wood), Passiflora mollissima (banana poka), Psidium (probably P. cattleianum, strawberry guava), Ricinus communis (castor bean), and “Chinese tallow tree, Sapium” (probably S. sebiferum). These species include not only plants of ornamental value, but also those valued as medicines and seasonings.
Recognition of the importance of identifying the sources and characteristics of potential immigrant plant species is heightened by the recognition that only a small fraction of the earth’s flora has ever been introduced into the United States. For example, the worldwide vascular-plant flora consists of some 250,000 species. Admittedly, a large fraction of those species are native to lowland tropical environments that have no counterpart in the United States—even in Florida, Hawaii, and Puerto Rico—and consequently pose little or no threat. But another fraction of the world’s flora, at least as large as all foreign species in the United States today, have yet to be introduced, and the potential for their naturalization cannot be ruled out without further evaluation.
Increased opportunity for collectors to reach heretofore inaccessible parts of arid southern Africa and Arabia present the likelihood that some native species from these regions will become naturalized in the arid United States. The zeal of some private parties for collecting succulents and other drought-tolerant species is extraordinary (Mack 2001). Some amateur collectors now have hundreds of species in their gardens, products of their own collecting trips for aloes and other succulents in Yemen, Madagascar, and southern Africa (Levick and Lyons 2000). Plants can escape from arid gardens. For example, the attractive perennial grass Pennisetum setaceum (fountain grass), a native of north Africa, is now widely sold in the United States (Isaacson 1996) and has become naturalized in Arizona and southern California (Hickman 1993). As recently as 10 years ago, it continued to escape from commercial nurseries in southern California into adjacent desert communities (R. N. Mack, personal observation).
Any estimate of the number of invasive plants that will arrive in the future is admittedly, and of necessity, rough; it is based entirely on past trends. Whatever the number of plant invasions now in the United States, which is itself disputed, they arose from all the plant species that have ever been introduced. There is no firm figure for these introductions, and we probably cannot derive a direct estimate. However, some 40,000 taxa are commercially available in the United States today (Issacson 1996), including about 2000-3000 naturalized species and more than 300 truly invasive species; so we can predict that entry of another 40,000 species might produce several hundred new plant invasions.
Although judgments about what is likely to occur can be based on historical or projected trends, the national database of plant pests intercepted at U.S. ports of entry is perhaps the best source of information on the number and identity of nonindigenous plant pests arriving in the United States. The Port Information