Africa's savannahs and the lions that roam there are disappearing at an alarming rate as ballooning human populations deprive the big cats of their natural habitat, a study released Tuesday showed.

About 75 percent of the continent's savannahs and two-thirds of the lion population have vanished over the past 50 years, according to researchers at Duke University in North Carolina.

"The word savannah conjures up visions of vast open plains teeming with wildlife," said Stuart Pimm of Duke's Nicholas School of the Environment.

"But the reality is that massive land-use change and deforestation, driven by rapid human population growth, has fragmented or degraded much of the original savannah."

Using new satellite data, the researchers were able to estimate that only 32,000 to 35,000 lions now live on Africa's savannahs, down from nearly 100,000 in 1960.

The situation is particularly dire in West Africa, where human populations have doubled over the past three decades, according to the study, which was published in the journal Biodiversity and Conservation.

Fewer than 500 lions remain in the region, spread across eight sites, it said.

The Duke researchers mapped areas still favorable for lions' survival by using high-resolution satellite imagery from Google Earth, along with human population density data and estimates of lion populations.

Existing maps based on low-resolution satellite imagery show large stretches of intact savannah, but the high-resolution images allowed the team to see that many of those areas are now dotted with small fields and small but widespread human settlements that encroached on the lions' habitat.

They found only 67 isolated stretches of savannah, defined as areas that receive between approximately 11 to 59 inches (28 to 150 centimeters) of rain annually, across the continent that had low enough human impacts and densities.

And only 10 of those areas were considered "strongholds" where lions had an excellent chance of surviving, many of them located within national parks.

"A 75 percent reduction in extent of African savannah is stunning and grim," said conservation scientist Thomas Lovejoy, a grants committee chair of the National Geographic's Big Cats Initiative that funded the study.

"It emphasizes the urgency for conservation of these great habitats and their magnificent species like lions."

The study comes after the US Fish and Wildlife Service announced last week it was launching a review on whether to list African lions under the Endangered Species Act.

Such a listing would prevent US hunters from bringing lion trophies from Africa back to the United States. Lion hunts are legal in some African countries.

The Asiatic lion was listed as an endangered species under the US law in 1970.