There have been relatively few attempts to quantitatively evaluate the number, identity, and origin of arthropods, pathogens, and undesirable plants that inadvertently arrive at the borders and ports of the United States. Interceptions of organisms whose entry into the United States is restricted are documented in the Port Information Network (PIN) database maintained by the USDA Animal and Plant Health Inspection Service (APHIS) since 1985. Roughly 53,000 interceptions of arthropods, pathogens, and noxious plants are made each year by APHIS inspectors, who endeavor to examine up to 2% of the cargo, baggage, and related materials arriving in the United States. PIN data, however, have recognized limitations and have rarely been made available to scientists outside APHIS. Despite those limitations, the PIN database is a potentially valuable resource; collaborative efforts of APHIS officials and scientists in different disciplines to analyze the data could do much to enhance our understanding of the pathways by which potential invaders arrive in the United States.
In contrast with arthropods and pathogens, introductions of most nonindigenous plants into the United States are intentional. A minority of these species has become invasive, but there is no consistent effort to monitor the fate of plants that arrive. In addition to the demand for nonindigenous plants for landscaping and gardening, Internet-based sales and seed exchanges have encouraged importation. Although imported plants do not, as a group, share any syndrome of traits that enhance their ability to become invasive, some activities might favor the importation of plants that are likely to become established. For example, renewed interest in medicinal plants, such as herbs, has resulted in the importation of plants that are troublesome in parts of the United States. Likewise, there is an interest in plants for erosion control that are hardy and spread readily; these characteristics increase the chances that they will become invasive.
China is likely to be a source of new invasive plant pests because of the dramatic increase in trading activity between China and the United States, because the two countries share physical and climatic environments and many related plant species, and because many collectors and nurseries are particularly interested in plants from China.
If nonindigenous plants, arthropods, and pathogens are introduced into a new environment in small numbers, they are not likely to survive, because of stochastic forces in the new locale. These forces are random demographic, environmental, catastrophic, and genetic events that can push small populations to extinction. The stochasticity can be overcome in some instances by factors that increase the chances that some members of a new population will survive; these factors include cultivation, the spatial distribution of the new immigrants, and multiple, sequential introductions that reinforce the size of the population and diversify the age distribution or genetic variability among its members. Information on such factors