Population and Environment in the U.S. Great Plains analyzes the recursive relationships between environment, population, and land use in the Great Plains, a semi-arid grassland covering roughly one-third of the United States, over a period of about one hundred and fifty years. The project team includes historians, sociologists, anthropologists, and demographers at the University of Michigan, Colorado State University, and the University of Saskatchewan, with consultants at other universities. Analysis draws on a variety of quantitative, qualitative, visual, and geospatial data, much of which is available from ICPSR.
Defining the Great Plains
Borders of the Great Plains
The Great Plains region is the vast natural grassland that stretches across central North America from the Rocky Mountains to the woodlands of the Midwest.
We define the limits of the Great Plains as follows:
North: The Canadian border
South: The 32nd parallel
West: 5,000 feet of elevation
East: The line of 700 mm of average annual precipitation
All regionalization is necessarily somewhat arbitrary, and we do not claim a monopoly on the definition of the Great Plains. In fact, precise borders elude consensus and each of our boundary lines is open to debate:
North/South: Arguably, the Great Plains extend north into Canada and south through Texas to Mexico. We have chosen northern and southern borders that maximize data uniformity and comparability.
West: The Rocky Mountains are commonly agreed to bound the Great Plains on the west, but exactly where in the Rockies to draw this line remains in dispute.
East: The only agreement on this boundary is that no authoritative line exists. Numerous people have attempted to define this border in both physiographical and cultural terms, using such demarcators as:
- The 100th, 98th, 95th, and 88th meridians
- The Missouri, Arkansas, and Mississippi Rivers
- Various levels of annual rainfall
Our boundaries depend on a combination of climatic, topographical, political, and cartographic criteria, and are ultimately drawn along county borders. According to these definitions, the Great Plains region contains about 475 counties in twelve states.
Characteristics of the Great Plains
As we have defined them, the counties belonging to the Great Plains share some environmental characteristics, but also differ from one another in terms of soil composition, temperature, precipitation, and elevation. The Great Plains region is just as hard to classify as it is to delineate, and can be subdivided according to a number of criteria:
East/West: Rainfall in the plains decreases from east to west. As a result, the eastern plains are quite arable, while the western plains are more suited to pastoral purposes. Natural vegetation in the east is dominated by tallgrass prairie and in the west by shortgrass steppe.
North/South: Average temperatures in the Great Plains increase from north to south, with mean annual temperatures ranging from less than 0°C to greater than 20°C.
High/Low: High plain rises more than 2,000 feet above sea level and slopes ever upward as it moves west.
This environmental diversity is mirrored in the diversity of attitudes toward the Great Plains, which have been romanticized as a fertile land of possibility and reviled as the Great American Desert. Therefore, the Great Plains both attract and repel population. The region thus includes major cities, areas with population densities of fewer than two people per square mile, and several gradations in between.
Across this diversity, however, the Great Plains counties are united by the flat or slightly rising treeless expanse that must have greeted all early visitors, extreme climates and dramatic weather patterns, and a preponderance of grasses in the natural vegetation.
The Great Plains Population and Environment project examines the recursive relationship between human impact and environmental change. We investigate both how human activities shape the environment and how the environment influences population dynamics. In our analyses, the human population and the broader environment each serve as both independent and dependent variables.
Changing land use, population, and environment drive our study, and four headlines emphasize the significance of this project:
- The limits of human action on the environment: how human intervention has been stymied by the characteristics of the land, and how farmers choose to work within environmental constraints instead of driving over them.
- The shift from local to global scales of human changes to the environment: the determinants and consequences of increasing farm size.
- human impacts on the environment: estimates of greenhouse gas emissions and carbon sequestration.
- Environmental impacts on population: how environment shapes family processes and migration patterns.
Through these interrelated approaches, we intend to obtain a fuller understanding of how environmental conditions have influenced demographic change, and how human land use has shaped the environment.
Over the period of our study, the white population of the U.S. Great Plains converted much of the native grassland to cropland, with significant environmental impact. However, our work has shown how the plowing and planting of the Great Plains, and the resulting environmental effects, were not monolithic. Moreover, human impact on the environment was limited by environmental constraints.
In On the Great Plains, Geoff Cunfer demonstrates that less than half of the land in the Great Plains has ever been used for crops, shifting the ground under the assertion that the region's agricultural transformation was total. Our research reveals that, while humans can decide where to plant crops, it is the environment that determines where crops will flourish. Furthermore, farmers in the plains responded to environmental constraints: almost all the variation in agricultural land use in the Great Plains is explained by environmental variables, especially precipitation, temperature, soil texture, and slope.
This evidence confirms our hypothesis that humans and the environment impacted one another: while crop-based agriculture did change the Great Plains environment, Great Plains farmers also responded to their environment and adapted their land-use practices accordingly. New sources from which we are currently extracting field-level data will allow us to further refine our understanding of this process.
Most studies of the relationship between population and environment focus either on the details of local processes or on the environmental effects of population change at the global level. Our project bridges this divide in three ways:
- Examining the complex articulation of the local to the regional and global over time.
- Investigating regional and global influences on local land-use decisions, primarily decisions about irrigation and farm size.
- Tracing the subsequent regional and global effects of those decisions.
Based on aggregate data, we hypothesize that the scale of population effects on the Great Plains has changed from local to global over the course of the twentieth century. In the early years of settlement, agriculture responded to the needs of the local population. Today, local population shapes the environment in urban, urbanizing, and recreational areas, but agricultural land use is now driven by global markets.
Over the course of the twentieth century, our data show that farm size in the Great Plains has grown as the population in agriculture has contracted, suggesting a shift from local to extra-local determinants of land use. This shift raises several important analytical questions about the relationships between farm size, population, economy, and the environment:
- What is the precise connection between increasing farm size and declining agricultural population?
- Is farm size driven by local, regional, national, or global forces?
- Is there a recursive relationship between farm size and the environment?
- Does the environment of the Great Plains drive decisions about farm size?
- Do larger farms have different impacts on the environment than small farms, for example by devoting more land to pasture (positive) or by using more pesticides and fertilizers (negative)?
We evaluate these questions by analyzing the economic context in which farm size grew over the course of the twentieth century. Our county-level data provide the intermediate scale for these analyses, while new data will fill in the picture at the field level and at the regional, national, and global scales.
Land-use transformations on the Great Plains since 1870 have altered the global carbon cycle. Our project contributes to the ongoing debate about environmental change by pioneering methods for measuring the effects of Great Plains agriculture on greenhouse gas production and carbon sequestration. In a region as large and significant as the Great Plains, it is crucial to measure the impact of the human population on the environment at a variety of scales, and to refresh those measurements as new data and new techniques become available. We have made fundamental progress in this area, first with the first accurate descriptions based on historical county-level data of the pace at which land was transformed from native grassland to cropland, and then with biogeochemical model results that show how land conversion and changing agricultural land use depleted (and then in some cases partly restored) soil carbon and nitrogen.
We are now both extending this approach to about 475 counties and refining our models with finer-scale data for a sample of these counties. These new analyses will allow us to engage in the crucial scientific and policy debate about the role of agricultural landscapes in producing greenhouse gases and sequestering carbon.
The other side of the population-environment equation includes the ways that environment shapes population. Our research thus far places the population of the Great Plains into two broad and overlapping contexts:
- The family settlement process: Family dynamics on the settlement frontier are shaped by the environment, the availability of resources, and a family and farm life cycle that develops out of the age and marital structure of the settlers.
- Migration: The various driving factors, notably drought, the rise of recreation, long-term economic change, and environmental hazards that draw and repel population and affect the family structures of migrants.
In both contexts, the overriding question is the same: how do people behave demographically in a complex social, economic, and environmental setting?
With the population growing in some parts and among some ethnic groups and socioeconomic classes in the Great Plains, and shrinking in other parts and among other ethnic groups and socioeconomic classes, we foresee valuable new research that relates the environment to rural and metropolitan populations.
Sources and Methods
County level Data about population and land use for the U.S. Great Plains from 1870 to the present.
Our county-level social, demographic, and agricultural data come from:
- The United States Census of Population
- The United States Census of Agriculture
- Vital statistics records from the United States Department of Health
- The Inter-university Consortium for Political and Social Research
Environmental data for the U.S. Great Plains from 1895 to the present.
To the social, demographic, and agricultural data we have added environmental data, including:
- Soil quality
- Natural amenities
- Distance between counties
Environmental information comes from:
- PRISM precipitation and temperature data
- VEMAP wind and humidity data
- SSURGO and STATSGO soils databases
We have interpolated these data to historical county boundaries, allowing us to link population dynamics and land use to environmental features and thus examine the impact of environment on land use and population change through both family formation and migration at the county level.
Great Plains Farm Family Survey.
A set of roughly 150 interviews with farm families in five areas of the Great Plains, conducted from 1997 to 1999, which provide valuable information about:
- Farm family incomes
- Responses to conservation measures
- The ways in which farmers see and use government programs
Through these interviews we attempt to demonstrate how families make demographic and land-use decisions in response to social, economic, political, and environmental forces. Linking these interviews to county- and individual-level census data will allow us to deal specifically with regional, environmental, and ethnic differences in the study population.
Century and DayCent Ecosystem models.
These generalized ecosystem models simulate carbon and nitrogen dynamics and greenhouse gas fluxes in grassland, forest, savanna, and crop systems. Our historical land use data provide parameters for these models, allowing us to simulate the outcomes of land use management practices on Great Plains agroecosystems. As land use is tied both directly and indirectly to population change, these models will reveal the effects of population on the ecosystem itself, in turn producing data about the environment of the Great Plains, which we can use in conjunction with our social and demographic data to analyze the interactions between environmental and social processes.
By examining the impact of agricultural practices on trace gas fluxes and soil carbon levels, we have developed carbon and nitrogen budgets for the counties of the Great Plains and determined best management standards for Great Plains grassland systems. Our data allow us to use the Century and DayCent models to determine the environmental consequences of Great Plains agriculture and of the recent and ongoing conversion of agricultural land to residential lawns and other non-agricultural uses.
Field-level land use data from 1930 to the present.
The interpretation of aerial photographs and satellite images by the Environmental Spatial Analysis Laboratory at the University of Michigan will produce data about land use practices at the field level for a sample of fifty-two counties representing the shortgrass, mixed grass, and tallgrass plains. Within each of the fifty-two sample counties, we will sample an average of eight sites over five time points, dating back to the 1930s.
These field-level data will allow us to test our hypotheses about the limits of human impact on the environment and about the stability of agricultural land use in the Great Plains by revealing land-use changes at the field level that may have been masked by county-level aggregation. For example, the county-level data analyzed by Geoff Cunfer in On the Great Plains demonstrate that land in crops reached a maximum in the 1930s and has changed little since. The field-level data will reveal whether individual fields and parcels were converted from one use to another. If we find that land use changed often in many places, then human choices were more important than environment in driving land use; if land use remained generally stable across many decades, then environmental drivers come to the fore.