We present a method for categorising and comparing alien or invasive species in terms of how damaging they are to the environment, that can be applied across all taxa, scales, and impact metrics.

Despite intensive research on effects of alien species during the past decade, invasion science still lacks capacity to predict impacts and thus provide timely advice to managers on where limited resources should be allocated. This capacity has been limited in part by the context-dependent nature of impacts, research highly skewed toward certain taxa and habitat types, and the lack of standardized methods for observing and quantifying impact. We review different strategies, including specific experimental and observational approaches, for detecting and quantifying ecological impacts of alien species. These include a four-way experimental plot design that allows comparing impact studies of different organisms. Furthermore, we identify hypothesis-driven parameters that should be measured at invaded sites to maximize insights into the nature of impact. We also present strategies for recognizing highimpact species. Our recommendations provide a basis for developing systematic quantitative measurements to allow comparisons of impact across alien species, sites and time.

Non-native species cause changes in the ecosystems to which they are introduced. These changes, or some of them, are usually termed impacts; they can be manifold and potentially damaging to ecosystems and biodiversity. However, the impacts of most non-native species are poorly understood, and a synthesis of available information is being hindered because authors often do not clearly define impact. We argue that explicitly defining the impact of non-native species will promote progress toward a better understanding of the implications of changes to biodiversity and ecosystems caused by non-native species; help disentangle which aspects of scientific debates about non-native species are due to disparate definitions and which represent true scientific discord; and improve communication between scientists from different research disciplines and between scientists, managers, and policy makers. For these reasons and based on examples from the literature, we devised seven key questions that fall into 4 categories: directionality, classification and measurement, ecological or socio-economic changes, and scale. These questions should help in formulating clear and practical definitions of impact to suit specific scientific, stakeholder, or legislative contexts.Definiendo el Impacto de las Especies No-NativasResumenLas especies no-nativas pueden causar cambios en los ecosistemas donde son introducidas. Estos cambios, o algunos de ellos, usualmente se denominan como impactos; estos pueden ser variados y potencialmente dañinos para los ecosistemas y la biodiversidad. Sin embargo, los impactos de la mayoría de las especies no-nativas están pobremente entendidos y una síntesis de información disponible se ve obstaculizada porque los autores continuamente no definen claramente impacto. Discutimos que definir explícitamente el impacto de las especies no-nativas promoverá el progreso hacia un mejor entendimiento de las implicaciones de los cambios a la biodiversidad y los ecosistemas causados por especies no-nativas; ayudar a entender cuáles aspectos de los debates científicos sobre especies no-nativas son debidos a definiciones diversas y cuáles representan un verdadero desacuerdo científico; y mejorar la comunicación entre científicos de diferentes disciplinas y entre científicos, administradores y quienes hacen las políticas. Por estas razones y basándonos en ejemplos tomados de la literatura, concebimos siete preguntas clave que caen en cuatro categorías: direccionalidad, clasificación y medida, cambios ecológicos o socio-económicos, y escala. Estas preguntas deberían ayudar en la formulación de definiciones claras y prácticas del impacto para encajar mejor con contextos científicos, de las partes interesadas o legislativos específicos.