Logging has been increasing in recent years, moving away from the river edges into the interior of the forests where the orangutans live, Cheryl Knott said in a telephone interview.
Knott studies orangutans in Indonesia's Gunung Palung National Park, home to about 2,500 of the animals, about one-tenth of those in the world. Orangutans live only in Indonesia and Malaysia, said Knott, whose work is sponsored by the National Geographic Society.
While the government of Indonesia has a commitment to protect orangutans, sending in national police periodically, the loggers return when the police leave, she said.
Knott said she hopes to raise awareness internationally "that we really do have a crisis here.... We could wake up in 20 years and they would be extinct."
Orangutans, like other great apes, are close relatives to humans. Researchers have learned a lot about them in recent years.
Knott said one colony was observed to use primitive tools, a skill passed on to their offspring. And she said the group she studies makes unique sounds under some circumstances.
Orangutans are almost totally arboreal, living in the trees, traveling through the trees and eating fruit from the trees, she said.
She said several hundred trees have been removed in her study area. Loggers cut them into manageable logs with chain saws, drag those logs to a nearby river and float them to market.