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A small but growing group of scholars is calling for conservation science to turn its attention to cities, characterized previously in these pages as the places “where people live and work” and “where half the world lives” (Miller & Hobbs 2002; Cox 2010). Although there are many motivations to conserve urban biological diversity (cf. Dearborn & Kark 2010), one question that has not been explicitly answered is what will conservation science gain from focusing on urban areas? What is special or necessary about conservation in the city?

These questions were most poignantly suggested to one of us (E.W.S.) while standing under the Cross-Bronx Expressway in the city of New York, one of the busiest freeways in the United States, beside the river that gave the borough and the highway its name. Through most of the 20th century the Bronx River was regularly described as an open sewer, but after 30 years of community effort—hauling away cars, gathering garbage, removing invasive plants—the waterway has regained the semblance of a river, if a straightened, corseted, and asphalt-ceilinged one. Just three blocks upriver from the expressway is the headquarters of my employer, the Wildlife Conservation Society. One of the oldest and most prestigious conservation organizations in the United States, the society is dedicated to conserving landscapes and wildlife in many of the world's less inhabited areas. In contrast, before me was the antithesis of a wild place: an ecosystem that, in the popular vernacular of conservation, had been “hammered,” which was literally surrounded by people. Conservation there seemed so unlikely and so expensive that no conservation priority-setting had ever identified the South Bronx, New York City, as a conservation target. Yet here were groups of people cheerfully committed to a scientifically informed process for its conservation. Why?

The obvious reason is because they live there. Conservation in the city is about conserving parts of the natural world (we use nature and natural world as synonyms of biological diversity at all levels, from genes to ecosystems) in the places where people live, and people increasingly live in cities. Only since 2007 have more people lived in urban areas than rural ones. The urban population in 2007 was larger than the entire world's population in 1900, and urbanization is predicted to increase for at least another century (United Nations 2009). One can argue that if one seeks to conserve the places where nature is most intact, one should focus on areas as far from cities as possible—in the wildest places (Sanderson et al. 2002). But if one is arguing that conservation should seek to restore the human relationship to nature, then the cities are the most important places for conservation exactly because the majority of people live there.

Conservation in the city, as the Bronx River Alliance and other groups have found, is not easy. In fact, cities are arguably the hardest places in the world to put conservation into practice. Species extirpations, land-use change, habitat fragmentation, biotic homogenization, modified disturbance regimes, zoonotic diseases, air, water, and soil pollution, competition from human commensals, and climate change—the entire litany—threaten nature in cities to differing degrees. The Bronx River is a laboratory for cumulative effects. Remarkably, despite this recipe for ruin, parts of nature persist, even thrive. A beaver returned to the Bronx River in 2007 after the species had not been recorded there for more than 200 years, and alewife herring, reintroduced to the river, found their way home to spawn after 3 years at sea. Conservation professionals need to find out how elements of nature can persist in the Bronx River, and in other urban settings, because the answers may be transferable to other cities and to other places outside cities, where human influence is on the rise. It is precisely because urban conservation is so challenging that scientists need to tackle it head on.

Conservation work in human-dominated environments leads one to heed calls for better integration of social and natural sciences (cf. Mascia et al. 2003). We need to know what human behaviors are conducive to the maintenance of biological diversity, what motivates beliefs and actions consistent with conservation of nature, and how to integrate conservation objectives with other societal goals—all issues high on the agenda in urban conservation. Our own work on the Mannahatta Project helped build a natural-history consciousness for New Yorkers through a detailed examination of the historical landscape ecology of Manhattan and adjacent waters (Fig. 1) (Sanderson 2009). To our immense delight, expressing ecology in the geographical terms of the city—its blocks and streets—has been broadly adopted by the public and provides a useful context for understanding the cityscape's current state, while motivating discussions about its future form. The Mannahatta Project demonstrates that not only have economics, architecture, culture, and law shaped the city, but ecology, history, and geography have as well (Sanderson 2010).

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Figure 1. The left side of this image from the Mannahatta Project (http://www.welikia.org) shows how Manhattan Island may have looked on 12 September 1609, when Henry Hudson and a small group of sailors entered the bay that is now known as New York Harbor and set in motion the events that eventually led to New York City as it is today. On the right is a photograph of Manhattan Island in 2008. (Reconstructed image by Markley Boyer, on the basis of data from the Mannahatta Project, Wildlife Conservation Society; photograph by Yann-Arthus Bertrand, Corbis.)

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Conservation in the city also challenges one to cross the frontier between strict conservation—understanding how to maintain the nature that exists—and restoration or reconstruction (cf. Saunders & Hobbs 1993)—understanding how to create the conditions for nature to thrive in the future. Conservation science, in our view, should aim not only to prevent the absence of nature, but also work to create the presence of diverse, functional ecosystems that are self-sustaining and sustain humans as part of them. For areas of the city where hills have been flattened, streams paved over, and wetlands filled, reconstruction more than conservation is warranted. Metropolitans are waiting for conservation professionals, the people whose business it is to study and understand how humans interact with nature, to help them build a new sustainable notion of and form for cities that are compatible with and complementary to the natural world.

Ultimately, conservation in cities is not just about the cities themselves; it is also about conservation of nature elsewhere. Cities are not isolated from other parts of the world. Rather, they depend on ecosystems near and far to supply resources they cannot produce themselves, and those resource-extraction activities often have dramatic effects on nature far from the city line. Thus, it is interesting to consider the density-dependent effects of human resource use in cities. Evidence continues to mount that on a per capita basis, urban dwellers use fewer, not more, resources than people living in areas of lower population density (City of New York 2007; Owen 2009). If people are drawn to the city because of economic opportunity and higher quality of life, but use fewer resources as a result of better public transportation, smaller living spaces, shared walls, and other efficiencies, then cities may prove to be as important as protected areas for the conservation of nature in the long run.

We are only scratching the surface of what rewards conservation science might reap in the city in an age of extreme population. Because conservation in cities is so difficult, it has often been ruled out, usually with the meek assumption that conservation efforts are inevitably limited by money, time, and, as in the case of triage, potential for recovery (Brooks et al. 2006; Botrill et al. 2008). We do not accept those limitations; we do not assume that some parts of the world are lost forever, especially the places where people are. We call for conservation of the whole world, for all places to receive care and consideration, for all the world to be considered part of “the natural world.” Conservation everywhere includes conserving the cities where people work, where half the world lives. Conservation in cites is challenging, but gratifying in equal measure, because it pushes us to exceed our professional expectations of ourselves, to integrate our activities into the mainstream of society, and to examine what we are trying to conserve and restore—not only aspects of biological diversity, but also the special and necessary human connections to the natural world.

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