ABSTRACT
Estimating the full extent of environmental damage caused by exotic species and the number of indigenous species extinctions they have caused is dif¢cult because little is known about the estimated 750,000 species in the United States-half of these species have not been described.4 Nonetheless, about 400 of the 958 species listed as threatened or endangered under the Endangered Species Act are at risk primarily because of competition with and predation by nonindigenous species.5 In other regions of the world, as many as 80% of the endangered species are threatened and at risk due to the pressures of nonnative species.6 Many other species not listed are also negatively affected by alien species and/or ecosystem changes caused by alien species. Estimating the economic impacts associated with nonindigenous species in the United States is also dif¢cult; nevertheless, enough data are available to quantify some of the impacts on agriculture, forestry, and public health. In this chapter, we assess, as much as possible, the magnitude of the environmental impacts and economic costs associated with the diverse invasive species that have become established within the United States. Although moving native species to other national locations where they did not exist previously can also have signi¢cant impacts, this assessment is limited to alien species that did not originate within the United States or its territories.
