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The world's remaining lions are in trouble. There are simply too many humans hungry for the same land the majestic cats roam. The more the human population grows, the more the lion population plummets. Only fences can keep one species from killing the other, according to a leading lion researcher.

In fenced reserves such as South Africa's Kruger National Park, which is as large as the state of New Jersey, "the population of lions is doing just fine," Craig Packer, an ecologist at the University of Minnesota, told NBC News from his research site in Tanzania.

"However, that is just a small proportion of the total African population of lions. The vast majority of lions live in unfenced reserves and … the trends are pretty disturbing," he added.

Counting lions is notoriously difficult work. Current best estimates put the African population at around 35,000. Nearly half of the known populations in unfenced areas are declining to the point that within 20 to 30 years, they will be just 10 percent of their potential population, Packer said.

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Humans pushing lions to the brink

The reasons for the lion decline are many, most driven by humans, the researcher noted. The biggest is conversion of suitable lion habitat to farm land to support a human population that is projected to quadruple in Africa by the end of this century.



There's also a rise in human-lion conflict. "Lions kill cattle and eat people and people don't like that so there are a lot of retaliatory killings," Packer explained. The killings used to be done with spears and bows and arrows. Today poisoning is ubiquitous, he said.

Other factors behind the loss of lions include poorly managed trophy hunting and the bushmeat trade, which robs lions of their prey and often leads to lions inadvertently getting caught in traps.

The August issue of National Geographic Magazine features a 50-page spread on African lions. National Geographic

Efforts to promote coexistence between humans and lions such as the Laikipia Predator Project in Kenya have met small-scale success, Packer said, but large-scale lion conservation will only come by raising the funds to fence them in.

Wakeup call

He knows that the concept of fences goes against the romantic vision of untrammeled landscapes where wildlife roams free. And, in some places, chain-link barriers are inappropriate, he said.



David Quammen writes in the August issue of National Geographic Magazine that even Packer "wouldn't put a fence across any valuable route of wildlife dispersal or migration."

But without the fences around large portions of the remaining lion populations, combined with funding for patrols and repairs, the future of the majestic cats is grim, Packer said.

"I'm just trying to encourage people to wake up," he told NBC News. "The future is now. The challenges are enormous. And if we don't start thinking about finding a way to fence these areas, there will be more and more habitat loss."

Notes: The August issue of National Geographic Magazine features a 50-page spread on lions, including two articles written by David Quammen and images from wildlife photographer Nick Nichols.

John Roach is a contributing writer for NBC News. He also contributes to National Geographic Online, though was not involved in the lion coverage for the August issue of the magazine. To learn more about him, visit his website.