Century is a model of terrestrial biogeochemistry based on relationships between climate, human management (fire, grazing), soil properties, plant productivity, and decomposition. The grassland version of the Century model was tested using observed data from 11 temperate and tropical grasslands around the world. The results show that soil C and N levels can be simulated to within ±25% of the observed values (100 and 75% of the time, respectively) for a diverse set of soils. Peak live biomass and plant production can be simulated within ± 25% of the observed values (57 and 60% of the time, respectively) for burned, fertilized, and irrigated grassland sites where precipitation ranged from 22 to over 150 cm. Live biomass can be generally predicted to within ±50% of the observed values (57% of the time). The model underestimated the live biomass in extremely high plant production years at two of the Russian sites. A comparison of Century model results with statistical models showed that the Century model had slightly higher r2 values than the statistical models. Data and calibrated model results from this study are useful for analysis and description of grassland carbon dynamics, and as a reference point for testing more physiologically based models prediction's of net primary production and biomass. Results indicate that prediction of plant and soil organic matter (C and N) dynamics requires knowledge of climate, soil texture, and N inputs.

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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.

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