Since the Industrial Revolution increased international trade and human movement have resulted in the accidental movement of many species worldwide at an unprecedented scale. There has been much interest in identifying which regions have acted as major donors of invasive species and why. Darwin proposed that more species from the Northern hemisphere have invaded the Southern hemisphere than vice versa because “the northern forms have existed in their own homes in greater numbers, and have consequently been advanced through natural selection and competition to a higher stage of perfection or dominating power, than the southern forms”. More recently, it has been suggested that Old World plants have more frequently invaded the New World than vice versa because they share traits for quick colonization of disturbed habitats and are pre-adapted to invade. These are two examples of the many species trait hypotheses that have been proposed to explain historical patterns of global invasions. Alternatively, these patterns may simply reflect the direction and level of human transport. Here, we evaluate the extent to which global species flows are best explained by the biodiversity of donor regions or the worldwide network of human transport, using ants (Formicidae) as model system.
Results/Conclusions
To test if global flows of the 241 species of alien ants are simply driven by the available species pools of the donor regions, we compared the observed species flows to expected flows based on the native species richness of each region. Observed species flows differed significantly from expected flows based on native species richness. For example, Australia and North America have exported less species than expected and Europe has exported more species than expected. To test if these patterns can be explained by the level and direction of human transport, we analysed flows of transported species representing the level of species introductions through human transportbefore the establishment stage, using border interceptions from air and maritime ports in five countries on three different continents. We found that expected species flows based on human transport were consistent with observed species flows (establishments) to these countries. Our results imply that the direction of human-mediated transport is able to generate the observed species flows in contrast to species trait hypotheses based on the evolutionary superiority of regional faunas.