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PAKAN RABAA, Indonesia — This village in West Sumatra, a lush province of volcanoes and hilly rainforests, had a problem with illegal loggers .

They were stealing valuable hardwood with impunity. At first, a group of local people put a fence across the main road leading into the forest, but it was flimsy and proved no match for the interlopers.

So, residents asked a local environmental group for camera traps or some other equipment that might help. In July , they got more than they expected: a treetop surveillance system that uses recycled cellphones and artificial intelligence software to listen for rogue loggers and catch them in the act.

“A lot of people are now afraid to take things from the forest,” Elvita Surianti, who lives in Pakan Rabaa, said days after a conservation technologist from San Francisco installed a dozen listening units by hoisting himself nearly 200 feet into the treetops. “It’s like the police are watching from above.”