Resource use by giant tortoises introduced as “ecological analogs” to Pinta Island, Galápagos

Background/Question/Methods

Loss of key plant-animal interactions (e.g., disturbance, seed dispersal, and nutrient cycling) due to large herbivore extinctions has diminished ecosystem functioning.  Introduction of ecological analog species could provide mitigation but rarely are any candidate analog species a perfect match for the extinct form targeted for replacement.  How morphologically and physiologically different can an analog be from the extinct species and still provide similar services? Pinta Island in the Galápagos Archipelago provides a pertinent opportunity to explore the issue.  The species of Galápagos giant tortoise native to Pinta (Chelonoidis abingdoni) was a saddle-backed form that is extinct in the wild due to decades of overexploitation.  In May 2010, 39 non-native Galápagos giant tortoises (sterilized hybrids) of two general phenotypes (saddle-backed and domed) were introduced to Pinta by the Galapagos National Park Service.  This first release of giant tortoises to Pinta, designed to implement changes in the ecosystem, precedes planned future releases to create a reproductive population.  Through behavioral observations and resource selection models we examined how and why analog tortoises used resources on the island in relation to their phenotype during the first year following release.

Results/Conclusions

Saddlebacked tortoises consistently selected areas with high densities of adult arboreal Opuntia cactus: immediately after release, tortoises were 1.2 x more likely to use high density cactus areas than low density cactus areas, and 590 x more likely a year after release.  Saddlebacked tortoises used the cactus as their primary food source, spending 57% of their foraging time on this single species.  Domed tortoises were more likely to use higher elevations, where the vegetation is much denser and the Opuntia cactus is absent, than saddlebacked tortoises, a pattern that is matched in the ecosystems these morphs typically occupy in their native range.  We conclude that due to the lack of niche equivalency, phenotype does influence the efficacy of candidate ecological analogs.  In the case of Pinta Island, because of their use of mid-elevations and high consumption of cactus, saddlebacked tortoises are likely a better fit for the niche left empty by the original Pinta tortoise, which apparently favored mid-elevations, and will better recreate lost ecological services.