Widely held among ecologists are the beliefs that non-native species are the second-greatest threat to biodiversity and that non-native species are unnatural and undesirable. The literature of the past two decades has seen vigorous debates around these perceptions, with some ecologists noting that non-native species can provide benefits to people and to native ecosystems and calling for an end to value-laden language in reference to non-native species in scientific contexts. I conducted a systematic review to test the hypothesis that the largely negative view of non-native species would lead to confirmation bias in research quantifying their effects. Specifically, I predicted that authors would tend to accentuate negative effects and downplay positive effects of non-native species in the abstracts of their papers relative to the findings presented in the results sections. Using a systematic searching and assessment procedure, I identified 74 peer-reviewed articles published in 2006, 2011, and 2016 that investigated the effects of non-native animals on abundance of a native species or diversity of a native community. For each paper, I compared the number of positive, negative, and non-significant outcomes reported in the results section to the outcomes as presented in the abstract.
Results/Conclusions
My sample revealed no evidence of confirmation bias in either direction: the number of abstracts that overemphasized negative findings was equal to the number of abstracts that overemphasized positive findings, and this pattern was consistent across time. However, papers reporting more consistently negative effects of non-native animals tended to be published in more prestigious journals (as measured by higher impact factor), providing some evidence for publication bias. I also found that researchers refer to most non-native species as “invasive”—a term specifically defined as an introduced species with significant negative impacts—regardless of whether the study actually documented negative effects of the animal in question.