The use of exotic species as taxon substitutes to restore lost ecological interactions is currently hotly debated. Aldabrachelys gigantea giant tortoises have recently been introduced to three islands in the Mascarene archipelago (Ile aux Aigrettes, Round Island and Rodrigues) to resurrect herbivory and seed dispersal functions once performed by extinct giant tortoises. However, potential unintended impacts by frugivore substitutes on native ecosystems, e. g. whether they will facilitate the germination of exotic plant species, are largely unknown. We investigated whether A. gigantea introduced to Rodrigues in 2006 could enhance the germination percentage of four widespread fleshy-fruited exotic species on the island. Using germination trials to forecast unintended impacts that could arise from the introduction of a frugivorous taxon substitute enables conservation managers to limit potential adverse negative interactions before they occur.
In germination trials that ran over 4 months, we investigated the effects of ingestion (gut passage and deposition in faeces) by sub-adult and adult A. gigantea on the germination percentage of four exotic fleshy-fruited plant species introduced to Rodrigues. We fed fruits of these plant species to sub-adult and adult A. gigantea to test how variation in age and size of the frugivore would affect seed germination. Feeding of distinctly coloured plastic pellets together with the fruits allowed us to test for individual tortoise effects on seed germination.
Ingestion by A. gigantea increased the percentage of seeds germinating of Mimusops coriacea and Lantana camara, but not percentage of germination of Veitchia merrillii or Wikstroemia indica. Seeds were more likely to germinate following ingestion by sub-adult rather than adult tortoises, which may be a consequence of the shorter gut passage time observed for sub-adults. Our results demonstrate that introduced frugivorous taxon substitutes could facilitate germination of exotic and invasive plants and highlight the need for conservation managers to weigh the risk of taxon substitutes potentially facilitating the germination and recruitment of exotic fleshy-fruited plants against the benefit of restoring lost seed dispersal functions of threatened indigenous plants. Our findings also highlight the importance of considering age and size variation in frugivores, in particular in long-lived taxa such as giant tortoises, when studying ingestion effects on the germination performance of plants.