Abstract
AbstractAimA 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.LocationGlobal.MethodsWe 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.ResultsIn 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 conclusionIt 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.
Highlights
The intercontinental exchange of thousands of introduced species has become a hallmark of the Anthropocene (Capinha et al, 2015; Corlett, 2014), and the number of new species introductions has exploded with the increasing globalization of human activities (Seebens et al, 2017)
We found that our view of alien species flows changed dramatically when we only considered species for which the “flow” between their native and their recipient region represented the actual introduction pathway with a higher degree of certainty
The invasion biogeography of ants was dominated by the 26 most extreme cosmopolitan species which had established on all six continents and could potentially arrive from any continent, via bridgehead effects
Summary
The intercontinental exchange of thousands of introduced species has become a hallmark of the Anthropocene (Capinha et al, 2015; Corlett, 2014), and the number of new species introductions has exploded with the increasing globalization of human activities (Seebens et al, 2017). This research has shown that some donor regions were overrepresented relative to what would be expected based on the available source pools of these regions (i.e. the number of native species that could potentially be exported) These asymmetric exchanges of alien species among continents may be linked to different human activities, such as accidental transport, pet and horticultural trade or environmental factors. Given that the bridgehead effect is such a widespread phenomenon, we hypothesize that it has the potential to distort our view of global species exchanges between donor and recipient regions, which classically map flows as direct introductions from the native to the introduced range To still be able to address this question and assess whether secondary introductions distort our view of global species exchanges, we used the number of continents where a species has established as an approximation of the likelihood of being introduced secondarily. We tested whether the global flows estimated by mapping species exotic in one continent (hereafter “Exo species”) to six continents differed (hereafter “Exo species”) and tested whether these potential flows can be linked to global trade flows
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