growing conditions, most crops fail to become invasive or even naturalized (that is, persistent but in low numbers) in new ranges.

The species that do arrive in the United States and become invasive, including the pests of nonindigenous crops, collectively affect the safety of our supply of food, fiber, timber, and water and the health of our domesticated animals and even humans. The cost of such invasions is probably incalculable. The aggregate figure for crop and timber losses and for the use of herbicides and pesticides to fight invasive species exceeds $100 billion per year, but this figure does not include the direct and indirect economic and ecological costs of invasions in ecosystems that are less intensively managed, such as forests, pastures, wetlands, and other wilderness areas (Pimentel et al. 2000).

As a result of the magnitude of the problem, scientists, crop producers, environmentalists, and public agencies are exploring ways to combat plant pests, beginning with preventing their entry into the United States. What might seem to be a straightforward endeavor is actually difficult. Regulating the arrival of nonindigenous species remains one of the most challenging tasks facing plant regulatory agencies today, because a major component of the global economy involves the transport of agricultural products, including the transfer of living organisms. Not only are agricultural products (such as grain, animal products, lumber, plant fibers, and cut flowers) being moved worldwide in unprecedented volume, but the use of imported germplasm that would give rise to these products in vast new markets is also increasingly widespread.

TROUBLE ON THE BORDER

The Animal and Plant Health Inspection Service (APHIS) of the United States Department of Agriculture (USDA) is responsible for preventing the introduction of plant and animal pests along human-mediated pathways. It carries out this mandate by prohibiting importation of known pests and by regulating the importation of commodities (such as fruits and vegetables), plants, and plant propagation materials that might harbor pests. APHIS agents at seaports, borders, and airports intercept restricted items such as food carried by tourists, and detect known pests that contaminate commodities, packing materials, and shipping containers. Although its monitoring activities in 1999 resulted in two million interceptions, which prevented the introduction of an estimated 53,000 plant pests (USDA 2000), the agency has the capacity to examine less than 2% of the cars, trucks, ships, and airplanes that bring products and people into the United States.

APHIS devotes much effort to evaluating the possibility that harmful pests will be contaminants in imported commodities and the significance of the economic or environmental damage that could result if these pests were introduced. Such evaluations are conducted on the basis of the probabilities of transportation and establishment and constitute the agency’s assessment of the risk of introductions. Risk assessment is the basis of APHIS decisions to permit or restrict the



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