Evaluation of trial reintroductions of two extinct in the wild reptile species on Christmas Island

Conservation reintroductions play a vital role in the recovery of threatened species, and clear goals and objectives are essential for evaluating their effectiveness. In this study, we assessed short‐term success (<18 months) of trial reintroductions of the Extinct in the Wild blue‐tailed skink (Cryptoblepharus egeriae) and Lister's gecko (Lepidodactylus listeri) on Christmas Island. Our evaluation criteria focused on body condition, reproduction, habitat suitability, survival and population growth. In 2018 and 2019, 170 C. egeriae and 160 L. listeri were translocated from a local captive breeding facility to a 2600 m2 outdoor fenced enclosure designed to exclude a predatory snake. Despite body condition declining immediately following release for both species, it had improved by 6 months post‐release. We also detected successful reproduction in both species. Apparent survival was high for C. egeriae but low for L. listeri, and population growth was only evident in C. egeriae. We were unable to determine whether low survival of L. listeri in the release site was due to high post‐release dispersal (beyond the exclosure) or mortality. Both species selected habitats that contained high rock and log cover and avoided areas with low ground cover. Appropriate assessment criteria, as utilized in this study, enable objective and timely evaluations of reintroduction success, thereby facilitating the improvement and refinement of reintroduction protocols. Our study showed that C. egeriae can establish (in the short‐ to medium‐term) in a site from which a principal threat has been excluded and undergo rapid population growth, whereas under current conditions L. listeri cannot. However, we also demonstrate that such medium‐term success may not lead to long‐term success, as the rapid increase in C. egeriae population was reversed between 29 and 31 months after release because the barrier used to exclude an invasive predator, the wolf snake (Lycodon capucinus), was breached.