The tiger Shere Khan was lord in Rudyard Kipling's Jungle Book, but his modern-day descendants are king no more: The big cats have seen their central Indian forests dwindle and fracture.
The remaining tigers are only surviving by moving through critical—but unprotected—corridors of land that link distant populations, a new study says.
Using hair and fecal samples, Sandeep Sharma, of the Smithsonian Conservation Biology Institute, and team studied genes from 273 individual tigers that live in four distinct locations within India's 17,375-square-mile (45,000-square-kilometer) Satpura-Maikal region.
Tigers once roamed across Asia from Turkey to the Russian Far East, but have vanished from over 93 percent of that range. (See tiger pictures.)
The 20th century was especially tough on the now-endangered beasts, when three subspecies became extinct, leaving six—all of which are at risk. (See a National Geographic magazine interactive of big cats in danger.)
At a glance the region's tigers seem to live in four populations, each occupying its own territory in what's called a designated tiger conservation landscape, or TCL. Those are Kanha-Phen, Pachmari-Satpura-Bori, Melghat, and Pench.
But the genetic study suggests otherwise: Corridors of woods and undeveloped land up to 125 miles (200 kilometers) long actually link Kanha and Pench into a single genetic unit, and Satpura-Melghat into a second.
That means the four populations of tigers are breeding as two much larger populations—and keeping their genetic diversity alive in the process.
Corridors also aid tiger survival on the ground, Sharma said, making the cats more likely to withstand many types of threats. (Related:"Tigers Making a Comeback in Parts of Asia.")
"If one of two connected populations drops, say because of poaching or some other factor, the other can expand and repopulate the area," he explained. But if these corridors aren't protected as wildlife habitat by the government or other entities, the land may be developed and leave the tigers in "islands."
If this happens, "eventually they are doomed."
Tiger Family Tree
Sharma and colleagues looked at the tiger population tree in the Satpura-Maikal region, which has seen dramatic declines in tiger habitat. (Read "A Cry for the Tiger" in National Geographic magazine.)
The team found two distinct periods in which tigers' genetic populations divided rapidly, and each was tied to known historical events.
"One was about 700 years ago, and that's [around] the time when Mughal invaders came into the region and they started clearing river valleys and intensified agriculture in those valleys," he said, noting that the major threat facing tigers at that time was habitat loss.
The second period was about 200 years ago, Sharma said, when the British Empire not only felled trees to fulfill its enormous need for timber, but also introduced a vast arsenal of firearms that dramatically increased the number of tigers killed by hunters.
"You can really see these two distinct patterns of genetic subdivision in this population," said Sharma, whose study appeared July 30 in Proceedings of the Royal Society B: Biological Sciences.
Sharma and colleagues also used their data to look back in time some 2,000 years and compare the present situation with ancient patterns of tiger gene flow.
Only tigers in those populations still connected by these corridors are maintaining similar levels of gene flow [to what] we saw historically," he explained.
In areas "that have lost the corridors, the gene flow has significantly decreased."
Living With Tigers
By illuminating the past and present the study provides a roadmap for where future conservation efforts must be focused—keeping the fragile links open between different population groups, according to the authors.
Today, however, these tiger corridors have no legal protection. They are simply forest landscapes, used by local peoples and subject to development, including mining in one of India's prime coal regions.
Earlier this year the Indian Ministry of Environment and Forests gave Coal India Limited permission for coal-mining development in the crucial Satpura-Pench wildlife corridor.Officials stated publicly that the mine is an underground rather than open facility, and thus shouldn't interfere with the tigers' migratory corridor.
But Sharma is unconvinced, suggesting that mining brings with it settlements, roads, and infrastructure, which can be a major threat to the corridors just at the time when hard genetic data have shown that tigers are using them to travel and reproduce.
Conservationist Luke Dollar, a National Geographic Society Emerging Explorer who manages the society's Big Cats Initiative, said the tough decisions faced here are common where big cats prowl.
"India has half of all the remaining tigers on Earth, but it's also a perfect example of what we face in big-cat conservation, whether it be here or in Africa," he said. "The cats and people are colliding in a struggle for space and existence."
Dramatic Interventions
Where populations become very isolated, dramatic human interventions may be necessary to save inbred cats, Dollar added.
For instance, in 1995 Texas cougars were released to breed with and revitalize a Florida panther population so small and inbred that Dollar described them as "walking dead."
Protecting wildlife corridors is the best way to avoid such drastic measures and offers a better chance of success as well.
"In conservation it's not individuals or individual populations that we worry about if we're going to play the long game," he said.
"We worry about the overall genetic integrity of the species, which is exactly where corridors are critical as the mechanism for genetic exchange that can maintain a robust population."
"Floating in a Human Sea"
Sharma stressed that tigers need to be managed not with a myopic approach, as isolated populations, but as one big population connected by corridors.
"India has the second largest human population in the world, and these tigers are floating in a human sea," he said.
"We can't create new tiger habitat, and there is no hope outside these areas. The only hope is these corridors. If you cut them down, and fragment these populations, eventually they will only exist in history books."
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Eric Schmidt (right) in conversation with Alan Rusbridger at Big Tent Activate Summit 2013 
