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  • 1. Blake, S.
    et al.
    Wikelski, M.
    Cabrera, F.
    Guezou, A.
    Silva, M.
    Sadeghayobi, Elham
    Uppsala University, Disciplinary Domain of Science and Technology, Biology, Department of Ecology and Genetics, Plant Ecology and Evolution.
    Yackulic, C. B.
    Jaramillo, P.
    Seed dispersal by Galápagos tortoises2012In: Journal of Biogeography, ISSN 0305-0270, E-ISSN 1365-2699, Vol. 39, no 11, p. 1961-1972Article in journal (Refereed)
    Abstract [en]

    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 (12days) tortoises moved an average of 394m (SE 34) and a maximum of 4355m over the longest recorded retention time (28days). 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.

  • 2.
    Sadeghayobi, Elham
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Biology, Department of Ecology and Genetics, Plant Ecology and Evolution.
    Blake, Stephen
    Wikelski, Martin
    Gibbs, James
    Mackie, Roderick
    Cabrera, Fredy
    Digesta retention time in the Galapagos tortoise (Chelonoidis nigra)2011In: Comparative Biochemistry and Physiology A, ISSN 1095-6433, E-ISSN 1531-4332, Vol. 160, no 4, p. 493-497Article in journal (Refereed)
    Abstract [en]

    The retention time of food in the digestive tract of animals has important implications for digestive physiology. Retention time impacts digestive efficiency and among herbivores affects plant-animal interactions including herbivory and seed dispersal. Poorly studied yet iconic Galapagos tortoises are large-bodied generalist herbivores and ecosystem engineers which migrate seasonally. Potentially variable digesta retention times due to strong seasonal and attitudinal temperature gradients may influence tortoise seed dispersal abilities and rates of herbivory. We fed captive adult tortoises living in semi-natural conditions on Galapagos with inert particles and seeds from locally available fruits to determine whether seed size and ambient temperature influenced retention time. Median retention time varied from 6 to 28 days, with a mode of 12 days. Seed size had no effect on any of our measures of retention time, but ambient temperature was inversely correlated with retention times. Long retention time facilitates long distance seed dispersal by Galapagos tortoises, which may improve effectiveness. The effect of temperature, which may double from hot lowlands to cold highlands through the seasonal cycle, on tortoise digesta retention time will strongly influence seed dispersal efficiency and may influence patterns of food selection and migration in this species.

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