rail lines (Forcella and Harvey 1988). Freight and the packing material used in freight consisted of whatever material was at hand. Straw was a common packing material for fragile goods. And straw, in addition to often being grain stubble, can be a heterogeneous collection of pasture species, all of which can be represented by viable seeds in the straw. Dewey (1896) commented on the likelihood that species spread in the United States via straw; he noted the suspicious circumstances by which some newly arrived species in Denver first appeared near the back door of a crockery shop. Much less is known about the entry of nonindigenous species along the long common borders of the United States with Canada and Mexico. It is known, however, that some species (such as the annual grass Bromus tectorum) immigrated to the Pacific Northwest on both sides of the international border as accidental immigrants, as seeds in grain, or attached to livestock (Mack 1981). Current inspection standards limit the movement of weeds in grain, although many of those species (such as Agropyron repens, quackgrass; Bromus spp.; and Chenopodium spp.) are still accidentally moved through the United States (Westbrooks 1993).
Accidental entry accounts for only a small fraction of the newly naturalized plant species in the United States. With changes in production, shipping, and inspection practices, the rate of accidental introduction of nonindigenous plants has decreased. Seawater has replaced soil for ship ballast, eliminating a mode by which soil and soil-based nonindigenous species are transported. But as a result, the arrival of waterborne hitchhikers has been facilitated substantially (Carlton and Geller 1993). Changes in growing and production techniques—including the introduction of herbicides and the implementation of quality-control and inspection mechanisms associated principally with the 1939 Federal Seed Act and the 1957 Federal Plant Pest Act—have resulted in increased proficiency of APHIS inspectors in reducing the frequency of introduction of invasive plant seeds as contaminants of imported cereal crops, forage grasses, and produce and as hitchhikers on live animals (Westbrooks 1993). Entry of soil now requires a special permit and is thus inspected or treated, so soil itself is unlikely to be a source of entry of plants.
With the arrival of the Europeans, the long isolation of North America ended as geographic and ecological barriers to insect colonization were breached by commerce. From 1640 to 1980, the number of immigrant species of insects and mites resident in the 48 conterminous states rose steadily. Since passage of the Plant Quarantine Act in 1912, all plants and most plant products entering the United States have been subject to inspection, and action has been taken to prevent entry of arthropod pests. Before 1920, numbers of accidentally introduced insects had tended to increase exponentially; as the 1912 act took effect, the increase slowed to a linear rate (U.S. Congress 1993).