The origin and distribution of the flora of Aldabra

Abstract

This paper is concerned with an analysis of the flora of Aldabra and the neighbouring islands of Cosmoledo, Astove and Assumption, which together form the Aldabra group. The flora includes 206 species of which 166 are indigenous and 40 are introduced. Only the indigenous flora is considered in detail. On the basis of geographic distribution these 166 species can be amalgamated into the following eight groups: Pantropical, Indo-Pacific, widespread-African, East African, Madagascan, Mascarene, Seychelles and Endemic, which indicate that the flora has been derived from a complete range of sources from widespread to very local. As assessment of the contributions made by each geographical group to the indigenous flora shows that about 55 % of the species are of widespread distribution and 45 % are found only in the region of East Africa and Madagascar. Furthermore, the majority of species of widespread distribution are associated with coastal habitats, whereas the majority of species of local distribution are associated with inland habitats. One hundred and sixty-six species is an exceptionally large number for a coral island since few others support more than a handful of indigenous species and these are invariably associated with coastal habitats usually being species of widespread distribution. Plenty of coral islands have produced endemics but none on the same scale as Aldabra which had 31 endemic species. A brief survey of possible means of plant introduction is given and these can be roughly grouped into sea, wind and animals, the first being associated mainly with the coastal and widespread species and the last two being associated mainly with the inland and more regional species. From this analysis of the flora of Aldabra several important facts emerge. Aldabra can support a large number of species and this is due mainly to the large terrestrial area which provides a variety of habitats. The flora includes a high percentage of local species and although still dominated by the widespread species the difference in numbers between widespread and local species is considerably less than on most other coral islands. Aldabra has a large number of species found only in inland habitats due again to the large terrestrial area and finally Aldabra has a very high percentage of endemism, almost 20 % of the indigenous flora.