ABSTRACTAimEffective policy and management responses to the multiple threats posedby invasive alien species (IAS) rely on the ability to assess their impacts beforeconclusive empirical evidence is available. A plethora of different IAS risk and/or impact assessment protocols have been proposed, but it remains unclearwhether, how and why the outcomes of such assessment protocols may differ.LocationEurope.MethodsHere, we present an in-depth evaluation and informed assessment ofthe consistency of four prominent protocols for assessing IAS impacts (EICAT,GISS, Harmonia+and NNRA), using two non-native parrots in Europe: thewidespread ring-necked parakeet (Psittacula krameri) and the rapidly spreadingmonk parakeet (Myiopsitta monachus).ResultsOur findings show that the procedures used to assess impacts mayinfluence assessment outcomes. We find that robust IAS prioritization can beobtained by assessing species based on their most severe documented impacts,as all protocols yield consistent outcomes across impact categories. Additiveimpact scoring offers complementary, more subtle information that may beespecially relevant for guiding management decisions regarding already estab-lished invasive alien species. Such management decisions will also strongly ben-efit from consensus approaches that reduce disagreement between experts,fostering the uptake of scientific advice into policy-making decisions.Main conclusionsInvasive alien species assessments should take advantage ofthe capacity of consensus assessments to consolidate discussion and agreementbetween experts. Our results suggest that decision-makers could use the assess-ment protocol most fit for their purpose, on the condition they apply a precau-tionary approach by considering the most severe impacts only. We alsorecommend that screening for high-impact IAS should be performed on a morerobust basis than currentad hocpractices, at least using the easiest assessmentprotocols and reporting confidence scores.