An isolated forest in the Chiew Larn reservoir. A Thai government project to supply hydroelectric power to the area transformed 150 forested hilltops into islands.
In 1987, the government of Thailand launched a huge, unplanned experiment. They built a dam across the Khlong Saeng river, creating a 60-square-mile reservoir. As the Chiew Larn reservoir rose, it drowned the river valley, transforming 150 forested hilltops into islands, each with its own isolated menagerie of wildlife.
Conservation biologists have long known that fragmenting wilderness can put species at risk of extinction. But it’s been hard to gauge how long it takes for those species to disappear. Chiew Larn has given biologists the opportunity to measure the speed of mammal extinctions. “It’s a rare thing to come by in ecological studies,” said Luke Gibson, a biologist at the National University of Singapore.
Over two decades, Dr. Gibson and his colleagues have tracked the diversity of mammals on the islands. In Friday’s issue of the journal Science, they report that the extinctions have turned out to be distressingly fast.
“Our results should be a warning,” said Dr. Gibson. “This is the trend that the world is going in.” Tropical forests are regularly cleared for logging, farming and cities. In most cases, the only original tree cover is reduced to isolated patches. Many of the original species of plants and animals may still survive in those fragments, but they experience new stresses. The edges of the fragments are no longer dim and humid, for example.
