Invasive ant and trade flows from continents to countries worldwide

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
population 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. 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. 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. 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.