African savannahs once teeming with wildlife have been fragmented by human development, driving a decline in the lion population, a Duke study found.

Published in the journal Biodiversity and Conservation, the study found that two-thirds of the lion population have disappeared in the last 50 years. The study also found that 75 percent of Africa’s savannahs have disappeared within the same time frame, as well. Stuart Pimm, co-author of the study and Doris Duke professor of conservation ecology at Duke’s Nicholas School of the Environment, credited massive land-use change, deforestation and rapid human population growth as factors that have degraded the original savannah in a press release Tuesday.

“Only 25 percent remains of an ecosystem that once was a third larger than the continental United States,” Pimm said in the release.

It is estimated that 100,000 lions roamed the savannah in 1960 with only 32,000 to 35,000 currently remaining. Lion populations in West Africa have had the steepest decline.

Using satellite imagery from Google Earth, human population density data and estimates of the local lion populations, the researchers were able to estimate where lions have the highest chance of survival. They found that only 10 areas were identified as strongholds—spots that offer lions an excellent chance of survival. Many strongholds exist within national parks, however, and none exist in West Africa, where human populations have nearly doubled in the last 20 to 30 years.

“Giving these lions something of a fighting chance will require substantial increases in effort,” Andrew Jacobson, a research associate in Pimm’s lab, said in the release. “The next 10 years are decisive for this region, not just for lions but for biodiversity, since lions are indicators of ecosystem health.”