Socioeconomic legacy yields invasion debt.

Globalisation and economic growth are widely recognised as important drivers of biological invasions. Consequently there is an increasing need for governments to address the role of international trade in their strategies to prevent unintended species introductions. Yet, many of the currently most problematic alien species are not recent arrivals but were introduced decades or even longer ago. Hence, current patterns of alien species richness may better reflect past rather than contemporary human activities, a phenomenon which might be called “invasion debt”. Here, we show that across 10 taxonomic groups (vascular plants, bryophytes, fungi, birds, mammals, reptiles, amphibians, fish, terrestrial insects and aquatic invertebrates) in 28 European countries, current numbers of established alien species are indeed more closely related to indicators of socio-economic activity from the year 1900 than to those from 2000, although the majority of species introductions occurred during the second half of the 20th century. The strength of the historical signal varies among taxonomic groups with those possessing good capabilities for dispersal (birds, insects) tending to have reduced lag-times. Nevertheless, our results suggest a considerable inertia for the majority of the taxa analyzed. 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 the next decades.