Hawaii's extinction crisis has claimed another victim: the copper-striped blue-tailed skink (Emoia impar), a once-common lizard that has now been declared extinct by the U.S. Geological Survey (USGS).

The skink was last seen in Hawaii in the 1960s. Extensive surveys between 1988 and 2008 failed to turn up any sign that it exists in the islands. It can, however, still be found on other Pacific island chains.

"No other landscape in these United States has been more impacted by extinction events and species invasions in historic times than the Hawaiian Islands, with as-yet unknown long-term cascading consequences to the ecosystem," USGS director Marcia McNutt said March 20 in a prepared statement. "Today we close the book on one more animal that is unlikely to ever be reestablished in this fragile island home."

It's unclear why the skink was extirpated in the Hawaiian islands but in a paper published this week in Oryx, USGS biologist Robert Fisher and Ivan Ineich of the Paris-based National Museum of Natural History point to a few possible factors, including evidence that the invasive big-headed ant (Pheidole megacephala) was preying on the lizards. According to the Offshore Islet Restoration Committee, big-headed ants, just one of the invasive ant species plaguing Hawaii, eat native plants and insects and have been shown to swarm over seabird chicks.

The researchers also say the introduction and spread of two lizards that look like the blue-tailed skink may have confused people looking for it. The first, the delicate skink (Lampropholis delicata), spread into the extirpated species's habitat. The other, a "sibling" species often referred to as the azure-tailed skink (Emoia cyanura), not only has a similar name as the blue-tailed skink, it also looks so much like the Hawaiian species that it could have been mistaken for its already missing relative. (E. cyanura has since become extinct in Hawaii.)

Fisher and Ineich call this a "cryptic extinction"—a species extinction that goes unnoticed for decades because it is easily confused with similar species. "The extinction of native Hawaiian bird species is well documented, partly because their presence and sounds had been so distinctive to humans," said Ineich. "But without regular field surveys, we tend to overlook the disappearances of smaller, secretive species, along with the causes of their extinction."

The researchers plan to continue studying at-risk species in the Pacific Islands.

Hawaii has more flora and fauna on the U.S. endangered species list than any other state. Dozens of others are awaiting protection.

Previously on Extinction Countdown:

Photo: A copper-striped blue-tailed skink (Emoia impar) photographed in Samoa during a USGS field survey. By Chris Brown, courtesy of USGS