The global spread and invasion capacities of alien ants

Abstract The ecological and economic impacts of biological invasions are usually highly conspicuous, but these are the outcome of a global, multistage process that is obscured from view. For most taxa, we lack a large-scale picture of the movements of alien species, the biases and barriers that promote or inhibit their spread at each stage, and blind spots in our ability to detect species during their spread. For instance, countries rely heavily on customs interceptions to prevent new species introductions, but their efficacy for detecting invaders remains unclear. To address these gaps, we synthesize and provide data at unprecedented geographic resolution on the global diversity and distribution of alien ants, a pervasive group strongly impacting humans and ecosystems. From >145,000 records spanning 602 regions, we identify 522 ant species exhibiting human-mediated spread, doubling recent estimates of their diversity. We show that movements of alien ant species across regions globally are non-random and, moreover, that these flows differ by the extents to which species invade—ranging from arrival to indoor establishment, naturalization, and harmful status. Importantly, we find that almost two-thirds of the 309 species that naturalize globally—most of which are ground- and litter-dwelling—are absent from customs interceptions, which record disproportionately high numbers of arboreal species. Our results illustrate the vast, yet uneven extent of ant invasions globally, and suggest that most alien species bypass biosecurity controls while spreading successfully worldwide. This raises doubts on the efficacy of current customs interceptions procedures and highlights a need for radically new approaches. Significance statement Biological invasions impact humans and ecosystems tremendously. Invasions are difficult to address because little is known about large-scale patterns of spread, species’ capacities to invade ecosystems, and the efficacy of existing biosecurity measures. This paper tackles these issues for alien ants, one of the most damaging groups of invasive animals. An unprecedented dataset reveals that 522 ant species exhibit human-mediated spread, 270 more than previously known. Species are grouped under four levels of invasion capacity corresponding to different invasion barriers. Geographic sources and sinks for the groups differ globally. Two-thirds of species with naturalization capacity have never been recorded at customs interceptions, which fail to detect many litter-dwelling species. Novel detection and control strategies for inconspicuous alien organisms are needed.