Almost half of Africa's lions are facing extinction and fencing them in -- and keeping humans out -- may be their only hope for survival, conservationists say. Writing in the journal Ecology letters, researchers said nearly half of Africa's wild lion populations might decline to near extinction during the next 20-40 years without urgent conservation measures, including fenced reserves. "It is clear that fences work and unfenced populations are extremely expensive to maintain," Craig Packer of the University of Minnesota said. Conservation costs are lower and lion population sizes and densities are greater in reserves secured by wildlife-proof fences, Packer said. Separating lion and human populations will be essential for the species' survival, researchers said. "These findings highlight the severity of the lion conservation crisis today and the limited choices we have to ensure a future for the species," Luke Hunter, president of big-cat advocacy group Panthera, said. "No one wants to resort to putting any more fences around Africa's marvelous wild areas, but without massive and immediate increases in the commitment to lion conservation, we may have little choice," he said. It is estimated that fewer than 30,000 lions remain in Africa in just 25 percent of the species' original natural habitat, a Panthera release said Wednesday.