The past few tens of millennia were hard times for the "megafauna" of the world. Hundreds of big-bodied species—from the mammoths of North America to the 3-meter-tall kangaroos of Australia to the 200-kilogram-plus flightless birds of New Zealand—just disappeared from the fossil record. A new, broad analysis continues the century-long debate over the loss of the big animals, coming down on the middle ground between blaming migrating humans for wiping them all out and climate change alone for doing them in.
As in most contentious scientific debates, uncertainties in the data have fueled the dispute over what took out the megafauna. Typically, researchers would try to pin down exactly when, say, the mammoths of North America died out, when the climate changed the fastest as the world came out of the last ice age, and, most difficult, when humans from Asia first arrived on the scene. If the extinction in a particular area seemed to coincide with severe climate change or with the arrival of humans, one or the other could be blamed. If it seemed to have been the humans, researchers assumed the new arrivals must have hunted down too many mammoths, brought a lethal disease with them, or altered the environment somehow, perhaps by too much burning.
But the case-by-case tactic has not yet carried the day for either side. So zoologists Graham Prescott and David Williams and their colleagues at the University of Cambridge in the United Kingdom decided to take a broader approach. In a paper published today in the Proceedings of the National Academy of Sciences, they lay out their analysis of the extinction of 110 genera of megafauna on five landmasses in relation to the timing of four kinds of climate change and the arrival of humans.
The Cambridge group compiled dates from previous studies for the arrival of humans and the extinction of megafauna on each landmass: Australia, Eurasia, New Zealand, North America, and South America. And they took the temperature record locked in an Antarctic ice core as a guide to global climate change. Then they compared how well climate change and human arrivals, alone or in combination, could predict the timing and severity of extinctions on the five landmasses. To sort out the importance of timing uncertainties, they tested 320,000 different extinction scenarios. "We tested a lot of models across a huge range of human arrival times and extinction times," Prescott says. "It seems likely that both climate and human factors played a role" in most cases.
"What they found makes sense," says mammalian paleoecologist Anthony Barnosky of the University of California, Berkeley. "It makes a clear case for there being an interaction. It shows what happens when two bad things happen at once." Barnosky and environmental scientist Barry Brook of the University of Adelaide in Australia have found such a human-climate synergy operating in megafaunal extinctions when severe climate change coincided with human arrivals. A similar synergy is happening today, they say, as global warming intensifies and the human population continues to grow.
But others have concerns about the latest study—for example, the way it lumps together events occurring as much as 10,000 years apart to test for coincidence. "When you have such a challenging problem, what are you willing to ignore in the details to get the big picture?" asks ecological statistician Andrew Solow of Woods Hole Oceanographic Institution in Massachusetts. "I'm worried that too much of the detail was omitted. This is a first step."