In 2009, the Christmas Island blue-tailed skink and Lister’s gecko were headed for imminent extinction. Parks Australia acted quickly to collect remaining
wild individuals in order to establish captive breeding programs on Christmas Island and at Taronga Zoo, Sydney, which have been highly successful.
A Threatened Species Recovery Hub project team is working closely with Parks Australia to help secure a future for the two lizards beyond captivity.
Jessica Agius, Jon-Paul Emery and John Woinarski report on the latest developments and the challenge of protecting
these ancient and unique island reptiles.
Many species occur only on islands, a characteristic that has long fascinated biologists. But while evolution in isolation has led to new species, it has also rendered these species highly vulnerable to changes in their environment, such as the arrival of new predators. In Australia, our three most recent extinctions have all been island endemics.
The Australian external territory of Christmas Island, which lies 1550 km off the north-western coast of mainland Australia, is a case in point. The island had an intriguing reptile fauna, comprising one endemic blind snake, two endemic geckos, two endemic skinks, and one native skink that also occurs elsewhere. These endemic species existed on the island for several million years, and all of the lizards remained common up to the 1970s, but four then declined precipitously. These declines now seem most likely to have been caused by the inadvertent introduction of the Asian wolf snake (Lycodon capucinus) from stowaways on freight shipping.
Observing the decline in wild populations, in 2009 and 2010 Parks Australia managed to secure enough individuals of the blue-tailed skink (Cryptoblepharus egeriae) and Lister’s gecko (Lepidodactylus listeri) to establish captive breeding populations. Shortly after, they became extinct in the wild. The opportunity came too late for the Christmas Island forest skink (now extinct) and coastal skink (now extirpated from Christmas Island). However, the captive populations of the blue-tailed skink and Lister’s gecko have flourished. The 66 skinks collected have swelled in 10 years to over 1500 captive individuals, and Parks Australia is exploring options beyond captivity.
Jessica Agius and supervisor David Phalen analyse samples for pathogens. Image: Jon-Paul Emery
The first of these explorations was trialling blue-tailed skinks in a 2600 m2 semi-wild enclosure on Christmas Island, which was surrounded by a 1m-high barrier designed to exclude wolf snakes, rats and giant centipedes. In April 2017, Parks Australia released 139 blue-tailed skinks into the exclosure and hub PhD candidate Jon-Paul Emery from the University of Western Australia monitored the new population. Jon-Paul recorded a gradual decline in their numbers, and by September 2017 none were detected. While there was no obvious cause of decline, monitoring also recorded large numbers of introduced giant centipedes. Despite efforts to remove them through active searches and trapping prior to and post release, centipede numbers could not be suppressed. As the centipedes had been on Christmas Island for at least 100 years, they had not previously been considered the primary threat to the skinks, but they are ferocious predators capable of preying on vertebrates much larger than themselves.
Jon-Paul undertook an experiment exposing skinks in smaller enclosures to a density of centipedes that matched that of the exclosure for three months and found that it reduced the survival of the skinks by over 30%. Christmas Island National Park staff consequently made great efforts over the next six months to eradicate centipedes from the reintroduction site, in preparation for another trial. Parks staff and Jon-Paul also added an estimated 20 tonnes of logs and branches, 10 tonnes of rock, ceramic tiles and wooden pallets to the site, to improve its habitat suitability.
Jon-Paul during monitoring of the first trial reintroduction. Image: Jon-Paul Emery
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