ORIGINAL PAPER How often are invasion-induced ecological impacts missed? Alisha Dahlstrom Davidson • Chad L. Hewitt Received: 12 February 2013 / Accepted: 19 October 2013 Ó Springer Science+Business Media Dordrecht 2013 Abstract Managers and policy makers depend on empirical research to guide and support biosecurity measures that mitigate introduced species’ impacts. Research contributing to this knowledge base gener- ally uses null hypothesis significance testing to determine the significance of data patterns. However, reliance on traditional statistical significance testing methods, combined with small effect and sample size and large variability inherent to many impact studies, may obscure effects on native species, communities or ecosystems. This may result in false certainty of no impact. We investigated potential Type II error rates and effect sizes for 31 non-significant empirical evaluations of impact for introduced algal and crus- tacean species. We found low power consistently led to acceptance of Type II errors at rates 5.6–19 times greater than Type I errors (despite moderate to large effect sizes). Our results suggest that introduced species for which impact studies have statistically non-significant outcomes (often interpreted as ‘‘no impact’’) may potentially have large impacts that are missed due to small sample or effect sizes and/or high variation. This alarming willingness to ‘‘miss’’ impacts has severe implications for conservation efforts, including under-managing species’ impacts and discounting the costs of Type II errors. Keywords Significance testing  Type II errors  Algae  Crustacea  False certainty  Biosecurity Introduction Fully describing and measuring impacts of introduced species is a process faced with several major chal- lenges (Thiele et al. 2010; Simberloff et al. 2013). One difficulty results from the taxonomic, spatial and temporal variety of impacts, let alone an agreed definition of what comprises ‘‘impact’’, resulting in the unfortunate truth that we undoubtedly acknowledge only a small fraction of the true impacts (Carlton 2002; Parker et al. 1999). Once we have elucidated an impact, how to categorize that impact also faces substantial debate (Simberloff 2011; Davis et al. 2011; Simberloff et al. 2011). Yet the need to better understand impacts in order to prevent introductions and protect ecological, economic and social values Electronic supplementary material The online version of this article (doi:10.1007/s10530-013-0570-4) contains supple- mentary material, which is available to authorized users. A. D. Davidson  C. L. Hewitt Australian Maritime College, University of Tasmania, Locked Bay 1396, Launceston, TAS 7250, Australia A. D. Davidson (&) Wayne State University, 5047 Gullen Mall, Detroit, MI 48202, USA e-mail: alisha.dahlstrom@gmail.com C. L. Hewitt Australian Rivers Institute, Griffith University, Nathan, QLD 4111, Australia 123 Biol Invasions DOI 10.1007/s10530-013-0570-4