Seed Dispersal by Chelonians: From Individuals to Communities

Plants are sessile for the most of their life cycle, but their gametes (pollen) and their propagules (seeds) can move from short to quite long distances across the landscape. The mechanism for the latter is called seed dispersal. Seed dispersal is the link between the end of reproduction and the beginning of vegetative growth, and is an important function that determines plant population persistence by influencing reproduction, population- and community dynamics. Many plant species rely on animal-mediated seed dispersal (zoochory), which thus ultimately shapes much of the world’s biodiversity. Chelonians represent one of the oldest vertebrate lineages, and have been considered one of the early frugivores and seed dispersers in evolutionary time. However, the role of chelonians as seed dispersers has been largely neglected and underestimated until very recently, despite repeated calls for the study of chelonian frugivory and seed dispersal (FSD). In my thesis, I assessed the role of chelonians as seed dispersers, from species to communities. I began by performing a review and synthesis of chelonian FSD in Chapter 1, where I mainly focused on: i) the taxonomical and geographic distribution of chelonian FSD, ii) the taxonomical distribution and traits of plants dispersed by chelonians, and iii) chelonian seed dispersal efficiency. My work is the first to provide an overview of the role of chelonians as frugivores and seed dispersers, and it highlights their importance not only from the individual and population perspective, but also from the community perspective. In Chapter 2 I then focused on Aldabra giant tortoises (Aldabrachelys gigantea) to assess their role in the seed dispersal community of Aldabra Atoll in relation to other frugivores. I provide the third study to date that evaluates the role of chelonians as seed dispersers at the community level using network analyses, and also provide the first evaluation of the role of tortoises and other dispersers in the plant–frugivore community of Aldabra Atoll. To my knowledge, my study on the Aldabra seed dispersal network is the first of its kind performed in the Western Indian Ocean – a globally important biodiversity hotspot. Furthermore, because the plant–frugivore community of Aldabra is intact and representative of the preanthropogenic assemblages that other islands used to have, the Aldabra network can serve as a template for the conservation and restoration of plant–animal interactions in these islands. In Chapter 3, I experimentally examined whether tortoise size and/or seed size affect their gut retention times. Gut retention time is one of the main traits that structure the spatial extent to which seeds can be dispersed. My study is one of the few that simultaneously evaluates both the effect of tortoise size and seed size on seed gut retention time. I demonstrate that both small and large tortoises can retain seeds for long periods, and thus spread seeds far and wide, further supporting studies that have highlighted the capacity of giant tortoises for restoration. Finally, in Chapter 4 I studied the thermoregulatory ecology of Aldabra giant tortoises, including looking at how environmental temperature may affect their role as seed dispersers, and then applied the findings about their thermoregulatory ecology in the wild to the management and husbandry of captive tortoises in Chapter 5. My work is the first to assess the thermoregulatory environment and ecology of Aldabra giant tortoises across temperature gradients. I successfully applied the knowledge gained through the latter work to inform the evaluation and management of the thermoregulatory environment of tortoises in Zurich Zoo, and provide methodological procedures that can be applied to other captive ectothermic species to provide an adequate thermal environment.