Ecological Armageddon

A new study has found that mammals vanish entirely from forest fragments after 25 years.

As tropical forests worldwide are increasingly cut into smaller and smaller fragments, mammal extinctions may not be far behind, according to a new study. “Our results should be a warning. This is the trend that the world is going in.”

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.

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. The small size of the surviving populations also creates problems. Over the course of a few generations, a small population can accumulate harmful mutations that make them less fertile or more vulnerable to diseases.

Scientists have hypothesized that many species will gradually decline in forest fragments until they become extinct. Reducing a vast carpet of jungle to isolated patches thus creates a so-called “extinction debt” that nature will sooner or later collect.

Just five years after the dam was built, they could see a difference. Several species were more rare on the islands than on the mainland. Researchers returned to the same 12 islands in 2012 and repeated the survey. The first survey had found seven species of mammals. Traps on the island found only a single species: the Malayan field rat. This was a startling find for two reasons. One was the drastic crash in diversity. The other was that the Malayan field rat wasn’t on the islands when they first formed.

Malayan field rats thrive around villages and farms and other disturbed habitats. The rats trapped must have come from the surrounding rain forests, where they still remain scarce. When they swam to the islands, they found fragmented forests that they could dominate. “I thought, ‘Wow, what if this trend holds?’ And it did.” On most of the islands, all the native species were gone, replaced by the rats. Only on a few islands did some species still cling to existence. All the islands were suffering massive extinctions in about 20 years. “No one expected to see such rapid extinctions.”

“This study confirms for mammals what we’ve long known for birds.” Records of birds from forest fragments in the Amazon show species going extinct at a comparable rate. The fast pace of extinction in forest fragments gives an urgency to conserving the large swaths of tropical forest that still remain. “Our study shows we may need to do that very quickly.”

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