Kurudi the gorilla feeds on wild celery deep in the rainforests of Rwanda. She uses her long careful fingers to open the plant’s succulent inside.

Someone watches: biologist Jean Paul Hirwa. He is hiding behind some stinging nettles. He notes her meal on his tablet computer.

A large adult male gorilla, known as a silverback, sits next to Kurudi. He looks at the scientist. But Mr. Hirwa isn’t nervous. He speaks gorilla. He makes a low hum—“ahh-mmm.” To a gorilla, that means, “It’s OK. No reason to worry.”

Do gorillas have reason to worry? Not as much as they used to—and that’s because people worried about them. A few decades ago, some thought mountain gorillas would go extinct by the year 2000. But the animals were removed from the “critically endangered” list last fall. Now they’re just “endangered.” “Endangered” still means trouble, of course. But it’s trouble with hope attached.

To get to this point, people used what some call “extreme conservation.” Researchers have monitored every single gorilla in the rainforest. Once in a while, veterinarians clean gorillas’ infected wounds. People have saved the gorillas’ homes by sending money into nearby communities. Now locals no longer feel the need to destroy forests for farming to make enough money to survive.

Instead of shrinking, the number of mountain gorillas has grown. A decade ago, there were 680. Now there are just over 1,000. They live in Congo, Uganda, and Rwanda. Mr. Hirwa and the two great apes are all part of the world’s longest-running gorilla study. The project started in 1967. “The gorillas are still here,” he says. “We celebrate that as a victory.”