The giant tortoise, Aldabrachelys, and its bearing on the biogeography and dispersal of terrestrial biota in the Western Indian Ocean

Abstract

The evolution of species of giant tortoises and the nature of processes governing their dispersal in the Western Indian Ocean have been matters of interest for some time. Historical records indicate that these animals were once widely distributed in the region, from Madagascar to the Seychelles in the north and to the Mascarene islands in the south. New data on the status of Aldabrachelys and related or supposed species, and controversies regarding genetic diversity and distribution have prompted an evaluation of the islands and submerged Banks in the region that, during the later Cenozoic, might have provided refuges and/or stepping-stones for the spread of populations in what is now open ocean. Given the apparent stability of the crust in much of the area for at least the Quaternary Period, sea-level, driven by climatic change, and transport by ocean currents, appear to be the principle factors controlling the access not only of populations of giant tortoises but also other terrestrial biota to potential refuge areas in the region. The distributions of living and fossil tortoise communities are discussed. Four types of terrestrial oceanic refuges for migrants have been identified. At the present day, with the exception of Aldabra, high limestone islands offer only relatively small areas for colonization and all would have been drowned during interglacial sea-level high-stands. Although they have the potential for some increases in land area during the early stages of transitions to low-stand, as sea-level fell significantly they would have become waterless pinnacles, only generally accessible to organisms capable of flight. Volcanic islands have the same general limitations, but may have remained as smaller entities when sea-level was higher than at present, but providing only limited land access and restricted vegetation. Low islands and sand cays, typically of more recent origin, could not have existed during periods of high sea-levels, but many are situated on extensive submerged Banks that would have formed relatively large land areas when sea-level was low. Major Banks, most of which are regarded as fragments of continental crust, are currently submerged but, during glacial periods would together have offered land areas in excess of 130,000km2, providing a diverse range of terrestrial environments and vegetation. Access to all of these was nevertheless restricted by the distribution of ocean currents, the only plausible natural means of dispersal, which probably varied during glacial interludes, but also by the abilities of the animals to survive long periods adrift in the ocean. The distribution of these tortoises, in both the fossil record and historical data, provides a model for the dispersal of other terrestrial biota in the region, although the present distribution and genetic characteristics of the tortoises probably reflects recent human intervention.