Biological invasions have steadily increased over recent centuries. However, we still lack a clear expectation about future trends in alien species numbers. In particular, we do not know whether alien species will continue to accumulate in regional floras and faunas, or whether the pace of accumulation will decrease due to the depletion of native source pools. Here, we apply a new model to simulate future numbers of alien species based on estimated sizes of source pools and dynamics of historical invasions, assuming a continuation of processes in the future as observed in the past (a business-as-usual scenario). We first validated performance of different model versions by conducting a back-casting approach, therefore fitting the model to alien species numbers until 1950 and validating predictions on trends from 1950 to 2005. In a second step, we selected the best performing model that provided the most robust predictions to project trajectories of alien species numbers until 2050. Altogether, this resulted in 3,790 stochastic simulation runs for 38 taxon-continent combinations. We provide the first quantitative projections of future trajectories of alien species numbers for seven major taxonomic groups in eight continents, accounting for variation in sampling intensity and uncertainty in projections. Overall, established alien species numbers per continent were predicted to increase from 2005 to 2050 by 36%. Particularly, strong increases were projected for Europe in absolute (+2,543 ± 237 alien species) and relative terms, followed by Temperate Asia (+1,597 ± 197), Northern America (1,484 ± 74) and Southern America (1,391 ± 258). Among individual taxonomic groups, especially strong increases were projected for invertebrates globally. Declining (but still positive) rates were projected only for Australasia. Our projections provide a first baseline for the assessment of future developments of biological invasions, which will help to inform policies to contain the spread of alien species.

Berlin Brandenburg Institute of Advanced Biodiversity Research Berlin Germany

Bio Protection Research Centre Lincoln University Christchurch New Zealand

Centre for Invasion Biology Department of Botany and Zoology Stellenbosch University Stellenbosch South Africa

Centro de Estudos Geográficos Instituto de Geografia e Ordenamento do Território IGOT Universidade de Lisboa Lisbon Portugal

Chair IUCN Species Survival Commission Invasive Species Specialist Group Rome Italy

Department of Biology Chemistry Pharmacy Institute of Biology Freie Universität Berlin Berlin Germany

Department of Biology University of Fribourg Fribourg Switzerland

Department of Biosciences Durham University Durham UK

Department of Botany and Biodiversity Research University of Vienna Vienna Austria

Department of Community Ecology Helmholtz Centre for Environmental Research UFZ Halle Germany

Department of Ecology Faculty of Science Charles University Prague Czech Republic

Department of Genetics Evolution and Environment Centre for Biodiversity and Environment Research University College London London UK

Department of Invasion Ecology Institute of Botany Czech Academy of Sciences Průhonice Czech Republic

Ecology Department of Biology University of Konstanz Konstanz Germany

Faculty of Forestry and Wood Sciences Czech University of Life Sciences Prague Praha Suchdol Czech Republic

Geobotany and Botanical Garden Martin Luther University Halle Wittenberg Halle Germany

German Centre for Integrative Biodiversity Research Halle Jena Leipzig Leipzig Germany

Institute for Environmental Protection and Research Rome Italy

Institute of Zoology Zoological Society of London London UK

Leibniz Institute of Freshwater Ecology and Inland Fisheries Berlin Germany

School of Natural and Environmental Sciences Newcastle University Newcastle upon Tyne UK

Senckenberg Biodiversity and Climate Research Centre Frankfurt am Main Germany

USDA Forest Service Northern Research Station Morgantown WV USA

Zhejiang Provincial Key Laboratory of Plant Evolutionary Ecology and Conservation Taizhou University Taizhou China

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Management Measures and Trends of Biological Invasions in Europe: A Survey-Based Assessment of Local Managers

Differences in Tri-Trophic Community Responses to Temperature-Dependent Vital Rates, Thermal Niche Mismatches and Temperature-Size Rule

Prioritising non-native fish species for management actions in three Polish rivers using the newly developed tool-dispersal-origin-status-impact scheme

Risk Perception: Chemical Stimuli in Predator Detection and Feeding Behaviour of the Invasive Round Goby Neogobius melanostomus

Plant invasion and naturalization are influenced by genome size, ecology and economic use globally

Potential sources of time lags in calibrating species distribution models

A latitudinal gradient in Darwin's naturalization conundrum at the global scale for flowering plants

Historical plant introductions predict current insect invasions

Unveiling the hidden economic toll of biological invasions in the European Union

The impact of land use on non-native species incidence and number in local assemblages worldwide

Adaptive photosynthetic strategies of the invasive plant Sphagneticola trilobata and its hybrid to a low-light environment

Identifying, reducing, and communicating uncertainty in community science: a focus on alien species

Building a synthesis of economic costs of biological invasions in New Zealand

Global economic costs of herpetofauna invasions

The global loss of floristic uniqueness

Survival, Growth, and Reproduction: Comparison of Marbled Crayfish with Four Prominent Crayfish Invaders