BioScience 

Abstract: H umans have long been fascinated by the dynamism of free-flowing waters. Yet we have expended great effort to tame rivers for transportation, water supply, flood control, agriculture, and power generation. It is now recognized that harnessing of streams and rivers comes at great cost: Many rivers no longer support socially valued native species or sustain healthy ecosystems that provide important goods and services (Naiman et al. 1995, NRC 1992).

Abstract: Biologists are nearly unanimous in their belief that humanity is in the process of extirpating a significant portion of the earth's spe­ cies. The ways in which we are doing so reflect the magnitude and scale of human enterprise. Everything from highway construction to cattle ranch­ ing to leaky bait buckets has been implicated in the demise or endan­ germent of particular species. Ac­ cording to Wilson (1992), most of these activities fall into four major categories, which he terms "the mind­ less horsemen of the environmental apocalypse": overexploitation, habi­ tat destruction, the introduction of non-native (alien) species, and the spread of diseases carried by alien species. To these categories may be added a fifth, pollution, although it can also be considered a form of habitat destruction. Surprisingly, there have been reIa­ tively few analyses of the extent to which each of these factors-much less the more specific deeds encomDavid S. Wilcove is a senior ecologist at the Environmental Defense Fund, Wash­ ington, DC 20009. David Rothstein re­ ceived his J.D. in 1997 from Northeastern

Abstract: [Extract] Aproximately 50,000 nonindigenous (non-native) species are estimated to have been introduced to the United States. Some of these are beneficial; for example, species introduced as food crops (e.g., corn, wheat, and rice) and as livestock (e.g., cattle and poultry) now provide more than 98% of the US food system, at a value of approximately $800 billion per year (USBC 1998). Other exotic species have been introduced for landscape restoration, biological pest control, sport, pets, and food processing, also with significant benefits. Some nonindigenous species, however, have caused major economic losses in agriculture, forestry, and several other segments of the US economy, in addition to harming the environment. One study reported that 79 exotic species had caused approximately $97 billion in damages during the period 1906–1991 (OTA 1993).

Abstract: A the many human activities that cause habitat loss (Czech et al. 2000), urban development produces some of the greatest local extinction rates and frequently eliminates the large majority of native species (Vale and Vale 1976, Luniak 1994, Kowarik 1995, Marzluff 2001). Also, urbanization is often more lasting than other types of habitat loss. Throughout much of New England, for example, ecological succession is restoring forest habitat lost from farming and logging, whereas most urbanized areas in that region not only persist but continue to expand and threaten other local ecosystems (Stein et al. 2000). Another great conservation challenge of urban growth is that it replaces the native species that are lost with widespread “weedy” nonnative species. This replacement constitutes the process of biotic homogenization that threatens to reduce the biological uniqueness of local ecosystems (Blair 2001). Urban-gradient studies show that, for many taxa, for example, plants (Kowarik 1995) and birds and butterflies (Blair and Launer 1997), the number of nonnative species increases toward centers of urbanization, while the number of native species decreases. The final conservation challenge of sprawl is its current and growing geographical extent (Benfield et al. 1999). A review by Czech and colleagues (2000) finds that urbanization endangers more species and is more geographically ubiquitous in the mainland United States than any other human activity. Species threatened by urbanization also tend to be threatened by agriculture, recreation, roads, and many other human impacts, emphasizing the uniquely far-reaching transformations that accompany urban sprawl. About 50% of the US population lives in the suburbs, with another 30% living in cities (USCB 2001). Over 5% of the total surface area of the United States is covered by urban and other built-up areas (USCB 2001). This is more land than is covered by the combined total of national and state parks and areas preserved by the Nature Conservancy. More ominously, the growth rate of urban land use is accelerating faster than land preserved as parks or conservation areas by the Conservancy (figure 1). Much of this growth is from the spread of suburban housing. It is estimated, for example, that residential yards occupy 135,000 acres in the state of Missouri (MDC 2002). This residential landscape represents nearly 1% of the total area of Missouri and is nearly three times the area occupied by Missouri state parks. Here I review the growing literature that documents how urban (and suburban) expansion harms native ecosystems. This knowledge can aid conservation efforts in two major ways. One is through the use of ecological principles—such as preserving remnant natural habitat and restoring modified habitats to promote native species conservation—to reduce the impacts of urbanization on native ecosystems. Rare and endangered species sometimes occur in urbanized habitats (Kendle and Forbes 1997, Godefroid 2001) and thus could be conserved there. Managing the large amount of residential vegetation (1% of the state area, as noted above) in ways that promote native plants and animals could also make a significant contribution to conservation.