Exploring the impact of reintroduced Chelonoidis hoodensis on Opuntia echios on Santa Fé Island, Galápagos

Abstract

The ecological balance of Santa Fé Island, Galápagos, is determined, to a large degree, by the interplay between introduced giant tortoises (Chelonoidis hoodensis) and the native arboreal cactus (Opuntia echios var. barringtonensis). Historically heavily impacted by invasive feral goats, the island has undergone significant ecological restoration. Notably, the original giant tortoise population on Santa Fé Island went extinct, leading to the introduction of an ecologically similar species, C. hoodensis, in 2017. This study aimed to elucidate the impact of reintroduced tortoises on O. echios populations, recognizing their pivotal roles as ecosystem engineers and keystone species. The locations and sizes of tortoise carapaces encountered during the study indicate that since their release, they have continued to disperse throughout the island. Prior to this study, accurately distinguishing between Santa Fé land iguana and tortoise scat for density measurements was challenging due to limited field ID information. Through this research, new methodologies were developed and applied that facilitate accurate field identification of tortoise scat, thereby enhancing our ability to measure population densities effectively. These data suggest that the reintroduction of giant tortoises is likely to aid in the restoration of Santa Fé Island by increasing the dispersal of O. echios.