The index, developed by researchers from the University of Adelaide and James Cook University, is called Species Ability to Forestall Extinction, an Adelaide release reported Thursday.
The SAFE index is based on previous studies into the minimum population sizes needed by species to survive in the wild and measures how close species are to that minimum.
"The idea is fairly simple -- it's the distance a population is (in terms of abundance) from its minimum viable population size," Corey Bradshaw of the University of Adelaide's Environment Institute said.
SAFE is designed to complement the International Union for Conservation of Nature Red List of Threatened Species, not replace it, he said.
"Our index shows that not all Critically Endangered species are equal. A combined approach -- using the IUCN Red List threat categories together with the SAFE index -- is more informative than the IUCN categories alone, and provides a good method for gauging the relative 'safety' of a species from extinction," he said.
Of the 95 mammal species considered in developing the SAFE index, the researchers said, more than one in five are close to extinction and more than half are at "tipping points" that could take their populations to the point of no return.