Title in English

Contribution to the ecological study of orchids in the Comoros archipelago: floristic diversity, biogeography, ecological status, and conservation challenges

Title in French

Contribution à l’étude écologique des orchidées de l’archipel des Comores : diversité floristique, biogéographie, statut écologique et enjeux de conservation

Maoulida Mohamed ABDILLAHI^1,2,4*; Frédéric BIORET²; Vincent BOULLET²; Étienne VENNETIER³

1. Laboratory of Geobotany and Phytosociology, INRAPE, National Research Institute for Agriculture, Fisheries and the Environment, P.O.Box 404, M'dé, Ex-CEFADER, Comoros

2. University of Brest, EA 7462 Geoarchitecture Territories, Urbanization, Biodiversity, Environment, Brest, France

3. Teacher, LPO Gustave Eiffel, Kahani, 97680 Ouangani, Mayotte

4. BP. 906. Specialization in Plant Biology and Ecology, Faculty of Sciences, University of Antananarivo

INTRODUCTION

Located in the Indian Ocean, the Comoros archipelago consists of four volcanic islands: Grande Comoros, Anjouan, Mohéli, and Mayotte (Figure 1). The first three are under Comorian authority, while Mayotte is under French administration. It has undergone several biological migrations, generally originating from the East African coast and neighboring islands (Madagascar, the Mascarene Islands, etc.).

This archipelago and all the islands of the southwestern Indian Ocean are among the world's biodiversity hotspots (Dorr, L. J., 1997; Labat J. et al., 2010), particularly for orchids. Orchids form the royal family of the plant kingdom (Stewart and Griffiths, 1995; Gravendeel et al., 2004), with over 25,000 species, of which approximately 1,000 are found in the Indian Ocean region. More than 1,000 species, distributed among 65 genera, are native to Madagascar and the surrounding islands: the Comoros, Mascarene Islands, and Seychelles (Benny et al., 2019), and are found in numerous climatic zones around the globe. These entomophilous flowering plants are highly evolved. For example, in the genus Aerangis, a total of 58 species, including three natural hybrids, are currently recognized. They are distributed throughout tropical and southern Africa, Madagascar, the Comoros, Réunion, and Sri Lanka. Thirty-one species and two varieties are found in continental Africa, none of which are present in the islands of the Indian Ocean. Most sub-Saharan African countries are home to at least one species, with the largest number found in Tanzania, where 19 species have been recorded, followed by Kenya and Malawi, with 10 species each (La Croix 2022). In addition, 27 species have been identified in the western Indian Ocean region, namely Madagascar, the Comoros, and Réunion; all are endemic.

Twenty of these are endemic to Madagascar, three to the Comoros only, three to both the Comoros and Madagascar, and one, Aerangis punctata, to both Madagascar and Réunion. No species have been reported on the other islands of the western Indian Ocean. The orchids of the Comoros archipelago include endemic, native, and threatened species. The La Grille and Karthala massifs are home to the majority of Comorian endemic species (more than 50% of orchids including 45 endemic species) (Rakotoarivelo F. et al., 2011).

However, knowledge of this flora has many gaps. No complete and up-to-date list of orchids from the entire archipelago exists to date. Works on the orchids of the Comoros Islands are old and partial: the work of Schlechter, R. (1915) in "Kritische Aufzählung der bisher von Madagaskar, den Maskarenen, Komoren und Seychellen bekanntgewordenen Orchidaceen", that of PERRIER DE LA BÂTHIE (1939-1941) in the Flore de Madagascar et des Comores, and concern a small amount of material from old collections by Boivin & Humblot. The only recent work available to date is quite fragmentary and concerns only the study of a single genus (e.g., Jumellea) or a single species (e.g., Jumellea bosseri, Jumellea pailleri) (Bosser, J., 2002; Cribb, P. & Hermans, J., 2009; Neirynck and Herremans, 2006; Pailler et al., 2008; Pailler, 2009). Few studies have been conducted on the taxonomy of the genera. Since the last comprehensive revision by Perrier de la Bâthie (1939), eight new species of Jumellea have been legitimately described (Summerhayes 1952; Senghas 1964, Senghas 1967; Bosser 1970; Wood 1982; Cribb 2009; Herremans 2001; La Croix 2000; Pailler 2009).

Currently, only two orchid species (Jumellea anjouanensis and Angraecum humblotianum) are listed as threatened by the IUCN for Grande Comore, Anjouan, and Mohéli, both categorized as "endangered" (EN). All orchids are partially protected, without species specificity, on the list of protected species published in decree no. 01/031/MPE/CAB of May 14, 2001, concerning the protection of wild fauna and flora species in the Comoros (Louette, 2004).

On Grande Comore and Anjouan, terrestrial and marine protected areas are being developed, and Mohéli has a protected area, the Mohéli National Park. In Mayotte, a preliminary list of orchids already exists, categorized according to the IUCN (Barthelat et al., 2006; IUCN French Committee, 2013; Hervouet & Barthelat, 2014). Although the marine natural park was created in 2010 and protects the lagoon, its area is limited by the high tide mark (the upper limit of the public maritime domain) and therefore does not include terrestrial habitats. The existing species on the islet of Mbouzi have been protected since the creation of this 82-hectare islet as a nature reserve in 2007. To address this, this work consists of taking stock of the diversity and biogeography of orchids in the Comoros archipelago and determining the ecological status of each species.

Figure 1: Category (Endemism, Indigenous and Introduction)

MATERIALS AND METHODS

Three main methods were used in this work: literature reviews, ecological surveys, personal observations, and conservation status assessment:

- The literature reviews consisted of consulting all documents relating to our topic held in the libraries of national institutions (INRAPE library, CNDRS library) and international databases: the IRNAPE herbarium, the University of the Comoros herbarium, and international herbaria (National Museum of Natural History in Paris, Sonnerat, Tropicos-Misouri Botanical Garden, National Botanical Conservatory of the Mascarene Islands in Réunion and Mayotte).

- The spot ecological surveys and personal observations were carried out in two phases: June and July 2018 (first phase); January and March 2019 (second phase), during which field observations were made using the linear survey method (Duvigneaud, 1980) as well as the collection of samples and their placement in a herbarium at INRAPE.

- The assessment of the conservation status of the species was carried out according to the IUCN criteria and categories described in the two guides: the categories and criteria of the IUCN Red List, Version 3.1 (2012) (www.uicn.fr/La-Liste-rouge-des-especes.html) and the guidelines for the application of the IUCN Red List criteria at the regional and national levels, Version 4.0 (2012) (www.uicn.fr/La-Liste-rouge-des-especes.html); but also based on the Red List of the Flora of Mayotte (Barthelat et al., 2006; IUCN France, 2018; Hervouet & Barthelat, 2014).

RESULTS AND DISCUSSION

A-Results

1-   Floristic Diversity and Biological Spectrum

a-   Floristic Diversity

A total of 148 orchid species were recorded throughout the Comoros archipelago, distributed across 33 genera (Table 1).

The most represented genera are Angraecum (21 species and subspecies), Eulophia (17 species), Cynorkis (16 species and varieties), Bulbophyllum (14 species), Habenaria (9 species), Jumellea (8 species), Aerangis (7 species), Polystachya (6 species), Aeranthes, Disperis, Nervilia, and Liparis (5 species each) (Figure 2). In contrast, the other genera Angraecopsis, Benthamia, Bolusiella, Calanthe, Cheirostylis, Corymborkis, Cymbidiella, Didymoplexis, Galeola, Goodyera, Graphorkis, Hetaeria, Lemurella, Malaxis, Microcoelia, Microterangis, Neobathiea, Oberonia, Oeoniella, Platylepis, Satyrium, Vanilla, and Zeuxine are less represented, with one to three species each. Furthermore, the genera Acampe, Benthamia, Bolusiella, Calanthe, Corymborkis, Didymoplexis, Galeola, Graphorkis, Lemurella, Neobathiea, Oberonia, Oeoniella, Platylepis, and Stichorkis are monospecific.

Table 1: List of orchid species recorded in the Comoros archipelago (endemic species in bold) in the biography and some field observations

Indication :

GC: Grand Comoros (Ngazidja); AN: Anjouan (Ndzuani); MO: Moheli (Mwali); MA: Mayotte (Maoré). MADA: Madagascar; AFR: Africa; REU: Reunion; MAU: Mauritius; SEY: Seychelles; MOZ: Mozambique.

END: Endemic; INDI: Native; INTR: Introduced; IND: Undetermined; ?: Presence doubtful.

b- Biological spectrum, epiphyte substrate, and habitat

Three biological forms can be distinguished according to Raunkiaer's (1906) biological spectrum in the orchids of the Comoros Archipelago (Table 2): epiphytes (on tree trunks, lithophytes, or saxicolous) represent approximately 54%, geophytes (terrestrial orchids) approximately 46%, and lianas only 1%.

It should also be noted that, depending on the climate and vegetation type, most of the recorded orchids are found primarily in humid and transitional forests or savannas (151 species), compared to 5 species in dry areas (forest, scrubland, and savanna). The humid climate and vegetation type of the sites play very important roles in the establishment of these species, most of which are epiphytes. However, some species with very high ecological plasticity in these islands can be found in both humid and dry forests (examples: Angraecum humblotianum, Bulbophyllum hyalinum, etc.). The Comoros, located in the tropics, boasts a wide diversity of flowers, including orchids. Most often, they grow on trees, but some also grow on rocks.

Several species of trees and shrubs are used as supports by epiphytic orchid species in the vegetation formations of the archipelago. In humid forests and mesophyll forests, the species found are Weinmannia comorensis (Cunoniaceae), Ophiocolea comorensis (Bignoniaceae), Aphloia theaeformis (Flacourtiaceae), and Tambourissa leptphylla (Monimiaceae). In dry forests, thickets, and fields, the species found are Vitex doniana (Verbenaceae) and Mangifera indica (Anacardiaceae).

2-Table : Biological spectra of orchids

2- Biogeographical Affinity of Species and Endemism

The flora of these islands shares similarities due to their origin (islands of the Indian Ocean). However, there is a disparity in the distribution of endemic species and their presence on each island (Table 3). Grande Comore has the highest proportion of endemic species despite its relatively young age.

Table 3 : Distribution of endemic species and their presence on each island

Figure 1 shows that of the 148 orchid species inventoried for all 4 islands of the archipelago, 70 species (i.e. 47%) are endemic, 72 are native or naturalized (i.e. 49%), 5 are undetermined (i.e. 3%) and only one introduced species (i.e. 1%).

Endemic species are distributed as follows:

- 34 species are endemic to the Comoros and Madagascar (i.e., 92%), 2 species are endemic to the Comoros and the Mascarene Islands (i.e., 5%), and only one species is endemic to the Comoros and Africa (i.e., 3%) (Fig. 2). Thus, the species are much more closely related between the Comoros and Madagascar than with Africa and the Mascarene Islands.

- 31 are endemic to the Comoros, of which seven (7) are strictly endemic to Grande Comore and two (2) strictly endemic to Anjouan.

Figure 2: Affinity of the Comoros endemic species with Madagascar, Africa, and the Mascarene Island

3- Ecological status of the species

The threats to the habitats of the studied species are both anthropogenic and natural in origin; however, it is the anthropogenic threat that contributes most to the destruction of these species' habitats and leads to a reduction in forest area, as the majority of the species are forest dwellers. Indeed, the IUCN classification takes into account relationships with neighboring populations to avoid inappropriate estimates, for example, estimates that could be overestimated for populations at the edge of their range (Feldmann, 2008). Literature reviews, field data, analysis of the distribution map of selected species, and the threats and pressures exerted on species and habitats allowed for an estimation of extinction risks for all selected species, given the lack of well-defined and structured terrestrial Protected Areas in the archipelago. The area of ​​occurrence is less than 400 km² and the area of ​​occupancy is less than 80 km². Fifteen (15) species are categorized as Critically Endangered (CR) (10%), 7 species as Endangered (5%), 24 as Vulnerable (VU) (16%), 9 as Near Threatened (NT) (6%), 27 as Least Concern (LC) (18%), and 68 species do not allow for a threat category to be proposed according to the Red List criteria due to a lack of data (DD) (46%) (Table 4). This does not mean that they are not threatened or of special interest, but rather that particular attention must be paid to them to better assess their status.

4-Table : Ecological status of orchids according to IUCN criteria

Discussion

1- Floristic Diversity and Endemism

The terrestrial ecosystems of the Comoros archipelago include rainforests in the mountainous regions of each island and dry forests. Altitude and soil have contributed to a high level of diversity, including several endemic and threatened species, among them many orchids. The number of orchid species inventoried during this study is very high (33 genera, 144 species, 1 subspecies, and 3 varieties of orchids, with 70 endemic taxa), compared to previous studies, as shown in Tables 1 and 4. It should be noted that Moulaert (1998) adopted the number of species from Perrier de la Bathie (1939, 1941), adding 3 species, and Pailler et al. (2018) cited 28 endemic species (Table 5). Since 1915, inventories carried out mainly in Mayotte, to a lesser extent in Grande Comore, and very little in Anjouan and Mohéli, have led to the discovery of several species, some of which are new to science. Revision by genus has also contributed to finding more and more new species. In addition, other species have been synonymized (e.g., Microterangis hariotiana, Solenangis cornuta, etc.), as mentioned by Kaith et al. (2006).

For example, two new genera have just been added to our list: Didymoplexis (with the species D. avaratraensis, a species that until now had only been found in Madagascar and which has just been rediscovered in Mayotte) and Zeuxine, an African genus (with the species Z. balli, which does not exist in Madagascar).

5-Table : Comparison of orchids inventoried from 1905 to 2006.

2-   

3-   Biogeography of Species

The insularity of the Comoros archipelago, its location in the tropics, allows it to have floristic affinities with neighboring countries. However, these affinities are stronger with Madagascar than with the African and Asian continents or the Mascarene Islands, as demonstrated by Rakotoarivelo et al. (2013) for the genus Jumellea Schltr. For example, the genus Disperis comprises nearly 80 species distributed in Africa, Madagascar, and the islands of the Indian Ocean (La Croix et al. 2002; Kurzweil and Manning, 2005; Govaerts, 2019), including 3 species native to the Comoros and Madagascar. None of these species are shared between the Comoros and Africa. The flora of the archipelago is very similar to that of Madagascar (with variations specific to each island) and can be considered a less diverse version of the main island (Louette et al., 2004). According to Pascal et al. (2001), one of the striking features of Mayotte's flora is the considerable proportion of Malagasy plants, which are three times more numerous than plants of African origin.

         From a botanical point of view, Mayotte is merely an extension of Madagascar. Recently, some thirty tree species previously considered endemic to Madagascar have been found in Mayotte, reinforcing the dominance of Malagasy plants on the island (Pascal, 2002). This could be due to the island's biogeographical isolation from Africa, which is a continent.

Other factors may also play a role, such as prevailing winds, cyclones (which move from Madagascar to the Comoros), ocean currents, and animals. Indeed, oceanic islands formed by submarine volcanism, such as the Comoros Archipelago, exhibit a distinct originality in terms of plant biodiversity (biogeographical endemism), but also a relative poverty compared to the continental landmasses from which colonizing organisms originate (Abdillahi, 2017). For example, as the closest landmass to Réunion Island, Madagascar provided the majority of the seeds for the native plants currently inhabiting the island. A study conducted on Dombeyoidae clearly illustrates the multiple Malagasy origins of their species (Pechon et al., 2010).

3- Risk of Species Extinction

For a preliminary assessment, the results obtained in this study are broadly similar to those obtained for Réunion Island for 130 species assessed, including 9 critically endangered (CR) species (6%), 18 endangered (EN) species (12%), 9 vulnerable (VU) species (6%), 9 near threatened (NT) species (6%), 57 of least concern (LC) species (38%), and 43 with sufficient data (DD) species (27.7%) (http://www.reunion.developpement-durable.gouv.fr/IMG/pdf/Dossier de presse Flore vasculaire de La Réunion_cle567cb3). The destruction and degradation of natural habitats represent the main cause of the decline of plant species in Réunion.

Urban and agricultural development is responsible for the progressive disappearance of numerous species. Furthermore, some plant species are subject to intense harvesting pressure.

Due to their beauty, several orchid species are harvested for commercial purposes (http://www.reunion.developpement-durable.gouv.fr/IMG/pdf/Dossier de presse Flore vasculaire de La Réunion_cle567cb3). Throughout the islands of the Comoros archipelago, the original vegetation has undergone extreme decline. The proportion of land covered by forests fell from 6.6% in 1990 to 1.7% in 2012, meaning that nearly 500 hectares of forest disappear each year (FAO, 2010 and 2012) across the three islands. In Mayotte, natural formations now cover only 4 to 5% of the land area. Fragile ecosystems, such as dry forest, survive only in fragments. They have given way to slash-and-burn farming systems or the expansion of urban areas (Keith et al. 2006; Abdillahi, 2009 and 2017).

CONCLUSION

The Comoros archipelago is rich in plant biodiversity, particularly orchids. One hundred and forty-five species, one subspecies, and three varieties, distributed across 33 genera, were inventoried during this study. The most represented genera are Angraecum, Eulophia, Cynorkis, Bulbophyllum, Habenaria, Jumellea, Aerangis, Polystachya, and Aeranthes. Epiphytic and geophytic species are distributed almost equally (54% and 48%, respectively), with lianas representing only 1%. The majority of these orchids are found in humid and transitional forests and/or savannas, with very few in dry areas.

Seventy (70) species (47%) are endemic, 72 are native or naturalized (49%), 5 are undetermined (3%), and only one is an introduced species (1%), with variations between the islands. Grande Comore is the island richest in endemic species. These species are more closely related to Madagascar (47%) than to the African continent (3%), the Asian continent (1%), or the Mascarene Islands (2%).

Within this orchid flora, fifteen (15) species are critically endangered (CR) (8%), seven (7) are endangered (EN) (5%), 25 (25) are vulnerable (VU) (16%), nine (9) are near threatened (NT) (6%), 27 (27) are of least concern (LC) (18%), and 68 (46%) are of data deficient (DD). The habitats of these species are subject to various anthropogenic and natural pressures and threats, the former being the most significant.

This article aims to provide objective data to support conservation actions that could be integrated into management and conservation plans for the orchids of the archipelago, including the delimitation of these species' habitats into protected areas and raising awareness among national and local authorities and the public about the biological and ecological importance of orchids. Further studies will be needed to further explore the ecology, diversity, and biogeography of these species in order to gain a comprehensive understanding of the orchids on each island of this archipelago.

Acknowledgments

We thank all those who supported us in conducting this field study and writing this article, in particular the Phytosociology and Geoarchitecture Laboratory at the University of Western Brittany and the SCAC service of the French Embassy in Moroni.

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https://inpn.mnhn.fr/espece/cd-nom/706149

http://floremaore.cbnm.org/index.php?option=com_content&view=article&id=3&Itemid=12

https://sites.google.com/site/biodiversitedescomores/home/espces-en-danger

science.mnhn.fr/institution/mnhn/collection/p/item/p00527425?listIndex=17&listCount=21

http://wcsp.science. kew.org

www.orchidspecies.com

www-mi-aime-a-ou.com

www.africanorchids.dk

www.ville-ge.ch/musinonfo/bd/cjb/africa/details.php

www.efloras.org/florataxon.aspx?flora_id=12&taxon_id=250080643

www.tropicos.org/Name/23513125?projectid=17

www.lesîlesdelalune.com

http://www.reunion.developpement-durable.gouv.fr/IMG/pdf/Dossier de presse Flore vasculaire de La Reunion_cle567cb3