One of the great mysteries surrounding the end of the last Ice Age is the disappearance of large Pleistocene mammals. Before the continental ice sheets melted, North America had a diverse array of large mammals, similar to what is found in Africa today. But by about 11,000 years ago, mastodons and mammoths, short-faced bears and giant beavers, and many other large mammals had gone extinct.

For decades researchers have been hotly debating the reasons for this mass extinction. Some argue that humans over-hunted these mammals. Others argue that dramatic changes in the environment after the continental ice sheets melted led to the extinctions.

Russell Graham, a paleontologist with the Denver Museum of Natural History, is firmly on the side of climatic change as the key factor.Graham argues that a "flickering" climate during the Pleistocene Epoch maintained what he calls a "patchy heterogeneous environment" that disappeared along with the ice sheets.

The Pleistocene lasted for about two million years, and the climate was not uniformly cold during that time. North American warmed and cooled abruptly at times, and these changes maintained a fairly diverse environment.

But towards the end of the Pleistocene, Graham thinks that a final climatic flicker triggered dramatic changes in habitat that set off a wave of extinctions. The climate became warmer and more stable after the last Ice Age, and habitat in places like the American Mid-West began to change. Extensive short grass prairies gradually replaced a varied mix of habitats, including the open spruce parklands favoured by mastodons.

Graham thinks the large Pleistocene mammals began to die out because they could not adapt to these changes. He and another researcher, working with the Illinois State Museum, tested this theory with an electronic database called FAUNMAP. They entered data from about 3,000 paleontological and archeological sites in the lower 48 States that met their selection criteria.

Using Geographical Information System (GIS) mapping techniques, they were able to document where different mammals lived during the last 40,000 years. They wanted to determine how different species dispersed as the climate changed. Did they move individually, or as part of larger communities?

They found that individual species dispersed at different times, in different directions and at different rates. This supported their theory about a patchy environment because it explained why animals that used to live together in similar environments are not found together today.

As the environment changed after the glacial retreats, animals that had once lived in the same habitats had to move to find suitable new environments. With Faunmap researchers can track how these communities split apart after the Pleistocene. Most small mammals dispersed to new areas, and few of them went extinct.

It was the large mammals that died off one after another. Large animals need lots of room, and Graham see their extinction as "a simple case of habitat destruction. The animals disappeared when their habitat disappeared and they had nowhere to go."

The researchers also wanted to lay to rest the over-hunting argument, and they used advances in radio-carbon dating for this task. In order to figure out exactly when different Pleistocene species went extinct, they dated bones from sites where they thought the species might have occurred for the last time.

Older methods of radio-carbon dating indicated that Pleistocene mammals were still around as recently as 8-9,000 years ago. But relatively large samples of bone had to be used for these tests, increasing the likelihood that contaminants in the bone would skew the findings.

Now, Advanced Mass Spectrometry is used for radio-carbon dating. With this new procedure, very small samples can be used to date individual amino acids. These tests dated the bones at 11,000 years old.

The AMS tests also found that the distinctive fluted points used by the Clovis hunters were younger than had earlier been thought, dating to 10,800-10,900 years old. While this was only a difference of a few hundred years from previous theories, it was a critical shift in time frames. It meant that most of the large Pleistocene mammals were extinct even before the Clovis hunters were around to pursue them.

Mastodons and mammoths, the largest of all the Pleistocene species, died off a few hundred years later than animals like horses and camels. Graham assumes that these larger mammals were somewhat buffered from environmental changes.

Proponents of the human overkill theory often point to the fact that Clovis points are regularly found in the same sites as mammoth and mastodon bones.

But Graham has a different interpretation. He thinks these hunters were "stepping on the bones of the last mastodon," meaning that these mammals were already on their way to extinction by the time the Clovis people arrived on the scene.

With the preciseness of the new radio-carbon dating techniques, researchers have been able to determine that these mass extinctions occurred within a relatively short period of time, about 250 years. Graham points out that today we are once again seeing species go extinct at a rapid rate, and he thinks that we should not forget what happened 11,000 years ago.

"We're seeing a similar scenario today and humans are hastening the process. Paleontology has a lot to say about conservation biology. We need to protect corridors so that animals can move around."

He also warns that we should not assume that species will go extinct at a gradual rate. "I think we're going to cook along okay with a few species dropping out along the way until we hit a threshold level, and then there will be a major collapse. I think we are going down the curve very fast."

Graham is now working on a second version of Faunmap that will include data from Alaska and Canada. For more information on this project, check the Illinois State Museum web site at www.museum.state.il.us/research/faunmap. In the Yukon, the Beringia Center is an excellent source for information on this chapter of northern history.