Predicting Invasions of Nonindigenous Plants and Plant Pests

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trated by the protracted legal battles in Australia over Echium plantagineium, given the antithetical sobriquets of “Patterson’s Curse” and “Salvation Jane” (Parsons and Cuthbertson 1992)! A similar situation may yet arise in the United States over the herbaceous species Hypericum perforatum (St. John’s wort). It is unquestionably an aggressive rangeland weed and was the target of a successful biological control program (Huffaker and Kennett 1959). But today the plant is valued by some as an herbal remedy and is touted as a boon to some local economies.

Furthermore, the “zero-tolerance” policies used by many regulatory agencies might require pesticide applications, inspections, or other treatment before plant material from areas known to be infested by a nonindigenous insect or pathogen is permitted to enter uninfested areas. Such policies are incompatible with biological control, nullify economic injury or action thresholds that are integral components of integrated pest-management programs, often lead to increased pesticide use, and can result in considerable costs to producers, inspection agencies, and other officials in affected areas.

Even attempts to disrupt the life cycle of the nonindigenous pest without classical biological control can have environmental consequences. For example, establishment of white pine blister rust had profound ecological impacts in many northern states, but the effects of programs to eradicate the native Ribes species (the alternative host of the pathogen) and discourage regeneration of white pine have not been addressed.

Collectively, those examples show the need for careful evaluation of biological control agents, inasmuch as protocols for screening the evolutionary potential of invaders–weighing genetic variation, natural selection, and ecological opportunity—are still in early stages of development (Ewel et al. 1999). The potential for such unintended consequences and the public’s diverse reaction to nonindigenous species need to be assessed as much as possible before the species’ release.

KEY FINDINGS

Of the country’s persistent nonindigenous species, only about 10 % are considered invasive. But appearances can deceive, and many of the remaining 90% might be considered innocuous only because their harmful effects have not been documented or even investigated.
Effects or impacts of invasive species are often hard to measure and even harder to predict, because scientific uncertainty arises in quantifying each step in the invasion process. Moreover, data are lacking on species composition and species abundance in many ecosystems before the arrival of invasive species.
Impacts occur on various scales—biological, spatial, temporal. Effects become harder to predict as one moves from the individual to the genetic, popu-

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