Effects of an ecological replacement herbivore on cactus recruitment on an arid island

Abstract

Recognized for their high biodiversity and endemism, island ecosystems are particularly vulnerable to anthropogenic factors such as invasive species and extinction of endemic keystone species. The Galápagos Islands, with some of the highest endemism rates in the world, are an important case study of these challenges. Native herbivores are increasingly at risk of extinction worldwide, leading to the loss of vital ecological functions such as seed dispersal and nutrient cycling. On Santa Fé Island, human-driven extinction of the giant tortoise is a prime example of this, while the introduction of feral goats (Capra hircus) further degraded island plant communities. Together, these changes altered the distribution and regeneration of endemic species like Opuntia echios var. barringtonensis, a dominant species and keystone resource for native herbivores. Recent conservation efforts have eradicated goats and reintroduced a related tortoise species, Chelonoidis hoodensis, to restore ecological functions and support ecosystem recovery. This study aimed to examine the role of C. hoodensis introduction in influencing the distribution and density of O. echios seedlings and saplings on Santa Fé Island that is recovering from more than a century of heavy feral goat degradation. I hypothesized that the presence of C. hoodensis would positively correlate with higher densities of O. echios seedlings and saplings; I hypothesize this because giant tortoises were historically one of the primary seed dispersers of O. echios. To examine the relationship between tortoise activity and O. echios demographics on Santa Fé Island, nine 50 × 50 m plots were established across varying tortoise activity zones and subdivided into 25 10 × 10 m subplots. Adult cactus height and DBH were recorded in large plots, while seedling and sapling densities and tortoise activity (via direct observation and scat) were documented in smaller subplots. Higher tortoise activity correlated with higher seedling density, suggesting a positive effect of ecological replacements on Opuntia regeneration. Continued monitoring is needed to assess long-term vegetation impacts given that the island vegetation is still recovering from feral goats and tortoises are still expanding. Ecological replacements, like the introduction of C. hoodensis on Santa Fé Island, offer a promising strategy for restoring lost functions in degraded ecosystems.