A 'mammoth' slide

Changing Ecosystems

Imagine having a giant sloth looking straight at you in your bedroom upstairs. Or a mastodon roaming around in a leisurely manner, literally dwarfing a herd of modern-day elephants. Because of these iconic creatures, the biodiversity in north American plains 20,000 years ago was as rich as African Serengeti plains. Where have these creatures gone?

Roughly 15,000 years ago, at the end of the last ice age, North America’s vast assemblage of large animals — mammoths, mastodons, camels, horses, ground sloths and giant beavers — began their precipitous slide to extinction.

And when their populations crashed, emptying a land whose diversity of large animals equalled or surpassed wildlife-rich Serengeti plains then or now, an entirely novel ecosystem emerged as broadleaved trees once kept in check by huge numbers of big herbivores claimed the landscape.

Soon after, accumulation of woody debris sparked a dramatic increase in the prevalence of wildfire, another key shaper of landscapes.

Ecological upheavals

This new picture of the ecological upheaval of the North American landscape just after the retreat of the ice sheets is detailed in a study published on November 19 in the journal Science.

The study, led by researchers from the University of Wisconsin-Madison, uses fossil pollen, charcoal and dung fungus spores to paint a picture of a post-ice age terrain different from anything in the world today.

The work is important because it is “the clearest evidence to date that the extinction of a broad guild of animals had effects on other parts of these ancient ecosystems,” says John W Williams, a UW-Madison professor of geography.

The research also details changes on the ice age landscape following the crash of keystone animal populations can provide critical insight into the broader effects of animals disappearing from modern landscapes.

The study was led by Jacquelyn Gill, a graduate student in Williams’ lab. She and hear colleagues analysed pollen, charcoal, and Sporomiella, a particular fungus that grows in the dung of large herbivores.

The new work, says Gill, informs but does not resolve the debate over what caused the extinction of 34 genera or groups of large animals, including icons of the ice age such as elephant like mastodons and ground sloths the size of sport utility vehicles.

“Our data are not consistent with a rapid, ‘blitzkrieg’ overkill of large animals by humans,” notes Gill, nor was their decline due to a loss of habitat. However, the work does seem to rule out a recent hypothesis that a meteor or comet impact some 12.9 thousand years ago was responsible for the extinction of ice age North America’s signature large animals.

The study was conducted using lake sediment cores obtained from Appleman Lake in Indiana, as well as previous data collected by other researchers. The key to their conclusion was a detailed study of the dung fungus that requires passage through a mammalian intestinal tract to complete its life cycle. It helps them reconstruct a picture of sweeping change to the ice age environment.

A gradual process

The decline of North America’s signature ice age mammals was a gradual process taking about 1,000 years. The decline in the huge numbers of ice age animals is preserved in the fossil record when the fungal spores disappear from the record altogether: “About 13.8 thousand years ago, the number of spores drops dramatically. They’re barely in the record anymore,” Gill explains.

Like detectives reconstructing a crime scene, the group's use of dung fungus spores helps establish a precise sequence of events, showing that the crash of ice age mega-fauna began before plant communities started to change and before fires appeared widely on the landscape.

“The data suggest that mega-faunal decline and extinction began at the Appleman Lake site sometime between 14.8 thousand and 13.7 thousand years ago and preceded major shifts in plant community composition and the frequency of fire,” notes Williams.  

Absent the large herbivores that kept them in check, such tree species as black ash, elm and ironwood began to colonise a landscape dominated by coniferous trees such as spruce and larch. The resulting mix of boreal and temperate trees formed a plant community unlike any observed today.

“As soon as herbivores drop off the landscape, we see different plant communities,” Gill explains, noting that mastodon herds and other large animals, occupied parkland like landscape, typified by large open spaces and patches of forest and swamp. “Our data suggest that these trees would have been abundant sooner if the herbivores hadn’t been there to eat them.”

Dynamics of extinction

While both the extinction of North America’s ice age mega-fauna and the sweeping change to the landscape are well-documented phenomena, there was, until now, no detailed chronology of the events that remade the continent’s biological communities beginning about 14.8 thousand years ago. Establishing that the disappearance of mammoths, giant beavers, ground sloths and other large animals preceded the massive change in plant communities, promises scientists critical new insight into the dynamics of extinction and its pervasive influence on a given landscape.

The findings also destroyed a long-held belief that Clovis hunters (early human hunters in North America who used arrow points made out of stone) and environmental shifts had led to the decline of megafauna in North America. It seems that the sequence is the other way around.

The slow-paced extinction of mega-fauna 14,800 to 13,700 years ago preceded the Clovis people, and it was a cause—not a result—of vegetation changes and increased fires.