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Socotra Archipelago (Yemen)
Abstract
The Socotra Archipelago contains islands of high conservation importance. Since 2003, the Socotra Archipelago is a UNESCO Man and Biosphere Reserve and since 2008 it was officially listed as a UNESCO Natural World Heritage Site. In addition, the Socotra Archipelago contains currently the only Ramsar Site of Yemen (Detwah Lagoon), and it has been declared as one of 200 WWF Global Ecoregions for its unique terrestrial ecosystems (xeric shrublands), the status indicated as critical/endangered by WWF. Even the freshwater ecosystems that harbor several endemics have been given a special status, as Socotra is recognized as one of the globally outstanding Freshwater Ecoregions of the World. Socotra is also part of the Horn of Africa Biodiversity Hotspot as recognized by Conservation International, and listed as a Centre of Plant Diversity by Plantlife International, ranked among the richest yet most threatened biodiversity areas on the Planet. The Archipelago was declared as its own Ecologically or Biologically Significant Marine Area (EBSA) at the Convention of Biological Diversity (CBD) meeting in 2016, and the same year as an Endemic Bird Area by BirdLife International, which listed several Important Bird Areas (IBAs) for the Archipelago. A number of endemic plants and vertebrates from Socotra have been included in the IUCN Red List, but updates are needed and very few of the many endemic invertebrates have been listed. However, the biodiversity of Socotra is increasingly affected by impacts that affect ecosystems globally, such as unsustainable resource use and global warming which are now visible in the islands. We discuss the main threats and list some general conservation needs in the Socotra Archipelago.
... Accordingly, the goat population in Socotra is in a semiferal state, posing a significant challenge to the delicate balance of the ecosystem. Compared with the average number of goats in other climatically similar regions, the fivefold greater number of goats kept by one family on Socotra leads to fundamental overgrazing of the island's vegetation and prevents the natural regeneration of tree species, as mentioned by several studies (Hamdiah et al. 2022;Lvončík et al. 2020;Maděra and Van Damme 2022;Rezende et al. 2022;Maděra et al. 2019a, b;Habrová and Pavliš 2017). ...
... While in 1966, the number of goats was estimated at approximately 19,000 (Brown 1966), it was already 220,000 in 1999 (Scholte et al. 2007). According to the last census, there are 480,000 goats on Socotra (Maděra and Van Damme 2022), corresponding to an average of 1.33 goats per hectare; this number does not include sheep, cattle or camels (Maděra and Van Damme 2022). Goat populations have historically fluctuated dramatically, mainly as a function of rainfall. ...
... While in 1966, the number of goats was estimated at approximately 19,000 (Brown 1966), it was already 220,000 in 1999 (Scholte et al. 2007). According to the last census, there are 480,000 goats on Socotra (Maděra and Van Damme 2022), corresponding to an average of 1.33 goats per hectare; this number does not include sheep, cattle or camels (Maděra and Van Damme 2022). Goat populations have historically fluctuated dramatically, mainly as a function of rainfall. ...
We examined the socioeconomic importance of goat farming on the island of Socotra. The study included 154 participants from various areas. These data were collected via a questionnaire and statistically analyzed using the Bayesian approach and Kruskal–Wallis test to offer insights into the subject matter. The collected data included important quantitative parameters, such as feeding, watering, herd composition, milk production, and mortality, as well as subjective parameters, including the breeders’ reasons for keeping goats. Our study revealed that the main reason for keeping goats on Socotra Island is livelihood purposes (60%), followed by cultural reasons (22%), whereas 18% of respondents keep goats as a companions. In terms of herd composition, generally a Socotri family keeps an average of 107 individuals per herd, of which 59% are adult females, 32% are young under six months, and 9% are adult males. Reproduction patterns indicate that the goats breed twice a year, primarily in May and October. For milk production, goats on Socotra produce an average of 368 ml/day on an annual basis, and the highest production is observed in the mountains. The study also revealed that goats are primarily slaughtered for social reasons, such as family attendance and weddings (55%). According to the respondents, the main cause of mortality in goats was disease (40%). The results of this study have implications for understanding the management of goat farming on the island and conserving the island’s biodiversity.
... While these interactions are well-documented in other regions, island ecosystems like the Socotra Archipelago (Yemen) provide unique opportunities to study plant-plant interactions due to distinct biodiversity, isolation, and environmental pressures. This archipelago is globally recognised as a hotspot of biodiversity, characterised by exceptional species richness and a high degree of endemism (Van Damme and Banfield 2011; Maděra and Van Damme 2022). Socotra Island, the largest in the archipelago, is home to several iconic endemic tree species, including frankincense (Boswellia spp.), myrrh (Commiphora spp.), and the Socotran dragon's blood tree (Dracaena cinnabari Balf.f.). ...
... The Socotra Archipelago, located in the western Indian Ocean, 380 km south of Yemen and 100 km from the Horn of Africa, is recognised as a global biodiversity hotspot and centre of plant diversity (Maděra and Van Damme 2022). Separated from southern Arabia in the Oligo-Miocene period, the archipelago's altitudes range from sea level to 1550 m (Haggeher Mountains), with landscapes dominated by limestone plateaus and igneous rock (Beydoun and Bichan 1970;Cheung and DeVantier 2006;Culek 2013). ...
Question
Plant–plant facilitation is a critical ecological mechanism in arid environments, influencing biodiversity and ecosystem resilience globally. Shrubs often serve as nurse species, enhancing tree regeneration and sheltering plant communities, particularly in overgrazed or degraded habitats. In this study, we examine the role of shrubs as nurse species in the Socotra Archipelago (Yemen), a biodiversity hotspot in which several endemic tree species, including the iconic frankincense ( Boswellia spp.), myrrh ( Commiphora spp.), and Socotran dragon's blood ( Dracaena cinnabari ) trees, are threatened. This is largely due to a lack of natural regeneration caused by the combined effects of overgrazing by goats, sheep, and climatic events such as extreme droughts and cyclones. To aid conservation of threatened trees in arid regions, nature‐based solutions are urgently needed to help tree regeneration. Effective nurse plants have this potential, particularly in arid environments. We therefore examined the role of thorny, poisonous, and/or unpalatable shrubs as nurse plants in sheltering threatened plant communities, with a focus on woody species in an arid insular context.
Study Area
The Socotra Archipelago (Yemen) situated in the western Indian Ocean, east of the Horn of Africa. It is a biodiversity sanctuary and a UNESCO Natural World Heritage Site.
Methods
Vegetation surveys were conducted in 144 paired plots under six common shrub species and adjacent open areas. Community data, environmental variables, and functional traits were analysed using RLQ and fourth‐corner analyses, while Linear Mixed Models (LMMs) evaluated the effects of environmental variables and nurse species on key functional traits based on Community Weighted Means (CWMs). Facilitation effects were quantified using the Relative Interaction Index (RII).
Results
Our analysis revealed significant variations in species composition and functional traits between under‐canopy and open‐area plots. Certain shrubs, such as Cebatha balfourii , facilitated significantly higher species richness under its canopy compared to open areas. Elevation and grazing pressure influenced these interactions, with notable effects on functional traits like the occurrence of legumes and tree growth forms. Buxus hildebrandtii was less effective in supporting species richness, while C. balfourii, Lycium sokotranum , and two Cissus species exhibited significant positive facilitation. The LMMs confirmed the importance of environmental variables and nurse shrub characteristics in shaping plant community dynamics.
Conclusions
The results highlight differences in the facilitative potential of the studied species, with some showing a stronger ability to act as nurse shelters through microhabitat amelioration and protection from herbivory. The presence of tree species under shrubs is confirmed, and the role of these nurse species in supporting diverse plant communities provides critical insights for conservation strategies, supporting biodiversity resilience and sustainable management in degraded landscapes like Socotra Island and other arid environments. Future efforts should focus on leveraging nurse shrubs to mitigate environmental pressures and enhance ecological restoration, in particular to help conserve range‐restricted and threatened plant species.
... This number increased dramatically to 220,000 by 1999 [3]. The most recent census indicates that Socotra now hosts 480,000 goats [4]. ...
... The island's population is estimated to be 100,000, subject to seasonal fluctuations. The exact figure remains uncertain due to the absence of a census in Socotra [4]. Socotra's climate is heavily influenced by the East African-Indian monsoon system, characterized by a biannual migrating intertropical convergence zone that shifts between its northern hemisphere winter position and southern Indian Ocean summer position [17]. ...
Parasites play a significant role in biodiversity, yet research on these organisms remains limited, particularly in tropical and subtropical regions. Parasites are also an essential aspect of domestic animal husbandry, and their prevalence depends on various factors, such as husbandry conditions and the environment. However, no studies have been conducted on parasites affecting domestic animals on Socotra Island. This study aimed to investigate parasites in selected goat populations on the island using intravital fecal diagnosis. A total of 406 samples from adult goats across different locations (lowlands and highlands) and seasons (dry and rainy) were collected, fixed in 10% formalin, and transported to the Czech Republic for coprological examination using flotation and ether sedimentation methods. Statistical analyses, including partial canonical correspondence analysis (pCCA), the Monte Carlo permutation test, and the chi-squared test, revealed a high prevalence of gastrointestinal parasite infections, with 89% of goats infected. Eimeria spp. had the highest prevalence (76%). Co-infection was common, with 55.9% of goats infected with multiple parasites. Seasonality influenced parasitism, with other Protostrongylidae, Muellerius spp., and Trichuris spp. predominating during the rainy season, and Eimeria spp. and gastrointestinal strongylids predominating during the dry season. This first study on Socotra Island, Yemen, provides crucial insights into effective intervention strategies for controlling goat parasite infections.
... Germination energy (in %) was defined as the number of germinated seeds determined on day 4 after setting up the germination test from the total number of seeds [42,43] (including all seeds, full and empty). ...
... However, general factors impacting the Socotran terrestrial ecosystems are well understood, such as overgrazing and climate change impacts through recent cyclone effects [13,42,43]. In situ conservation and local seed germination in local nurseries on Socotra combined with replantation in nature (protected from browsing and replanted within the same area to avoid population mixing) are realistic additional strategies for improving natural protection and natural regeneration. ...
The endemic Boswellia species (Burseraceae) on Socotra Island (Yemen) are of great local significance due to their various local ethnobotanical uses. However, despite the fact that these trees are endangered, little is known about their biology. We tested seed germination rates in controlled experiments (trials of 21 days) for two subsequent years and for nine endemic taxa of Boswellia occurring on Socotra Island. For this, seeds were collected island-wide from a wide range of localities and for several populations per species. We observed differences in germination among Boswellia species, among species and localities and among both years, which indicates that the development of seeds is strongly affected by external ecological factors. Although we noted a large variation in seed germination (relatively high in Boswellia socotrana), and half of the species showed relatively low mean daily germination, our study indicated that all endangered endemic Frankincense Tree taxa of Socotra harbor the potential for in situ conservation through recruitment, given that known impacts can be reduced in local replantation areas (e.g., grazing).
... Socotra is an archipelago composed of four islands located in the Indian Ocean and is home to diverse vascular plants; about 37% are endemic [14][15][16]. Thus, it is not surprising that the Socotra Archipelago was stated a UNESCO Biosphere Reserve in 2003 [17]. The natural character of Socotra Island is the result of long-time isolation from the mainland [18], as well as the geological processes, together with the adaptation of the organisms to extreme weather conditions [19]. ...
Background: Dracaena cinnabari is a monocot species that does not form annual tree rings; thus, its age can only be estimated. This species is threatened by low natural regeneration, with an evident absence of younger individuals most likely caused by overgrazing; therefore, knowing trees’ ages is important for possible conservation strategies; Methods: Data collection was conducted on the Firmihin Plateau on Socotra Island (Yemen) in 2021, and the diameter at breast height (DBH) of 1077 individuals was measured, the same as those established on monitoring plots 10 years before the current measurement. The 10-year radial stem increment and DBH obtained in 2011 served as a basis for the linear model from which the equations for the age calculation were derived. Results and Conclusions: A direct model of age estimation for D. cinnabari was developed. According to the fit model, the age in the first (10.1–15 cm) DBH class was estimated to be 111 years, while that in the last DBH class (90.1–95 cm) was estimated to be 672 years. The results revealed that the previously used indirect methods for D. cinnabari age estimation were accurate.
Societal Impact Statement
Conserving frankincense trees (Boswellia) is crucial for both ecological and socio‐economic reasons. Surveying these trees in the field and using remote sensing unmanned aerial vehicles in the Socotra Archipelago, we found that Socotran frankincense trees are threatened by forest fragmentation, overgrazing, and increasingly frequent extreme climate events. A better understanding of the distribution and the threats of these important insular species will improve the conservation policy of the local authorities and benefit local communities in the Socotra Archipelago. At the same time, this work serves as a good practice example to guide conservation efforts for other culturally important threatened tree species around the world, therefore helping to sustain local livelihoods, fostering ecological resilience, and supporting socio‐economic stability.
Summary
Globally, frankincense trees (Burseraceae: Boswellia) are increasingly under threat because of habitat deterioration, climate impacts, and the olibanum trade. Despite harboring nearly half of the species in the genus, up‐to‐date insights are lacking for the insular endemic frankincense trees of the Socotra Archipelago UNESCO (United Nations Educational, Scientific and Cultural Organization) World Heritage Site (Yemen).
We combined georeferencing of individual trees in the field with remote sensing applying unmanned aerial vehicles (UAVs) to evaluate Boswellia distribution and (sub)population sizes in the entire Socotra Archipelago.
We counted 17,253 trees across all 11 taxa and we surveyed almost 55% directly in the field, collecting individual information on threats and health indicators. We estimate that the current total population sizes of the relatively common Socotran Boswellia taxa (Boswellia elongata, Boswellia popoviana, and Boswellia ameero) consist of a few thousand mature individuals with fragmented distribution of which a large proportion occurs in highly disjunct relictual stands, while the more range‐restricted species survive only through a few hundred (Boswellia nana and Boswellia samhaensis) to fewer than a hundred trees (Boswellia scopulorum). Our field data show that the Socotran frankincense trees are threatened by fragmentation and overgrazing resulting in a lack of natural regeneration, in combination with effects of extreme climate events (e.g., higher frequency and intensity of cyclones and prolonged drought) and potential future infrastructure developments; the species are less impacted by resin collection.
We provide recommendations to strategize urgent protection of the declining Socotran frankincense trees, and we update their conservation status, resulting in an endangered status for seven and a critically endangered status for four taxa.
The Socotra Archipelago (Yemen) is an interesting biodiversity hotspot, with a significant proportion of endemic species that have evolved to survive in an arid subtropical environment, inscribed as a World Heritage Site by UNESCO. The terrestrial ecosystems of Socotra face several threats, including climate change, overgrazing and soil degradation. Socotra Island has four endemic species of the genus Commiphora (Burseraceae). Little is known about their local distribution and ecology, yet these trees could be useful indicator species. Our study focuses on the distribution and niche characterisation of the four endemic Commiphora species of Socotra and how climate change may affect them. The aim is to improve insights into their habitats and to provide an essential basis for future local management plans and ecological restoration. We compared the current distribution with the forecasted potential distribution under a CMIP6 (Coupled Model Intercomparison Project) climate scenario, allowing us to define target conservation areas and assess potential local extinction risks. To achieve this, we collected distribution data in the field throughout Socotra Island, covering the current distribution ranges of the four species. To assess the potential distribution of these species, we applied three models (GAM, MaxEnt, RandomForest) using bioclimatic, topographic and soil variables. Forecasts under a climate change scenario were made using bioclimatic variables from the CMCC-CESM2 climate model for two different socioeconomic pathways. The distribution of three endemic Socotran Commiphora is mainly correlated to clay content in the soil and winter precipitation, while C. socotrana is affected by seasonal precipitation and temperature. Under different potential future climate scenarios, the distribution of C. ornifolia is predicted to remain stable or increase, while C. parvifolia distribution could increase, yet C. planifrons and C. socotrana are predicted to undergo a strong reduction of suitable areas and an upward shift in the mountains. Our results highlight that it is essential to conserve the unique terrestrial ecosystems in Socotra and to preserve these endemic trees which have a wide range of ecosystem services. Updates on the predicted extinction risk assessment are fundamental to understand conservation priorities and strategize future actions to ensure the persistence of Socotran myrrh trees and other endangered endemic tree taxa on the island.
The last remaining Dragon Tree Forest on Earth survives on a small plateau (Roqeb di Firmihin) in Socotra Island (Yemen). The Socotran Dragon’s Blood Tree (Dracaena cinnabari Balf. f.) is endemic to the Socotra Archipelago UNESCO World Heritage Site. Being a culturally important and endangered tree species, its conservation is a priority. Despite this, a complete inventory of the Firmihin Dracaena forest (14.9 km²) has never been attempted before. We applied the use of unmanned aerial vehicles (UAVs) for the first time in conserving the Socotran Dragon’s Blood Tree. A pair of UAVs (small drones) were used during field surveys in 2021 to spatially describe individual tree positions, tree density, mortality, and the forest age structure. Aerial images were processed into a single orthophoto image of high spatial resolution (8 cm/pixel) used for detailed analysis. We applied image-enhancement techniques, used object-based classification, and corrected every entry manually during the inventory process. In total 35,542 individual living trees and 2123 uprooted trees were inventoried. The mean age of the forest, based on crown age (derived from crown size) was estimated at an average of ca. 300 years (291.5 years) with some individuals older than 500 years. Our analysis reveals that the trajectory of recent cyclones and the average direction of fallen trees in Firmihin are correlated, suggesting that intensified winds (as a result of global warming) catalyze the decline of the overmature forest. Our study illustrates the use of UAVs in collecting crucial data for the conservation and threat assessment of endangered tree species in Socotra, and regular drone inventories could be applied (e.g., after future cyclone events or landslides) to better evaluate the status of these vulnerable island ecosystems.
A study of the water fleas (Crustacea: Cladocera) in man-made lakes in the northeast part of the United Arab Emirates revealed five species: Ceriodaphnia cf. cornuta Sars, 1885; Daphnia (Ctenodaphnia) arabica Neretina, Al Neyadi et Hamza, 2022; Moina cf. micrura Kurz, 1875; Anthalona mediterranea (Yalim, 2005); Coronatella anemae Van Damme et Dumont, 2008. The morphologies of the four taxa are described in detail, except that of D. Arabica, which has been described previously. The phylogenies of the C. cornuta and M. micrura species groups were reconstructed based on sequences of the COI mitochondrial gene and the possible divergence age of the Arabian clades was estimated based on molecular clocks with paleontological calibration. We concluded that the C. cornuta complex was differentiated in the Upper Jurassic to Lower Cretaceous. The splitting off of clades containing the Arabian population took place during the Oligocene to Miocene. The M. micrura species group was differentiated in the Upper Cretaceous, and the splitting off of clades including Arabian populations took place around the Oligocene. Therefore, the clades (of different hierarchical orders) in the Arabian Peninsula are very old compared to clades of similar rank in northern Eurasia, which usually have Late Pleistocene history. Most of our sampled water bodies were newly constructed man-made reservoirs. As revealed, the phylogroups are locally distributed, and we hypothesized that they are colonists from surrounding natural water bodies inhabited by the relicts of older fauna that survived after the great climate aridization and then occupied newly available (man-made) biotopes.
The topical collection ‘Twenty years of biodiversity research and nature conservation in the Socotra Archipelago’, in short ‘Socotra biodiversity research and nature conservation’ was conceived at the 18th Friends of Socotra annual meeting and Socotra conference which took place at the Orto Botanico di Palermo, Palermo, Italy, 26–29 September, 2019. In total, 13 research papers are included in the collection, which covers a selection of the latest scientific progress on the fauna and flora of the terrestrial and aquatic environments of the Socotra Archipelago UNESCO World Heritage Site (Yemen). Topics include conservation, taxonomy, ecology, biology and biogeography. The focus is mainly on biodiversity conservation and aimed at identifying current challenges, trends and processes that may impact on local ecosystems and livelihoods, based on analysis of data collected over the last decades. With this collection, we wish to emphasise the importance of taking into account science-based conservation approaches in future strategic steps towards safeguarding the ecosystems of Socotra.
Dragonflies are important bioindicators of aquatic ecosystem health. We discuss the distribution and richness of the 22 dragon- and damselfly species known from the Socotra Archipelago UNESCO World Heritage Site (Yemen) in the western Indian Ocean. We compiled 841 records from literature, field data and museum collections, covering a surveying period of 123 years (1896–2019). Distribution of the only endemic, the Socotra Bluet Azuragrion granti (McLachlan, 1903), an indicator of pristine lotic waters, is updated. Relatively higher dragonfly species richness occurs in eastern Socotra where suitable aquatic habitats are present all year. We observed a significant drop of the number of taxa over several decades in areas in the Hadiboh Plain where human development impacts are strongest. We discuss the indigenous people’s perception of dragonflies to assess the potential of these invertebrates as local flagships for aquatic biodiversity conservation. Our knowledge of the Socotran Odonata remains incomplete, yet they are now among the best studied animal groups in the archipelago. A new record of an Oriental Anax indicates that the island’s dragonfly fauna may yet harbour surprises. Our compilation of odonate records is the first for Socotra and serves as a baseline for future monitoring of the archipelago’s vital aquatic habitats. We converted all records into a geodatabase which we uploaded to an online, freely available web mapping server and mobile application to facilitate research and conservation of the Socotran dragonflies ( www.dragonfliesofsocotra.com ).
Graphic abstract
The Socotra Archipelago is a unique hotspot of biodiversity, particularly for terrestrial reptiles (28 endemic out of 31 native species) whose ecology is known only in general terms. We describe the ecological preference and the daily activity of 21 species of terrestrial reptiles, geckos, lizards, skinks, snakes and chameleon, from the largest island of the archipelago. The database includes 6950 records collected by 260 standardized transects from 2007 to 2013. The sister species Haemodracon riebeckii–H. trachyrhinus with very distinct body sizes, overlapped in their distribution but segregated by habitat and substrate. Three pairs of sister species with great phylogenetic affinity and similar morphology (Pristurus insignis–P. insignoides; Pristurus guichardi–P. obsti; Hemidactylus dracaenacolus–H. granti) were only partially segregated by distribution, habitat, substrate and body size, in the order of decreasing frequency. On the contrary, the other, phylogenetically more distant species of geckos, widely distributed and less similar in morphology, overlapped completely in their habitat use and distributional range. Such patterns suggest that strictly related species reduced their competition by segregation in different habitats and temporal niches and by non-overlapping distribution within the relatively small island of Socotra. On the other hand, the activity rhythm was similar within each genus. The unique biodiversity of Socotra endured up to present, thanks to a human impact lower than for most other inhabited islands, but the pace of change has accelerated recently. Conservation measures are urgent, particularly for the species of endemic geckos with a distribution range as small as 10 km2.
We report on the first finding of a widespread exotic pest species, the red palm weevil Rhynchophorus ferrugineus (Olivier) (Coleoptera: Curculionidae) on date palms Phoenix dactylifera L. (Arecaceae) on Socotra Island, Yemen. Our initial surveys reveal that the pest species is present in date palm plantations on the north-eastern coast of the island, within 5 km of the capital Hadiboh. More surveys are needed to determine the wider distribution of this palm pest on Socotra, and what impact current invasions are having on date production. This invasion, together with the impact of the dubas bug, Ommatissus lybicus de Bergevin (Hemiptera: Tropiduchidae), which seems well established on the island, is likely to have a significant negative impact on livelihoods, since dates are the most important locally produced food, after milk and meat. The invasion provides ample evidence of the risks associated with trade, and the need to strengthen biosecurity measures to protect traditional food production and biodiversity from further pest incursions in this UNESCO World Heritage Site.Graphic abstract
Unsustainable overgrazing is one of the most important threats to the endemic and endangered population of dragon’s blood tree (Dracaena cinnabari) on Socotra Island (Republic of Yemen). However, there is a lack of information about the exact population size and its conservation status. We estimated the population size of D. cinnabari using remote sensing data. The age structure was inferred using a relationship between crown projection area and the number of branch sections. The conservation importance of each sub-population was assessed using a specially developed index. Finally, the future population development (extinction time) was predicted using population matrices. The total population size estimated consists of 80,134 individuals with sub-populations varying from 14 to 32,196 individuals, with an extinction time ranging from 31 to 564 years. Community forestry controlled by a local certification system is suggested as a sustainable land management approach providing traditional and new benefits and enabling the reforestation of endemic tree species on Socotra Island.
Dragon’s blood tree, a flagship endemic species of Socotra, is threatened with extinction due to lack of natural regeneration, likely because of goat herbivory and/or climatic factors. Loss of dragon’s blood tree would result in loss of other native flora, heightening the importance of formulating a conservation strategy for it. Although artificial afforestation might be used to offset the lack of natural regeneration, it would have to overcome the same threats faced by naturally occurring seedlings. Moreover, there is no published information on the growth dynamics of seedlings in plantations in situ on Socotra. To fill this information gap, we compared seedling growth (total plant height, leaves number, stem height, stem diameter) over an 8-year period after planting at three sites that differed in the degree to which goats were excluded and in whether they were watered regularly over the period. In addition to developing a new classification of the growth stages, which will enable better tracking of population dynamics, we found that continuous goat exclusion was necessary to prevent seedling mortality. Also, although seedling growth overall was slow, growth parameters of regularly irrigated seedlings ranged from 156% to 446% of those not regularly watered, suggesting that this treatment can speed seedlings’ escape from goat browsing. For the dragon’s blood tree and likely for other taxa within this group, extremely slow growth and long generation times suggest that conservation and restoration efforts will require a commitment to active management that will last decades, rather than a short-term approach.
Data on the prehistoric biota of the Socotra Archipelago (Yemen) are scarce. We report on the first extinct vertebrate ever recorded from this western Indian Ocean island group. An exceptionally preserved adult specimen of Egyptian fruit bat Rousettus aegyptiacus was found in a calcite flowstone in Hoq Cave on the NE coast of Socotra Island. Accelerator mass spectrometry (AMS) radiocarbon dating on bone fragments estimates the fossil at ca. 7600–7400 cal a BP (radiocarbon age 6669 ± 70 14C a BP) corresponding to an early Holocene wet period in southern Arabia and the Horn of Africa. Morphometric analysis of the skull compared to Recent specimens from the neighbouring mainlands finds the fossil morphologically most similar to Egyptian (Nile Valley and Libyan Desert), not to East African or Middle Eastern populations. The species is currently absent from the Horn of Africa (Somalia and Eastern Ethiopia). Although potentially a stray individual, we also discuss the palaeoenvironmental implications under the hypothetical scenario that the fruit bat is a remnant of an extinct resident population. If so, the extinction of an important pollinator and seed disperser may have impacted the Socotran terrestrial ecosystems.
The first record of Triops Schrank, 1803 (Crustacea: Branchiopoda: Notostraca: Triopsidae) from Socotra Island is presented herein. Besides Madagascar and the current note, the genus is unknown from Indian Ocean islands. A brief morphological exploration indicates that the Socotran Triops cf. granarius (Lucas, 1864) (trachyaspis form) belongs to an African clade within this widespread Old World species complex and not to T. granarius s.str. from East Asia. Detailed morphological and molecular revision of the poorly studied Middle Eastern and Northern African populations in comparison to East Asian populations are needed to understand this group and the phylogenetic position and status of the Socotran tadpole shrimp. Found in a single locality (the archeological site Eriosh) in the rapidly changing coastal plains on Socotra, together with other large branchiopods (endemic Anostraca and unidentified Spinicaudata), the insular Triops population and the regionally neglected temporary lake habitat deserve a special protection status from a biodiversity conservation perspective. Triops cf. granarius is hereby suggested as a local flagship species for the conservation of temporary pool habitats on the island.
We describe a new species of stygobiont atyid shrimp of the genus Halocaridinides Fujino & Shokita, 1975 from subterranean brackish and freshwater environments on Socotra Island (Yemen). This is the first atyid shrimp discovered in the Socotra Archipelago. This Socotran endemic is the second species of Halocaridinides recorded from an Indian Ocean island and the first to occur in non-anchialine environments. The new species is characterized by the absence of eye pigmentation, a reduced rostrum and the presence of a single spine in the uropodal exopod diaeresis. Morphological variability in the robustness of the first and the second chelipeds, particularly obvious in the chelae, may indicate local
speciation or phenotypical plasticity. At both extremes of the variation, animals with most slender (tenuis type) or robust (robustus type) morphologies occur. We discuss the Indo-Pacific distribution of the genus and the potential origin of the new Socotran species. An amended diagnosis for the genus and a key to all species are included.
A complete list of bat records available from the Socotra Archipelago was compiled from the literature and from new records, based on field studies and examination of museum specimens. The record reviews of the particular species are complemented with distribution maps, summaries of distributional status, notes on field data and on taxonomic status. From the Archipelago, at least 54 records of four bat species belonging to four families are known; viz. Rhinopoma cystops Thomas, 1903 (34 record sites), Rhinolophus clivosus Cretzschmar, 1828 (6), Asellia italosomalica De Beaux, 1931 (9), and Hypsugo lanzai Benda, Al-Jumaily, Reiter et Nasher, 2011 (5). While the latter three species are known only from the island of Socotra, R. cystops was documented also from two other larger islands, Abd El Kuri and Samha. The Socotran populations of R. clivosus were found to be morphologically and genetically unique and therefore are here described as a separate subspecies, Rhinolophus clivosus socotranus subsp. nov.
Altogether, 1,564 species taxa (identifi ed at least to the genus level) from 25 orders of Hexapoda have been recorded from the Socotra Archipelago to date. In addition, members of Diplura and Ephemeroptera are known from the archipelago as well, but they have not been identifi ed so far. The most speciose order in Socotra are beetles (Coleoptera) with 540 species. The total number of endemic taxa described from the archipelago is 662 species, which represents 42% of all known species. The vast majority of species is known from the main Socotra Island (1,520 species, 635 endemic taxa). The diversity of insects in Socotra is briefl y commented on and compared with the nearest archipelago – the Seychelles.
We describe Carex socotrana Rěpka & Maděra, a new endemic species found in the Hajhir Mountains on Socotra Island. It differs from the morphologically similar African continental species C. steudneri Boeckeler in having a shorter stem, smaller leaf length and width, completely smooth leaf blades and margins, and pistillate scales without a whitish membranous margin and with distinctive awns at the apex. The spike clusters are smaller and more scattered on the stem, and the perigynium and its beak are smaller than in C. steudneri. So far only one small and one large population have been found near the highest mountain peak, Mount Scand. The new taxon is 1370 km from the closest known site of C. steudneri.
Socotra Island (Republic of Yemen) is inhabited by descendents of a primeval south-Semitic language group for some 3000 years. The main way of living is traditional pasturage husbandry and in the lesser extent also fishery. Pasturage is practised in a similar extensive way as on continental Yemen. Increasing anthropogenic pressure on the natural resources is thus above all driven by livestock grazing and by excessive wood depletion used for construction and fuelwood. As a matter of fact, grazing practice influenced plant communities and notably contributed to the contemporary distribution and structure of tree populations around the island, including endemic Commiphoras, Boswellias, Dendrosicyos socotrana and Dracnena cinnabari. Consequently, the most extensive vegetation type is low grazed forest dominated by some Euphorbiaceae representatives as Croton sp. or Jatropha unicostata, species unpalatable to the livestock. Preservation of the biological richness of Socotra Island will not be, therefore, possible without restrictions aimed at pasturage husbandry, at least in the key structural segments of the ecological network. Precise geobiocoenological differentiation is needed for recognizing the detailed state of natural conditions of the island.
Woody vegetation dynamics and Dracaena cinnabari regeneration have been studied for five years in the conditions of Socotra Island. Woody plants were measured regularly inside and outside the exclosure area, and the growth and survival of D. cinnabari seedlings were observed. In the exclosure of about 1000m2 a total of 49 species were identified, including 23 endemics, growing in the average density of 3.82 specimens per m2. The fenced area was overgrown relatively rapidly by dense grass cover – reaching approx. 2.7t/ha. Species growth dynamics inside and outside the exclosure shows that grazing had a marked impact, leading to the elimination of trees and shrubs. All grazed species grew noticeably in the exclosure, in the average of 50cm in 5years. D. cinnabari as the dominant flagship species of Socotra has been studied with regards to regeneration dynamics. Observations indicate that probability of its seedlings survival increases with their age. No seedlings germinated from the seeds sown in the experiment, however, outplanted seedlings performed relatively well. Field observations show that D. cinnabari seed germination is triggered when the seed reaches a protected micro-habitat with a developed humus layer and high relative humidity in the soil lasts for at least two days.
The study describes main biotopes of Socotra Island. The biotopes were distinguished and described during complex field observations on more than 250 localities of Socotra between years 1999-2004. Classification of biotopes is based on differences in physiognomy, structure and species composition of the vegetation component of present biocoenoses. Groups of biotope types differ, above all, in the physiognomy and structure of vegetation. Biotope types are divided according to composition of dominant species. In this manner, 13 groups of biotopes and 39 biotope types were distinguished and described
http://www.degruyter.com/view/j/jlecol.2013.6.issue-3/jlecol-2014-0004/jlecol-2014-0004.xml?format=INT
Some misunderstandings persist in the biological literature, concerning the geological evolution of the Socotra Archipelago. The aim of this paper is to interpret new information about the Gulf of Aden geology, from the view of possible methods of terrestrial biota species migration to the Socotra Islands. An overview of the Socotra Platform with the Socotra Archipelago topography is given. Present-day geological publications are mostly oriented towards tectonic structure of the Gulf and its tectonic evolution, and thus information concerning the elevation of the land surface and the sea level was necessary to deduce. The first biogeographically relevant emergence of a land mass in the area of present-day Socotra Archipelago commenced during the late Eocene Epoch (38-34 Ma BP). Some islands persisted after later transgressions of the sea, before the time of the opening of the Gulf of Aden rift (ca 20-17 Ma), accompanied by substantial uplift and large-scale uplift of the land. This was the last time when terrestrial biota could, relatively easily, reach the area of the Socotra Archipelago on land from the African mainland, and also with medium probability from present-day Arabia. The total evaporation of the Red Sea from 11-5 Ma BP enabled the migration of terrestrial species from and to Arabia via Somalia. Nevertheless, channels in Guardafui and Brothers basins made important, but perhaps nonfatal, barriers. The last and most important uplift of Haggier Mts. on Socotra occurred at the end of the Miocene Epoch (9-6 Ma BP). That was probably the time of the last Tertiary emergence of the Socotra Platform, potentially enabling some species to migrate across narrowed abovementioned channels. Great changes in sea level occurred during the Quaternary Period, periodically exposing the surface of the Socotra platform. Two channels persisted, preventing the invasion of modern species onto the Socotra Archipelago. Channels within the Brothers basin between Abd al-Kuri Isl. and other islands of the Archipelago formed some barriers to dispersal, and probably led to important biota differences in the scope of the Archipelago. Finally, a scenario of the “facilitation” provided by tsunami and sea currents for the immigration of biota onto the Archipelago is presented.
The Socotra archipelago is an ancient continental fragment situated in the Western Indian Ocean that has been isolated for at least 18 million years (Ma). Although often described as the ‘Galapagos of the Indian Ocean’, compared to the Galapagos and other island systems, this Arabian Sea archipelago is relatively poorly studied and often overlooked in works on island biology. Nevertheless, the Socotra archipelago is characterised by relatively high floristic diversity and endemism with local adaptations and radiations , making it a good example of a system suitable for the study of island biogeography . We discuss origins, affinities, disjunctions and adaptations in the flora and explore underlying environmental and evolutionary processes that may have helped the Socotra archipelago to its present uniqueness.
ABSTRACT
The paved (tarmac) roads had been constructed on Soqotra island over the last 15 years. The vegetation along the roads was disturbed and the erosion started immediately after the disturbance caused by the road construction. Our assumption is that biotechnical measurements should prevent the problems caused by erosion and improve stabilization of road edges. The knowledge of plant species which are able to grow in unfavourable conditions along the roads is important for correct selection of plants used for outplanting. The vegetation succession was observed using phytosociological relevés as a tool of recording and mapping assambblages of plants species along the roads as new linear structures in the landscape. Data from phytosociological relevés were analysed and the succession was characterised in different altitudes. The results can help us to select group of plants (especially shrubs and trees), which are suitable to be used as stabilizing green mantle in various site conditions and for different purposes (anti-erosional, ornamental, protection against noise or dust, etc.).
The climate of Socotra, influenced by the Indian Ocean Monsoon, is poorly known, hampering understanding
of its paleoclimate and (endemic) biodiversity. Mean annual rainfall and temperature, measured
in a network of meteorological stations from 2002 to 06, were 216 mm and 28.9 �C. Combined with cloud
cover information from satellite images, this data provides clear ideas on inter- and intra-annual
variability. Precipitation derived from the northeast (NE) winter monsoon influences especially the NE
plateaus and windward side of the Haggeher Mountains because of orographic effects. The southwest
(SW) summer monsoon concentrates at the southern half of the island and generally produces less
rainfall. During the SW summer monsoon, clouds cover the highlands and plateaus south of the
Haggeher Mountains, creating fog. Preliminary measurements suggest that at higher altitudes, fogderived
moisture may constitute up to two-thirds of total moisture, amounting up to 800 mm. The
predominant SW aspect of the enigmatic dragonblood tree underlines the importance of fog. Long-term
weather observations by Socotri put these short-term meteorological observations into a longer
perspective. Socotri informants also described the drought years when livestock populations crashed,
after which windows of opportunities for the regeneration of dragonblood and other grazing-sensitive
trees may have occurred.
Based on field studies by the authors in March-April 2002 and March 2003, the altitudinal zonation of the woodland and forest vegetation in the moister northern part of Socotra Island is described and analysed. The phytosociological surveys along an altitudinal transect in the Haghier Mts from the coastal plain to the summit areas supplemented by surveys on the escarpments of the Hamadero and Ma'alah Plateau revealed a major divide between (semi)deciduous lowland and low montane communities (below c. 750 m) on the one hand and high montane communities at altitudes predominantly between c. 850 and 1500 m on the other hand. Differing in their substrates, climatic situation, floristic affinities and physiognomy, the communities of these two altitudinal ranges are classified in two different alliances, the low montane Crotonion socotrani and the high montane Crotonion sulcifructi all. nov., which can both be included in the new order Acridocarpo socotrani-Crotonetalia socotrani. Within the Crotonion socotrani the typical association (Crotonetum socotrani), usually heavily degraded, dominates the coastal plains and lower foothills, followed by an altitudinal form of which two associations can be distinguished. The slopes above the foothills are covered by forests with emergent Sterculia africana var. socotrana (Adenio sokotrani-Sterculietum socotranae ass. nov.), then above 500 m by a colourful forest with Boswellia ameero (Ruellio insignis-Boswellietum ameero ass. nov.). The high montane Crotonion sulcifructi includes a more widespread association, the Trichodesmo scottii-Cephalocrotonetum socotrani ass. nov., and an association particularly rich in epiphytic bryophytes, the Leucado hagghierensis-Pittosporetum viridiflorum ass. nov., which is restricted to the moistest sites.
Socotra Island (Yemen), a global biodiversity hotspot, is characterized by high geo-morphological and biological diversity. In this study, we present a high-resolution vegetation map of the island based on combining vegetation analysis and classification with remote sensing. Two different image classification approaches were tested to assess the most accurate one in mapping the vegetation mosaic of Socotra. Spectral signatures of the vegetation classes were obtained through a Gaussian mixture distribution model, and a sequential maximum a posteriori (SMAP) classification was applied to account for the heterogeneity and the complex spatial pattern of the arid vegetation. This approach was compared to the traditional maximum like-lihood (ML) classification. Satellite data were represented by a RapidEye image with 5 m pixel resolution and five spectral bands. Classified vegetation relevés were used to obtain the training and evaluation sets for the main plant communities. Postclassification sorting was performed to adjust the classification through various rule-based operations. Twenty-eight classes were mapped, and SMAP, with an accuracy of 87%, proved to be more effective than ML (accuracy: 66%). The resulting map will represent an important instrument for the elaboration of conservation strategies and the sustainable use of natural resources in the island.
We present here a synthesis of the evolution of rifted continental margin systems in the Gulf of Aden. These margins are volcanic to the west of the Gulf of Aden, where they are influenced by the Afar hotspot, and non-volcanic east of longitude 46° E. The combined use of magnetics, gravity, seismic reflection, field observations (tectonic, stratigraphic and sedimentological) and oil well data allowed us to obtain better constraints on the timing of continental rifting and seafloor spreading. From the Permo–Triassic to the Oligocene, the Arabian–African plate was subject to distributed extension, probably due, at least from the Cretaceous, to tensile stresses related to the subduction of the Tethysian slab in the north. In Late Eocene–Early Oligocene, 34–33 Ma ago, rifting started to localise along the future area of continental breakup. Initially guided by the inherited basins, continental rifting then occurred synchronously over the entire gulf before becoming localised on the northern and southern borders of the inherited grabens, in the direction of the Afar hotspot. In the areas with non-volcanic margins (in the east), the faults marking the end of rifting trend parallel to the inherited grabens. Only the transfer faults cross-cut the inherited grabens, and some of these faults later developed into transform faults. The most important of these transform faults follow a Precambrian trend. Volcanic margins were formed in the west of the Gulf, up to the Guban graben in the southeast and as far as the southern boundary of the Bahlaf graben in the northeast. Seaward dipping reflectors can be observed on many oil industry seismic profiles. The influence of the hotspot during rifting was concentrated on the western part of the gulf. Therefore, it seems that the western domain was uplifted and eroded at the onset of rifting, while the eastern domain was characterised by more continuous sedimentation. The phase of distributed deformation was followed by a phase of strain localisation during the final rifting stage, just before formation of the Ocean–Continent Transition (OCT), in the most distal graben (DIM graben). About 20 Ma ago, at the time of the continental break-up, the emplacement of the OCT started in the east with exhumation of the subcontinental mantle. Farther west, the system was heated up by the strong influence of the Afar hotspot, which led to breakup with much less extension. In the Gulf of Aden (s.str), up to the Shukra El Sheik fracture zone, oceanic spreading started 17.6 Ma ago. West of this fracture zone, oceanic accretion started 10 Ma ago, and 2 Ma ago in the Gulf of Tadjoura. Post-rift deformation of the eastern margins of the Gulf of Aden can be seen in the distal and proximal domains. Indeed, the substantial post-rift uplift of these margins could be associated with either the continental break-up, or activity of the Afar hotspot and related volcanic/magmatic activity. Uplift of the northern proximal margin was still active (e.g. stepped beach rocks exposed at 60 m of 2 Ma; 30 m of 35,200 years; 10 and 2 m) and active volcanoes can be inferred at depths of between 70 and 200 km beneath the margin (at 5–10 km distance from the coast). On the distal margin, heat flow measurements show a high value that is associated with post-rift volcanic activity and the development of a volcano (with flows and sills) shortly after the formation of the OCT. The Afar hotspot is therefore important for several reasons. It allows the localisation of deformation along the Red Sea/Aden system and the rapid opening of the Gulf after the continental break-up; its influence also seems to persist during the post-rift period.
Understanding the factors which affect the distribution of alien plants in arid islands is complicated
by the complex and stochastic nature of the invasion process per se, the harsh environmental conditions, and the low number of researchers and sampling effort. We present the results of the most comprehensive inventory to date of alien vascular plant species occurring in Socotra Island, a global biodiversity hotspot just beginning to be developed. A floristic survey was conducted between 2006 and 2008 in 36 grid cells of 10 9 10 km. We integrated this data from this survey with those from scientific literature. We recorded 88 alien plant species. Tree and herbaceous species were the most common growth forms. Species from Asia and edible species were prevalent. We identified 80 species considered weeds worldwide with[50 % adapted to arid conditions. We used a two-part model to analyze the spatial distribution of naturalized and alien plant species in relation to environmental and anthropogenic factors. Altitude and human-related factors play a significant role in the distribution of both naturalized and invasive species. Notably, the latter can potentially spread mainly in the alluvial basal areas. This study underpins the knowledge about alien species and their spatial distribution in Socotra Island. It provides a baseline for plant invasion management and contributes data for the analyses of invasion processes on islands worldwide.
The Socotra Archipelago (Yemen) is globally recognized for its outstanding biodiversi-ty and endemism, designated on this basis a UNESCO World Heritage Site in 2008. The island underwent long geological and political isolation, ensuring preservation of unique ecosystems un-til the start of the new millennium. Now, Socotra Island is undergoing rapid development, out of balance with conservation. Major causes for biodiversity loss in other global insular ecosystems such as habitat fragmentation and degradation, pollution, invasive species and the impact of tour-ism, are becoming pressing issues that deserve close attention. Unsustainable resource use, the loss of traditional land management and illegal trade in biota are worrying phenomena that further increase the pressures on Socotra's ecosystems. We provide the first comprehensive review of potential human impacts on Socotra before the 21 st century, an updated discussion of some of the principal threats to its biodiversity in recent times, discussing local examples within a historical context of known extinction processes on islands, and underline the importance of traditional knowledge in the protection of Socotran ecosystems.
At first view, soil erosion on Socotra Island, Yemen would seem to be a minor problem. This appraisal is based on the fact
that on the one hand the island is poor in soil resources, and on the other hand research on the island has to date focussed
on biodiversity. However, results of soil investigations on Socotra showed that in the Homhil Protected Area land degradation
in terms of erosion due to soil structure deterioration and humus loss has increased drastically: within 3 years a loss of
about 40 m3 in a single gully head was estimated. Soil loss inevitably involves uprooting of trees and a decrease in soil fauna. Biodiversity
is, of course, the most important argument for protecting the unique floral and faunal richness of the island, but what would
terrestrial biodiversity be without soils? The present approach relies on a “down-to-earth” system of soil monitoring, based
on both modern and ancient knowledge and oriented towards current environmental and political objectives. It should be understood
as a first step towards conserving soils and vegetation in a Protected Area of this tropical island.
KeywordsAncient indigenous knowledge-Soil conservation-Biodiversity-Socotra island-Yemen
Many endemic tree species have important scientific, ecological and economic value but the scarcity of information about their
biological and ecological features makes it difficult to develop conservation strategies for them. A four-step approach is
presented to address this problem, based on the analysis of data collected in a limited-duration field study: (1) Data collected
are used to analyse the ecological niche, population structure and regeneration status of the species in question. (2) Several
IUCN Red List (RL) parameters, useful for assessing the species’ risk of extinction, are measured, including population counts,
number of locations, extent and area of occurrence. (3) The IUCN RL parameters are used together with the other information
gathered to set preliminary conservation priorities. (4) The analysis of utilization pattern is used to develop conservation
actions specific to the environmental and socio-economic context. To test the applicability of this approach Boswellia spp. of Socotra island were analysed. Ground-rooted species (B. ameero, B. elongata and B. socotrana) were the most abundant and widespread and, according to the spatial analysis, were characterised by a geo-altitudinal zonation.
However, the Weibull functions fitted on their stem diameters, and the absence or presence of only a small number of saplings
highlighted a poor regeneration status. In the absence of conservation actions, these species will probably be subject to
a progressive decline because of uncontrolled grazing. Of the four cliff-rooted species, which grow in sites that are less
accessible to livestock, two (B. popoviana and B. dioscorides) were of lower conservation priority and may become the most numerically abundant. Conversely, the other two (B. nana and B. bullata), which may be threatened by stochastic events because of their reduced populations and small number of locations, were considered
of very high priority. Different conservation actions were then identified for each species. In particular, for Boswellia species producing gum, the conservation-through-use action was discussed as a potential option.
KeywordsIUCN Red List–Ecological niche modelling–Random Forest–Soil parameters–Weibull function
ABSTRACT. We update the knowledge on the 30 terrestrial reptile species of Socotra, an archipelago well known for its highly endemic flora and fauna. We use the records obtained during several surveys throughout all the islands of the archipelago, carried out between 2007 and 2010, as well as the bibliographic records. We recorded all the known species of terrestrial reptiles for the islands (Chamaeleo monachus, Haemodracon riebeckii, Haemodracon trachyrhinus, Hemidactylus dracaenacolus, H. granti, H. homoeolepis, H. flaviviridis, H. forbesi, H. inintellectus, H. oxyrhinus, H. robustus, H. pumilio, Pristurus abdelkuri, P. guichardi, P. obsti, P. insignoides, P. insignis, P. sokotranus, P. samhaensis, Mesalina balfouri, M. kuri, Hakaria simonyi, Trachylepis
socotrana, Pachycalamus brevis, Leptotyphlops filiformis, L. macrurus, L. wilsoni, Typhlops socotranus, Ditypophis vivax, Hemerophis socotrae). We report for the first time the occurrence of Hemidactylus robustus in Samha Island. H. homoeolepis is
excluded from the fauna of Abd al-Kuri Island.
The Soqotra archipelago is one of the most isolated landmasses in the world, situated at the mouth of the Gulf of Aden between the Horn of Africa and southern Arabia. The main island of Soqotra lies not far from the proposed southern migration route of anatomically modern humans out of Africa approximately 60,000 years ago (kya), suggesting the island may harbor traces of that first dispersal. Nothing is known about the timing and origin of the first Soqotri settlers. The oldest historical visitors to the island in the 15th century reported only the presence of an ancient population. We collected samples throughout the island and analyzed mitochondrial DNA and Y-chromosomal variation. We found little African influence among the indigenous people of the island. Although the island population likely experienced founder effects, links to the Arabian Peninsula or southwestern Asia can still be found. In comparison with datasets from neighboring regions, the Soqotri population shows evidence of long-term isolation and autochthonous evolution of several mitochondrial haplogroups. Specifically, we identified two high-frequency founder lineages that have not been detected in any other populations and classified them as a new R0a1a1 subclade. Recent expansion of the novel lineages is consistent with a Holocene settlement of the island approximately 6 kya.
We assessed seven decades of change in the largest known population of the endangered endemic Boswellia elongata Balf. F. (Burseraceae) on Socotra Island (Yemen). To quantify the population change we evaluated tree number and locations on digitized images from various sources in the period 1956–2017 and combined this with direct field measurements of the population between 2011 and 2017. Our study reveals that the Homhil Nature Sanctuary B. elongata population shows a continuous decline since 1956. The steady but slow natural decline was strongly accelerated by two catastrophic cyclones in November 2015, when 38% of the trees were directly destroyed by strong winds. During the following 2 years 29% of the remaining trees died additionally. The remaining population has a bell-shaped size distribution; most trees are around 40 cm in diameter (range 18 to 70 cm). Tree ring analysis of 11 dead trees with a diameter of 29 to 44 cm without bark, resulted in estimated tree ages between 80 and 101 years. We estimate that similar-sized trees showing strong signs of senescence have a maximum age of a little over 100 years. The age structure of the Homhil population is, therefore, unbalanced with large sized trees prevailing. Natural regeneration is absent for decades. Viable seeds are available and have been shown to germinate, but the development of seedlings into saplings is a bottleneck. If the decline continues at the current rate, only 30 trees will remain there in 2036. Protection, planting and awareness activities are needed to keep this unique frankincense tree in Homhil Nature Sanctuary.
The importance of populations of the Socotra dragon’s blood trees (Dracaena cinnabari Balf.f., 1882) for horizontal precipitation capture was estimated for the first time. Dragon’s blood trees (Asparagaceae) inhabit semiarid cloud forests with low levels of rainfall that strongly depend on additional moisture from fog and clouds. Our estimation was based on a detailed description of the aboveground biomass structure of stands of the dragon’s blood trees on Socotra Island. The aboveground biomass was described using non-destructive methods. An example of a well-preserved forest was measured in terms of the basic biometric characteristics of each tree, and the trees were divided into three classes based on crown projections. For each class, we took detailed measurements of one representative tree. All measured and counted growth characteristics were converted to the stand level. The tree population at the whole island level was investigated using remotely sensed data. The estimation of horizontal precipitation was performed using detailed and precise evaluation of the available climate data in half-hour intervals over the period of June 2009 and June 2010 and the ambulatory direct measurements (12.6.2009–26.6.2009). The mean annual horizontal precipitation below the crown of the dragon’s blood tree was estimated to be 792 mm in the highest (951–1545 m a.s.l.), 373 mm in the middle (601–950 m a.s.l.) and 46 mm in the lowest (180–600 m a.s.l.) altitudinal zones. Our model showed that the horizontal precipitation exceeded 40% of the total annual precipitation. The decline in the dragon’s blood tree forest could therefore cause the loss of additional water from the island’s hydrological cycle.
Socotra, the largest of four islands in the Socotra archipelago (Yemen), is characterized by high geo-morphological and biological diversity. Traditional management of land resources has been impacted by cultural shifts, globalization and urban expansion. In addition, the island has been affected by climate impacts, such as droughts and recent cyclones causing significant economic and environmental damage. Our study presents an analysis of the dynamics of the vegetation on the island for the period 2001-2019 and highlights the status of different landscapes. We used Earth Observation techniques and the Normalized Difference Vegetation Index to assess land productivity dynamics basing on vegetation indexes derived from average resolution satellite images. The results show that although the arid landscapes of Socotra seem resilient, over 30% of the central mountainous areas of the Hageher (hosting a unique and high biodiversity) are decreasing in productivity , whereas in the lowlands (where plant diversity is relatively lower) productivity has increased significantly for the same time period, up to 40% in some areas. Given the difference in biological diversity of the sites, this study can help prioritize conservation areas and activities, which are crucial for Socotra's terrestrial environment. The methodology applied could be replicated for continuous monitoring in the future.
We reviewed and compared the occurrence and biogeographic origins of three arthropod groups, darkling beetles (Coleoptera, Tenebrionidae), spiders (Araneae) and butterflies (Papilionoidea) on the Socotra archipelago (Yemen). More than half of the darkling beetles (80.3%) and spiders (51.5%) are endemic to the Socotra archipelago, in contrast with only 5 species (18.5%) of butterflies. The number of non-endemic taxa is higher among those distributed widely in the Afro-Oriental region, less of taxa from Afro-Arabian or African origin. These species colonised the Archipelago by natural means, but we document also introduction through human activity. The exact distribution and evolutionary patterns still need thorough research (e.g. molecular, phylogenetic, and phylogeographic analyses).
There are seven endemic species of Boswellia Roxb. ex Colebr. on Socotra Island, Yemen. Boswellia socotrana Balf. f. is a culturally, economically, and ecologically important species on the island. The name Odina aspleniifolia Balf. f. has been considered as a synonym, but there are morphological differences between the two taxa sufficient to justify their distinction at subspecific rank. Therefore, O. aspleniifolia is transferred to Boswellia as B. socotrana subsp. aspleniifolia (Balf. f.) Lvončik. A lectotype is designated for O. aspleniifolia. The distribution and ecology of both subspecies are discussed, as is their conservation status.
The two new species Boswellia bullata and B. dioscoridis from Socotra (Yemen) are described and illustrated.
Among the most prominent examples for the disjunct distribution of xeromorphic-sclerophyllous plants in Macaronesia and eastern Africa–Arabia, referred to as the Rand flora biogeographic pattern, is the “dragon tree group” within Dracaena (Asparagaceae). However, little is known about the evolutionary origin of this iconic group of semi-desertic trees. Here, we use exceptionally well-preserved fossils from western Anatolia to demonstrate range and ecological shifts of the lineage probably leading to some of the modern dragon trees. Compression fossils of apical leaf rosettes and detached leaves of Dracaena tayfunii spec. nov. were compared to modern Dracaena using the architecture of leaf-bearing branches, leaf morphology, and highly diagnostic leaf epidermal features observed with light and electron scanning microscopy. The palaeoecology of Dracaena was inferred using the depositional setting and associated plant taxa. The ca. 16 million-year-old (Ma) Dracaena from western Anatolia shows a character combination restricted today to the Macaronesian dragon tree, Dracaena draco: (1) Leaves are arranged in terminal rosettes; (2) leaves are ensiform, oblong, with a conspicuously dilated base (pseudo-sheath); and (3) leaf epidermis is strongly papillate with sunken stomata overarched by papillae of four neighbouring epidermal cells. Depositional setting, taphonomy, and the fossil plant association indicate that the Miocene Dracaena either grew in seasonally dry swamps within a complex fluvial–lacustrine environment or on adjacent slopes under a humid, warm climate. Hence, semi-desertic modern dragon trees allied to D. draco displaying distinct xeromorphism may have originated from a western Eurasian mesic lineage that had evolved xeromorphic characteristics by the Miocene. The morphology of this mesic ancestor later enabled the lineage to colonize and survive in the semi-desertic environments where it is found today. The new fossil species of Dracaena represents a classic example of pre-adaptation and niche shift.
Due to its remarkable flora and fauna, the Socotra Archipelago has been the focus of a number of ecological studies dating back to the beginning of the nineteenth century. Its unique biodiversity and – until very recently at least – relatively intact ecosystems have earned it the status of a World Heritage Site, and the islands are frequently referred to as the “Galápagos of the Indian Ocean”. Despite its small size, the archipelago is one of the major island biodiversity hotspots in the world with respect to its endemic fl ora, with roughly 37% of plant species and 15 genera unknown from elsewhere. The objective of this book is to summarise the existing state of knowledge on the vegetation in an ecological context. Apart from providing descriptive accounts of the various vegetation units, the book also deals therefore with the factors, predominantly abiotic, affecting the distribution and abundance of species. As with other isolated islands or archipelagos, Socotra is an ideal model system, relatively unaffected by many complicating factors, to test fundamental ecological theories from a wide variety of biological disciplines (including biogeography, evolutionary biology and colonisation).
Basement rocks crop out in three main uplift areas; elsewhere they are overlain by a veneer of Cretaceous and Tertiary plateau limestones. The oldest basement rocks are amphibolite facies meta-sediments and meta-igneous rocks which have been intruded by synkinematic granites and late-kinematic gabbros; younger weakly metamorphosed tufts and mudstones also occur. Post-kinematic igneous activity gave rise to a sequence of volcanic rocks, hornblende/ biotite and peralkaline granites, gabbros and minor intrusions which make up the bulk of the Haggler mountains. Of these the large occurrence of peralkaline granites is considered in detail. Following a long period of basement peneplanation, sediments were deposited which consist of Lower and Middle Cretaceous limestones, with some basal sandstones, followed without apparent break by cliff-forming shelf limestones of Palaeocene-Eocene age. Oligo-Miocene chalky deposits are only preserved in structural depressions. Vertical movements were predominantly responsible for the structure which is simple in the east but dissected by WNW-ESE block faulting in the west. Correlation with southern Arabia and Somalia suggests that the basement metamorphic association is Pre-Cambrian and the post-kinematic igneous rocks are early Palaeozoic; the sedimentary formations broadly correlate with those of southern Arabia and the Oligo-Miocene deposits in particular are most closely related to those of Dhufar.
Present study has produced first detailed land‐cover map of Socotra Island. A Landsat 7 ETM+ dataset was used as a main source of remotely sensed data. From numerous reference points (more than 250) coming from the ground data verification the set of training fields and the set of evaluation fields were digitised. As a classification method the supervised maximum likelihood classification without prior probabilities was used in combination with rule‐based post‐classification sorting, providing results of sufficient accuracy and subject resolution. Estimates of the area and degree of coverage of particular land‐cover classes within Socotra Island have brought excellent overview on state of island biotopes. Overall accuracy of the map achieved is more than 80%, 19 terrestrial land‐cover classes (including three types of Shrublands, three types of Woodlands, two types of Forests and Mangroves) have been distinguished. It consequently allows estimates of the current and potential occurrence of endemic plant populations, proposals of management and conservation plans and agro‐forestry planning.
Narrative of the journey
- Forbes Ho
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