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.

Aim To provide the first comparative overview on the current numbers of alien species that invade representative European terrestrial and freshwater habitats for a range of taxonomic groups. Location Europe.Methods Numbers of naturalized alien species of plants, insects, herptiles, birds and mammals occurring in 10 habitats defined according to the European Nature Information System (EUNIS) were obtained from 115 regional data sets. Only species introduced after ad 1500 were considered. Data were analysed by ANCOVA and regression trees to assess whether differences exist among taxonomic groups in terms of their habitat affinity, and whether the pattern of occurrence of alien species in European habitats interacts with macroecological factors such as insularity, latitude or area. ResultsThe highest numbers of alien plant and insect species were found in human-made, urban or cultivated habitats; if controlled for habitat area in the region, wetland and riparian habitats appeared to support relatively high numbers of alien plant species too. Invasions by vertebrates were more evenly distributed among habitats, with aquatic and riparian, woodland and cultivated land most invaded. Mires, bogs and fens, grassland, heathland and scrub were generally less invaded. Habitat and taxonomic group explained most variation in the proportions of alien species occurring in individual habitats related to the total number of alien species in a region, and the basic pattern determined by these factors was fine-tuned by geographical variables, namely by the mainland-island contrast and latitude, and differed among taxonomic groups.Main conclusions There are two ecologically distinct groups of alien species (plants and insects versus vertebrates) with strikingly different habitat affinities. Invasions by these two contrasting groups are complementary in terms of habitat use, which makes an overall assessment of habitat invasions in Europe possible. Since numbers of naturalized species in habitats are correlated among taxa within these two groups, the data collected for one group of vertebrates, for example, could be used to estimate the habitat-specific numbers of alien species for other vertebrate groups with reasonable precision, and the same holds true for insects and plants.

Until now, analytical studies on European urban floras have mostly concentrated on the central and north-western parts of the continent. In this paper, factors determining species richness of urban flora were studied for the city of Rome, Italy, based on a comprehensive floristic survey carried out between 1985 and 1994, and updated in 2005. All species were recorded in grid cells of 1.6 km(2) and classified into native and alien (the latter divided into archaeophytes and neophytes). The grids were classified with respect to the prevailing habitat type, area available to vegetation, level of disturbance and geographical position within the city. Data were analysed using minimal adequate models. Total species number was determined by habitat and its interaction with position on the north-west gradient; other variables explained much less variance. Holding other variables constant, the average species number per grid cell was highest in archaeological sites and parks, followed by woodlands and rivers, and grasslands and recent developments. Residential areas and the historical centre were poorest in species number. Towards the north of the city, species richness in corresponding habitats increases because of higher landscape heterogeneity and closer association with diaspore pools in the surroundings. Native species make up on average 84% of the total species numbers, and trends opposite to those for the total number of species were found for the proportional representation of aliens. The occurrence of alien and native species in the flora of Rome is driven by similar factors, but factors that increase representation of aliens decrease that of natives and vice versa. The representation of neophytes and native species in grid cells was easier to explain (74% of variation accounted for) than that of archaeophytes (27%); this result reflects that in terms of ecology and response to factors, archaeophytes take an intermediate position between native plants and neophytes. Proportional representation of neophytes decreased with increasing area available to vegetation, reflecting that semi-natural vegetation is better developed where less fragmented

Aim We examine how two categories of non-native species (archaeophyte and neophyte, introduced before and after  1500, respectively) have had different impacts on β diversity across European urban floras. Our goal is to use the unique biological perspective provided by urban areas, and the contrasting historical and geographical perspectives provided by archaeophytes and neophytes, to infer how non-native species will impact upon β diversity in the future.Location Twenty-two urban areas located in seven European countries. MethodsWe used the β -sim dissimilarity index to estimate the level of β diversity for 231 unique pair-wise combinations of 22 urban floras. We examined bivariate plots of dissimilarity by geographical separation of city centres to evaluate distance decay of similarity for native species, archaeophytes and neophytes.Results Based on average percentages, 52.8% (SD = 8.2%) of species in the urban floras were identified as non-native with 28.3% (SD = 6.9%) classified as neophytes and 24.5% (SD = 4.9%) as archaeophytes. Relative to native species, across urban floras, archaeophytes were associated with higher compositional similarity and weaker distance decay patterns, whereas neophytes were associated with lower compositional similarity and stronger distance decay patterns.Main conclusions Across European urban floras, archaeophytes and neophytes occurred in similar numbers but archaeophytes were consistently associated with lower β diversity and neophytes with higher β diversity. Thus, the impact of non-native species on β diversity can be determined, at least in part, through their historical and geographical associations with anthropogenic activities. If archaeophytes represent the long-term biogeographical outcome for human commensal species, neophytes could develop similar patterns. The consequences, however, are likely to be more substantial ecologically and geographically due to the increasing numbers of neophytes and their global anthropogenic associations. Nevertheless, at present, our findings suggest that, based on occurrence information, neophytes have not achieved this state with European urban floras retaining regionally distinct assemblages of neophytes.