Three species of reptile on Australia's Christmas Island have been declared extinct in the wild, according to a study released on Tuesday, with scientists baffled as to the cause.
Lister's gecko, the blue-tailed skink and the Christmas Island forest-skink were downgraded from "critically endangered" to "extinct in the wild" in the latest report by the International Union for the Conservation of Nature (IUCN).
"The extinctions... are an intriguing 'whodunnit', as their cause remains unclear," said John Woinarski, professor of conservation biology at Charles Darwin University in northern Australia.
The reptile population on Christmas Island, an Australian territory just south of Indonesia, has been declining rapidly since the 1970s, the IUCN said.
While scientists speculate that a snake introduced in the 1980s or environmental changes following the introduction of the Yellow Crazy Ant could be to blame, "the reason for the decline remains unclear," according to the report.
Scientists tried in vain to establish a captive breeding programme for the forest skink and it has now been declared extinct in the wild.
Lister's gecko and the blue-tailed skink both have "well-established" captive breeding populations but are now also extinct in the wild.
"In this case, the extent and severity of decline was revealed too late to save these Christmas Island reptiles," said Woinarski.
The forest skink was last seen in the wild in 2010 and the last remaining specimen died in captivity in 2014, Craig Hilton-Taylor, head of the IUCN's "red list" of endangered species, told reporters.
"It's all a rather sad picture, but the red list also gives us hope and shows us that conservation can work," added Hilton-Taylor.
The IUCN also sounded the alarm over the western ringtail possum, which was downgraded from vulnerable to critically endangered due to a fall in species numbers by 80 percent over the past 10 years.
Scientists suspect climate change is to blame as the possum is "susceptible to heat stress" and its food source has shrunk.
"A drying climate is pushing the ringtail possum to the brink of extinction," said the report.
Hilton-Taylor said: "We are currently losing species at the fastest rate in human history."
However, there was better news for two species of kiwi in New Zealand that were upgraded from endangered to vulnerable due to conservation efforts and predator control.
The IUCN's updated "red list" was released on Tuesday in Tokyo.
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