Abstract
Many of the most problematic alien species are not recent arrivals but were introduced several decades ago. Hence, current patterns of alien species richness may better reflect historical rather than contemporary human activities, a phenomenon we call "invasion debt".
We show that across vascular plants, bryophytes, fungi, birds, mammals, reptiles, amphibians, fish, terrestrial insects and aquatic invertebrates in 28 Europe, current numbers of alien species established in the wild are more closely related to indicators of socio-economic activity from the year 1900 than to those from 2000. The strength of the historical signal varies among taxonomic groups: those with good dispersal (birds, insects) are more strongly associated with recent socioeconomic drivers.
The consequences of the current high levels of socio-economic activity on the extent of biological invasions will thus probably not be completely realized until several decades into the future.