The Succulents of Socotra – Giants and Dwarfs for Island Biogeography?

The islands of the Socotra Archipelago (Abdalkuri, Samhah, Darsa, Socotra), from 80 to 220 km in distance from Cape Guardafui as the easternmost point of the Horn of Africa, bear a variety of endemic and non-endemic succulents. Climatologically, the Horn of Africa, Arabia and Socotra are situated in the northern arid subtropical belt which means high drought stress to the vegetation in annual phases. Open formations of vegetation are characterised by chamaephytes and nano-phanerophytes, by succulent dwarf shrubs to giant candelabrum or caudex species. Patterns of ecomorphological and ecophysiological adaptations are shown from which life forms, succulence, surface reduction, water potentials, salinification and CAM metabolic pathway are discussed to indicate main tendencies in minimizing water loss. Those succulents are remnants of an old-African drought flora type or a result of dispersal events and further speciation. Some of the islands’ flora seems to depend upon the time of splitting of the continent Gondwana in late Cretaceous. The giant caudiciform succulents Adenium socotranum, Dendrosicyos socotranus , and Dorstenia gigas , the tree Dracaena cinnabari and some monotypic taxa e.g. are regarded as such palaeoendemics. Other succulents might have originated in more or less younger dispersal events and speciated to neoendemics further ( Echidnopsis spp., some Euphorbia spp.) with closer vicariant species. The succulent species are presented in their vegetation types, with references to their ecological traits. The application of a general concept for gigantism or dwarfism cannot be corroborated among succulents in the Socotra Archipelago. Anthropogenic influence has resulted in a degradation of vegetation, which hence means an already long-lasting ecological urgency for the population.