St Helena and Ascension BioProvince
This BioProvince (as defined by Armen Takhtajan) comprises the volcanic islands of St Helena and Ascension situated in the South Atlantic. St Helena formed some 14 million years ago, while Ascension is no more than about one million years old. Both were created by a build-up of volcanic material and their origins are closely linked to the volcanic activity associated with the Mid Atlantic Ridge. The area experiences a mild, maritime, tropical climate tempered by the trade winds. Because of its extreme isolation even over the past 14 million years few plant species have managed to colonise St Helena but this immense period has given rise to an extremely high level of endemism for such a relatively small island. For this reason the area has been classed as a floristic BioRegion in its own right. In total there are about 50 endemic vascular plants which seem to be derived from just 25 ancestral stock. Generic endemism is also comparatively high with some ten endemic flowering plant genera including Commidendrum, Lachanodes, Melanodendron, Petrobium, Pladaroxylon (Asteraceae), Mellissia (Solanaceae), Nesiota (Rhamnaceae), Nesohedyotis (Rubiacea), Trimeris (Campanulaceae) and Trochetiopsis (Malvaceae).
Ascension Island supports very few species and is still in the early stages of geological and ecological development. There are thought to be fewer than 30 indigenous vascular plant species but of these ten are endemic including four flowering plants, Sporobolus durus, S. caespitosus (Poaceae), Oldenlandia ascensionis (Rubiaceae) and Euphorbia origanoides (Euphorbiaceae), and six ferns, Asplenium ascensionis (Aspleniaceae), Anogramma ascensionis and Pteris ascensionis (Adiantaceae), Dryopteris ascensionis (Dryopteridaceae), Marattia purpurascens (Marattiaceae) and Xiphopteris ascensionis (Grammitidaceae).
Many of the BioProvince’s endemics appear to have their origins in Africa where the prevailing winds and currents come from, and some of these are thought to be relicts (palaeoendemics) of an ancient South African flora that died out when the climate became dryer some 10 million years ago.
The following accounts for this BioProvince have been written or will be written with particular reference to endemic and locally important species. Accounts available are displayed in green or yellow. Those displayed in red are either in the pipeline or awaiting expert contributions.
|Endemic Vascular Plant Flora|