Sawmill Sink is a challenging place to dive. It's a flooded sinkhole on the island of Abaco in the Bahamas. Below 30 feet is a layer of opaque water saturated in poisonous, acidic hydrogen sulfide. But in 2004, a retired military diver named Brian Kakuk figured out how to penetrate the murk and dive into the lightless depths of the hole. Along ledges, he found a trove of bones, including those of dozens of animal species that have vanished from the island of Abaco and, in many cases, become extinct altogether.
A new report in the Proceedings of the National Academies of Science describes the discovery, and the authors believe it has implications for one of the great mysteries in science — the wave of extinctions in the Americas at the end of the Pleistocene as the climate changed, and human beings arrived. It may also help us think about our present and future impact on an increasingly stressed biosphere.
There are few issues in paleontology more controversial than the Pleistocene extinctions. Numerous species, including mammoths, saber-toothed cats, horses, camels and giant sloths, vanished about the time human beings arrived. That could be correlation rather than causation, however: It was climate change that made it possible for humans to cross into the Americas (over a land bridge that existed when the seas were low).
The Bahamas offers a special laboratory for separating the climate factors from the human factors. During the Ice Ages, when sea levels were about 250 feet lower, Abaco was a much larger, heavily wooded island pocked with sinkholes and inhabited by a great diversity of animals. But there were no people. Abaco remained uninhabited by people until 1,000 years ago — long after the major climate changes, said David Steadman, curator of ornithology at the Florida Museum of Natural History and the lead author of the new paper.
"We can't find any evidence of people in the Bahamas more than 1,000 years ago," he said.
The research shows, in effect, two separate extinction spasms: One associated with climate change more than 10,000 years ago, and the second with the arrival of humans. The second one was slightly worse.
The divers retrieved more than 5,000 fossils from Sawmill Sink. The scientists were able to identify 95 different vertebrate species represented among those bones. Of those, 39 had gone extinct on Abaco. It appears 17 of those disappeared during the climate change some 10,000 years ago, and then another 22 species vanished roughly 1,000 years ago, approximately when humans arrived. Some of those species can still be found elsewhere, but 10 of them are completely extinct, Steadman said.
The first wave of extinctions occurred when the pine forests of the Ice Age period gave way to warmer, more tropical forests on the much smaller island. But some species proved remarkably adaptable to climate change and stuck around. Then the people arrived, probably on boats riding prevailing currents from Hispaniola.
Among the late extinctions were a species of the Cuban crocodile and a species of large tortoise.
"It looks like within no more than a century of humans arriving in the Bahamas, they wiped out the tortoise and the crocodile," Steadman said.
Steadman and co-author Janet Franklin, a professor of geography at Arizona State University, have long collaborated on research on island ecosystems, not only in the West Indies but also in Polynesia. In both regions, humans arrived relatively late. And they were not from hunter-gatherer cultures, but rather had already adopted agriculture.
The human impact on island fauna was therefore not simply limited to hunting. In the Bahamas there is dramatic evidence of fires about 1,000 years ago, Steadman said. The hypothesis is that people cleared land, using fire, to create space for their crops. This loss of habitat on already shrunken islands added stress to many native species.
The discovery of the fossils had been previously announced, but the new paper provides the most comprehensive description of the Ice Age ecosystem.
"We have sort of the big picture of the whole ecosystem," Franklin said. "A whole lot of birds, and bats and lizards and snakes and crocodiles and tortoises. So we can really picture what the wildlife was like, and what the ecosystem was like, in the Bahamas in the last Ice Age.”
She was struck by how adaptable and resilient many of the species proved to be.
“It was remarkable that quite a few species persisted in the Bahamas through the end of the Ice Age," she said.
The scientific community is looking closely at how climate change will continue to threaten native species. But the authors of the new paper conclude their report by saying that researchers should not overlook the immediate, direct impacts from human activity: "For the indigenous species of terrestrial vertebrates that remain, we fear that direct human activities, such as habitat alteration and introduction of invasive species, threaten their future more than climate change."
Read more from Joel Achenbach at Achenblog.