Abstract

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.