The 'known unknowns' of invasive species impact measurement

There are thousands of case studies documenting invasive species' impacts and these have led to the development of over 30 hypotheses that describe how invasions occur and their impacts manifest. The proliferation of invasion hypotheses over the decades has spurred several efforts to identify conceptual overlap and increase clarity of impact mechanism in the field. What is still lacking, however, is a comprehensive accounting of the evidence base on invasive species impacts, especially in regard to the biological scales, temporal scales, and taxonomic groups that receive research. Identifying the 'known unknowns' of empirical invasion impact research serves a critical function in the effort to evaluate support for existing hypotheses and generate novel hypotheses. We built a taxonomically and geographically comprehensive database of over 1500 research articles that report measures of invasive species' ecological impacts published over the past 18 years (1999-2016). We found that, field-wide, published measures of invasive species' impacts are highly skewed toward those measured at the population or community-level with scarce information on impacts at other biological scales (e.g., physiology, behavior). We also show that existing impact evidence stems most often from one-off studies of single invasive species. Yet, even for species that receive consistent attention, impacts have rarely been documented across more than one biological scale, beyond very short time periods, or in several ecosystems. In order to predict and anticipate how impacts manifest in a variety of temporal and biological contexts, the evidence base informing existing invasion hypotheses must become more integrative both within and across publications.