

Amidst predictions of global warming-driven global extinctions, a dietary analysis of ancient teeth suggests that animals may prove more adaptable than expected.

The tale of the teeth, collected at two sites in Florida and spanning a transition between extreme temperatures during an ice age climate cycle, runs counter to the standard narrative of animals as unable to adjust their behavioral patterns.

"One of the main assumptions is that species niches are conserved. Here we're showing that the diets vary and change," said Larisa DeSantis, a Florida Museum of Natural History zoologist. "These niches are not the same. The animals were not doing the same thing constantly through time."

A prominent study published in Nature in 2004 predicted that about one-quarter of all species would be "committed to extinction" by 2050 if the planet's temperature increased by about 6 degrees Fahrenheit. Such an increase falls in the middle of the Intergovernmental Panel on Climate Change's calculations of global temperature change in the next century.

Some researchers call the one-quarter extinction prediction excessive. Others think it's conservative. The IPCC says that a rise of several degrees could put a quarter of all species at risk of extinction, with a jump of a few more degrees threatening up to three-quarters of Earth's animals.

Those predictions, however, are based on models in which creatures don't alter their habits when weather changes disrupt their traditional food chains. And though the authors of the teeth analysis warned against extrapolating their findings, which documented a gradual shift spanning hundreds of thousands of years, to the fast-warming climate of the present, the research suggests a limit to our own predictions.

"I don't think you can use this study as a model for what's going to happen to a given species. But it does say that if we have global warming, then there will be changes to animals, and those changes will be complex," said study co-author Robert Feranec, a vertebrate paleontologist at the New York State Museum. "It's hard to understand what global warming is going to do."

The 115 fossil teeth in the study came from 11 large mammal species, some still found in Florida and others long departed: horses, deer, pronghorns, tapirs, two types of llama, two types of peccary, and three types of mammoth. The animals' bones had come to rest in two ancient lakebeds. The first group dated to approximately 1.9 million years ago, when America was locked in the frigid grip of an ice age. The second group of fossils dated from about 1.3 million years ago, a period of glacial retreat.

Different plants have different ratios of carbon isotopes — variations of carbon with different atomic masses. Those carbon ratios are recorded in the teeth, hair and tusks of the animals that eat them, so the researchers were able to deduce the animals' diets by analyzing the chemical composition of their teeth. When it was cold, the animals' diets were dominated by grass. When it warmed, they ate a mixture of grass, shrubs and trees. The tapirs — a now-endangered, pig-like animal with a prehensile snout that typically lives on land — apparently took to water.

The findings "build upon what we know are inadequacies in how ecologists predict changes in species under modern climate change," said Jessica Hellmann, a University of Notre Dame ecologist who was not involved in the study.

"Most of the models that we use to project future change assume that species will continue doing in the future what they do today. As the authors point out, these models don't account for flexibility in the tolerances of some species," she said.

How much temperatures fluctuated between 1.9 million and 1.3 million years ago isn't known. According to Feranec, it may have resembled fluctuations known to have occurred in Florida near the end of the last ice age. At about 9 degrees Fahrenheit, those fluctuations were comparable to those predicted at the upper ranges of modern climate change.

But whether modern animals will adapt as easily as their ice age forebears is an open question. The researchers cautioned that human-driven climate change is happening much faster than glacial transitions.

"It may move too fast for animals to switch what they're doing," said Feranec. His caveat was echoed by Patrick Gonzalez, a University of California, Berkeley forest ecologist who has served as an expert reviewer for the IPCC.

"Current global warming is occurring in a short period of approximately 50 to 150 years. This extremely rapid pace may not leave enough time for substantial species adaptation," said Gonzalez.

Even if animals can adapt to temperature shifts, the combination of climate change and competition with billions of resource-hungry, habitat-developing humans may prove too great, warned Feranec.

The last wave of extinctions came at the end of the last ice age, when the planet warmed and humans spread out of Africa and around the world.

"We lost 35 genera of large mammals in North America at that time," said Feranec. "If we can use that as any kind of model, then whenever you have a large human footprint and rampant global warming, you seem to end up with large-scale extinction."

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Citation: "Effects of Global Warming on Ancient Mammalian Communities and Their Environments." By Larisa R. G. DeSantis, Robert S. Feranec, Bruce J. MacFadden. Public Library of Science ONE, Vol. 4 Issue 6, June 2, 2009.

Image: Larisa DeSantis

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