A new study published in Conservation Letters by Tapia Aguilera & Gibbs provides evidence that reintroducing giant tortoises to Española Island in the Galapagos archipelago is transforming the island’s plant communities. The researchers found that the tortoises are engineering the island’s ecosystem by reducing woody plants and facilitating more grasslands. The tortoises could be making the island habitable for other rare species, such as the Waved Albatross, that only nests on Española Island.
The local giant tortoise, Chelonoidis hoodensis, had fallen to a population of just fourteen individuals in the 1960s. They’ve since been a conservation success story, with over two thousand tortoises reared and released on the island, but what effect are they having on their habitat?
To find out, Tapia Aguilera & Gibbs conducted exclosure experiments over eight years to compare plant communities with and without tortoises. They established fenced plots to exclude the tortoises, as well as unfenced control plots, to study the effects of the tortoises’ presence. Measurements taken over the eight years found that herbaceous plant cover and numbers of regenerating woody plants decreased in areas with tortoise access, while grass cover increased compared to the fenced exclosures without tortoises. This evidence suggests the tortoises are browsing on herbaceous plants and saplings while their trampling activity clears space for grasses to establish.
Tapia Aguilera & Gibbs also analysed aerial imagery of the island taken over 15 years to examine landscape-scale impacts of the tortoises. Comparing areas with varying tortoise densities, they found that in places with 1-2 tortoises per hectare (a hectare is two American football pitches, or one and a third ROTW football pitches), there were decreases in woody plant cover over time. This landscape-scale shift toward reduced trees and shrubs in areas where tortoises had reestablished indicates the tortoises are creating a more open, savannah-like habitat. The creation of grassier areas across the landscape mirrors the plot-level changes observed in the exclosure experiments.
The study’s findings suggest that trophic rewilding programs involving megaherbivores like tortoises could help restore critical ecosystem functions on islands where native species have been severely reduced. Specifically on Española Island, the return of tortoises may assist in regenerating the endangered prickly pear cactus Opuntia megasperma. The tortoises like the taste of the cactus and eat any parts of it they can.
“Tortoises consumed virtually every cladode (pad) that fell to the substrate. This would effectively eliminate vegetative reproduction in cactus… Similarly, tortoises consumed almost all cactus fruits deposited, which would greatly expand the scope for seed dispersal away from the parent plant, where bird predation on seeds is most intense (Grant & Grant, 1981). We expect that seed dispersal by tortoises provides a critical vector for sexual reproduction in cacti (Gibbs et al., 2014), resulting in more cactus recruits in the larger landscape, whereas intense consumption of cactus parts by tortoises renders asexual reproduction by cactus inviable.”Tapia Aguilera & Gibbs 2023.
The tortoises may also aid habitat restoration for the critically endangered Waved Albatross. The albatross is a big bird which requires open areas free of woody vegetation to nest and take flight. By limiting shrub encroachment, the tortoises create suitable grassy nesting grounds for this Galapagos endemic bird found only on Española.
Tapia Aguilera & Gibbs conclude that, for Española Island, rewilding with Giant Tortoises has been a success, but caution against assuming that the circumstances here apply everywhere. They write:
“Given the increasing scale and ambition of rewilding projects involving large-bodied herbivores on islands many of which are predicated on restoration of ecosystem services those herbivores might have once provided, more such studies examining other herbivores on other islands in the context of a broader range of ecosystem services are needed.”Tapia Aguilera & Gibbs 2023.
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Tapia Aguilera, W. and Gibbs, J.P. (2023) “Rewilding giant tortoises engineers plant communities at local to landscape scales,” Conservation Letters. Available at: https://doi.org/10.1111/conl.12968.