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Status and Trends of the Nation's
Biological Resources

Regional Trends of Biological Resources — Grasslands

USGS photo by F.L. Knopf: cover photo - antelope on a grassland Reprinted from: Mac, M.J., P.A. Opler, C.E. Puckett Haecker, and P.D. Doran. 1998. Status and trends of the nation's biological resources. 2 vols. U.S. Department of the Interior, U.S. Geological Survey, Reston, Va. 964pp.
"The prairie, in all its expressions, is a massive, subtle place, with a long history of contradiction and misunderstanding. But it is worth the effort at comprehension. It is, after all, at the center of our national identity." — William Least Heat Moon (1991)
Grasslands rank among the most biologically productive of all communities (Williams and Diebel 1996). Their high productivity stems from high retention of nutrients, efficient biological recycling, and a structure that provides for a vast array of animal and plant life (Estes et al. 1982). Grasses have contributed the hereditary material for the principal human food crops—rice, wheat, corn, and other grains. Worldwide production of such grain crops exceeds all other food crops combined. Grasslands also contribute immense value to watersheds and provide forage and habitat for large numbers of domestic and wild animals. Nevertheless, current levels of erosion in North America exceed the prairie soil's capacity to tolerate sediment and nutrient loss, thus threatening a resource essential to sustain future generations (Sampson 1981). Added to this threat is the potential for overgrazing by livestock and for other human activities to reduce the social and aesthetic values of grasslands and to restrict the commodities that grasslands can produce (National Research Council 1994), and the likelihood that severe degradation may be irreversible.

In North America, the prairie communities (about 1.5 million square kilometers) contain the majority of the continent's native grasslands (Fig. 1). Environmental features that describe the native North American grasslands embody similarity in vegetation, an abiotic environment to which the vegetation and structure respond, and the nature of the animal communities.

Figure 1: Distribution map of eleven grass species found in the central plains states. Inset map of U.S. showing study area.
Fig. 1.   The central grasslands, delineated by Küchler's potential vegetation types (Küchler 1964).

North American grasslands are similar in the general uniformity of their vegetation, dominance of grasses and grasslike plants, lack of shrubs, and absence of trees (Weaver 1968). The Great Plains grassland evolved in the rain shadow of the Rocky Mountains, where seasonal precipitation occurs mostly in spring and summer. From the Rocky Mountains east to the Mississippi River, the amount of precipitation increases and the frequency of droughts decreases (Simms 1988). Along a north-south gradient from central Texas to south-central Canada, the growing season becomes shorter, the average temperature decreases, and a greater proportion of annual precipitation occurs as snow. These broad-scale environmental gradients significantly influenced the evolutionary composition and distribution of prairie communities (Steinauer and Collins 1996; Weaver et al. 1996).

Many small and large grazing animals evolved on the North American prairie (Van Valkenburgh and Janis 1993), each with life-history and behavioral traits well adapted to the open character of prairie. For example, prairie dogs markedly affect the nutrient cycling, soil formation, and composition of grassland animal and plant communities (Miller et al. 1994). Moreover, it is important to realize that grasslands and their associated wildlife reflect events of the distant past (Knopf and Samson 1997). The incursion of animals—bison, elk, and others—into North America across land-bridges that once connected the Asian and North 9 American continents is but one example of the role of past events. Thus, understanding how events of the distant past influenced both the isolation and interchange of plants and animals that interact with the more recent landscape is an essential step in interpreting any particular ecological community, including grasslands.

Understanding the biological resources of the Great Plains is difficult. The exact size of remaining grasslands in North America is unknown and difficult to estimate, and the community has undergone significant change since it was first described by the early explorers and surveyors (Samson and Knopf 1994). Agriculture, urbanization, and mineral exploration have had both local and regional effects on biological resources. Invasions of nonindigenous plant species after fire suppression in the eastern, central, and southern prairies, as well as water developments in the western plains, have drastically altered grassland landscapes. Establishment of woodlots, shelterbelts, and tree-lined river and stream corridors within the prairie has contributed to a significant and ongoing loss of genetic diversity in North American grasslands (Knopf 1986).

This chapter highlights the status and trends of the main bodies of North American grasslands; the tall-grass prairie, the mixed-grass prairie, and the short-grass prairie. We feature the animals and plants dependent on native grassland and attempt to provide insight into the relationship between remaining native grassland and biological resources by reviewing available, current information and by describing threats.

This resource is based on the following source: 
Samson, Fred B., Fritz L. Knopf, and Wayne R. Ostlie.  1998.  Grasslands.
     Pages 437-472 in M. J. Mac, P. A. Opler, C. E. Puckett Haecker, and P. D. 
     Doran, eds.  Status and Trends of the Nation's Biological Resources, 
     Vol. 2.  U.S. Department of the Interior, U.S. Geological Survey.

This resource should be cited as: 

Samson, Fred B., Fritz L. Knopf, and Wayne R. Ostlie.  1998.  Grasslands.  
     Pages 437-472 in M. J. Mac, P. A. Opler, C. E. Puckett Haecker, and P. D. 
     Doran, eds.  Status and Trends of the Nation's Biological Resources, 
     Vol. 2.  Jamestown, ND: Northern Prairie Wildlife Research Center Online.  
     http://www.npwrc.usgs.gov/resource/2000/grlands/grlands.htm 
     (Version 21JAN2000).

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