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Seed dispersal by Galápagos tortoises

Authors

  • Stephen Blake,

    Corresponding author
    1. Max Planck Institute for Ornithology, Vogelwarte Radolfzell, Schlossallee 2, D-78315 Radolfzell, Germany
    2. Whitney R. Harris, World Ecology Center, University of Missouri–St Louis, B216 Benton Hall, One University Boulevard, St Louis, MO 63121-4400, USA
    3. Charles Darwin Foundation, Puerto Ayora, Isla Santa Cruz, Galápagos, Ecuador, Casilla 17-01-3891, Quito, Ecuador
    4. 247 Illick Hall, State University of New York College of Environmental Science and Forestry, 1 Forestry Drive, Syracuse, NY 13210, USA
    5. Department of Biology, Washington University, Campus Box 1137, 1 Brookings Drive, St Louis, MO 63130, USA
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  • Martin Wikelski,

    1. Max Planck Institute for Ornithology, Vogelwarte Radolfzell, Schlossallee 2, D-78315 Radolfzell, Germany
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  • Fredy Cabrera,

    1. Charles Darwin Foundation, Puerto Ayora, Isla Santa Cruz, Galápagos, Ecuador, Casilla 17-01-3891, Quito, Ecuador
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  • Anne Guezou,

    1. Charles Darwin Foundation, Puerto Ayora, Isla Santa Cruz, Galápagos, Ecuador, Casilla 17-01-3891, Quito, Ecuador
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  • Miriam Silva,

    1. Charles Darwin Foundation, Puerto Ayora, Isla Santa Cruz, Galápagos, Ecuador, Casilla 17-01-3891, Quito, Ecuador
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  • E. Sadeghayobi,

    1. Department of Vegetation Ecology, Uppsala University, Villavägen 14, SE-752 36 Uppsala, Sweden
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  • Charles B. Yackulic,

    1. Department of Ecology and Evolutionary Biology, Princeton University, Guyot Hall, Princeton, NJ 08544, USA
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  • Patricia Jaramillo

    1. Charles Darwin Foundation, Puerto Ayora, Isla Santa Cruz, Galápagos, Ecuador, Casilla 17-01-3891, Quito, Ecuador
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Stephen Blake, Max Planck Institute for Ornithology, Vogelwarte Radolfzell, Schlossallee 2, D-78315 Radolfzell, Germany.
E-mail: sblakewcs@gmail.com

Abstract

Aim  Large-bodied vertebrates often have a dramatic role in ecosystem function through herbivory, trampling, seed dispersal and nutrient cycling. The iconic Galápagos tortoises (Chelonoidis nigra) are the largest extant terrestrial ectotherms, yet their ecology is poorly known. Large body size should confer a generalist diet, benign digestive processes and long-distance ranging ability, rendering giant tortoises adept seed dispersers. We sought to determine the extent of seed dispersal by Galápagos tortoises and their impact on seed germination for selected species, and to assess potential impacts of tortoise dispersal on the vegetation dynamics of the Galápagos.

Location  Galápagos, Ecuador.

Methods  To determine the number of seeds dispersed we identified and counted intact seeds from 120 fresh dung piles in both agricultural and national park land. To estimate the distance over which tortoises move seeds we used estimated digesta retention times from captive tortoises as a proxy for retention times of wild tortoises and tortoise movement data obtained from GPS telemetry. We conducted germination trials for five plant species to determine whether tortoise processing influenced germination success.

Results  In our dung sample, we found intact seeds from > 45 plant species, of which 11 were from introduced species. Tortoises defecated, on average, 464 (SE 95) seeds and 2.8 (SE 0.2) species per dung pile. Seed numbers were dominated by introduced species, particularly in agricultural land. Tortoises frequently moved seeds over long distances; during mean digesta retention times (12 days) tortoises moved an average of 394 m (SE 34) and a maximum of 4355 m over the longest recorded retention time (28 days). We did not find evidence that tortoise ingestion or the presence of dung influenced seed germination success.

Main conclusions  Galápagos tortoises are prodigious seed dispersers, regularly moving large quantities of seeds over long distances. This may confer important advantages to tortoise-dispersed species, including transport of seeds away from the parent plants into sites favourable for germination. More extensive research is needed to quantify germination success, recruitment to adulthood and demography of plants under natural conditions, with and without tortoise dispersal, to determine the seed dispersal effectiveness of Galápagos tortoises.

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