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In the land of giants: habitat use and selection of the Aldabra giant tortoise on Aldabra Atoll

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Abstract

With habitat loss and fragmentation among the greatest threats to biodiversity, a better understanding of the habitat use of keystone species is critical in any conservation management strategy. Aldabra Atoll, in the Seychelles archipelago, has the largest population worldwide of giant tortoises. This endemic species (Aldabrachelys gigantea) could be vulnerable to habitat fragmentation and loss induced by climate change related reduction in rainfall. Here, we assess habitat use and selection by A. gigantea in its natural environment on Aldabra. We quantified the habitat areas of A. gigantea based on the first high-resolution terrestrial habitat map of Aldabra, produced for this purpose using satellite imagery. The resulting map was combined with 4 years of movement data to assess A. gigantea habitat use and selection at landscape and home range scales. Grassland or ‘tortoise turf’ habitat was most preferred by A. gigantea on Aldabra, at the landscape scale across seasons, followed by open mixed scrub. These two habitats cover only 30 km2 (19.2%) of the surface of the atoll (total area: 155.5 km2). At the home range scale, there was no significant preference shown and habitat was used randomly. Our results suggest that Aldabra’s grassland habitat, despite its small area, is of great importance to A. gigantea. Conservation management actions for A. gigantea on Aldabra and elsewhere should therefore focus on the protection and maintenance of this habitat.

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Data availability

The habitat map that supports the findings of this study is openly available in Dryad at  https://doi.org/10.5061/dryad.8pf04d3. Data are governed by a Creative Commons Attribution 4.0 International copyright (CC BY 4.0). Data are fully public but should be appropriately referenced by citing this paper. Although not mandatory, we additionally suggest that data users contact and collaborate with data contributors (contact information available through the Dryad website).

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Acknowledgements

Aldabra Atoll is managed by the Seychelles Islands Foundation (SIF). We thank SIF’s staff on Aldabra for persistently searching for the GPS-tagged tortoises in inhospitable terrain to download their GPS telemetry data. We are also thankful to the SIF Aldabra and Head Office staff for logistical and administrative support; Philip Haupt for the preparation of the satellite images and discussion; Heather Richards, Christina Quanz and Wilna Accouche for feedback on the results of the classification process; Justin Prosper, Lindsay Chong-Seng, Christina Quanz and Christian Fleischer for the collection of the habitat reference points in 2009 and Environment Trust Fund for funding the flights to Aldabra; Franz Kuemmeth from e-Obs for his generous support with the custom-built GPS tags; and to Global Environment Fund (project ID 3295) for funding the acquisition of the GeoEye satellite imagery, then made available by SIF and Philip Haupt at Rhodes University. We are grateful to the Zurich-Aldabra Research Platform based at the University of Zurich who provided financial and research support, the ‘Forschungskredit’ of the University of Zurich for financial support. G. Schaepman-Strub’s contribution was supported by the University of Zurich Research Priority Program on Global Change and Biodiversity (URPP GCB).

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Correspondence to R. Walton.

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Communicated by Indraneil Das.

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Walton, R., Baxter, R., Bunbury, N. et al. In the land of giants: habitat use and selection of the Aldabra giant tortoise on Aldabra Atoll. Biodivers Conserv 28, 3183–3198 (2019). https://doi.org/10.1007/s10531-019-01813-9

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Keywords

  • Habitat mapping
  • Home range
  • Megaherbivore
  • Remote sensing
  • Seychelles
  • Western Indian Ocean