Predicting overall effects of increased UV-B levels on US agriculture is difficult because of the scarcity of appropriate studies. The impact might be minimal. Genetic variability in resistance to UV-B has been documented in crop species (Teramura 1983, Teramura and Sullivan 1991, D'Surney et al. 1993) and weed species (such as Plantago lanceolata; McCloud and Berenbaum 1994); this allows for the possibility of adaptation in the case of weeds and selective cultivar use in the case of crop plants. Moreover, the nearly universal adoption of the Montreal protocols for global reductions in chlorofluorocarbon production suggests that ozone depletion could diminish, rather than accelerate, in the future.

Increased Frequency of Biological Invasions

Biological invasions constitute a form of global change as devastating ecologically and economically as more widely recognized global climatic changes (Vitousek et al. 1996). The growth of agriculture, industry, and development over the last 200 years has led to redistribution of species in all major taxa. In the United States, for example, over 5,000 species of alien plants have been introduced and established, infesting over 100 million acres of terrestrial and aquatic habitats; the acreage occupied by these plants is increasing 8–20% per year. The proportion of the flora that is made up of alien weeds varies by state from less than 10% to, in the case of Hawaii, almost 50% (Pimentel et al. 2000).

The impacts of alien species on agriculture and forestry are enormous (Niemela and Mattson 1996). Many of the introduced species are themselves pests; life-history attributes that are conducive to colonizing new habitats—including exploitative reproductive traits, broad resource requirements, small bodies, and high vagility—predispose these species to pest status. That problem is compounded by the fact that many species that are not pests in their native range become important pests in their new, acquired range because factors that historically regulated their population, such as natural enemies, are not present. Moreover, introduced species can act as disease vectors and facilitate the entry of new pathogens into nonnative agroecosystemso

Introduced species can reduce yields by removing crop biomass directly (as in the case of alien insects) or by competing with crop plants for water, minerals, or light for photosynthesis. Introduced species can also cause problems by altering trophic dynamics and ecosystem structure.

Forest ecosystems in North America have been radically altered, and dominant species driven to near extinction, by such invasive pests as chestnut blight, gypsy moth, and balsam woolly adelgid (Niemela and Mattson 1996). On rangeland, toxic species can poison livestock (even, on occasion, pollinator species); in agricultural and forest ecosystems, intro-

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