Putting the spotlight on the Christmas Island Giant Gecko (Cyrtodactylus sadleiri)

Heading up the R2R fauna team, Brent Johnson

Image: Sophie Arnall

The Range to Reef fauna team has recently completed a comprehensive investigation into the distribution and abundance of this interesting endemic survivor on Christmas Island. Although now listed as Endangered under the EPBC Act this large arboreal gecko continues to persist as the last native reptile found outside captivity on the island.

Building on from the important ecological studies undertaken by PhD researcher Melissa Wynn, and with the co-operation of Parks Australia, Range to Reef expanded the areas investigated to cover all major habitat types found on Christmas Island. The nocturnal nature of this species requires long hours of walking measured transects through the landscape at night using powerful head torches to locate individual geckos. Multiple transects at each randomly selected reference point were each sampled over a minimum of three nights to enhance the confidence in calculating the estimated abundance of geckos. Once spotted, laser rangefinders calculated the height and distance from the transect of each gecko for later statistical analysis. The teams also recorded the presence of pest species such as black rats, wolf snakes and giant centipedes using the same methodology. All these introduced species are thought to predate on the native gecko.

Giant geckos were found across much of Christmas Island in variable densities, with the central plateau and northern zones appearing to be highest. Giant geckos were recorded in high abundance in both primary and secondary forest types suggesting that these species will utilise previously disturbed habitats. Analysis of the results will investigate what influence habitat is having on distribution and abundance.

Giant gecko and spider

Image: Judy Dunlop

Little is known about what other factors may influence giant gecko distribution and abundance. Christmas Island has a complex history in relation to invasive species and invasive species management and in particular, the highly invasive Yellow Crazy Ant (Anoplolepis gracilipes) has been implicated in declines of red crabs (Gecarcoidea natalis) and other native species across the island. It is possible that factors other than habitat type may drive the patterns of giant gecko distribution and abundance seen today.

Following a comprehensive statistical analysis of the existing data further investigations may be required to fill in any gaps in our knowledge on the abundance and distribution of this species on Christmas Island.

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