(via wind, birds, and so on) of pathogens have occurred occasionally but are rare for other taxa. The role of humans as the chief dispersers of nonindigenous species into the United States will continue.
The rate of introduction of invasive plants, arthropods, and pathogens will increase in the future because of continuing growth in international travel and trade.
Most arthropods and pathogens arrive into the United States accidentally. Attributes that enable these organisms to escape detection and to survive in transit facilitate their arrival.
Arrival rates of nonindigenous arthropods and pathogens are probably related to the abundance of the species in their native habitats, the geographic areas of their native habitats, their association with plant materials that are likely to be imported into the United States, and the volume of trade between donor countries and the United States
Most plants that arrive in the United States are intentionally introduced. New pathways that facilitate the arrival of invasive plant pests include importation of nonindigenous species for herbal medicines, Internet-assisted exchanges of seeds or plants, and commerce in species new to horticulture. Such informal exchanges are largely unregulated and unmonitored.
China is a potentially major donor of future invasive plants, arthropods, and pathogens. This new situation reflects the recent radical increase in trade after a long hiatus and the similarity of physical environments in China and the United States. Such trade is the chief source of introduced plants that could become invasive—plants for ornamental horticulture. For example, there is strong interest in the importation of new horticultural taxa from China. This interest could lead to the arrival of new invasive plant species and provide increased opportunities for hitchhiking arthropods and pathogens to arrive in the United States. Many plant-feeding insects and pathogens from China would have a high probability of encountering susceptible families or genera of hosts in the United States.
The Port Interception Network database maintained by APHIS since 1985 is an important source of information about the arrival of nonindigenous species. Although the detail and comprehensiveness of its data are limited, they do identify the species arriving at specific U.S. ports and borders, and the origin of the commodities associated with them. Collaborative efforts between APHIS officials and scientists to expand the information content of the PIN database and to use the data to assess patterns in arrivals and traits associated with frequently introduced species would prove informative.
Europe was historically the primary source of invasive nonindigenous plant, arthropod, and pathogen species in the United States, and nonindigenous species continue to be intercepted at high rates on some commodities from Europe.