The most recent geologic period in Earth's history has been labeled the Quaternary, extending back approximately two to four million years. This period has been divided into two geologic epochs–the Pleistocene, which ended approximately 12,000 years ago, and the Holocene since that time. Over the course of the Quaternary, dramatic climatic shifts occurred that, to a great extent, shaped the rolling landscape and vegetation of today's Great Plains. Since the Late Pleistocene, three major trends have dominated the environmental history of the region. First, between about 18,000 and 10,000 years ago, glaciers began to waste, and climatic amelioration significantly altered the biota and landscape; second, during the early and middle portions of the Holocene, the climatic trend was toward increased dryness and warmth; and third, there followed a period of greater moisture and cooling in the late Holocene. It should be noted, however, that the climate is "time-transgressive," meaning it does not change simultaneously everywhere, and therefore its effects on Plains biota would have varied over the years and across space.
In recent simulation models large-scale climatic shifts have been shown to be the result of changing atmospheric circulation patterns caused by shifts in the orientation of the earth's axis. At 18,000 years ago, the Laurentide ice sheet, while cooling temperatures across North America, caused a split in the jet stream. A deflection of the southern branch of the jet stream brought moister conditions to what is now the southwestern United States, while anticyclonic winds, generated by the ice sheet, brought prevailing easterlies and dry air to the northwest, instead of the usual moisture-laden westerlies. Shifts in the jet stream also appear to have been a major factor in changing climatic patterns during the Holocene.
Individual faunal species and communities were greatly affected by the Pleistocene climatic and vegetation shifts. Late Pleistocene faunal communities have been referred to as "disharmonious," "intermingled," and "nonanalog assemblages" to describe the heterogeneous nature of the species composition. R. Dale Guthrie, a mammalian biogeographer, described these communities as a "patchy mosaic" that allowed for increased numbers and diversity of herbivores. In the Great Plains a dry and cool wooded parkland existed, with a hardwood forest of oak, aspen, and cottonwood to the east and a coniferous forest to the west. In the open areas, sagebrush was dominant. With glacial retreat, the eastern forest followed the ice northward into southern Manitoba and Saskatchewan, while the western coniferous forest moved upslope into the Rockies.
During the late Pleistocene, faunal communities were compressed, and members of such diverse habitats as alpine tundra and xeric grasslands were living together in the same community. A patchy mosaic of Ice Age plants and animals has also been applied to describe this time period. With glacial retreat came a collapse in the Ice Age communities and a reduction in biotic diversity. More than one-half of North America's large mammals became extinct. Less-nutritious Holocene vegetation communities, which had defensive responses to grazing and browsing, replaced this mosaic of highly nutritious late Pleistocene vegetation. The change was probably too abrupt for the adaptive abilities of many herbivores. Paleoindian hunters may have been an additional cause of the mammals' extinction.
The shift from Pleistocene to Holocene environments was the most dramatic period of climatic change in recent earth history. Profound reorganization of biotic communities resulted, with a pattern of diverse patchy environments being replaced by a pattern of zonal communities–from "plaids" to "stripes," according to Guthrie. The megafaunal species that survived, such as the bison, were significantly smaller in size, but they were able to expand their range to become the predominant species in the Great Plains.
Increased warmth and generally drier conditions characterized the early Holocene. Active stream alluviation occurred throughout the region, with wetlands and marshes common in the Southern Plains. The trend toward increased warmth and decreased precipitation peaked around 8500 B.P. The dominance of the Pacific westerlies and the seasonal peak in solar radiation were important factors during this climatic episode, often referred to as the Hypsithermal or Altithermal. Drought-resistant grasslands expanded, and lake levels dropped in a time-transgressive trend that began first in the Northern Plains and then extended later to the Central and Southern Plains. For example, on the prairies of Manitoba thermal maxima occurred about 7,000 years ago, with summer temperatures as much as 3.5ºF warmer and annual precipitation about one inch less than today.
By about 4,500 years ago the onset of modern, or Neoglacial, conditions began, but again this was not uniform across the region. Between 3,000 and 4,000 years ago, the mid- Holocene prairies of northern Alberta and Manitoba were replaced by boreal forests. In the Nebraska Sandhills, increased groundwater, which changed the mid-Holocene marshlands to lakes, signaled the beginning of the Neoglacial in the Central Plains around 3,700 years ago.
While the magnitude of the climatic shift from the mid-Holocene thermal maxima to the Neoglacial was nowhere as severe as the Pleistocene-Holocene shift, significant changes did take place. In the Colorado Front Range, expansion of alpine glaciers occurred, while in the Southern Plains increased moisture permitted soil formation, and an open oak woodland became established in central Texas.
The increased effective moisture of the Late Holocene, particularly between a.d. 1550 and 1880, appears to have had a positive effect on bison populations. It has been proposed that high forage production led to increased population density, larger herds, and decreased migration. Plains groups appear to have taken advantage of this trend, as is evident by large bison kill sites, such as the Glenrock and Vore Sites in Wyoming, and abundant evidence of bison from Central Plains archeological sites.
During the last hundred years the climate has shifted to a warm-dry regime. How long this will last is unknown. It is important to know about Plains paleoenvironments because they provide an understanding of how Holocene biotic communities developed and suggest models for likely future climate change and for the thoughtful management of ecosystems.
Kenneth P. Cannon National Park Service Lincoln, Nebraska
Guthrie, R. Dale. Frozen Fauna of the Mammoth Steppe: The Story of Blue Babe. Chicago: University of Chicago Press, 1990.
Porter, Stephen C., ed. The Late Pleistocene, vol. 1 of Late-Quaternary Environments of the United States, ed. H. E. Wright, Jr. Minneapolis: University of Minnesota Press, 1983.
Wright, Herbert E., Jr., ed. The Holocene, vol. 2 of Late-Quaternary Environments of the United States. Minneapolis: University of Minnesota Press, 1983.