Canyons,
Cultures, and Environmental Change:
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Toward a Land Use History of North America: A Context for Understanding Environmental Change
Author: Thomas D. Sisk
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Hiker atop a pinnacle in Grand Staircase - Escalante National Monument. Photo © 1999 Ray Wheeler |
We inhabit a changing planet. Most of us realize that our lives are lived on a scale that is insignificant compared to geological time – continents break up and drift apart, mountains rise and are worn away by the elements, and all of human civilization is dwarfed by the vastness of Earth’s history.
What many of us do not recognize is that, like the continents and the mountains, the Earth’s living ecosystems are undergoing constant changes as well. While typically faster than the movement of continents, ecological change occurs at a pace that can be difficult to detect over the span of a human lifetime. Many areas that were covered with moist forests when humans moved into western North America are now shrub-dominated deserts (Thompson et al. 1993), and large expanses of arid grassland seen by the pioneers during the westward expansion of the United States have given way to shrubs and woodland (Hastings and Turner 1965). More recently, popular portrayals of the Lewis and Clark expedition have shown Americans how dramatically the Great Plains have been changed by settlement and agriculture, while portions of the northern Rockies still resemble the landscapes described by these early explorers at the beginning of the 19th century (Ambrose 1996).
Many changes in the patterns of North American land forms and vegetation – collectively referred to as landcover change – have resulted from or been influenced heavily by human activities. In most places across the continent, our activities have become the dominant driver of environmental change. Yet the extensive and influential role of human activity is often overlooked in the literature, as well as in our perception of our surrounding landscapes. Why? Throughout human history, each new generation has accepted the state of the world that they inherited from their ancestors. Environmental impacts that preceded them were viewed as natural, or at least normal, so changes occurring over several generations often went undetected (Reichman and Pulliam 1996). Even today, as we strive to minimize or reverse many of the obvious environmental impacts that are occurring during our own lifetimes, more gradual changes, even the critically import ones like climate change and the loss of biological diversity, are seldom fully appreciated.
The importance of compiling a land use history
A better understanding of the rate and direction of change in the Earth’s ecosystems is important for its own sake, but it is also increasingly vital for interpreting current environmental trends and guiding the management of our natural resources. Scientists and resource managers no longer assume that nature exists in a static, unchanging "natural" condition interrupted only by the work of humans. Instead, we view nature as a dynamic system of which humans are a part, recognizing that a variety of forces – ranging from natural disturbances to climatic change, deforestation, and the conversion of native habitats to agriculture – are constantly interacting to determine the pace and direction of change (Pickett and White 1985). This perception of nature has important implications for our understanding of how nature works and our formulation of appropriate responses to emerging environmental problems.
Setting management objectives
The perception of ecosystems as dynamic entities, without a single climax or "natural" state, can make the work of land and resource managers more complex. For example, the authorizing legislation for the National Park Service mandates conservation of "natural and historical objects and the wild life therein…unimpaired for the enjoyment of future generations." It is not clear, however, (except in a few cases, such as historic battlefields) whether the "objects" mentioned are those that were present at the time the first descriptions of the landscape were recorded, at the time the park was created, or during some other unspecified period. Similarly, how can biological resources be preserved "unimpaired" if change is a characteristic of the ecosystem itself? Whether we are concerned with forests or wildlife, wetlands or the oceans, identifying appropriate management objectives has become a challenging task, one that is almost impossible to accomplish without an understanding of the natural rate of change and range of variation within natural systems (National Research Council 1992).
Policy makers, and those who advise them, must ask not only whether environmental change is occurring, but also how it fits into the historical context. Certain changes, such as the sustainable harvest of fish and wildlife populations or well-planned logging practices, may fall within the typical range of variation for the ecosystem and require no mitigating management response. Other trends, such as the filling of wetlands and the rapid liquidation of old growth forests, may have no historical precedent, suggesting that decisive action may be necessary to prevent unacceptable ecosystem degradation. Recognizing the difference between these situations constitutes a critical new challenge for environmental scientists, yet without reliable historical records the differences may be impossible to distinguish, making it difficult to identify an appropriate course of action. Increasingly, applied ecologists are trying to understand the effects of management alternatives in the context of background rates of change in ecological systems, often referred to as the natural range of variation or NRV. A comprehensive history of changes in land use and land cover could identify this variability, enabling policy makers and resource managers to make more informed decisions as they face increasingly complex choices regarding the use and conservation of the resources entrusted to their care.
Understanding global change
Technical advances during the past 25 years have unequivocally shown that the entire Earth is undergoing rapid ecological change, and the most obvious and pronounced change is caused by human land use (Vitousek 1994). Perhaps the most remarkable evidence of such change has emerged from the deployment of LANDSAT and a host of other satellites and airborne sensors. The images acquired through remote sensing technology give us an unprecedented perspective on current land use, and they allow us to track land-use changes during the latter part of the 20th century. Trends detected during the relatively short period of space-based measurements, however, can be difficult to interpret. While the devastating implications of tropical deforestation and other extreme trends are obvious from recent LANDSAT images (Skole and Tucker 1993), our understanding of more subtle patterns of landcover change, such as the reforestation of the eastern deciduous forests and the spread of shrubs across the arid Southwest, often require a perspective that reaches back before the space age. Longer timelines can be compiled from historical archives. For example, evidence of early landcover change from photographs, accounts from 18th and 19th century land surveys, and paleoecological evidence from fossil pollen and fire scars can be assembled and analyzed through a multidisciplinary land-use history program. By integrating many sources of historical data and developing the scientific and statistical tools for analyzing change, the land-use history program will increase the value of information obtained from modern remote sensing programs by making it possible to interpret these data in the context of a comprehensive, albeit far less detailed, time line stretching back before the European settlement of North America and, in many cases, back to the last ice age or earlier.
Uncovering cause and effect
A descriptive approach to landcover change will identify historic trends and current conditions, but the prediction of future changes in land cover and their effects requires an improved understanding of the cause-and-effect relationships that link human activities with changes in land use and land cover. Much of the impact that humans have had on the environment can be viewed as a series of unplanned experiments, with particular perturbations generating measurable responses in the form of contractions in the ranges of some species and expansions in the ranges of others. Within the context of these temporal dynamics, species extinctions and the spread of non-native species may be seen as the extreme cases, where biological elements are lost or introduced.
Many of these "experiments" have been run repeatedly throughout human history. As civilizations have expanded and declined they have left their marks on landscapes, and environmental scientists are developing ingenious techniques for assembling the data needed to assess the results. By analyzing the relationships between changes in land use and land cover, at multiple temporal and spatial scales, it is possible to more confidently distinguish human-induced change from background climatic variation and other natural variability. A better understanding of the cause-and-effect relationships that underlie changes in land cover will lead to better predictive models for land-use planning and improved assessment of the likely outcomes of alternative land-use scenarios.
Increasing environmental awareness
Understanding landcover change and adopting a dynamic perspective on North American ecosystems have led many specialists to a deeper understanding of the environment and the place of humans in it. A comprehensive land-use history for North America offers many opportunities for extending this understanding to a broader audience, allowing citizens to explore the complex and often striking long term shifts in land use and land cover that characterize the regions where they live.
Part of the reason that many people have difficulty appreciating the importance of environmental change is that our perception of the world is constrained by the scale at which we live our lives. We typically are preoccupied with events that occur over time scales of a few minutes to a few months, and spatial scales ranging from our front lawns to a city block or farm field. Ongoing environmental changes, occurring over extensive areas at rates that vary over many orders of magnitude, are often overlooked when examined at these human scales. Graphical presentations of landcover and land-use change over longer time periods and larger areas could provide a powerful introduction to the changing face of North America. The subsequent chapters of this publication provide vivid examples of how maps and graphics can reveal striking patterns over different temporal and spatial scales, challenging us to expand and sharpen our perception of environment. Emerging digital technologies offer unlimited opportunities for extending this approach to museum displays, films, on-line productions, and other educational resources.