Rewilding Giant Tortoises Engineers Plant Communities

A groundbreaking study conducted by Galápagos Conservancy scientists has shed light on the transformative effects of reintroducing giant tortoises to island ecosystems. The research, published in the leading biodiversity conservation journal Conservation Letters, demonstrates that the restoration of these megafauna has profound consequences for plant communities and wildlife at both local and landscape scales.

Trophic rewilding, which involves reintroducing large-animal populations to revive top-down interactions and reverse environmental degradation, has gained increasing attention as a strategy for restoring ecosystems. While the importance of megafauna restoration is well recognized, the impact on ecosystems once population restoration occurs has yet to be studied.

The study by Galápagos Conservancy scientists Washington Tapia Aguilera and James P. Gibbs focused on a population of giant tortoises reintroduced to the arid island of Española in Galápagos. These tortoises have recovered from a low of 15 individuals to 3,000 today. Through careful monitoring and extensive vegetation mapping, the researchers discovered significant changes in plant communities have occurred in response to the restoration of the tortoises. At the local scale, the researchers used fenced exclosures to examine the effects of giant tortoise activity on plant community composition over eight years. The results indicated that the presence of reintroduced giant tortoises led to a shift in the ecosystem towards grasses, with a reduction in woody plants through browsing and trampling.

These local impacts were manifested at the landscape scale. By assessing the extent of woody vegetation over 15 years in areas with varying densities of re-established tortoises, the researchers found that a threshold density of 1-2 tortoises per hectare halted the incursion of woody plants, resulting in reduced dominance of trees in areas where tortoises had become established.

These findings have significant implications for ecosystem restoration and conservation efforts around the world. The restoration of giant tortoise populations has shaped plant communities and triggered cascading effects on many components of biodiversity on the island. By understanding the ecological consequences of reintroducing mega-reptiles like giant tortoises, conservationists can better inform and implement successful rewilding initiatives elsewhere.

This study represents a crucial step in understanding trophic rewilding and its potential to restore ecosystems. It highlights the importance of considering the unique characteristics of tropical island ecosystems which were once dominated by large-bodied reptiles and the transformative effects that restoring these megafauna can have on these fragile environments.

About: Washington Tapia Aguilera is a conservation scientist specializing in the Galápagos Islands and Director General of Conservando Galápagos, Galápagos Conservancy's operating arm in the archipelago. James P. Gibbs is Acting President of Galápagos Conservancy and a distinguished professor at the State University of New York's College of Environmental Science and Forestry.

The study was conducted by Galápagos Conservancy in collaboration with the Galápagos National Park Directorate with funding from NASA.