Assessing plant diversity after giant tortoise reintroduction on Santa Fé Island, Galápagos

Abstract

Invasive species pose significant threats to island ecosystems, often leading to the loss of native biodiversity and altered ecosystem functions. On Santa Fé Island (SFI) in the Galápagos, the eradication of invasive goats has been followed by the introduction of the giant tortoise species Chelonoidis hoodensis from Española Island as an ecological replacement for the extinct native tortoise species. This study investigated the impact of this introduction on plant diversity since giant tortoise reintroduction in 2015. We established 20 permanent quadrats (PQs) along gradients of tortoise activity on SFI and conducted vegetation surveys using the line-intercept method, along with observations of tortoise presence and scat. We used Shannon's diversity index to measure plant diversity and performed non-metric multidimensional scaling (NMDS) with Bray-Curtis dissimilarity to compare plant diversity. An activity index was created from tortoise and scat counts to estimate tortoise activity in each plot. Our results, revealed no significant correlation between tortoise activity and plant diversity (r = 0.0406, p = 0.9). Our findings suggest that the introduction of C. hoodensis has not yet significantly influenced plant diversity on SFI. The lack of observed impact could be due to the relatively short timescale since introduction, the predominance of juvenile tortoises, and the prior recovery of vegetation post-goat eradication. This study highlights the complexity of ecological replacement strategies and underscores the need for long-term monitoring and adaptive management to fully understand and enhance their effectiveness in conservation efforts.