The mainland of Equatorial Guinea has been much less explored than the island territories. The first known collections were made by the German Gustaf Mann (1836-1916), expeditionary of the Baikie’s Niger Expedition. Although he focused his work on Bioko, he also collected fifty specimens in Litoral province (Corisco, Elobey, and in the Kongui river [Congüe]) in 1862. These specimens are preserved mainly in the K herbarium.

It took almost half a century for the explorations in this area to continue, carried out by another German explorer, Günther Tessmann (1884-1969), who in 1908 collected 100 specimens in Litoral province and especially in Wele Nzas (hosted at K), at the Nkolentangan (Alén), Uelleburg and Bebai stations. In 1928, the Spanish forestry engineer Fernando Nájera, commissioned by the “Dirección General de Marruecos y Colonias”, visited the continental area to study the forest richness of Spanish Guinea. During the four months that he stayed in Río Muni, he studied and identified around 60 types of wood. These collections were probably lost from the Madrid “Escuela de Ingenieros de Montes” due to a fire. The result of Nájera’s study was published by the “Instituto Forestal de Investigaciones y Experiencias” (Nájera 1930).

The history of Spanish botanical collections in the continental region begins with the pharmaceutical inspector Lope del Val Cordón (1896-1967), who visited the area between June and July 1939, collecting 650 specimens in the four continental provinces. The first botanist to take a decided interest in the flora of Equatorial Guinea was Emilio Guinea López (1907-1985). He made only one trip to the continent between June and September 1945. Guinea indicates that he collected 3,000 specimens. This number is difficult to confirm due to the complicated numbering system that he used. What we know for sure is that more than 1000 vouchers are preserved in MA. A part of these specimens is found in Lisbon (LISC), where Emilio Guinea traveled to identify this material, and perhaps in London (K). The itinerary of his trip shows that he traveled through most of the continental region, with long walking journeys through the most intricate of the tropical forest. Although he collected in the four provinces, it is noteworthy his systematic and intense tours to the Wele Nzas and Kie Ntem provinces. A first advance of his work was published in the Anales del Jardín Botánico de Madrid (Guinea 1945). The following year he published what is still today the main work on the flora of Río Muni: Ensayo Geobotánico de la Guinea Continental española (Guinea 1946). Guinea was occasionally accompanied by J. Gómez Moreno. It will be almost fifty years after (1946), before floristic studies were resumed in the continental region starting in 1990. The Spanish-Portuguese Manuel Fidalgo do Carvalho, who had already collected in Bioko, moved his residence from Malabo (Bioko) to Bata on April 10, 1991, although he had already made some collections in Río Muni in December 1990. Carvalho remained on the continent until September of 1997. During his time in Río Muni, he made 1896 botanical collections. The abundant material that he collected from each specimen is very remarkable, always from fertile plants, and perfectly prepared and preserved. A complete series of his collections is held in MA, with numerous duplicates in major European and North American herbaria (AAU, B, BM, BR, COI, FI, G, H, K, LG, LISC, MO, NY, P, S, UPS, US, WAG, etc.) as well as in the National Herbarium of Equatorial Guinea (BATA).

At the same time of Guinea’s collecting work, in the 90s, the CUREF (“Conservación y Utilización Racional de los Ecosistemas Forestales de Guinea Ecuatorial”) project, funded by the European Union, started, with special emphasis on the study of plants. The botanical leader of the project was Jean Lejoly (BRLU) with the invaluable help of Stanislaw Lisowski (POZG). Several European botanists collected with the support of this project: Tariq Stévart (BRLU, MO), Ingrid Parmentier (BRLU), Bruno Senterre (BRLU) and Stephan Porembski (ROST) who collaborated in the training of a group of Guinean botanists from the INDEFOR (“Instituto de Desarrollo Forestal”), under the newly created BATA herbarium: Crisantos Obama, Pablo Esono, Norberto Nguema, Ildefonso Ndong, Domingo Ngomo, Francisca Eneme, and Diosdado Obiang Mbomio. In addition, there were six additional plant collectors who sometimes accompanied the CUREF-INDEFOR collecting team: Pierre van Asbroeck (BRLU) who collected in 1998 with Lejoly; N. Biedinger (BRLU) in 1998 with Porembski and N. Nguema, and Elad Maurice Epah (BRLU) in 1998 with Lejoly.

In 1997, with the financial support, of the “Agencia Española de Cooperación Internacional y Desarrollo” first, and later the Spanish Ministerio de Ciencia (in their different denominations), the Real Jardín Botánico, CSIC, began a project to study the flora of Equatorial Guinea, including Río Muni, Bioko and Annobón. It was directed by Mauricio Velayos (1955-) from the Real Jardín Botánico, CSIC. Since then, apart from M. Velayos, a team of researchers participated and directed the exploration and collection trips through the four continental provinces: C. Aedo (MA), F.J. Cabezas (MA, currently at MACB), M. de la Estrella (MA, currently at COA), P. Barberá (MA, currently at MO) and M. Fero Meñe (MA, currently at UNGE). In total, 6,800 specimens were collected under the Flora of Equatorial Guinea project.

Other people also participated in plant collecting campaigns organized by the Real Jardín Botánico: Raquel Pérez-Viso, who lived in Bata between 1998 and 2000 and collected intensively in Río Muni (4,500 specimens, especially from the Centro Sur Province). Myriam López Serrano, a pharmacist from Granada, focused on the study of the traditional use of medicinal plants of the Centro Sur Province. Encarnación Gómez Marín and Laureano Merino Cristóbal collected 160 plant specimens for the publication of the book Plantas Medicinales de Guinea Ecuatorial. Up to 48 more botanists visited Equatorial Guinea sporadically and collected in the country, but all together collected less than 100 specimens.

Most recent published data from Río Muni are those of Sosef & al. (2017). In this study, the reported plant species richness for the whole of Equatorial Guinea is 3,049 species, increasing the estimated figure up to 3,821 species. This approach was based on the study of 15,341 records reflecting a Collection Density Index (CDI) for the country (number of collections/100 km²) of 54.69 (Campbell 1989). When CDI is calculated, the figures reveal a moderate level of knowledge of the region and more precisely from the continental region. These authors suggest a minimum of a CDI of 100 before one can state that the botanical inventory of a country or a region is of an acceptable level. If we calculate the CDI of the four provinces of Río Muni, only the Litoral Province is near the level of the CDI of 100, being Kie Ntem and Wele Nzas the provinces with the lowest botanical knowledge (Table 1).

If we plot the collection localities from the 23,517 specimen records (Fig. 1) we observed that plant-collecting efforts are geographically biased and some inaccessible places remain poorly known, thus botanical exploration in Río Muni is still one of the main tasks for future biodiversity studies.

Due to their biological importance, floristic inventories of tropical African underexplored areas are an urgent need. Despite of the huge development of communication infrastructures in the continental region of Equatorial Guinea, a large percentage of its territory is still inaccessible (Fig. 1). Besides, some of the published data are not supported by herbarium specimens, thus so much of the information available is not reliable. Many of the analyses, conservation plans, or management strategies that have been developed in African countries have been made with outdated bibliographic data and assuming complete biodiversity inventories, which are far from being so. Thus, it is quite common in these regions (tropical Africa) that when new collections are added, species numbers change drastically. Similar results are usually observed with new taxonomic treatments or systematic analyses. Endemic or threatened species are usually known from a few collections, which require new data to properly evaluate their real taxonomic status or threat level. The Río Muni Vascular Plants catalogue is a compendium of numerous cases of the above-mentioned problems.

The checklist of Vascular Plants from Río Muni comprises 178 families, 1,020 genera and 2,707 species and infraspecific taxa (Appendix 1). Pteridophytes represent the 5% of the species (142) and the 12.35% of the families of Río Muni (22). With these figures, the floristic knowledge of Río Muni, in terms of number of species, increases in 26.14% from the most recent studies (Table 2). The number of taxa considered as native is 2,411 (89.96%), 32 species (1.2%) are introduced, 36 cultivated (1.35%), and 158 (5.92%) should be considered as naturalized since they are able to establish new populations.

The most diverse families are Rubiaceae (294 species), Fabaceae (290), Orchidaceae (168) and Poaceae (105). The 10 most species-rich families are completed by Euphorbiaceae (87), Apocynaceae (85), Cyperaceae (79), Annonaceae (68), Acanthaceae (65) and Melastomataceae (61). These 10 families alone comprise about half of the species (49.22%) of Río Muni (Fig. 2).

This floristic pattern of family diversity is similar to the one found in Annobon island (Velayos & al. 2014). Such pattern is observed even when total species-richness figures are 10-times bigger (Annobon 24 species of Fabaceae, Río Muni 290) and it highlights the importance of the flow of species across the territories of the Gulf of Guinea. This idea is supported by the low number of endemic species observed in these territories.

Although the botanical exploratory level of Río Muni is relatively high when compared to the Central African countries, the number of species recorded so far is still far from the observed in neighboring Cameroon or Gabon, with 6,883 and 5,236 species respectively (Sosef & al. 2017). Besides the differences in area among the three of them, these figures are pointing out the need of continuing with the collection effort in Río Muni. The number of species in the continental region and the entire Equatorial Guinea would likely increase if plant collecting efforts were extended to the poorly explored areas in the near future.

The number of threatened taxa (VU, EN and CR) is 144 (5.31% of the total taxa) of which 48 are under extinction risk, i.e., considered Endangered or Critically Endangered (Table 3). Two endemic species, Polyscias aequatoguineensis and Scleria mongomoensis are categorized as Endangered and should be regarded as prioritary species for Conservation plans and strategies. Nonetheless, a revaluation of the real situation of these species should be even more important to do since preliminary data indicate that the real threatened status of Vascular Plants in Equatorial Guinea is far from well known. As an example, we focus our discussion below on endemic or subendemic plants from Río Muni.

Of all threatened taxa, only three are non-native to Río Muni namely Terminalia neotaliala, Gossypium hirsutum (both naturalized) and Memecylon erythranthum (introduced); and only 11 species could be considered as strictly endemic to Río Muni (Table 4). This low number of endemic taxa is reflecting the absence of natural barriers in the territory.

From the 13 endemic/subendemic species, three of them (Scleria mongomoensis, Polyscias aequatoguineensis, and Grossera angustifolia) have been recently evaluated by the IUCN as EN (Larridon & Bauters 2020; Paradis & al. 2021; Ruiz de Diego & al. 2022). Guyonia obamae records from Río Muni have been identified as G. arenaria, and the species was preliminary assessed as NT (Lachenaud & al. 2018), nonetheless its taxonomic situation is still unclear.

We confirm that 12 out 13 of the endemic species evaluated should be considered as threatened (Table 4). Only for the subendemic Polystachya riomuniensis, since the distribution range of the species is quite large, the Extent of Occurrence (EOO) criteria states it as Least Concern while the Area of Occupancy (AOO) indicates the taxon as EN. We consider a more realistic conservative approach until taxonomic studies confirm the identity of these 12 species or new collecting campaigns confirm the presence of these taxa beyond the current distribution ranges.

Scaphopetalum obiangianum is a shrub 20-150 cm tall, known from the understory of rain forest, at around 700 m in elevation (Leal 2007). The species is endemic to Equatorial Guinea and is only known from one collection made by Leal in 2005 in Monte Mitra, which thus represents a single occurrence in the Litoral Province. The AOO is estimated as 4 km², below the upper threshold for Critically Endangered status under subcriterion B2. The EOO is the same as the AOO. This occurrence therefore represents the entire known population of the species. It is located within a protected area (Monte Alen National Park), and no specific threats have been identified at the present time. Therefore, Scaphopetalum obiangianum is assigned a preliminary status of VU D2.

Begonia aequatoguineensis is an herb up to c. 5 cm high, known from primary forest at c. 450 m altitude (Sosef & Nguema Miyono 2010). The species is endemic to Equatorial Guinea and is only known from one collection, which thus represents a single occurrence, made by Nguema Miyono at West of Mendung, in the Centro Sur Province. The AOO is estimated as 4 km², below the upper threshold for Critically Endangered status under subcriterion B2. The EOO is the same as the AOO. This occurrence therefore represents the entire known population of the species. It is located within the protected area Monte Alén National Park and no specific threats have been identified at the present time. Therefore, Begonia aequatoguineensis is assigned a preliminary status of VU D2.

Asplenium carvalhoanum is an epiphyte, known from primary rainforest, endemic to Equatorial Guinea (Herrero & al. 2001). The species is only known from one collection, made by Carvalho in Cogo, between Emangós and Ncó villages, in the Litoral Province, which thus represents a single occurrence. The AOO is estimated as 4 km², below the upper threshold for Critically Endangered status under subcriterion B2. The EOO is the same as the AOO. This occurrence therefore represents the entire known population of the species. It is not located within a protected area and is threatened by logging and by small-scale harvesting. The most serious plausible threat is the logging. We infer a future decline in the extent and quality of its habitat, therefore Asplenium carvalhoanum is thus assessed as CR B2ab(iii).

Macropodiella uoroensis is endemic to Equatorial Guinea, known from the rapids of the Uoro river, at Centro Sur province. The species is only known from one collection made by Rial & Laso in 1994, representing a single occurrence. The AOO is estimated as 4 km², below the upper threshold for Critically Endangered status under subcriterion B2. The EOO is the same as the AOO. This occurrence therefore represents the entire known population of the species. It is not located within a protected area and is potentially threatened by water pollution due to increase in urbanization. We infer a future decline in the extent and quality of its habitat hence, Macropodiella uoroensis is assessed as CR B2ab(iii).

The geographical analyses of threatened species from Río Muni revealed the value of the National System of Protected Areas of Equatorial Guinea (Fig. 3). A total of 618 records out of the 23,517 compiled correspond to threatened species (CR, EN or VU), 224 of them are outside protected areas, which means that 36.25% of the threatened plants of Río Muni do not have any protection at all. If we consider those Near Threatened taxa, there are 721 total occurrences, 276 outside protected areas (38.28%). Monte Alén National Park is by far the most important Protected Area in Río Muni, hosting 49.23% of the occurrences of threatened species in this continental region. Two protected areas, Río Campo, and Montes Temelón do not include any occurrence from threatened species (Fig. 3).

The conservation relevance of the system of protected areas of Equatorial Guinea comes to the fore when we consider the endemicity of the flora. Figure 4 shows the spatial distribution of the 13 endemic and subendemic species. Twenty-one of these occurrence records (46.6%) are included in protected areas. In this sense, the situation of Scleria mongomoensis, Macropodiella uoroensis, Polyscias aequatoguineensis, Grossera angustifolia and Asplenium carvalhoanum is particularly delicate since none of their localities has any protection status. As aforementioned, all of them, but Polyscias aequatoguineensis, could be also threatened under the IUCN category of Critically Endangered.

Expanding the area of the Monte Alén National Park and establishing a monitoring program for these species to establish corridors between protected areas or new areas of special interest for the flora are necessary strategies to achieve a sustainable management of plant diversity in Equatorial Guinea. Connecting the different protected areas or increasing the surface area of some of them seems to be the best alternative to include the greatest number of threatened species present in this region. Finally, we emphasize that coastal meadows are especially under protected from human impacts.