Bridgehead effects distort global flows of alien species

Aim A major goal of invasion biology is to understand global species flows between donor and recipient regions. Our current view of such flows assumes that species are moved directly from their native to their introduced range. However, if introduced populations serve as bridgehead populations that generate additional introductions, tracing intercontinental flows between donor and recipient regions misrepresents the introduction history. Our aim was to assess to what extent bridgehead effects distort our view of global species flows. Location Global. Methods We separately mapped “flows” of 252 alien ant species established on one to six continents, representing a gradient of relatively certain to completely unreliable flows. To assess the importance of bridgehead effects in distorting our view of global species flows, we first quantified the proportion of cosmopolitan species per country. A high proportion of such species would indicate that exclusively mapped flows from the native range to these countries are unreliable. We then tested if the global flows obtained mapping species exotic in one continent to six continents differed and tested if these flows can be linked to global trade flows. Results In 83% of countries, more than 50% of alien ants were established on six continents, indicating that flows to these countries are unreliable. Flows of species established on a single continent were linked to global trade flows, while flows including cosmopolitan species were not linked to global trade. Main conclusion It is crucial to account for bridgehead effects when assessing the biogeography and intercontinental flows of alien species. This is urgent for improving our understanding of how species are moved around the planet.