Management of nonindigenous species is a crucial aspect of maintaining native biodiversity and normal ecosystem functions. We attempt to guide researchers in developing projects that will be of use to conservation practitioners, tangibly improving applied conservation measures. We advocate a directed approach for conservation research to aid in prioritizing nonindigenous species for intervention by resource managers. This approach includes outlining what needs to be known to make such relative judgments about the impacts of nonindigenous species and the most promising methods by which to obtain such information. We also address active measures that should be taken once priorities have been set, highlighting the roles of risk assessment and research in improving control efforts. Ultimately, a better match between research and practical conservation needs should result in more effective reduction of the effects of nonindigenous species on native species. Dirección de la Investigación para Reducir los Efectos de Especies ExóticasResumen: El manejo de especies exóticas es un aspecto crucial para el mantenimiento de la biodiversidad nativa y de las funciones normales de un ecosistema. Intentamos guiar a los investigadores para desarrollar proyectos que serían de uso para los practicantes de la conservación, mejorando tangiblemente las medidas de conservación aplicada. Apoyamos una estrategia dirigida de investigación para la conservación para ayudar a priorizar especies exóticas para la intervención de los manejadores de recursos. Esta estrategia incluye delinear lo que se necesita saber para hacer juicios relativos sobre los impactos de especies exóticas y los métodos más promisorios para obtener dicha información. También nos abocamos a las medidas activas que deberían ser tomadas una vez que se establezcan las prioridades, subrayando los papeles de la evaluación de riesgo y la investigación para mejorar los esfuerzos de control. A fin de cuentas, la mejor conjun- Byers et al. Reducing Invader Impact 631 ción de la investigación con las necesidades de conservación práctica debe resultar en una reducción más efectiva de los efectos de las especies exóticas sobre las especies nativas.

The major challenge to stewardship of protected areas is to decide where, when, and how to intervene in physical and biological processes, to conserve what we value in these places. To make such decisions, planners and managers must articulate more clearly the purposes of parks, what is valued, and what needs to be sustained. A key aim for conservation today is the maintenance and restoration of biodiversity, but a broader range of values are also likely to be considered important, including ecological integrity, resilience, historical fidelity (ie the ecosystem appears and functions much as it did in the past), and autonomy of nature. Until recently, the concept of “naturalness” was the guiding principle when making conservation‐related decisions in park and wilderness ecosystems. However, this concept is multifaceted and often means different things to different people, including notions of historical fidelity and autonomy from human influence. Achieving the goal of nature conservation intended for such areas requires a clear articulation of management objectives, which must be geared to the realities of the rapid environmental changes currently underway. We advocate a pluralistic approach that incorporates a suite of guiding principles, including historical fidelity, autonomy of nature, ecological integrity, and resilience, as well as managing with humility. The relative importance of these guiding principles will vary, depending on management goals and ecological conditions.

Preamble: Executive Order (EO) 13112—defines an invasive species as “an alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health.” In the Executive Summary of the National Invasive Species Management Plan (NISMP) the term invasive species is further clarified and defined as “a species that is non-native to the ecosystem under consideration and whose introduction causes or is likely to cause economic or environmental harm or harm to human health.” To provide guidance for the development and implementation of the NISMP, the National Invasive Species Council (NISC) and the Invasive Species Advisory Committee (ISAC) adopted a set of principles outlined in Appendix 6 of the NISMP. Guiding Principle #1 provides additional context for defining the term invasive species and states “many alien species are non-invasive and support human livelihoods or a preferred quality of life.”

Invasions by non-native plants threaten the preservation of many plant and animal species and communities throughout North America. These pest species compete with and displace native plants and animals and may substantially alter ecosystem functions (e.g., fire occurrence and frequency, nutrient cycling). Awareness of these threats among wildland managers has greatly increased in the last decade. In a recent poll of National Park superintendents, 61% of 246 respondents indicated non-native plant invasions were moderate or major problems at their parks. Likewise, over 60% of Nature Conservancy stewards nationwide polled in 1992 indicated weeds were among their top 10 management problems, listing nearly 200 problem species. Over 12% indicated weeds were their worst problem. Weed control programs are now in place in wildlands across the continent, employing techniques ranging from manual removal, mechanical methods, prescribed fire, judicious use of herbicides, the release of biological control agents, and encouragement of native competitors. The most successful endeavors follow an adaptive management strategy in which plans based on the goals of the preserve are developed, weeds that interfere with those goals are identified and prioritized, and control measures are selected and implemented where appropriate. Emphasis is placed on preventing new weeds from becoming established and on early detection and elimination of incipient infestations. Managers must focus on the vegetation or community desired in place of the weeds and periodically re-evaluate whether their programs are moving them toward this objective. Control of weeds in wildlands poses unusual problems not ordinarily met in other systems which offer challenging research opportunities for weed scientists and ecologists.

We developed a protocol for categorizing nonnative plants according to their negative impacts on biodiversity in a large area such as a state, nation, or ecological region. Our objective was to provide a tool that makes the process of identifying, categorizing, and listing nonnative plants that cause negative impacts to biodiversity analytic, transparent, and equitable and that yields lists that are useful to researchers, land managers, regulators, consumers, and commercial interests such as the nursery industry. The protocol was designed to distinguish between species that cause high, medium, low, or insignificant negative impacts to native biodiversity within the state, region, or nation of interest. It consists of 20 multiple-choice questions grouped into four sections, which each address a major aspect of a species' total impact and when combined yield an overall “Invasive Species Impact Rank” or “I-Rank” (high, medium, low, or insignificant). The nonprofit organization NatureServe is now using this protocol to assess the estimated 3,500 nonnative vascular plant species that are established in the United States to create a national list prioritized by negative impact on biodiversity. The protocol and additional information are available on the Internet athttp://www.natureserve.org/getData/plantData.jsp, and over 500 completed species assessments are available through NatureServe Explorer (http://www.natureserve.org/explorer/).

We examined the role of nonnative honey bees (Apis mellifera) as pollinators of the invasive, nonnative plant species yellow star-thistle (Centaurea solstitialis), both introduced to the western United States in the early to middle 1800s. Using four different treatments (three exclosure types) at flower heads, we observed visitation rates of different pollinators. Honey bees were the most common visitors at each of three transects established at three study locales in California: University of California at Davis, Cosumnes River Preserve, and Santa Cruz Island. A significant correlation existed between honey bee visitation levels monitored in all these transects and the average number of viable seeds per seed head for the same transects. Selective exclusion of honey bees at flower heads using a 3 mm diameter mesh significantly reduced seed set per seed head at all locales. Seed set depression was less dramatic at the island locale because of high visitation rates by generalist halictid bees Augochlorella pomoniella and Agapostemon texanus that penetrated the 3mm mesh. The introduced megachilid bee Megachile apicalis occurred at all three locales as well (though in much lower numbers) and may contribute to pollination. In an ancillary study, seed set of plants with bagged heads was compared with that of plants without any bagged heads to test for resource shunting effects. These results showed that seed set differences observed between treatments within a single plant were not exaggerated due to resource shunting induced by the bagging technique. Yellow star-thistle may have low or variable levels of self-compatibility (as reflected by low seed set levels in small-mesh bags), increasing the importance of pollination in its breeding system. These results suggest that honey bees and yellow star-thistle may act as invasive mutualists, an association that may extend to other nonnative plant and pollinator species from Eurasia.

Available literature indicates that hurricanes do not generally produce long-term detrimental impacts to unmodified coastal systems and that they often provide net benefits along the U.S. Gulf Coast. While there is normally initial erosion from hurricanes, they also often result in a large influx of inorganic sediments, creating new wetlands and contributing to the maintenance of existing wetlands. The formation of washover deposits is disastrous where cultural development has occurred, but in natural areas these deposits are part of the natural cycle of shoreline development and contribute to habitat diversity and productivity. Abundant rainfall typically associated with hurricanes often results in large increases of sediment and nutrient inputs into coastal estuaries, leading to both short-term and long-term increases in productivity. Rainfall during tropical disturbances accounts for a significant part of total precipitation along the northern gulf. The immediate impact of hurricanes may be to reduce populations of some species but these populations generally recover rapidly. Overall, productivity in natural systems seems to be increased by periodic hurricanes. Hurricane impacts are often severe and long lasting in wetlands that have been modified by human impacts such as semi-or complete impoundments.

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