Global hotspots and correlates of alien species richness across taxonomic groups.

Human-mediated transport beyond biogeographic barriers has led to the introduction and establishment of alien species in new regions worldwide. However, we lack a global picture of established alien species richness for multiple taxonomic groups. Here, we assess global patterns and potential drivers of established alien species richness across eight taxonomic groups (amphibians, ants, birds, freshwater fishes, mammals, vascular plants, reptiles and spiders) for 186 islands and 423 mainland regions. Hotspots of established alien species richness are predominantly island and coastal mainland regions. Regions with greater gross domestic product per capita, human population density, and area have higher established alien richness, with strongest effects emerging for islands. Ants and reptiles, birds and mammals, and vascular plants and spiders form pairs of taxonomic groups with the highest spatial congruence in established alien richness, but drivers explaining richness differ between the taxa in each pair. Across all taxonomic groups, our results highlight the need to prioritize prevention of further alien species introductions to island and coastal mainland regions globally.\ud
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The transport of species across biogeographic barriers by humans is a key component of global environmental change1,2,3. Some of the species introduced to new regions will establish self-sustaining populations and thus become a persistent part of the local biota4. Numbers of these established alien species (EAS) are predicted to increase further as a result of increasing global trade, land-use intensification, urbanization and climate change5. Although patterns of EAS richness have been analysed for particular regions6,7,8 and taxa9,10,11,1213 individually, we still lack a global synthesis across a broad range of taxonomic groups. Such a synthesis will be invaluable for (i) identifying geographical hotspots and coldspots of EAS richness, both across and within taxonomic groups, and for (ii) identifying and assessing potential correlates and drivers of EAS richness across different taxonomic groups\ud
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Here, we assess global patterns and correlates of EAS richness across eight taxonomic groups by integrating comprehensive published (vascular plants12, birds13, fishes14, ants15 and spiders16) and so far unpublished databases (amphibians, mammals and reptiles) (Fig. 1). As a spatial framework, we use the 609 regions (186 islands or archipelagos, and 423 mainland regions) from level 4 of the Biodiversity Information Standards framework (TDWG)17, representing countries or states and provinces within larger countries, and major islands and archipelagos. We identify the global hotspots (high richness) and coldspots (low richness) of EAS across the taxonomic groups while accounting for differences in area and sampling effort. ‘Sampling effort’ consists of published inventory completeness estimates of native species of amphibians, birds and mammals18, vascular plants19 and native genera of ants20 as a proxy (see Methods). We also explore additional macroecological and socioeconomic correlates behind cross-taxon EAS richness patterns.\ud
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We expect regions with higher gross domestic product per capita (GDPpc) or with higher population densities to receive more alien species introductions across taxa (that is, to experience higher colonization pressure through trade and transport), resulting in higher EAS richness7,8,10,21. We also test whether EAS richness patterns follow the latitudinal gradients often observed for native biota, with higher richness in regions with higher mean annual temperature and precipitation22,23. We expect island regions to have higher EAS richness than mainland regions, as islands are thought to be more prone to the establishment of alien species12,24,25. In addition, we expect more isolated oceanic islands to have greater EAS richness, as they have been shown to receive more introductions, at least for birds9. We also expect coastal regions (as points of introduction) to have higher EAS richness than landlocked regions. Finally, we assess the degree of spatial congruence of EAS richness among taxonomic groups and explore the variables that might explain differences in spatial species-richness patterns among groups