duced species can alter drainage patterns and accumulation of biomass and create fire hazards. Cheatgrass, a fire-adapted annual, has invaded over 5 million hectares of shrub-steppe habitat in the Great Basin and, by eliminating native perennials, contributed to the frequency of fires, which increased by a factor of 20 (Vitousek et al. 1996).

Introduced species, via introgression and hybridization with native species, can also interfere with evolutionary processes (Darmency 1997), with many possible outcomes, including extinction of native species, introduction of pesticide-resistance genes into susceptible genomes, and transfer of agriculturally undesirable traits into native species. All told, the management of alien weeds alone in the United States exacts a cost of $3.6–4.5 billion per year (Westbrooks 1998).

As for the impact of biological invasions on the future of pesticide use in the United States, suffice it to say that some of the most controversial examples of pesticide use in the last half-century have involved attempts to eradicate introduced species. An ill-fated campaign launched in 1957 to eradicate Solenopsis invicta, the red imported fire ant, culminated in expensive failure, public concern, and political concern about the safety of the chemical control agents used (Kaiser 1978, Hinkle 1982). Efforts to contain Mediterranean fruit flies (Ceratitis capitata) in California in 1989-1990 with malathion-laced baits have also led to a public outcry (Myers et al. 1998). Pesticides have often been used initially to eradicate localized populations of introduced species because they kill more quickly than do most biological control methods. Such use, while often highly desirable in agricultural contexts, might be less appropriate for eradicating normative species in preserves, refuges, and wildlands, where use of chemicals is inconsistent with management practices and can pose unacceptable risks to nontarget species, and in urban green spaces, where pesticide use can present unacceptable risks of exposure to large numbers of people.

In view of the increasing globalization of trade, accidental introductions will probably increase in frequency in the near term. Possible consequences of this accelerated pace of invasion are increased frequency of pesticide failure due to resistance acquisition and increased demand for narrow-spectrum chemicals or biologically based approaches.

Loss of Biodiversity

Whether accelerated biodiversity loss contributes a separate form of global change or is the consequence of the combined effects of many human-caused forms of global change is debatable. What appears to be clear, however, is that extinction rates globally are higher than rates recorded in the recent past (Pimm et al., 1995; Purvis et al., 2000; Wilson 1993). Effects of biodiversity loss on agriculture vary with site and system. In general, how-

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