What Killed Ice-Age Animals? Lakes and Trees Provide Surprising Clues

Researchers seeking to find out what happened to the ecosystem with the extinction of ancient large plant-eating animals may have raised intriguing new questions about what killed the animals off in the first place.

A series of profound changes to the surrounding landscape likely occurred as a result of the dying of such creatures as mammoths and mastodons, which began their decline in North America about 15,000 years ago at the end of the last ice age, according to a team of researchers at the University of Wisconsin at Madison. The extinction of these herbivores apparently prompted a proliferation of broadleaved trees and, ultimately, the accumulation of woody debris that contributed to a dramatic increase in wildfires.

The researchers also determined that the decline of these animals apparently was gradual, meaning it was not likely they died from some sudden event.

Taken together, the evidence appears to rule out several popular theories about what caused their mass extinction, including the impact of a meteor or comet, a ``blitzkrieg'' of human hunting, or a loss of habitat due to climate changes.

The researchers stressed, however, that they were not trying to find out how the animals died, only what happened as a result of their death.

"The lesson of the research is that species extinctions have ecological consequences," said John W. Williams, professor of geography at the University of Wisconsin at Madison, an expert on ancient climates and ecosystems, and the study's senior author.

"To us, the most interesting thing is that we have a whole set of large herbivores – in all, 34 genera (groups) – that all go extinct," he added. "What are the consequences of losing all these animals in a short amount of time? We showed major changes to the landscape in terms of composition and function of plant communities that our data say is linked to the extinction of these animals."

The researchers, led by graduate student Jacquelyn Gill, analyzed fossil pollen, charcoal and dung fungus spores in ancient sediment beneath Appleman Lake in Indiana, a deep body of water left behind when the last ice age ended about 20,000 years ago.  They also included data obtained previously from sites in New York by co-author Guy S. Robinson of Fordham University.

To get an idea of the population density of the animals, the scientists examined levels of the fungus Sporormiella, commonly found in the dung of large-plant eaters. They also measured pollen embedded in the layers of sediment to get a picture of plant growth and density, and charcoal to determine when fires occurred. 

"The lakes become really good archives of changes in the local environment," Gill said. "They are nice little bowl-shaped lakes that are good records of ecological change. All of these things settle into the lake every year – and slowly, over time, the mud builds."

The researchers delved into roughly 40 feet of sediment and – based on the amounts of fungal spores in the layers of material – determined that the decline of the animals was a gradual process that took about 1,000 years. Radiocarbon dating helped determine age.  The decline in the huge numbers of ice age animals is preserved in the fossil record when the fungal spores disappeared. "About 13.8 thousand years ago, the number of spores drops dramatically," Gill said. "They're barely in the record anymore."

Through studying pollen deposits, they found that broadleaved trees such as oaks, black ash, elm and ironwood began to flourish after the animals declined and could no longer eat them, keeping them in check. Once the animals began to die, the broadleaved trees began to colonize a landscape earlier dominated by needle-leaved trees such as spruce and larch. The resulting mix formed a plant community unlike any seen today, the researchers said.

"We see broadleaved trees in low abundance when these mammals are present in high abundances, and broadleaved trees in high abundance when the dung fungus declines," Williams said. 

The findings apparently discount several well-known hypotheses of extinction, such as a meteor strike or that the animals died due to habitat loss from a changing climate. It is also unlikely they were wiped out by an onslaught of hunting by Clovis humans, a culture distinguished by the use of fluted spear points in hunting.

The decline of the animals predated the habitat changes and the arrival of the fluted spears, and the evidence of a gradual decline makes a meteor or comet impact much less likely, researchers said.

"We haven't eliminated humans, but the decline predates the tools," Gill said. "We can say that by the time humans figured out the big game hunting technique, the population of these big animals was already getting smaller. Also, it wasn't habitat loss – that was more a consequence of extinction than a cause."

Based on charcoal levels, the team also found an increase in the number of fires after the animals died, another important shaper of the landscape. 

"The landscape went from no fires to frequent fires," Williams said. "We don't fully know what the role of humans was in this. It could be that humans started to arrive and used fire. Or, another reason could have been that once the large animals declined, there were more trees and the trees served as fuel during lightening strikes, the ignition."

Williams described the period as "a most interesting one for scientists to study," calling it "a good model system for the kinds of climate changes we might see this century."

The study was published November 19 in the journal Science. In addition to Williams, Gill and Robinson, authors included Katherine B. Lininger of UW-Madison and Stephen T. Jackson of the University of Wyoming.

The work was funded by the National Science Foundation, the Wisconsin Alumni Research Foundation and the UW-Madison Center for Climatic Research in the Nelson Institute for Environmental  Studies.

—Marlene Cimons, NSF    

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