of farms, but not with total population or socio-economic indicators such as the number of tourists or gross domestic product. Plants are primarily used as ornamentals and for forage and food; native taxa-including many threatened ones-are mostly used for fuelwood, forage and utilitarian purposes. The islands where agricultural activities prevail present more useful taxa than the others. The agrobiodiversity and traditional practices (e.g., multiple cropping, selection of resistant varieties and storage of propagules) are crucial to cope with recurrent droughts and to ensure food security in Cabo Verde. Our inventory discloses previously ignored information on particular species; in particular, some species are no longer used.
This study improves the knowledge of the useful plants of Cabo Verde. Most of the useful species are introduced but do not represent serious conservation problems; of much more concern is the overuse of some native taxa and, in particular, of 38 endemics listed, most of them endangered. Only by ensuring the safeguard of native plant populations in Cabo Verde will the sustainable exploitation of these resources be possible and benefit the local economy.
Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/plants11101313/s1. Table S1. Useful plant species in Cabo Verde islands: taxonomic diversity, origin and uses. Table S2. Geographic, demographic and economic indicators for Cabo Verde.