Evaluation of assisted colonisation trials to the Cocos (Keeling) Islands for an Extinct in the Wild skink

Date: 01, Nov, 2020
Author(s):   Schubert, K.
Publisher: The University of Western Australia

Masters Thesis

Assisted colonisations are active interventions to prevent extinction of a threatened species when it is unfeasible to abate the cause of the threat. The Christmas Island blue-tailed skink (Cryptoblepharus egeriae) is Extinct in the Wild due to introduced predatory species and has been bred in captivity for a decade. Assisted colonisation trials to two of the Cocos (Keeling) Islands, Pulu Blan and Pulu Blan Madar (980 km south-west from Christmas Island), took place in September 2019 and March 2020, each involving the introduction of 300 skinks bred in captivity. The study aimed to evaluate the suitability of these two islands as potential sites to support a self-sustaining blue-tailed skink population, by monitoring skink survival, reproduction, and habitat use. Here I show that the trial on Pulu Blan met two of four pre-determined metrics for success: (1) signs of mating and reproduction at six-months post-release, and (2) survival to six-months of age of skinks born on the island. Survival was much lower on Pulu Blan Madar, where a previously undetected super-colony of yellow crazy ants (Anoplolepis gracilipes) appeared to kill skinks and drive their dispersal to the edge of vegetated habitat. I showed the Pulu Blan provided suitable habitat (forest with open canopy) for expanding the skink population, but on Pulu Blan Madar, the rapid appearance of A. gracilipes, and reinvasion by rats, showed the critical need for regular monitoring following skink translocations. Given the challenges in controlling invasive species on Christmas Island, and the high costs of maintaining a captive breeding program, assisted colonisation of blue-tailed skinks to the Cocos (Keeling) Islands is a feasible option for establishing self-sustaining populations of this species outside of captivity.