In a paper just published in the Journal of Science, Dr. Luke Gibson and colleagues from the University of Singapore report disturbing findings. A hydroelectric dam built in Thailand in 1987 created a large reservoir that flooded forests to such an extent that 150 hilltops were turned into small islands.
Dr. Gibson saw this islands as the perfect “natural laboratory” to study the effects of habitat fragmentation on extinction. He studied the isolated mammal populations of the islands for twenty five years. He found that there was an almost complete mass extinction of small mammals within five years on the smaller islands. The small mammals on the larger islands held out for just under twenty five years.
The mass extinctions were caused by a combination of an invasive species and the increased prevalence of deleterious mutations in the mammals. The populations of the invasive Malayan Field rat on the islands grew to such sizes that they displaced the native species. Harmful mutations accumulated because the population sizes dropped sharply after the initial flooding, lowering fecundity and resistance to disease.
Although not reported in Dr. Gibson’s study, large mammals like tigers, tapirs, and elephants completely disappeared from the islands.
The takeaway is that fragmentation is bad. More specifically, the biodiversity in small fragments has little to no chance of surviving. For this reason, conservation efforts should be focused on larger fragments in which biodiversity does not decrease as rapidly and is therefore possible to salvage.