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Drivers of diversity in Macaronesian spiders and the role of species extinctions

Authors

  • Pedro Cardoso,

    Corresponding author
    1. Azorean Biodiversity Group (CITA-A), Departamento de Ciências Agrárias, Universidade dos Açores, Terra-Chã, 9701-851 Angra do Heroísmo, Portugal
    2. Smithsonian Institution, National Museum of Natural History, 10th & Constitution NW, MRC 105, Room E-509, Washington, DC 20560-0105, USA
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  • Miquel A. Arnedo,

    1. Biodiversity Research Institute & Department of Animal Biology, Universitat de Barcelona, Av. Diagonal 645, 08028 Barcelona, Spain
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  • Kostas A. Triantis,

    1. Azorean Biodiversity Group (CITA-A), Departamento de Ciências Agrárias, Universidade dos Açores, Terra-Chã, 9701-851 Angra do Heroísmo, Portugal
    2. Biodiversity Research Group, Oxford University Centre for the Environment, South Parks Road, Oxford OX1 3QY, UK
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  • Paulo A. V. Borges

    1. Azorean Biodiversity Group (CITA-A), Departamento de Ciências Agrárias, Universidade dos Açores, Terra-Chã, 9701-851 Angra do Heroísmo, Portugal
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Correspondence: Pedro Cardoso, Smithsonian Institution, National Museum of Natural History, PO Box 37012, MRC 105, Room E-509, Washington, DC 20013-7012, USA.
E-mail: pcardoso@ennor.org

Abstract

Aim  To identify the biogeographical factors underlying spider species richness in the Macaronesian region and assess the importance of species extinctions in shaping the current diversity.

Location  The European archipelagos of Macaronesia with an emphasis on the Azores and Canary Islands.

Methods  Seven variables were tested as predictors of single-island endemics (SIE), archipelago endemics and indigenous spider species richness in the Azores, Canary Islands and Macaronesia as a whole: island area; geological age; maximum elevation; distance from mainland; distance from the closest island; distance from an older island; and natural forest area remaining per island – a measure of deforestation (the latter only in the Azores). Different mathematical formulations of the general dynamic model of oceanic island biogeography (GDM) were also tested.

Results  Island area and the proportion of remaining natural forest were the best predictors of species richness in the Azores. In the Canary Islands, area alone did not explain the richness of spiders. However, a hump-shaped relationship between richness and time was apparent in these islands. The island richness in Macaronesia was correlated with island area, geological age, maximum elevation and distance to mainland.

Main conclusions  In Macaronesia as a whole, area, island age, the large distance that separates the Azores from the mainland, and the recent disappearance of native habitats with subsequent unrecorded extinctions seem to be the most probable explanations for the current observed richness. In the Canary Islands, the GDM model is strongly supported by many genera that radiated early, reached a peak at intermediate island ages, and have gone extinct on older, eroded islands. In the Azores, the unrecorded extinctions of many species in the oldest, most disturbed islands seem to be one of the main drivers of the current richness patterns. Spiders, the most important terrestrial predators on these islands, may be acting as early indicators for the future disappearance of other insular taxa.

Ancillary