Ecological Impacts of Alien Species: Quantification, Scope, Caveats, and Recommendations

Despite intensive research on effects of alien species during the past decade, invasion science still lacks capacity to accurately predict impacts and thus provide timely advice to managers on where limited resources should be allocated. This capacity has been limited partly by the context-dependent nature of ecological impacts, research highly skewed toward certain taxa and habitat types, and the lack of standardized methods for detecting 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 for 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 high-impact species. Our recommendations provide a foundation for developing systematic quantitative measurements to allow comparisons of impact across alien species, sites and time.