Speculations on seed dispersal and the flora of the Aldabra archipelago

Abstract

The 263 angiosperm non-marine taxa now recognized for the islands of the Aldabra archipelago consist of 176 native taxa, of which 43 are endemic to the islands, and 87 cultivated plants and weeds that have been introduced by man, of which 4 are no longer extant. A number of so-called introduced plants, such as Casuarina equisetifolia and Cocos nucifera , could be native, for they are well known strand species. Sites that are likely landing places for the establishment of sea-borne plant propagules are also likely places for human settlement, especially if there are shade and/or food trees already established. The status of these plants is therefore uncertain. The endemic taxa are mainly of Madagascan and African affinities, as are also many of the inland, non-endemic, native taxa. The strand flora, however, generally have a wider, Indo-Pacific distribution. Wind dispersal, as is to be expected from the prevailing winds, is ineffective, whereas sea dispersal, although effective, provides problems of successful establishment. It is estimated that a minimum buoyancy period of 5 or 7 1/2 days in sufficient for propagules to reach Aldabra from Madagascar or Africa respectively. Dispersal by birds, despite the apparent shortage of suitable seed- or fruit-eating migrant birds, can be accounted for by the successful introduction of 1 taxon every 400 years. With a probability of 1 in 106 an annual migrant population of only 2500 birds is necessary. The evolution of the flora followed the final emergence of Aldabra from beneath the sea some 80000 years ago. The establishment of the strand flora was then followed by the less salt-tolerant inland flora. This flora must have been well established before the arrival of the giant tortoise in order to provide for its own regeneration as well as being able to withstand the requirements of the tortoise for shade and food. The establishment of the lagoon mangrove flora followed the breaching of the land rim some 4000-5000 years ago.