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When a Dam Turned a Forest Into Tiny Islands, Only Rats Were Happy

Although the rate of extinction the researchers observed is startling, it’s unfortunately not surprising

Water submerged tracts of this Thai forest, cutting off one part from another and creating forest islands. (Antony Lynam)

Conservationists have long recognized that breaking one large forest into smaller, isolated parts does not bode well for species survival. Isolating populations of animals can create genetic bottlenecks that leave them unable to adapt to changing circumstances, or cut off from resources. Just how badly these effects of fragmentation impact animals, however, is tricky to determine. Now, an accidental experiment in fragmentation in Thailand confirms researchers’ fears: breaking a forest into lots of little islands results in rapid and near-complete extinction of many of its animal residents.

It all started back in 1987, when a dam blocking Thailand’s Khlong Saeng river created an impromptu 60-square mile lake where once a forest stood, the New York Times reports. The lake filled in valleys and left only 150 of the tallest tips of forested hills jutting out of the reservoir. Whichever animals happened to be on those hill tops, in whatever mix, lived, while the others drowned. Cut off from their former habitat and isolated from others like them, these remaining animals began an experiment in survival.

Now, 25 years later, the results are bleak, conservationists say. One 25-acre island sported seven species of small mammal in 1993; now only one—a rat—remains. Researchers checked another 11 islands they had been monitoring over the years, and found that the rat generally reined supreme in those habitats, too. The Times explains why this is so disconcerting: 

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 Dr. Gibson 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.

The researchers performed surveys of several more islands, again finding either just rats, or, in best case scenarios, one or two forest species. Plus, the rats.

Although the rate of extinction the researchers observed is startling, it’s not surprising. In 2003, researchers observed the same trend for birds cut off from one another in Amazon rainforest fragments, the Times points out. Likewise, researchers in Chile found that tree and shrub species shifted in fragmented patches, and in Connecticut some amphibians disappeared from fragmented forests. The list goes on.

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