An impact assessment of species introductions require information from a number of sources on a number of areas such as the biology, ecology, and genetics of the alien species; the information will need to be readily available and understandable to those performing the risk assessment and to policy makers. Risk assessment must also include benefit assessment; an accurate accounting of the benefits derived from exotic species is essential.

Impacts of introduced species will fall into two broad categories: ecological, which includes biological and genetic effects and socio-economical (Table 1). However, these two categories are not independent and socio-economic changes brought about by alien species can in turn cause more ecological changes. Thus, a reduction in native species may be from direct interaction with an exotic species, or it may result from increased fishing pressure or changes in land use brought about by the presence of a newly established species.

Impacts may depend on the objective of the introduction. Analyses of the FAO database on introductions of aquatic species (DIAS) reveals that aquaculture development was the most often cited reason for fish introductions, and that government organizations were responsible for more introductions than any other group. Table 2 presents information that most of the ecological effects of introduced species reported were negative, however the socio-economic impacts were reported to be more often beneficial and there were more positive socio-economic benefits reported than negative ecological impacts.

Table 3 presents some popular conceptions regarding impacts of alien species and how the databases can provide information to support or refute the generalizations. The purpose of challenging the broad generalizations is not to replace one generalization with another, but to provide some estimate of their validity.