Ecological costs of waiting to restore ecosystem engineers: The case of giant tortoises in the Galapagos Islands

Abstract

In the Galápagos Islands, giant tortoises ( Chelonoidis spp.) engineer terrestrial ecosystems. Endemic tortoise lineages have been lost from Floreana and Pinta Islands, resulting in ecosystem shifts, with tortoise population restoration currently being considered using hybrids that include ancestry from the now extinct C. niger of Floreana Island. We combined long-term vegetation monitoring with simulation of vegetation responses to tortoises, and tortoise translocation scenarios to assess whether using C. niger hybrids as ecological replacements on Pinta Island could restore ecosystem function without compromising restoration on Floreana Island. Woody plants have increased by >100% on Pinta Island between 2011 and 2022 in the absence of ecologically effective numbers of tortoises, but re-establishment of tortoises at a moderate density (0.7 tortoises/ha) could reverse those trends and delay restoration on Floreana Island by ca. one decade while preventing continued ecosystem degradation on Pinta Island. We demonstrate the feasibility of restoring ecological replacements and the importance of rewilding ecosystem engineers before ecosystem shifts become irreversible.