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 130,000 km 2 ,

Ancient DNA elucidates the lost world of western Indian Ocean giant tortoises and reveals a new extinct species from Madagascar

“…4). This is in line with the geological history of Aldabra, which was evidently at least once completely submerged during the last 200,000 years (32)(33)(34)(35). Consequently, any giant tortoises that potentially occurred there before would have drowned.…”

Late Holocene spread of pastoralism coincides with endemic megafaunal extinction on Madagascar

“…In Madagascar, people often encourage ovicaprids to forage on a wide range of plant types in deference to the grazing preferred by zebu. The dispersal ability of giant tortoises may attest to the high tolerance of resource depression [42], but our findings underscore that the potential for direct competition must be considered during ongoing efforts to both reintroduce giant tortoises and maintain local livelihoods [30,43]. Future recovery and analysis of additional ovicaprid bones should be a priority, because even our limited dataset follows from sampling all known collections of ovicaprid bones from the region.…”

“…Sites marked in green include bones of only endemic animals, those in yellow include only introduced animal bone, and those in red include both. We used sites grouped along the Morombe coast[1][2][3][4][5][6][7][8][9][10], Tulear coast[11][12][13][14][15][16][17][18][19][20][21][22], Mangoky drainage[23][24][25][26], and Onilahy drainage[27,28] and sometimes the Far South[29][30][31][32][33][34][35][36][37][38][39][40][41][42][43] for comparisons of stable isotope values among taxa (electronic supplementary material, appendix 'Data Analysis -Radiocarbon & Stable Isotopes'). Specimens from two other west coast sites[44,45] contribute14 C data to our analyses but are not considered in comparisons of stable isotope data given their relatively wide geographical spread.…”

Last Interglacial changes in sea level on Aldabra, western Indian Ocean

“…Geology proved to have had a profound influence on the origins and dispersal of the Aldabra biota throughout the western Indian Ocean. The presence or absence of terrestrial species provides important evidence of repeated drowning and recolonization of the island, orchestrated by sea‐level change (Braithwaite, 2016a). The expedition laid the foundations for the establishment of a research station on Aldabra by the Seychelles Islands Foundation (inaugurated in 1979), and for the recognition of the site as a UNESCO World Heritage Centre in 1982.…”