Just because we've screwed up the climate doesn't mean we're to blame for everything.

megafauna.jpg Woolly mammoths, the only way we know ‘em.  Credit: Peter Bevan

This year may be remembered as the year the weight of climate change finally began to sink in. It only took climate scientists two decades of banging their heads against the wall to accomplish it.

While most observers call that cause for celebration, a few researchers are worried the climatologists have been too successful. They point to an increasing tendency to blame humans for ecological mysteries, a bias that’s shaping funding priorities and hampering attempts to better understand natural history.

Both terrestrial and marine biologists, for example, are fighting over the fate of large animals, or “megafauna.” The former group is grappling with the long-standing question of whether the first North American settlers catalyzed the extinction of mammoths, mastodons, and other sizeable terrestrial mammals. The latter debate involves the more recent decline of Steller sea lions.

In both cases, the latest evidence suggests scientists were too quick to embrace the anthropogenic causes.


Paleontologists have long puzzled over the problem of what happened to North America’s big land mammals 11,000 years ago, at the end of the Pleistocene Epoch. Since 1967, University of Arizona geoscientist Paul Martin’s “overkill” theory—that humans wiped out these animals—has held sway.

It seems a plausible idea, given that most of the largest mammals seemed to disappear soon after humans stormed the Bering Land Bridge, and it remains popular. Evolutionary biologist Jared Diamond embraced it in his popular books, 1992’s The Third Chimpanzee and again in Guns, Germs, and Steel: The Fates of Human Societies, which won the Pulitzer Prize in 1998. In a 2005 book, Twilight of the Mammoths: Ice Age Extinctions and the Rewilding of America, Martin extends his argument that “virtually all extinctions of wild animals in the past 50,000 years are anthropogenic.”

But not everyone is convinced.

Russ Graham, director of Penn State’s College of Earth and Mineral Sciences, cites a dearth of actual kill sites and other exceptions to the overkill chronology. “We’ve come a long way in the last 30 years as we’ve developed new techniques,” he says, referring to isotopic analysis of fossils, computer models and new archeological discoveries. “All they have is timing,” he says of overkill theorists.

Anthropologist Donald Grayson shares Graham’s suspicion that unwarranted enthusiasm for anthropogenesis is “playing a major role” in diverting attention from alternative explanations. In a chapter of a forthcoming book on the arrival of the first North Americans, Grayson attributes the bias to a political agenda: “One may applaud the intent, but it is hard to avoid the fact that overkill’s continued popularity in this context appears more closely related to the environmental movement than to any supporting evidence.”

In a recent paper in the Journal of Archaeological Science titled “A requiem for North American overkill,” Grayson and fellow archeologist David Meltzer point out that Martin’s theory was published five years after the appearance of Rachel Carson’s exposé of the dangers of chemical pollution, Silent Spring, and the same year as the founding of the Environmental Defense Fund.

“We suggest that the overkill argument captured the popular imagination during a time of intense concern over our species’ destructive behavior toward life on Earth. It retains that grasp today,” they wrote. “It is easy to show that overkill’s continued popularity is closely related to the political uses to which it can be put.”


Similar worries are circulating among marine mammalogists charged with teasing apart the forces responsible for the decline of Steller sea lions along the Aleutian Islands of western Alaska. Unlike the thriving Stellers of southeast Alaska, since the 1960s, the western population has plummeted by 75 percent. Fearing that fishing fleets competing for the sea lions’ favored prey are to blame, the US government granted more than $120 million to West Coast biologists over the last decade to investigate the collapse. It is one of the most expensive wildlife research programs in American history, and more ambitious than most projects dealing with other species facing greater risks.

For the first few years, many biologists favored a “junk food” hypothesis for the decimation of sea lion populations—dwindling stocks of high-energy herring removed by one of the world’s largest fishing fleets had left the sea lions with only pollock and other low-value prey. Greenpeace and other environmental groups joined the debate and convinced the federal government to impose strict fishing restrictions on the fleet.

The anthropogenic bias is understandable, Guenette adds, given what humans have been doing to the planet’s wildlife and ecosystem. “We have a lot of things to do with species decline,” she says, “but there are also bigger things at play.”

Then, in 2003, University of Alaska marine biologist Alan Springer and seven colleagues published a paper in the Proceedings of the National Academy of Sciences, proposing that mid-twentieth century whaling in the North Pacific was the real cause. By the 1970s, so many of the great whales had been removed that orcas (or killer whales), which had preyed heavily on the now-scarce great whales were forced to switch to smaller marine mammals, including Aleutian sea lions.

It seemed humans were definitely to blame.

But, just as further investigation undermined the Pleistocene overkill hypothesis, papers and conference talks over the past few months have cast doubt on anthropogenic explanations for the Steller mystery. In a Marine Ecology Progress Series paper, Sally Mizroch and Dale Rice of the National Marine Mammal Laboratory dismiss the idea that orcas ever fed heavily on larger whales and characterize Springer et al.‘s understanding of whaling in the region as “not very accurate.”

Similar attacks can be found in a recent Progress in Oceanography paper by seven marine mammal researchers and an upcoming paper scheduled to appear in Marine Mammal Science by University of British Columbia and Canadian government biologists.

One of the UBC researchers, Sylvie Guénette, says too many people were eager to point the finger at fishing. “It was a nice intellectual challenge, and it was clever, but I don’t think the numbers prove it,” he says of Springer’s hypothesis. The anthropogenic bias is understandable, she adds, given what humans have been doing to the planet’s wildlife and ecosystem. “We have a lot of things to do with species decline,” she says, “but there are also bigger things at play.”

The “bigger things” are probably natural climate fluctuations. At least, that’s the emerging consensus in both debates. The ecosystem computer models that Guénette and her colleagues are applying to the sea lion disappearance show that a combination of climate cycles, fishing, and predation is more likely the cause than any single culprit.

Meanwhile, for Graham, Grayson and other overkill skeptics, it’s becoming clear that climate was a leading factor in the extinction of most of the large mammals. Graham, for example, says he suspects changing habitats brought about by sequential Ice Ages gradually drove some species extinct. While humans may have played a role in killing off the last mastodons and mammoths, he says, the giant sloths, bears, tapirs, and other species were probably naturally removed before we arrived.

While natural climate change may have once been more powerful than anything our ancestors did, Graham says, that’s no reason to stop worrying about anthropogenic effects. What we’ve learned about the consequences of a changing ecosystem should give us pause regardless of its origins. If anything, Graham says, “It’s going to be really critical to understand how these climate changes are going to affect species in the future.”

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Originally published September 7, 2006

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