ecology of natural ecosystems. Although agricultural impacts and ecological impacts are rarely discussed in concert, we feel that the two are mutually illuminating; together they form a continuum that presents an array of challenges to managers and policy makers. First, we review the diversity of impacts and how they are measured from a strictly anthropocentric view, followed by a strictly ecological view, and then discuss what occurs when the two views are incongruent. In addition, we emphasize the utility of identifying trade and transportation pathways as a way to organize the diversity of potential invaders. Using vectors of introduction to define groups of invaders with similar biology and potentially similar impacts on natural systems may prove an efficient approach to identify targets for regulation and control. Often targeting the vector is easier than targeting each individual species.
We then focus on three aspects of predicting the risk of impacts from introductions of particular species: (1) predicting the successful establishment of invaders, (2) predicting the impact of species that establish themselves, and (3) assessing the uncertainty associated with these predictions. Progress in predicting establishment of certain invaders shows promise for informing policy on planned introductions, but relies heavily on detailed biological and geographical information about the species. Forecasting which invaders will then have the biggest impacts is even more challenging. Factors such as host range and dispersal ability can help gauge how big of a problem an invader will become. The prevalence of ecological idiosyncrasies, complex indirect effects and the possibility for synergistic effects among invaders, however, hinder our ability to predict ecological impacts. We finish with a discussion of prioritizing the management and control of invasive species based on impact, and the need for more consistent measures of impact.
To illustrate the main points that we emphasize in this paper, we have provided three detailed case histories of invasions. Through these case histories, we attempt to convey the importance that a biological perspective plays in assessing the degree of impact, as well as understanding the mechanisms behind that impact. We have intentionally chosen case histories that involve species native to the United States that have become problematic invaders in other parts of the world. This admittedly biased selection represents a suite of invaders less publicized within the United States that may feature prominently in current or future trade disputes.
Impact from an anthropocentric view is often equated with the economic losses caused by a non-indigenous species or by the cost of its control or eradication. For example, the Office of Technology Assessment report on non-indigenous species lists as ''high impact" those species that are significant pests of agriculture, rangelands, or forests and those that seriously foul waterways or power plants (U.S. Congress, 1993). Among several proposed measures of impact of non-indigenous species, Williamson (1998) quoted costs in pounds sterling for control of different weed species in the United Kingdom. Just as