Species living in rainforest fragments could be far more likely to disappear than was previously thought, says an international team of scientists. In a study spanning two decades, the researchers witnessed the near-complete extinction of native small mammals on forest islands created by a large hydroelectric reservoir in Thailand.

"It was like ecological Armageddon," said Luke Gibson from the National University of Singapore, who led the study. "Nobody imagined we'd see such catastrophic local extinctions."

The study, just published in the leading journal Science today, is considered important because forests around the world are being rapidly felled and chopped up into small island-like fragments.

"It's vital that we understand what happens to species in forest fragments," said Antony Lynam of the Wildlife Conservation Society.

"The fate of much of the world's biodiversity is going to depend on it."

The study was motivated by a desire to understand how long species can live in forest fragments. If they persist for many decades, this gives conservationists a window of time to create wildlife corridors or restore surrounding forests to reduce the harmful effects of forest isolation.

However, the researchers saw native small mammals vanish with alarming speed, with just a handful remaining - on average, less than one individual per island - after 25 years. "There seemed to be two culprits," said William Laurance of James Cook University in Australia.

"Native mammals suffered the harmful effects of population isolation, and they also had to deal with a devastating invader - the Malayan field rat."

In just a few years, the invading rat grew so abundant on the islands that it virtually displaced all native small mammals. The field rat normally favors villages and agricultural lands, but will also invade disturbed forests.

"This tells us that the double whammy of habitat fragmentation and invading species can be fatal for native wildlife," said Lynam.

"And that's frightening because invaders are increasing in disturbed and fragmented habitats around the world."

"The bottom line is that we must conserve large, intact habitats for nature," said Gibson. "That's the only way we can ensure biodiversity will survive."

'Near-complete extinction of native small mammal fauna 25 years after forest fragmentation' by Luke Gibson, Antony J. Lynam, Corey J. A. Bradshaw, Fangliang He, David P. Bickford, David S. Woodruff, Sara Bumrungsri and William F. Laurance was published on 27 September 2013 in Science and is available at http://www.sciencemag.org (doi: 10.1126/science.1240495).