Although research on human-mediated exchanges of species has substantially intensified during the last centuries, we know surprisingly little about temporal dynamics of alien species accumulations across regions and taxa. Using a novel database of 45,813 first records of 16,926 established alien species, we show that the annual rate of first records worldwide has increased during the last 200 years, with 37% of all first records reported most recently (1970–2014). Inter-continental and inter-taxonomic variation can be largely attributed to the diaspora of European settlers in the nineteenth century and to the acceleration in trade in the twentieth century. For all taxonomic groups, the increase in numbers of alien species does not show any sign of saturation and most taxa even show increases in the rate of first records over time. This highlights that past efforts to mitigate invasions have not been effective enough to keep up with increasing globalization.

ambientale, sapienza università di roma, roma, italy; f Dipartimento di scienze e tecnologie Biologiche ed ambientali, università del salento, lecce, italy; g istituto Beni culturali, regione emilia-romagna, Bologna, italy; h centro conservazione Biodiversità (ccB), Dipartimento di scienze della Vita e dell'ambiente (DisVa), università di cagliari, cagliari, italy; i scuola di Bioscienze e medicina Veterinaria, università di camerino, macerata, italy; j Dipartimento di scienze della Vita, università di modena e reggio emilia, modena, italy; k Dipartimento di scienze della terra, dell'ambiente e della Vita (DistaV), università di genova, genova, italy; l Dipartimento di Biologia, ecologia, e scienze della terra (DiBest), università della calabria, cosenza, italy; m Dipartimento di scienze della Vita e Biologia dei sistemi (DBios), università di torino, torino, italy; n comitato scientifico, museo regionale di scienze naturali efisio noussan, aosta, italy; o sezione di Botanica filippo Parlatore, museo di storia naturale, università di firenze, firenze, italy; p Dipartimento di Biologia, università di napoli federico ii, napoli, italy; q Dipartimento di scienze agrarie, alimentari e forestali, università di Palermo, Palermo, italy; r scuola di scienze agrarie, forestali, alimentari ed ambientali, università della Basilicata, Potenza, italy; s strada Val san martino superiore, torino, italy; t centro ricerche floristiche marche, Pesaro, italy; u Dipartimento di Pianificazione, Design, tecnologia dell'architettura (PDta), sapienza università di roma, roma, italy; v Department of Botany, national museum of natural history, smithsonian institution, washington, Dc, usa; w Via isonzo, massa, italy; x Dipartimento di scienze della terra, università di torino, torino, italy; y Via regazzoni Bassa, Padova, italy; z museo di storia naturale della calabria ed orto Botanico, università della calabria, cosenza, italy; aa Dipartimento di scienze della Vita, università di trieste, trieste, italy; ab fondazione museo civico di rovereto, trento, italy; ac sezione di Botanica ed ecologia Vegetale, Dipartimento di scienze e tecnologie Biologiche, chimiche e farmaceutiche (steBicef), università di Palermo, Palermo, italy; ad Dipartimento di scienze agrarie e forestali (Dafne), università della tuscia, Viterbo, italy; ae Via europa unita, schio, italy; af istituto per le Piante da legno e l'ambiente (iPla), torino, italy; ag laboratori di Botanica, Dipartimento di scienze delle Produzioni agroalimentari e dell'ambiente, università di firenze, firenze, italy; ah largo Brigata cagliari, Vercelli, italy; ai Dipartimento di scienze e tecnologie ambientali, Biologiche e farmaceutiche, università della campania luigi Vanvitelli, caserta, italy;

Our ability to predict the identity of future invasive alien species is largely based upon knowledge of prior invasion history. Emerging alien species-those never encountered as aliens before-therefore pose a significant challenge to biosecurity interventions worldwide. Understanding their temporal trends, origins, and the drivers of their spread is pivotal to improving prevention and risk assessment tools. Here, we use a database of 45,984 first records of 16,019 established alien species to investigate the temporal dynamics of occurrences of emerging alien species worldwide. Even after many centuries of invasions the rate of emergence of new alien species is still high: One-quarter of first records during 2000-2005 were of species that had not been previously recorded anywhere as alien, though with large variation across taxa. Model results show that the high proportion of emerging alien species cannot be solely explained by increases in well-known drivers such as the amount of imported commodities from historically important source regions. Instead, these dynamics reflect the incorporation of new regions into the pool of potential alien species, likely as a consequence of expanding trade networks and environmental change. This process compensates for the depletion of the historically important source species pool through successive invasions. We estimate that 1-16% of all species on Earth, depending on the taxonomic group, qualify as potential alien species. These results suggest that there remains a high proportion of emerging alien species we have yet to encounter, with future impacts that are difficult to predict.

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Aim Many Australian Acacia species have been planted around the world, some are highly valued, some are invasive, and some are both highly valued and invasive. We review global efforts to minimize the risk and limit the impact of invasions in this widely used plant group.Location Global.Methods Using information from literature sources, knowledge and experience of the authors, and the responses from a questionnaire sent to experts around the world, we reviewed: (1) a generalized life cycle of Australian acacias and how to control each life stage, (2) different management approaches and (3) what is required to help limit or prevent invasions.Results Relatively few Australian acacias have been introduced in large numbers, but all species with a long and extensive history of planting have become invasive somewhere. Australian acacias, as a group, have a high risk of becoming invasive and causing significant impacts as determined by existing assessment schemes. Moreover, in most situations, long-lived seed banks mean it is very difficult to control established infestations. Control has focused almost exclusively on widespread invaders, and eradication has rarely been attempted. Classical biological control is being used in South Africa with increasing success.Main conclusions A greater emphasis on pro-active rather than reactive management is required given the difficulties managing established invasions of Australian acacias. Adverse effects of proposed new introductions can be minimized by conducting detailed risk assessments in advance, planning for on-going monitoring and management, and ensuring resources are in place for long-term mitigation. Benign alternatives (e.g. sterile hybrids) could be developed to replace existing utilized taxa. Eradication should be set as a management goal more often to reduce the invasion debt. Introducing classical biological control agents that have a successful track-record in South Africa to other regions and identifying new agents (notably vegetative feeders) can help mitigate existing widespread invasions. Trans-boundary sharing of information will assist efforts to limit future invasions, in particular, management strategies need to be better evaluated, monitored, published and publicised so that global best-practice procedures can be developed.