Socioeconomic legacy yields an invasion debt

Abstract

Globalization and economic growth are widely recognized 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 species introductions. However, 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 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 alien species established in the wild are indeed more closely related to indicators of socioeconomic 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) more strongly associated with recent socioeconomic drivers. Nevertheless, our results suggest a considerable historical legacy for the majority of the taxa analyzed. The consequences of the current high levels of socioeconomic activity on the extent of biological invasions will thus probably not be completely realized until several decades into the future.

Fig. 1.

Trends in species introductions and socioeconomic indicators in Europe over the 20th century. (Left) Percentages of alien-species introduction events for different taxa recorded in the European countries covered by the DAISIE project (www.europe-aliens.org) (19) before 1900, between 1900 and 1950, and after 1950. Numbers in parentheses represent the overall number of species per taxon with known introduction dates. Species for which dates on their first record are missing were not considered. (Right) Average trends in three socioeconomic indicators: per capita GDP (in standardized 1990 International Geary-Khamis Dollars), population density, and the share of exports in GDP at three different time points (1900, 1950, 2000) for the 28 European countries used in this study.

Fig. 2.

Alien-species richness of 10 different taxonomic groups in 28 European countries as explained by current and historic socioeconomic models. Bars represent Akaike weights for spatial autoregressive models explaining the current distribution of established alien species across 28 European countries by either current or historical socioeconomic conditions. The predictors in the models are scores on the three axes of a principal component analysis using human population density, standardized per capita GDP, and share of exports in 1900 (gray), and 2000 (black) as input variables.