The low genetic diversity seems to suggest that the Alcedo population is relatively young, but all the volcanoes on Isabela are about the same age and thus presumably the tortoise populations should be about the same age, the researchers report.
The one unique thing about Alcedo among Galápagos volcanoes is that it experienced an explosive eruption in its past, according to the researchers. All other Galápagos volcanoes are constructed on basaltic lavas that slowly seeped out from the ground.
According to potassium-argon dating of the several-meters-thick layer of rhyolitic thephraparticulate matter from an eruptionon Alcedo, the researchers concluded that the catastrophic eruption took place about 100,000 years ago and that it probably wiped out most of the giant tortoise population there.
To test their theory, the scientists analyzed genetic data from tortoises that live near the volcano today. They found a clear signature of a dramatic population decline. The signature, a reduction in genetic variety, is known as a genetic bottleneck.
The bottleneck signature is absent from the other tortoise populations on Isabela, but none of these populations were impacted by the eruption, say the researchers.
Using another genetic technique, the researchers determined that four new lineages of tortoises descended from the survivors of the Alcedo eruption appeared within the last 88,000 years. "In other words, these lineages appeared, by mutation, after the volcano eruption," said Beheregaray.
The scenario that emerges, conclude the researchers, is of all the Alcedo tortoises getting blown off the volcano at the time of eruption except for the lone, lucky lineage, which could have been as small as a single pregnant tortoise, but probably a group of related individuals.
"The bottleneck test has been used extensively to test for population contractions, especially for endangered species that experienced recent contractions," said Beheregaray. "In our study we were able to show the signature of a relatively old population contraction and estimate the age since contraction and relate it to a well dated catastrophe."
Robert Rothman, a biologist at Rochester Institute of Technology in New York who is familiar with the group's work but not a member, said the research is interesting and the conclusions reasonable.
"There doesn't seem to be any reason why Alcedo would have been colonized later, so to link it to the major rhyolite eruption is a very reasonable conclusion," he said.
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National Geographic Magazine Photos:
1930 image of bather "riding" on the back of a turtle in Australia
David Doubilet image of a green turtle
National Geographic Guide to Animals and Nature: Go>>
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