Uncertainties propagate across the process of an impact assessment. The first source of uncertainty emerges due to natural variation associated with the occurrence of an alien species’ impact on native biota. Uncertainties arise at three key stages when information on the impact of an alien species is captured 1) the impact observation stage; i.e. when the impact is measured 2) the impact report stage; i.e. when the impact is communicated in some form of report and finally, 3) at the ICAT assessment stage; i.e. when the assessment is conducted. Any uncertainty that arises will be carried through to the subsequent stages, as illustrated through the encapsulation of uncertainties across the process.