Alien populations keep establishing at alarming rates and often have
highly detrimental impacts on recipient environments. Quantifying the
magnitude of their impact is essential for prioritization and management
and is commonly done by comparing ecological variables between invaded
and uninvaded states. Such estimates are highly uncertain and often
biased because they ignore the temporal dynamics of the system. This has
hampered the understanding and prediction of impacts, and hence
management. To address this, we propose a framework to quantify impacts
by contrasting the trajectory of ecological variables in presence of an
alien with that forecasted in the absence of the alien. We discuss how
trajectories in absence of the alien can be forecasted statistically and
how uncertainty in these forecasts can be accounted for when estimating
impacts. This framework readily allows for comparisons of alien species'
impacts across taxa and regions, as well as with impacts caused by other
stressors.