Challenges of Reaching Consensus on Assessing Which Non-native Plants Are Invasive in Natural Areas

Abstract

Received for publication 2 May 2002. Accepted for pub li ca tion 7 May 2002. The contributions of Hillary Cher ry, Eli Jimenez, Joan Dusky, and Linda Tyson, and the support of the Univ. of Floridaʼs In va sive Plants Working Group are much ap pre ci at ed. This re search was supported by a grant from the Florida Nurserymen and Growers Association, and the Flori da Ag ri cul tur al Ex per i ment Station, approved for publication as Journal Series No. R-08546. To whom reprint requests should be ad dressed. E-mail address: amfox@mail.ifas.ufl .edu Reaching consensus on which naturalized non-native species are invasive is important for several reasons. Agreement is necessary for widespread compliance and participation in efforts to stop further dispersal, and for obtaining assistance in and support for early detection and control programs. Mechanisms for predicting which species might become invasive after introduction may be expected to be somewhat open to debate, but, surely, it cannot be very diffi cult to agree on the in va sive ness of species that are already here. When economic impacts on commodity groups are the primary concern (e.g., ag ri cul tur al weeds) such agreement is usually attained easily. However, for species that have primarily ecological impacts the situation is much less straightforward. The problem typ i cal ly revolves around attempts to distinguish between “Invasive” and “Non-invasive” char ac ter is tics and impacts. Non-native species can have a continuum of ecological impacts in natural ecosystems, ranging from a relatively harmless, sporadic appearance of a species confi ned to disturbed areas, to total re place ment and disruption of indigenous com mu ni ties. Selection of a dividing point along this continuum of impacts is diffi cult because plants may shift their relative position with time, either becoming more disruptive as pop u la tions increase, or being less disruptive as na tive or introduced herbivores and pathogens take their toll. Superimposed on this dynamic range are the myriad values and experiences that infl uence peopleʼs perceptions of what level of ecological disruption is acceptable. This spectrum ranges from the view that nat u ral ecosystems should be protected from any non-native incursions (e.g., where a single plant is potentially displacing a native plant), to the view that nothing short of native species extinction is worthy of concern. Fortunately, people can agree about the ecological impacts of many naturalized, nonnative plant species because either their severe ecological disruption is well recognized, such as cheatgrass (Bromus tectorum L.), melaleuca [Melaleuca quinquenervia (Cav.) Blake], and hydrilla (Hydrilla verticillata (L.F.) Royle), or they are just opportunistic weeds of highly disturbed lands that pose little threat to natural ecosystems. Controversy haunts the middle ground, especially when the species are in an early stage of their invasion and/or have high economic value. In such situations, consensus is most likely to be achieved if there is a clear and scientifi cally based set of criteria on which logical decisions about the status of invasive species can be based.