Plants and rocks
Species diversity of plants on Aldabra is unusually high for an isolated oceanic island and the plant communities form a mosaic of highly varied vegetation types. A high proportion (twenty percent) of the one hundred and eighty angiosperm species found on Aldabra are endemic. Such endemism and species diversity can be associated with the large size of and geomorphology of the atoll. Being a very large and raised coral atoll allows for the development of inland habitats and its variable geology encourages variation of plant community across the wide-ranging forms of limestone substrate.The atoll's vegetation is dominated by three main types.
Mangrove occupies the broad lagoon edges and is formed of a community of four main species: Avicennia marina, Bruguiera gymnorhiza, Ceriops tagal and Rhizophora mucronata. A dense mass of evergreen Pemphis acidula, usually with no understorey plants but occasionally interspersed with Acrostichum aureum (a large fern) or Sideroxylon inerme, forms what is termed “Pemphis scrub”. Mixed scrub is the third main type and consists of a wide number of both evergreen and deciduous species that can be described as “open” or “closed” mixed scrub with relation to the shrub density.
Important to mention is a phenomenon termed “tortoise turf” which is of great interest to especially evolutionary ecologists. It is a community of twenty-two species of grasses, sedges and herbs that has evolved under intense grazing pressures from giant tortoises across eastern Grande Terre. It has a low growth height of no more than 2cm which makes it appear like an urban mowed lawn. These species are, remarkably, genetically dwarfed and maintain this low growth form even when tortoise grazing is stopped. In such an instance the dwarf species will, however, be rapidly displaced by faster and taller growing species such as the coarse Sporobolus virginicus which is common on Aldabra. The aforementioned tortoise extinctions across other Indian Ocean islands may have also seen undocumented associated extinctions of similar genetic dwarf plant species.
In terms of geomorphology, Aldabra is a classic raised coral atoll formed on an ancient volcanic submarine mount. The substrate of the atoll is a biogenic limestone base of which four main layers can be distinguished that were formed during separate periods of reef growth. Submergence and re-emergence of the atoll is associated with sea level changes of several hundred metres that are associated with glacial and interglacial periods and the locking up of vast volumes of water in the polar ice sheets. Aldabra is currently raised at an average of 8m above sea level and is undergoing mainly erosional geological processes.
This manifests in the highly rugged and pitted “champignon” limestone that dominates the atoll. Champignon takes its name from the mushroom-shape of the lagoon islets that is caused by wave erosion undercutting. Fossils studies related to this geological sequence of immersion and emersion have interestingly shown that the giant tortoise has successfully colonised Aldabra independently on at least three separate occasions.
