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do not appear to be scientific, often resting on undefended judgments by experts and on arbitrary algorithms for combining risks (Simberloff and Alexander 1998). Furthermore, these risk assessments are expensive; one conducted by the US Department of Agriculture (USDA) on risks associated with importing larch from Siberia into the United States (USDA 1991) cost $500,000 (Jenkins 1996). It is difficult to imagine finding funding sources sufficient to mount risk assessments for all the challenges that might appear even to an educated layperson to be justified on prima facie grounds.

Virtually every specialist in invasion biology who has examined the matter concludes that aspects of the ecological impact of a nonindigenous species are inherently unpredictable (for example, Hobbs and Humphries 1995), and many scientists argue that every species should be considered a potential threat to biodiversity and sustainability if it were to be introduced (for example, Ruesink and others 1995). That implies that every species proposed for deliberate introduction, whether or not it appears superficially to be innocuous, necessitates some formal risk assessment. The cost would be staggering if the USDA process (USDA 1991) were the model.

In addition, many parties introduce species not inadvertently, but deliberately. These range from the boy smuggling giant African snails to his grandmother, who released them in her yard in Miami (Simberloff 1997a), to such large industries as the pet and ornamental-pet trades, which lobby vigorously against many restrictions. In the United States, recommendations that all species proposed for introduction must be on “white lists”—lists of species whose invasive potential has been assessed and has been approved for introduction—have been systematically attacked by those interest groups. Rather, the major laws that restrict entry of species use “black lists”lists of species that have already been shown to be damaging or are strongly suspected of being dangerous; a species is prohibited only if it is on such a list (Schmitz and Simberloff 1997). Rarely is blacklisting forward-looking.

Thus, there will always be a flow of nonindigenous species. However, the flow can be lessened. Undoubtedly, increased public education as to the risks would lead to fewer deliberate and inadvertent introductions as people strive to be good environmental citizens. The Convention on Biological Diversity mandates that its signatories “as far as possible and as appropriate . . .prevent the introduction of . . .those alien species which threaten ecosystems, habitats, or species.” Bean (1996) suggests that this statement reflects widespread recognition that nations are obliged to attempt to prevent introductions, and he cites as an example New Zealand's 1993 Biosecurity Act, which subjects all incoming persons and goods to rigorous inspection and prevents the importation of any species not already cleared by government authorities for inclusion on a white list. He also notes the increasing international and national regulation of purging of ballast water and points out that the considerable legal framework and effort that many nations use to attempt to prevent agriculturally harmful introductions could be adapted and expanded to prevent ecologically harmful ones. The problem is educating the public sufficiently that they demand regulation of nonindigenous species.