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Decline of Ice-Age Mammals Led to Landscape Transformation, Says UW Researcher
November 20, 2009 — Some 15,000 years ago, as the ice sheets melted away and iconic animals such as mammoths and mastodons began their slide into extinction, the North American landscape transformed into a novel ecosystem dominated by hardwood and spruce trees.
This new picture of ecological upheaval following the end of the last ice age is detailed in a study, co-written by University of Wyoming Botany Professor Stephen T. Jackson and published today (Friday) in Science.
The study, led University of Wisconsin-Madison researchers John W. Williams, a professor of geography and an expert on ancient climates and ecosystems, and Jacquelyn Gill, a graduate student in Williams' lab, used fossil pollen, charcoal and dung fungus spores to paint a picture of post-ice age terrain different from anything in the world today.
The researchers' findings are "the clearest evidence to date that the extinction of a broad guild of animals had effects on other parts of these ancient ecosystems," says Williams.
The most marked effect was on the vegetation, particularly in the interior of the United States, where a unlikely combination of trees grew side by side in forests that stretched for hundreds of miles during a period in which humans also appeared for the first time in North America.
"This period stands out because the vegetation of much of the interior United States was very peculiar in composition," Jackson says. "We call that a no-analog period, meaning that vegetation of that time has no modern counterpart on the landscape and hasn't for the past 10,000 years."
The presence of spruce trees throughout this region of the United States was not a surprise, Jackson says. It was the presence of other species of trees that was.
"Today, if you go look for spruce trees in eastern or central North America, you find them growing with fir and paper birch and jackpine and alder. They are concentrated in those boreal forests on the Canadian Shield," he says. "But, during this late glacial period, spruce trees were growing with ash trees, hophornbeam, elm and oak trees.
"We have this unique mix of temperate broadleaf trees, and boreal spruce trees. There's no place you can go and find all of those species growing together in extensive forests," Jackson adds. "But, during the ice retreat, we find these strange forests growing all the way from Minnesota down into the Missouri Ozarks and all the way east into central Tennessee and then north to the eastern Great Lakes."
The decline of mastodons and other megafauna, combined with a climate that featured stark seasonal contrasts, led to the dramatic change in vegetation, the study concludes.
"As soon as the herbivores drop off the landscape, we see different plant communities," Gill says. "Our data suggests that these trees would have been abundant sooner if the herbivores hadn't been there to eat them."
While the extinction of North America's ice-age megafauna -- including ground sloths, camels and giant beavers -- and the sweeping change to the landscape are well-documented phenomena, Jackson and his colleagues' study presents the first detailed chronology of the events that reshaped the continent's biological communities nearly 15,000 years ago.
The group's work also seems to discredit a recent hypothesis that an extraterrestrial comet or meteor strike nearly 13,000 years ago caused the extinction of the signature animals during North America's last ice age.
"Our evidence places the initial decline of the megafauna at 2,000 years before the supposed impact event," Jackson says.
The study was supported by grants from the National Science Foundation to UW and the University of Wisconsin.