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as sediment pouuteci cree s ano rivers, filled in mill ponds and caused the closure of water-drven milis=on all but the largest streams. Sustainability is "6ftren'Ilk6ned to a three- legged stool, with the.legs representing social, economic and environmental systems and the seat representing the inescapable linkages among these systems.' Surely, the changes for the ,Robs,ns and their contemporaries, the inexorable downward spiral of dependence on slavery, the diminished production and polluted waterways, and the ever increasing fragility of the economic validate this tripartite metaphor. The Civil War and the Reconstruction that followed brought new changes to the land and inaugurated a period of regional economic depression that would extend up to World War II. The Robson land, like that of so many neighbors, was put into foreclosure. Although tenant farming and sharecropping continued in some places, much land was simply abandoned. Most of the land along the Wooden Bridge trail was abandoned between 1870 and 1910, and that fallow succession was repeated once again. Old field weeds were soon replaced with dog -hair thickets of pines. Through time, these pine thickets thinned to respectable forests. When Duke Forest was formally established in 1931, pine stands were about 40 to 60 years old ,,and broadleaved trees —oaks, sweetgums, n aples and hickories —were prominent beneath them. The waters of New Hope Creek that flowed beneath the Wooden Bridge in 1870 were red with sediment, and flash floods were common. But within a couple decades, the roots of regrowing forest trees and shrubs stabilized fragile soils and mitigated flows. Today, at least at that location, New Hope Creek runs clear and supports a diverse array of aquatic life. The fact that ecosystem change has repaired some of the impacts of those many years of unsustainable land use is reason for hope; but it is also true that no vestige of old- growth forest remains and that it will take many more decades, even centuries, to restore soils to their former productivity. DU,KENVIRONMENT 18 ;10 `0 //i Om 0 , "change continues on this i 1 " ,""1" � Once rurol laihd i$ „rapidly b ,pfnJ,n`4urban Forest is being replaced by ompldx, imperljious surfaces like roof tops, "parking; lots and roads that greatly alt°o'r local climate, the quality, quantity and timing of water flows, and wildlife habitat. Is all of this change sustainable? The word "sustainability" is tricky. "To sustain” is defined in many dictionaries as "to keep in existence, to maintain." To some this implies an idealistic sustainable endpoint —a destination. But if history tells us anything, it is that sustainability is a journey, not a destination, and that journey always occurs in the context of three kinds of change. First, the world is changing. The capacity for ecosystems to change is essential to their persistence. Forested landscapes and watersheds are constantly being disturbed and constantly undergoing change. Over the long term, change is essential to adaptation and survival. Second, we are changing. Each generation of human beings brings new technologies and values to the land. My interests and values are very different from those of my parents and grandparents, and the interests and values of my children and grandchildren are no less different from mine. And third, we are changing the world. This has always been true, but today there are more than 7 billion of us, and our individual effects on Earth's ecosystems are disproportionately magnified by the power of the technologies we employ to garner the things we need or think we need. We are today agents of unprecedented change. And we are hopeful that nature's change processes will mitigate our impacts, too. But history provides no guarantee that that will be the case. Our human population has increased over eightfold since the Robsons abandoned their land, and each of us individually consumes 10 times more energy and resources than did the Robsons and their peers. Furthermore, many of our insults to our planet's ecosystems have no precedent in either historic or prehistoric times. Sustainability, I would argue, is an inherently anthropocentric concept. For millions and millions of years, Earth's myriad ecosystems functioned wonderfully in our absence. It may be humbling, but it is good to remember that we are not an essential element to any of Earth's ecosystems. No other single organism has changed our planet to the extent that we have. But, were we to disappear tomorrow, ecosystems would continue to change and life would continue to evolve. Eventually, our portion of Earth's history would be reduced to a thin, albeit messy, layer in its geologic strata. I have a favorite Gary Larson cartoon. A stegosaurus stands at a lectern before an audience of other dinosaurs and says, "Friends the picture is bleak; climates are changing, mammals are on the rise, and here we sit with brains the size of a walnut." Dinosaurs are, unfairly I think, often depicted as the exemplar of unsustainability — unable to adapt, they were a cul de sac in the history of life. But, these remarkable beasts dominated Earth's ecosystems for a remarkable span of time -150 million years. Humankind has been around for about 1 /1,000th of that amount of time, yet many seem to think that the entire history of life that preceded us occurred solely Z for our benefit. That view has encouraged the widespread belief that we cannot severely damage Earth's capacity to sustain us or our children. But ttv j if ory of our interactions: with the land teiI's us We are fond of calling attention to two of human features, inteilrgence ars, ;- awareness, that set us apart from all the rest of creation; we have, afterail, brains the size of a grapefruit. We are hopeful that these traits will lead us on a more sustainable path, although there is not much evidence in our history to support that hope. But I believe our future will hinge much more on two other traits —the empathy to care for the well-being of others living now and in the future, and the humility to understand our proper place in the world and our dependence on the health and diversity of its ecosystems. Norman L. Christensen is founding de the Nicholas School and professor eme . in the Division of Environmental Sciet and Policy.