Conservation biologists and other environmentalists confront five obstacles in building support for regulatory policies that seek to exclude or remove introduced plants and other non-native species that threaten to harm natural areas or the natural environment. First, the concept of “harm to the natural environment” is nebulous and undefined. Second, ecologists cannot predict how introduced species will behave in natural ecosystems. If biologists cannot define “harm” or predict the behavior of introduced species, they must target all non-native species as potentially (...) “harmful”. an impossibly large regulatory task. Third, loss of species richness may constitute harm to an environment, but introduced organisms typically, generally, and significantly add to species richness in ecosystems. If species richness correlates with desirable ecosystem properties, moreover, such as stability and productivity, as some ecologists believe, then introduced organisms, by increasing species richness, would support those desirable properties. Fourth, one may plausibly argue that extinction constitutes environmental harm, but there is no evidence that non-native species, especially plants, are significant causes of extinction, except for predators in certain lakes and other small island-like environments. Fifth, while aesthetic, ethical, and spiritual values may provide a legitimate basis for invasive species policy, biologists often cite concepts such as “biodiversity” and ecosystem “health” or “integrity” to provide a scientific justification. To assert that non-native species threaten biodiversity or undermine ecosystem health, however, may be to draw conceptual entailments or consequences from definitions of “biodiversity” and “integrity” that arbitrarily exclude non-native species or make the presence of exotic species a per se indicator of decline. (shrink)

Within the last 20 years, the US has mounted amassive campaign against invasions bynon-indigenous species (NIS) such as zebramussels, kudzu, water hyacinths, and brown treesnakes. NIS have disrupted native ecosystemsand caused hundreds of billions of dollars ofannual damage. Many in the scientificcommunity say the problem of NIS is primarilypolitical and economic: getting governments toregulate powerful vested interests thatintroduce species through such vehicles asships' ballast water. This paper argues that,although politics and economics play a role,the problem is primarily one of scientificmethod. (...) Even if commercial interests werewilling to spend the necessary funds to controlNIS, and even if government were willing toregulate them, ecological theory is notadequate to provide clear direction for eithereffort. The paper argues there is nocomprehensive, predictive theory ofinvasibility, as part of a larger theory ofcommunity structure, that might guideecological decision making regarding NIS, andfor at least three reasons. (1) There is nofirm definition of NIS, native, exotic,and so on, and ecologists do not use the termsconsistently; as a result, biologists debatingvarious accounts of community structure andecological explanation often do not even makelogical contact with each other. (2) Thedominant theory used to understandinvasibility, island biogeography, has noprecise predictive power and is unable toclarify when NIS might promote biodiversity andwhen they might hinder it. (3) There are nofirm, empirical generalizations that revealwhen a colonizer or a NIS might be likely totake over a new environment, and when it mightnot succeed in doing so. As a result,scientists have only rough rules of thumb toshore up their arguments against NIS. Given theincompleteness of current ecological theory,the paper closes with several suggestions forways that study of NIS might enhanceunderstanding of basic commmunity structuresand vice versa. (shrink)