Just 15 animals formed the founding group for a long-term conservation program that eventually returned thousands of tortoises to the island. By 2020, more than 2,300 Española giant tortoises lived there, including animals born in the wild. Española Island in Ecuador’s Galápagos archipelago came close to losing its giant tortoise population. The recovery marked a major conservation achievement, although the return of the species did not automatically restore every part of its habitat.
The Evolutionary Applications study found evidence that released tortoises were reproducing naturally. This finding helped establish that released animals could survive over the long term. Gibbs and his colleagues examined the results of the decades-long effort. The researchers assessed tortoise survival, reproduction, movement, and interactions with vegetation. Their findings indicated that the reintroduced population faced a low risk of extinction, but limited food availability and dense woody vegetation could restrict its further growth and expansion across the island. These figures documented a growing contribution from natural reproduction, despite the limited genetic diversity of the founding population. Their analysis suggested that the population could remain viable without continued releases, although habitat conditions would still influence how widely the tortoises could spread. A larger tortoise population does not necessarily mean that the animals can use all available habitat. Cactus provides an important food source and helps supply moisture, making its availability significant for tortoise survival.
A 2014 study published in PLOS ONE by James P. A 2013 study published in Evolutionary Applications documented the small founding population. Researchers found 14 Española giant tortoises on the island, comprising 12 females and two males. A male from the San Diego Zoo was subsequently added to the group, bringing the breeding population to 15 animals. Among the sampled animals, the proportion born in the wild increased from none in 1994 to 3% in 2004 and 24% in 2007. The researchers also estimated that about half of the tortoises released since 1975 were still alive in 2007. The 2014 PLOS ONE study found that tortoises tended to occupy areas with abundant mature prickly pear cactus and relatively low densities of woody vegetation.
