ScienceDaily (Nov. 20, 2009) — Roughly 15,000 years ago, at the end of the last ice age, North America's vast assemblage of large animals -- including such iconic creatures as 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 equaled or surpassed Africa's 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, the accumulation of woody debris sparked a dramatic increase in the prevalence of wildfire, another key shaper of landscapes.
Mastodons graze on black ash trees in a pleistocene swamp. A new study by researchers at the University of Wisconsin-Madison shows that the disappearance of North America's large herbivores not long after the retreat of the ice sheets that covered much of the continent triggered a dramatic reshaping of the landscape. (Credit: Illustration by Barry Roal Carlsen
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 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 and an expert on ancient climates and ecosystems who is the study's senior author. What's more, he says, the detailing of 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. Other co-authors are Stephen T. Jackson of the University of Wyoming, Katherine B. Lininger of UW-Madison and Guy S. Robinson of Fordham University.
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.
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Science 20 November 2009:
Vol. 326. no. 5956, pp. 1100 - 1103
DOI: 10.1126/science.1179504
Pleistocene Megafaunal Collapse, Novel Plant Communities, and Enhanced Fire Regimes in North America
Jacquelyn L. Gill,1,2 John W. Williams,1,2 Stephen T. Jackson,3 Katherine B. Lininger,1 Guy S. Robinson4
Although the North American megafaunal extinctions and the formation of novel plant communities are well-known features of the last deglaciation, the causal relationships between these phenomena are unclear. Using the dung fungus Sporormiella and other paleoecological proxies from Appleman Lake, Indiana, and several New York sites, we established that the megafaunal decline closely preceded enhanced fire regimes and the development of plant communities that have no modern analogs. The loss of keystone megaherbivores may thus have altered ecosystem structure and function by the release of palatable hardwoods from herbivory pressure and by fuel accumulation. Megafaunal populations collapsed from 14,800 to 13,700 years ago, well before the final extinctions and during the Bølling-Allerød warm period. Human impacts remain plausible, but the decline predates Younger Dryas cooling and the extraterrestrial impact event proposed to have occurred 12,900 years ago.
1 Department of Geography, University of Wisconsin, Madison, WI 53706, USA.
2 Center for Climatic Research, University of Wisconsin, Madison, WI 53706, USA.
3 Department of Botany, University of Wyoming, Laramie, WY 82071, USA.
4 Department of Natural Science, Lincoln Center Campus, Fordham University, New York, NY 10023, USA.
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