 |
Forest islands in Chiew Larn Reservoir – such as the one in foreground of this photo – form a useful system to examine the ecological impacts of fragmentation. Surveys of these islands by Gibson and colleagues show that extinctions in forest fragments can occur much more rapidly than previously thought.
photo: Antony Lynam
|
Abstract
Tropical forests continue to be felled and fragmented around the world. A key question is how rapidly species disappear from forest fragments and how quickly humans must restore forest connectivity to minimize extinctions. We surveyed small mammals on forest islands in Chiew Larn Reservoir in Thailand 5 to 7 and 25 to 26 years after isolation and observed the near-total loss of native small mammals within 5 years from <10-hectare (ha) fragments and within 25 years from 10- to 56-ha fragments. Based on our results, we developed an island biogeographic model and estimated mean extinction half-life (50% of resident species disappearing) to be 13.9 years. These catastrophic extinctions were probably partly driven by an invasive rat species; such biotic invasions are becoming increasingly common in human-modified landscapes. Our results are thus particularly relevant to other fragmented forest landscapes and suggest that small fragments are potentially even more vulnerable to biodiversity loss than previously thought.
________________________
Ecological Armageddon in Forest Fragments
An international team of scientists including the University of Adelaide's Professor Corey Bradshaw has found that species living in rainforest fragments could be far more likely to disappear than was previously assumed.
Published today in the leading journal Science, the researchers outlined a study spanning two decades in which they witnessed the near-complete extinction of native small mammals on forest islands created by a large hydroelectric reservoir in Thailand.
"Tropical forests remain one of the last great bastions of biodiversity, but they continue to be felled and fragmented into small 'islands' around the world," says co-author Professor Corey Bradshaw, Director Ecological Modelling at the University of Adelaide's Environment Institute.
"This study shows we need to be even more concerned than we thought – the speed at which there was near-total loss of native small mammals was alarming and shows that leaving fragments of forest behind is not nearly enough to protect these species."
"It was like ecological Armageddon," says Luke Gibson from the National University of Singapore, who led the study. "Nobody imagined we'd see such catastrophic local extinctions."
The study was motivated by a desire to understand how long species can live in forest fragments. If they persist for many decades, then 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 almost vanish at great speed, with just a handful remaining – on average, less than one individual per island – after 25 years.
As well as suffering the effects of population isolation, the small mammals also had to deal with a devastating invader – the Malayan field rat. In just a few years, the invading rat virtually displaced all native small mammals. The field rat normally favours 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," says Dr Antony Lynam, from the US-based Wildlife Conservation Society. "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," says Luke Gibson. "That's the only way we can ensure biodiversity will survive."
Luke Gibson, Antony J. Lynam, Corey J. A. Bradshaw, Fangliang He, David P. Bickford, David S. Woodruff, Sara Bumrungsri & William F. Laurance. 2013. Near-Complete Extinction of Native Small Mammal Fauna 25 Years After Forest Fragmentation. Science. 341 (6153): 1508-1510.
