Equivalency of Galápagos Giant Tortoises Used as Ecological Replacement Species to Restore Ecosystem Functions

Abstract

Loss of key plant-animal interactions (e.g., disturbance, seed dispersal, and herbivory) due to extinctions of large herbivores has diminished ecosystem functioning nearly worldwide. Mitigating for the ecological consequences of large herbivore losses through the use of ecological replacements to fill extinct species' niches and thereby replicate missing ecological functions has been proposed. It is unknown how different morphologically and ecologically a replacement can be from the extinct species and still provide similar functions. We studied niche equivalency between 2 phenotypes of Galápagos giant tortoises (domed and saddlebacked) that were translocated to Pinta Island in the Galápagos Archipelago as ecological replacements for the extinct saddlebacked giant tortoise (Chelonoidis abingdonii). Thirty-nine adult, nonreproductive tortoises were introduced to Pinta Island in May 2010, and we observed tortoise resource use in relation to phenotype during the first year following release. Domed tortoises settled in higher, moister elevations than saddlebacked tortoises, which favored lower elevation arid zones. The areas where the tortoises settled are consistent with the ecological conditions each phenotype occupies in its native range. Saddlebacked tortoises selected areas with high densities of the arboreal prickly pear cactus (Opuntia galapageia) and mostly foraged on the cactus, which likely relied on the extinct saddlebacked Pinta tortoise for seed dispersal. In contrast, domed tortoises did not select areas with cactus and therefore would not provide the same seed-dispersal functions for the cactus as the introduced or the original, now extinct, saddlebacked tortoises. Interchangeability of extant megaherbivores as replacements for extinct forms therefore should be scrutinized given the lack of equivalency we observed in closely related forms of giant tortoises. Our results also demonstrate the value of trial introductions of sterilized individuals to test niche equivalency among candidate analog species.

Science for a wilder Anthropocene: Synthesis and future directions for trophic rewilding research

“…translocations within the Galapagos Islands (e.g., ref. 73). These studies document that large and giant tortoises are low-risk, high-impact rewilding candidates that provide key ecological functions as dispersers of large seeds, herbivores, and The term "present-natural" refers to the state that a phenomenon would be in today in the complete absence of human influence through time (111).…”

Advances in restoration ecology: rising to the challenges of the coming decades

“…Recent debates discuss the extent to which species might be substitutable, and hence functions performed by recently extinct species could be reinstated by the translocations of closely related extant species (e.g., Griffiths et al 2013, Hunter et al 2013). An extreme form of this approach is what has been dubbed ''rewilding'' (e.g., Donlan et al 2005), in which functional equivalents of long-extinct but presumably keystone species are introduced to ecosystems.…”

Turtles and Tortoises of the World During the Rise and Global Spread of Humanity: First Checklist and Review of Extinct Pleistocene and Holocene Chelonians.

“…There is also increasing evidence building of the major ecological impact that grazing herds of giant tortoises have on ecosystem characteristics, especially on islands (Gibbs et al , 2014Hunter et al 2013;Froyd et al 2014) (see Fig. 14).…”