from nonindigenous microorganisms and identifying their continent of origin (Martin et al. 2000).
Since the second half of the 19th century, most nonindigenous plants in the United States have been introduced deliberately. However, the origins of deliberate introductions of plants extend much further back into the history of agriculture in the United States. Regardless of the indigenous crop species available, the first colonists and their descendants had a strong preference for European crops, such as wheat, beets, onions, and brassicas. These species were repeatedly introduced; except for the weed species carried as contaminants in the seeds of these introduced crops, little harm occurred. One noteworthy exception, which is an example of how nonindigenous organisms can unexpectedly interact with indigenous ones, is the beet leafhopper (Circulifer tenellus), which was introduced into the United States on the tops of sugar beets. It was later found to transmit an indigenous virus to sugar beets from native plants in semiarid regions of California and the Rocky Mountains, causing devastating losses to the sugar beet industry (Duffus 1971).
The introduction of nonindigenous species was not restricted to crops. English settlers along the eastern coast of North America in the 17th century soon became dissatisfied with the quality of native forage. One 17th century settler in New England complained that the native forage “is so devoid of nutritive vertue, that our beasts grow lousy with feeding on it, and are much out of heart and lung” (Cronon 1983). Securing nutritious forage was a serious concern for settlers who had neither the time nor the inclination to experiment with native species as forage. By 1635, those planning to settle in Maryland were cautioned to bring a “good store of Claver grasse seede, to make good meadows” (Edwards 1948). The native species clearly had been dismissed as unsuitable.
Much American folklore to the contrary, the Intermountain West of the United States is only marginally suitable for livestock; the principal native grasses, all bunchgrasses, are intrinsically intolerant of persistent grazing (Mack and Thompson 1982). As a result, the abundance of these grasses was radically reduced by 1900, less than 50 years after the wholesale introduction of cattle and sheep. By 1900, many had concluded that “the native grass is gone, and experiment has not yet fully demonstrated the adaptability of any other grass to this soil and climate” (Anon. 1901 as cited in Mack and Thompson 1982). The call went out rapidly for species from Eurasia. Little or no effort was spent in exploring the native species; nonindigenous grasses—such as Agropyrum cristatum (crested wheatgrass), Bromus inermis (awnless brome), and even the highly invasive B. tectorum—were soon being evaluated as substitutes (Mack 1981). Only a few of