"Giant tortoises as drivers of the seed dispersal network of Aldabra"
Animal-mediated seed dispersal is an important ecological function in many ecosystems worldwide, especially in the tropics and sub-tropics. The application of network analyses to seed dispersal interactions has advanced tremendously in recent years, allowing researchers to address broad-scale questions on the community-level structure of seed dispersal. However, a detailed understanding of seed dispersal at a community level in species-rich ecosystems, especially if it includes experimental evaluations of all pairwise interactions, is a Herculean task. As a way to reduce the manifold complexities encountered in many ecosystems, evolutionary ecologists have a long tradition of using isolated island ecosystems, with their comparatively much fewer species, as field laboratories.
We are using the Aldabra Atoll in the Western Indian Ocean, and its vertebrate frugivores and fleshy-fruited plants, as a model system to investigate seed dispersal on multiple levels of ecological organisation, from understanding the structure of the entire seed dispersal network to predicting the outcome of pairwise frugivore–fruit interactions. The latter will focus on one of the likely central frugivore species in the network, the giant Aldabra tortoise (Aldabrachelys gigantea), aiming to elucidate its functional role as a seed disperser in detail. Aldabra is the last place on Earth where giant tortoises, a class of formerly widespread, major seed dispersers, can be studied in near-pristine conditions, providing the impetus for the focus on tortoise as a likely driver of a large proportion of the seed dispersal network.
The project has three intimately linked research foci: (1) The structure of the frugivore–plant interaction network of Aldabra; which and how many frugivores eat what and how many fruits? We will construct a robustly informed network by exhaustively sampling seed dispersal interactions using three methodologies: focal observations, camera traps, and fecal analysis. (2) Whereto are giant tortoises dispersing ingested seeds? To examine this question we will construct and experimentally field-test mechanistic, individual-based and spatially explicit models of tortoise-mediated seed deposition. (3) What is the effect of giant tortoise gut passage on seed germination and seedling establishment? This will be answered with seed germination and seedling establishment experiments.
This project is the first step in establishing Aldabra as a prime laboratory for high-resolution seed dispersal network studies.
