# Biological invasions: a global assessment of geographic distributions, long-term trends, and data gaps

Hanno Seebens, Laura A Meyerson, David M Richardson, Bernd Lenzner, Elena Tricarico, Franck Courchamp, Alla Aleksanyan, Emre Keskin, Hanieh Saeedi, Perpetra Akite, et al.

► To cite this version:

Hanno Seebens, Laura A Meyerson, David M Richardson, Bernd Lenzner, Elena Tricarico, et al.,

Biological invasions: a global assessment of geographic distributions, long-term trends, and data gaps.

Biological Reviews, 2025, 100, pp.2542–2583. 10.1111/brv.70058. hal-05397064

HAL Id: hal-05397064

https://hal.science/hal-05397064v1

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Biol. Rev. (2025), **100**, pp. 2542–2583.
doi: 10.1111/brv.70058

2542

Biological invasions: a global assessment of
geographic distributions, long-term trends,
and data gaps

Hanno Seebens<fnref n="1" />,<fnref n="2" />,*, Laura A. Meyerson<fnref n="3" />, David M. Richardson<fnref n="4" />,<fnref n="5" />, Bernd Lenzner<fnref n="6" />, Elena Tricarico<fnref n="7" />, Franck Courchamp<fnref n="8" />, Alla Aleksanyan<fnref n="9" />,<fnref n="10" />,<fnref n="11" />, Emre Keskin<fnref n="12" />, Hanieh Saeedi<fnref n="13" />,¹⁴, Perpetra Akite¹⁵, Jake M. Alexander¹⁶, Sarah A. Bailey¹⁷, Dino Biancolini¹⁸,¹⁹, Tim M. Blackburn²⁰,²¹, Hans Juergen Boehmer²²,²³, Alejandro Bortolus²⁴, Marc W. Cadotte²⁵, César Capinha²⁶,²⁷, James T. Carlton²⁸, Jo Anne Crouch²⁹, Curtis C. Daehler³⁰, Franz Essl<fnref n="6" />, Llewellyn C. Foxcroft<fnref n="5" />,³¹, Jason D. Fridley³², Nicol Fuentes³³, Mirijam Gaertner<fnref n="5" />,³⁴, Bella Galil³⁵, Emili García-Berthou³⁶, Pablo García-Díaz³⁷,³⁸, Sylvia Haider³⁹, Liam Heneghan⁴⁰, Kevin A. Hughes⁴¹, Cang Hui⁴²,⁴³, Ekin Kaplan<fnref n="6" />, Andrew M. Liebhold⁴⁴,⁴⁵, Chunlong Liu⁴⁶, Elizabete Marchante⁴⁷, Hélia Marchante⁴⁸, Alicia Marticorena⁴⁹, David W. Minter⁵⁰, Rodrigo A. Moreno⁵¹, Wolfgang Nentwig⁵², Aidin Niamir<fnref n="2" />, Ana Novoa<fnref n="4" />,⁵³, Ana L. Nunes⁵⁴, Aníbal Pauchard⁵⁵,⁵⁶, Sebataolo Rahlao⁵⁷,⁵⁸, Anthony Ricciardi⁵⁹, James C. Russell⁶⁰, K.V. Sankaran⁶¹, Anna Schertler<fnref n="6" />, Evangelina Schwindt⁶², Ross T. Shackleton<fnref n="5" />,⁶³, Daniel Simberloff⁶⁴, David L. Strayer⁶⁵,⁶⁶, Alifereti Tawake⁶⁷, Marco Thines<fnref n="2" />,⁶⁸, Cristóbal Villaseñor-Parada⁶⁹, Jean Ricardo Simões Vitule⁷⁰, Viktoria Wagner⁷¹, Victoria Werenkraut⁷², Karsten Wesche⁷³, Demian A. Willette⁷⁴, Rafael D. Zenni⁷⁵ and Petr Pyšek<fnref n="4" />,⁷⁶

<footnote n="1">¹Department of Animal Ecology & Systematics, Justus Liebig University Giessen, Heinrich-Buff-Ring 26, Giessen 35392, Germany</footnote>

<footnote n="2">²Senckenberg Biodiversity and Climate Research Centre, Senckenberganlage 25, Frankfurt 60325, Germany</footnote>

<footnote n="3">³Department of Natural Resources Science, University of Rhode Island, Woodward Hall, 9 Easy Alynni Avenue, Kingston, RI 02881, USA</footnote>

<footnote n="4">⁴Institute of Botany, Czech Academy of Sciences, Žánek 1, Průhonice CZ-25243, Czech Republic</footnote>

<footnote n="5">⁵Centre for Invasion Biology, Department of Botany and Zoology, Stellenbosch University, Private Bag X1, Matieland, Stellenbosch 7602, South Africa</footnote>

<footnote n="6">⁶Division of BioInvasions, Macroecology and Global Change, Department of Botany and Biodiversity Research, University Vienna, Rennweg 14, 1030, Vienna, Austria</footnote>

<footnote n="7">⁷Department of Biology, University of Florence, via Madonna del Piano 6, 50019, Sesto Fiorentino (FI), Italy</footnote>

<footnote n="8">⁸CNRS, AgroParisTech, Ecologie, Société & Evolution, Université Paris Saclay, Paris 91190, France</footnote>

<footnote n="9">⁹Department of Geobotany and Ecophysiology, Institute of Botany aft. A. Takhtajyan NAS RA, Acharyan 1, 0063, Yerevan, Armenia</footnote>

<footnote n="10">¹⁰International Scientific Educational Center of NAS RA, Baghramyan 24d, 0019, Yerevan, Armenia</footnote>

<footnote n="11">¹¹Interdisciplinary Research Center of Armenian State University of Economics, Nalbandyan 128, 0025, Yerevan, Armenia</footnote>

<footnote n="12">¹²Evolutionary Genetics Laboratory (eGL), Department of Fisheries and Aquaculture, Ankara University Agricultural Faculty, Ankara 06135, Turkey</footnote>

<footnote n="13">¹³Senckenberg Research Institute and Natural History Museum, Department of Marine Zoology, Biodiversity Information Section, Senckenberganlage 25, 60325, Frankfurt am Main, Germany</footnote>

<footnote>* Author for correspondence (Tel.: +49 (0) 641 99-35726; E-mail: hanno.seebens@allzool.bio.uni-giessen.de).</footnote>
<footnote n="14">¹⁴Goethe University Frankfurt, Department 15 - Life Sciences, Institute for Ecology, Evolution and Diversity, Max-von-Laue-Straße 13, 60438, Frankfurt am Main, Germany</footnote>

<footnote n="15">¹⁵Department of Zoology, Entomology and Fisheries Sciences, Makerere University, Makerere Hill Road, P.O Box 7062, Kampala, Uganda</footnote>

<footnote n="16">¹⁶Institute of Integrative Biology, ETH Zurich, Universitätsstrasse 16, CH-8092, Zurich, Switzerland</footnote>

<footnote n="17">¹⁷Great Lakes Laboratory for Fisheries and Aquatic Sciences, Fisheries and Oceans Canada, 867 Lakeshore Road, Burlington, Ontario L7S 1A1, Canada</footnote>

<footnote n="18">¹⁸National Research Council of Italy, Institute for BioEconomy (CNR-IBE), Via dei Taurini, Rome 19 00118, Italy</footnote>

<footnote n="19">¹⁹IUCN SSC Invasive Species Specialist Group, Viale Cesare Pavese, Rome 305 00144, Italy</footnote>

<footnote n="20">²⁰Centre for Biodiversity and Environment Research, Department of Genetics, Evolution and Environment, University College London, London, WC1E 6BT, UK</footnote>

<footnote n="21">²¹Institute of Zoology, Zoological Society of London, London, NW1 4RY, UK</footnote>

<footnote n="22">²²Geobotany Section, Institute of Earth System Sciences, Leibniz University Hannover, Nienburger Strasse 17, 30167, Hannover, Germany</footnote>

<footnote n="23">²³School of Geography, Earth Science & Environment, University of the South Pacific, Suva, Fiji</footnote>

<footnote n="24">²⁴Instituto Patagónico para el Estudio de los Ecosistemas Continentales (IPEC-CONICET), Puerto Madryn 9120, Chubut, Argentina</footnote>

<footnote n="25">²⁵Department of Biological Sciences, University of Toronto Scarborough, 1265 Military Trail, Toronto, Ontario M1C 1A4, Canada</footnote>

<footnote n="26">²⁶Centre of Geographical Studies, Institute of Geography and Spatial Planning, University of Lisbon, R. Branca Edmée Marques, Lisbon 1600-276, Portugal</footnote>

<footnote n="27">²⁷Associate Laboratory Terra, Tapada da Ajuda, Lisbon 1349-017, Portugal</footnote>

<footnote n="28">²⁸Coastal & Ocean Studies Program, Williams College-Mystic Seaport, 75 Greenmanville Ave, Mystic, CT 06355, USA</footnote>

<footnote n="29">²⁹United States Dept. of Agriculture, Agricultural Research Service, Foreign Disease/Weed Science Research Unit, 1301 Ditto Avenue, Fort Detrick, MD 21702, USA</footnote>

<footnote n="30">³⁰School of Life Sciences, University of Hawaii at Manoa, Honolulu, HI 96822, USA</footnote>

<footnote n="31">³¹Scientific Services, South African National Parks, Private Bag X402, Skukuza, 1350, South Africa</footnote>

<footnote n="32">³²Department of Biological Sciences, Clemson University, 132 Long Hall, Clemson, South Carolina 29634, USA</footnote>

<footnote n="33">³³Departamento de Botánica, Facultad de Ciencias Naturales y Oceanográficas, Universidad de Concepción, Casilla 160-C, 4030000, Concepción, Chile</footnote>

<footnote n="34">³⁴Nürtingen–Geislingen University, Neckarsteige 6-10, 72622, Nürtingen, Germany</footnote>

<footnote n="35">³⁵The Steinhardt Museum of Natural History, Israel National Center for Biodiversity Studies, Tel Aviv University, Klausner 12, Tel Aviv 69978, Israel</footnote>

<footnote n="36">³⁶GRECO, Institute of Aquatic Ecology, University of Girona, Maria Aurèlia Capmany i Farnés 69, Girona 17003, Spain</footnote>

<footnote n="37">³⁷School of Biological Sciences, Zoology building, University of Aberdeen, Tillydrone Avenue, Aberdeen, AB24 2TZ, UK</footnote>

<footnote n="38">³⁸Instituto de Ecología Regional (IER), Universidad Nacional de Tucumán (UNT) - Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), 4107 Yerba Buena, Tucumán, Argentina</footnote>

<footnote n="39">³⁹Institute of Ecology, Leuphana University of Lüneburg, Universitätsallee 1, Lüneburg 21335, Germany</footnote>

<footnote n="40">⁴⁰Department of Environmental Science and Studies, DePaul University, 1110 W. Belden Avenue, Chicago, IL 60614, USA</footnote>

<footnote n="41">⁴¹British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Road, Cambridge, CB30ET, UK</footnote>

<footnote n="42">⁴²Centre for Invasion Biology, Department of Mathematical Sciences, Stellenbosch University, Stellenbosch 7602, South Africa</footnote>

<footnote n="43">⁴³National Institute for Theoretical and Computational Sciences, African Institute for Mathematical Sciences, Cape Town 7945, South Africa</footnote>

<footnote n="44">⁴⁴Faculty of Forestry and Wood Sciences, Czech University of Life Sciences Prague, Praha 6, Suchdol, Czech Republic</footnote>

<footnote n="45">⁴⁵USDA Forest Service Northern Research Station, Morgantown, WV 26505, USA</footnote>

<footnote n="46">⁴⁶The Key Laboratory of Mariculture, Ministry of Education, College of Fisheries, Ocean University of China, 5 Yushan Road, Qingdao, Shandong Province 266005, People's Republic of China</footnote>

<footnote n="47">⁴⁷Centre for Functional Ecology, Associate Laboratory TERRA, Department of Life Sciences, University of Coimbra, Calçada Martim de Freitas, Coimbra 3000-456, Portugal</footnote>

<footnote n="48">⁴⁸Research Centre for Natural Resources Environment and Society (CERNAS), Polytechnic Institute of Coimbra, Coimbra Agriculture School, Bencanta, Coimbra 3045-601, Portugal</footnote>

<footnote n="49">⁴⁹Herbario CONC, Departamento de Botánica, Facultad de Ciencias Naturales y Oceanográficas, Universidad de Concepción, Casilla 160-C, Concepción, Chile</footnote>

<footnote n="50">⁵⁰CABI, Bakeham Lane, Egham, Surrey, TW20 9TY, UK</footnote>

<footnote n="51">⁵¹Fundación Tortumar-Chile, Serrano 1112, PC 1100606, Iquique, Chile</footnote>

<footnote n="52">⁵²Institute of Ecology and Evolution, University of Bern, Baltzerstrasse 6, Bern 3012, Switzerland</footnote>

<footnote n="53">⁵³Estación Experimental de Zonas Áridas (EEZA-CSIC), Carretera Sacramento s/n, Almería 04120, Spain</footnote>

<footnote n="54">⁵⁴IUCN, The David Attenborough Building, Pembroke Street, Cambridge, CB2 3QZ, UK</footnote>

<footnote n="55">⁵⁵Laboratorio de Invasiones Biológicas (LIB), Facultad de Ciencias Forestales, Universidad de Concepción, Victoria, Concepción 631, Chile</footnote>
⁵⁶Institute of Ecology and Biodiversity (IEB), Victoria, Concepción 631, Chile

⁵⁷Oceanographic Research Institute, 1 King Shaka Avenue, Durban, KwaZulu-Natal, South Africa

⁵⁸School of Life Sciences, University of KwaZulu Natal, Westville campus, University Road, Westville, Durban 3629, South Africa

⁵⁹Department of Biology, McGill University, 1205 Docteur Penfield, Montreal, Quebec H3A 1B1, Canada

⁶⁰School of Biological Sciences, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand

⁶¹Kerala Forest Research Institute, Peechi 680 653, Kerala, India

⁶²Instituto de Biologia de Organismos Marinos (IBIOMAR-CONICET), Blvd. Brown 2915, Puerto Madryn 9120, Argentina

⁶³Swiss Federal Institute for Forest, Snow and Landscape Research WSL, Zürcherstrasse 111, Birmensdorf 8903, Switzerland

⁶⁴Department of Ecology and Evolutionary Biology, University of Tennessee, Knoxville, TN 37996, USA

⁶⁵Cary Institute of Ecosystem Studies, 1336 Glen Leven Road, Ann Arbor, MI 48103, USA

⁶⁶Graham Sustainability Institute, 625 E. Liberty Street, University of Michigan, Ann Arbor, MI 48104, USA

⁶⁷Locally Managed Marine Area Network International Trust, 41 Muktaben Place, Vatuwaqa, Suva, Fiji

⁶⁸Goethe University Frankfurt, Department of Biological Sciences, Institute of Ecology, Evolution and Diversity, Max-von-Laue Str. 9, 60438, Frankfurt am Main, Germany

⁶⁹Facultad de Ciencias, Universidad Católica de la Santísima Concepción, Alonso de Ribera 2850, 4090541, Concepción, Chile

⁷⁰Laboratório de Ecologia e Conservação, Setor de Tecnologia, Departamento de Engenharia Ambiental, Universidade Federal do Paraná, Brazil

⁷¹Department of Biological Sciences, University of Alberta, T6G 2E, Edmonton, Alberta, Canada

⁷²Instituto de Investigaciones en Biodiversidad y Medio Ambiente, Consejo Nacional de investigaciones Científicas y Técnicas - Universidad Nacional del Comahue, Río Negro, Argentina

⁷³Senckenberg Museum for Natural History Görlitz, Am Museum 1, 02826, Görlitz, Germany

⁷⁴Biology Department, Loyola Marymount University, Los Angeles, CA 90045, USA

⁷⁵Departamento de Ecologia e Conservação, Instituto de Ciências Naturais, Universidade Federal de Lavras, Lavras, Trevo Rotatório Professor Edmír Sá Santos, s/n, Caixa Postal 3037, Lavras, MG 37200-090, Brazil

⁷⁶Department of Ecology, Faculty of Science, Charles University, Viničná 7, Prague CZ-12844, Czech Republic

## ABSTRACT

Biological invasions are one of the major drivers of biodiversity decline and have been shown to have far-reaching consequences for society and the economy. Preventing the introduction and spread of alien species represents the most effective solution to reducing their impacts on nature and human well-being. However, implementing effective solutions requires a good understanding of where the species are established and how biological invasions develop over time. Knowledge of the status and trends of biological invasions is thus key for guiding research efforts, informing stakeholders and policymakers, for targeted management efforts, and preparing for the future. However, information about the status and trends of alien species is scattered, patchy, and highly incomplete, making it difficult to assess. Published reports for individual regions and taxonomic groups are available, but large-scale overviews are scarce. A global assessment therefore requires a review of available knowledge with careful consideration of sampling and reporting biases. This paper provides a comprehensive global assessment of the status and trends of alien species for major taxonomic groups [Bacteria, Protozoa, Stramenopila, Alveolata, and Rhizaria (SAR), fungi, plants, and animals] for Intergovernmental Panel of Biodiversity and Ecosystem Services (IPBES) regions.

The review provides irrefutable evidence that alien species have been introduced to all regions worldwide including Antarctica and have spread to even the most remote islands. The numbers of alien species are increasing within all taxa and across all regions, and are often even accelerating. Large knowledge gaps exist, particularly for taxonomic groups other than vascular plants and vertebrates, for regions in Africa and Central Asia, and for aquatic realms. In fact, for inconspicuous species, such as Bacteria, Protozoa, and to some degree SAR and fungi, we found records for very few species and regions. Observed status and trends are thus highly influenced by research effort. More generally, it is likely that all lists for alien species of any taxonomic group and region are incomplete. The reported species numbers therefore represent minima, and we can expect additions to all lists in the near future. We identified six key challenges which need to be addressed to reduce knowledge gaps and to improve our ability to assess trends and status of biological invasions.

**Key words:** Neobiota, invasive species, non-native, alien, biogeography, time series, worldwide, future projections, knowledge gaps, IPBES.
CONTENTS

<table><tbody><tr><td>I. Introduction</td><td>2546</td></tr><tr><td>II. Methods</td><td>2546</td></tr><tr><td>III. Acknowledgement of incomplete knowledge and data</td><td>2547</td></tr><tr><td>IV. A global overview</td><td>2548</td></tr><tr><td>(1) Historical trends and current status</td><td>2548</td></tr><tr><td>(2) Biases and information availability</td><td>2548</td></tr><tr><td>V. Africa</td><td>2551</td></tr><tr><td>(1) Bacteria and Protozoa</td><td>2551</td></tr><tr><td>(2) SAR</td><td>2551</td></tr><tr><td>(3) Fungi</td><td>2551</td></tr><tr><td>(4) Plantae</td><td>2551</td></tr><tr><td>(a) Historical trends</td><td>2551</td></tr><tr><td>(b) Current status</td><td>2552</td></tr><tr><td>(5) Animalia</td><td>2553</td></tr><tr><td>(a) Historical trends</td><td>2553</td></tr><tr><td>(b) Current status</td><td>2554</td></tr><tr><td>VI. The americas</td><td>2555</td></tr><tr><td>(1) Bacteria and Protozoa</td><td>2555</td></tr><tr><td>(2) SAR</td><td>2555</td></tr><tr><td>(3) Fungi</td><td>2556</td></tr><tr><td>(4) Plantae</td><td>2556</td></tr><tr><td>(a) Historical trends</td><td>2556</td></tr><tr><td>(b) Current status</td><td>2556</td></tr><tr><td>(5) Animalia</td><td>2558</td></tr><tr><td>(a) Historical trends</td><td>2558</td></tr><tr><td>(b) Current status</td><td>2558</td></tr><tr><td>VII. Asia & the pacific</td><td>2559</td></tr><tr><td>(1) Bacteria and Protozoa</td><td>2559</td></tr><tr><td>(2) SAR</td><td>2559</td></tr><tr><td>(3) Fungi</td><td>2559</td></tr><tr><td>(4) Plantae</td><td>2560</td></tr><tr><td>(a) Historical trends</td><td>2560</td></tr><tr><td>(b) Current status</td><td>2561</td></tr><tr><td>(5) Animalia</td><td>2562</td></tr><tr><td>(a) Historical trends</td><td>2562</td></tr><tr><td>(b) Current status</td><td>2562</td></tr><tr><td>VIII. Europe & Central Asia</td><td>2563</td></tr><tr><td>(1) Bacteria and Protozoa</td><td>2563</td></tr><tr><td>(2) SAR</td><td>2563</td></tr><tr><td>(3) Fungi</td><td>2564</td></tr><tr><td>(4) Plantae</td><td>2564</td></tr><tr><td>(a) Historical trends</td><td>2564</td></tr><tr><td>(b) Current status</td><td>2565</td></tr><tr><td>(5) Animalia</td><td>2566</td></tr><tr><td>(a) Historical trends</td><td>2566</td></tr><tr><td>(b) Current status</td><td>2567</td></tr><tr><td>IX. Antarctica</td><td>2568</td></tr><tr><td>X. Future trends of biological invasions</td><td>2568</td></tr><tr><td>XI. Ways forward to improve assessments on biological invasions</td><td>2569</td></tr><tr><td>XII. Conclusions</td><td>2571</td></tr><tr><td>XIII. Acknowledgements</td><td>2571</td></tr><tr><td>XIV. References</td><td>2572</td></tr></tbody></table>
# I. INTRODUCTION

The introduction of alien and invasive alien species (see definitions in Section II below) can have far-reaching consequences (Pyšek *et al.*, 2020) for the distribution of life on Earth (Leroy *et al.*, 2023; Aulus-Giacosa, Ollier & Bertelsmeier, 2024), biodiversity conservation (Pyšek *et al.*, 2012b) and native species survival (Blackburn, Bellard & Ricciardi, 2019; Bellard, Bernery & Leclerc, 2021), ecosystem functioning (Linders *et al.*, 2019; Pérez, Vilà & Gallardo, 2022), human well-being and plant and animal health (Lazzaro *et al.*, 2018), and the economy (Diagne *et al.*, 2021). The rate of new alien species introductions has risen continuously for centuries (Seebens *et al.*, 2017) and is expected to keep rising (Seebens *et al.*, 2021a). The ongoing accumulation and spread of alien species poses distinct challenges to assessing the current status of biological invasions. Comprehensive and regularly updated assessments of available knowledge about alien species distributions are essential for elucidating the underlying dynamics of biological invasions globally, for assessing current trends and data needs, and for informing and supporting managers and policymakers (Latombe *et al.*, 2017; Meyerson *et al.*, 2022).

The high dynamism of alien species introductions and spread complicates the task of providing up-to-date and accurate assessments of the status of biological invasions across the world. The establishment of new alien species is reported frequently worldwide (Seebens *et al.*, 2021b). For example, the regularly updated alien flora of the Czech Republic reported an increase of the total number of alien plants from 1378 in 2002 to 1576 in 2022 (Pyšek *et al.*, 2022). Fluctuations in alien species numbers result from a multitude of factors, such as the intensity of introductions of new species, environmental changes, or local disturbances (Pyšek *et al.*, 2020). However, changes may also arise from the deployment of new technologies (Meyerson *et al.*, 2022) and effective alien species management and eradication (Simberloff, 2011; Pluess *et al.*, 2012a,b; Spatz *et al.*, 2022). Further uncertainty arises from the highly variable intensity of research and the notorious inconsistency in reporting (Hughes *et al.*, 2021; Meyerson *et al.*, 2022). It is therefore necessary to conduct regular assessments of alien species' distributions, and their temporal trends, to integrate the most comprehensive available information, as well as taking data gaps into account.

Here, we present results of a comprehensive assessment of the historical trends and current distributions of alien species across geographic regions worldwide. This effort builds on the report of the Intergovernmental Panel of Biodiversity and Ecosystem Services of invasive alien species and their control (IPBES, 2023). This IPBES report presents a broad overview and assessment of the current situation of biological invasions, their impacts, management, and policy options. Chapter 2 of the report focuses on the trends and status of alien species (Seebens *et al.*, 2023), which has been updated and revised here by assessing: (i) the levels of invasions by established alien Bacteria,

Protozoa, SAR (monophyletic group consisting of Stramenopila, Alveolata, and Rhizaria; partly considered as Chromista), fungi, plants, and animals across all IPBES regions, both in terms of overall quantitative patterns and individual examples; (ii) the trends in the accumulation of established alien taxa and associated invasion dynamics that have resulted in their current distributions; and (iii) taxonomic and geographic data gaps, where future research efforts should be directed. We provide overviews of the trends and status of established alien species at the global scale, including an assessment of gaps (Section IV), for individual IPBES regions (Sections V–IX), and future trends (Section X), followed by recommendations for improving the level of knowledge (Section XI).

# II. METHODS

We define key terms in this article as used in IPBES (2023) (Table 1). The information provided herein was collated in two ways: through comprehensive literature reviews and by generating a database of alien species occurrences to provide

<table><caption>Table 1. Definitions of key terms following IPBES (2023).</caption><thead><tr><td>Key terms</td><td>Definitions</td></tr></thead><tbody><tr><td>Trends</td><td>Temporal changes in established alien species numbers over decades to centuries.</td></tr><tr><td>Status</td><td>Currently observed and reported distribution patterns of alien species.</td></tr><tr><td>Gap</td><td>Absence of information on alien species occurrences, also called the Wallacean shortfall (Lomolino, 2004). A gap usually arises from low sampling effort, but might also be due to other reasons, such as the lack of accessibility of information, underreporting, unresolved taxonomies, or linguistic barriers.</td></tr><tr><td>Alien species</td><td>Species transported beyond the limits of their native range through human agency and that occur in the new region outside cultivation or captivity. We here refer to 'established' alien species, representing those alien species that form self-sustaining populations in the wild (Richardson et al., 2000). This corresponds to category 'C3' in the framework proposed by Blackburn et al. (2011), and is a synonym of the term 'naturalised' (e.g. Richardson et al., 2000; Pyšek et al., 2017). For the sake of readability, we use 'alien species' in the main text although the numbers and examples refer to established alien species.</td></tr><tr><td>Invasive alien species</td><td>A subset of established alien species that spread and have a negative impact on biodiversity and local ecosystems following the definition of IPBES (2023) and IUCN (2000). We note that neither spread nor negative impact have agreed-upon quantitative definitions.</td></tr></tbody></table>
a quantitative basis for assessing the distribution of alien species.

The database underlying this article represents a collection of standardised checklists of alien species for regions worldwide and has been generated by applying a published workflow called ‘Standardisation and Integration of Alien Species distribution data’ (SINAS) (Seebens et al., 2020). This workflow standardises information available in checklists (i.e. lists of species per region) and integrates data into a single combined database. The R code for applying the workflow has been published previously (Seebens et al., 2020); the generation of the final database is thus fully transparent and reproducible. Standardisation included the harmonisation of taxonomic names according to the backbone taxonomy of the Global Biodiversity Information Facility (GBIF, www.gbif.org), the harmonisation of key terms according to Groom et al. (2019), matching the spatial resolution of all checklists, and the standardisation of years of first records according to rules defined in the workflow and outlined in Seebens et al. (2020). Once all the data were standardised, the checklists were combined, and duplicates removed. Conflicting entries from different databases that could not be resolved were retained and are flagged as such in the database.

To ensure transparency and achieve comprehensiveness, we sought published and freely accessible global databases of alien species checklists. Seven databases (Table 2) met these criteria; these together collate more than 4000 individual sources of information including scientific publications, reports, and regional databases. Integrating these sources resulted in the single database underlying this article that is called ‘SINAS database 2.5’. The integration achieved by applying the SINAS workflow to the seven databases resulted in the largest currently available single database of alien species, with 175,980 occurrences in 264 regions of 37,591 established alien species worldwide. The database is freely available (https://doi.org/10.5281/zenodo.10038256) and

follows the data principles of being findable, accessible, interoperable, and reusable (FAIR).

As the spatial delineation of the study, we adopted the classification of IPBES regions and subregions (IPBES, 2021), as used in their recent biodiversity reports (IPBES, 2019, 2023). This classification recognises five IPBES regions (‘Africa’, ‘Asia & the Pacific’, ‘The Americas’, ‘Europe & Central Asia’, and ‘Antarctica’) and 18 subregions (3–5 subregions for each region except Antarctica, which is not subdivided). We do not cover Antarctica in the same detail as the other IPBES regions as it has far fewer reported alien species. We used the IPBES classification for providing overviews in the text and tables, while the maps shown in the main text keep the regional and national categorisation of the underlying databases.

To provide overviews of status and trends, we grouped taxa according to their taxonomic classification following the GBIF backbone taxonomy with two exceptions: We included the group algae, which is a paraphyletic group including red algae (Rhodophyta), green algae (Chlorophyta, Charophyta), cryptophytes (Chryptophyta), and haptophytes (Haptophyta). The GBIF taxonomy includes the kingdom Chromista, which is an outdated classification. Instead, we used the supergroup SAR, which is a monophyletic group consisting of Stramenopila, Alveolata, and Rhizaria.

### III. ACKNOWLEDGEMENT OF INCOMPLETE KNOWLEDGE AND DATA

We acknowledge the likelihood that none of the alien species checklists used here can be considered complete or unbiased. Reasons for the lack of data include low sampling effort, lack of detection, underreporting, inaccessible literature and reports, lack of expertise, inconsistent use of definitions

<table><caption>Table 2. Databases of alien species used for generating the database underlying the figures and tables herein. In these databases, regions correspond to countries, administrative units in case of large countries, and islands, and a ‘regional record’ denotes a record of an established alien species in such a region.</caption><thead><tr><td>Database</td><td>Content used here</td><td>Citation and source</td></tr></thead><tbody><tr><td>Global Naturalised Alien Flora (GloNAF)</td><td>Regional records of alien vascular plants</td><td>van Kleunen et al. (2019) https://idata.idiv.de/DDM/Data/ShowData/257</td></tr><tr><td>Global Avian Invasions Atlas (GAVIA)</td><td>Regional records of alien birds</td><td>Dyer et al. (2017b) https://doi.org/10.1038/sdata.2017.41</td></tr><tr><td>Distribution of Alien Mammals (DAMA)</td><td>Regional records of alien mammals</td><td>Biancolini et al. (2021) https://doi.org/10.6084/m9.fishshare.13014368</td></tr><tr><td>Alien amphibians and reptiles</td><td>Regional records of alien amphibians and reptiles</td><td>Capinha et al. (2017) https://doi.org/10.1111/ddi.12617</td></tr><tr><td>MacroFungi</td><td>Regional records of alien macrofungi</td><td>Monteiro et al. (2020) https://doi.org/10.15468/2qky1q</td></tr><tr><td>Alien Species First Records (FirstRecords)</td><td>First records of alien species in regions across taxonomic groups</td><td>Seebens et al. (2017) https://doi.org/10.5281/zenodo.4632335</td></tr><tr><td>GRIIS</td><td>Regional records of alien and invasive alien species across taxonomic groups</td><td>Pagad et al. (2022) https://doi.org/10.5281/zenodo.6348164</td></tr></tbody></table>
and concepts, time lags in the process of biological invasions, delays in reporting, taxonomic uncertainty, and language barriers, among others. Consequently, species numbers reported here are conservative minima, and actual numbers of alien species are likely higher in all cases.

Knowledge and data availability varied highly across taxa, regions, and over time. Data were especially scarce for certain taxonomic groups, particularly for invertebrates, fungi, SAR, protozoans, and prokaryotes. Maps and time series for these groups were highly influenced by the availability of reports rather than true distributions. Furthermore, while much information is available for some regions of the world, it is largely lacking for others, which influenced our presentation of status and trends. While we tried our best to provide information for all groups and regions, sections inevitably vary in comprehensiveness. We provided more detailed evaluations, including figures, only for data-rich groups and regions, but could not cover all taxonomic groups and regions in the same detail. We therefore assessed the reported numbers and distributions of species in the context of data availability. We included a section on knowledge gaps (Section IV.2), where we provided an overview of knowledge gaps across regions and taxonomic groups, but to avoid repetition, we did not address knowledge gaps for each region–taxon combination.

# IV. A GLOBAL OVERVIEW

## (1) Historical trends and current status

The number of alien species records has increased since 1700 CE consistently across all taxonomic groups and regions (Fig. 1). Between 1700 and 1850, the number of alien species was comparatively low and increased only slightly for all taxa. Numbers of alien species records rose markedly during the 19th century, and that trend continues today. The onset of the acceleration in alien species numbers varied slightly among taxonomic groups with a tendency of earlier onsets for mammals, birds, and vascular plants, and a later acceleration for invertebrates (Fig. 1). However, this may partly be an effect of lower research intensity and data availability for the latter group, which resulted in more delayed detections (Seebens et al., 2017; Muñoz-Mas et al., 2023). The time series of alien species numbers were very similar for all IPBES regions except Africa, which shows lower values, perhaps in part due to lower data availability. In addition to cumulative numbers, the rates of newly recorded alien species have risen continuously for all taxonomic groups and nearly all regions except for mammals, which peaked around 1950 (Fig. 1). The declining trend in rates of alien mammal introductions is likely due to more stringent regulation of trade, their higher detectability compared to smaller organisms, the comparatively small pool of potential candidate species, and some successful eradications (Simberloff et al., 2013; Seebens et al., 2017).

Currently observed numbers of alien species vary across IPBES regions, with the highest numbers recorded for Europe & Central Asia followed by The Americas, Asia & the Pacific, and Africa (Fig. 2, Table 3). Countries with particularly high numbers of alien species include the USA and Australia, which are also geographically large areas, and New Zealand, France, and the UK. Hawaii stands out as being a group of remote islands with invasion levels comparable to countries, such as Japan or Belgium, which are distinctly larger in area.

Despite the larger area, Asia & the Pacific harbour similar numbers of alien amphibians and reptiles to Europe & Central Asia (Table 3), which is possibly a result of stringent biosecurity measures in some areas such as Australia, New Zealand, and Japan (Brenton-Rule, Barbieri & Lester, 2016; Chapple et al., 2016; García-Díaz et al., 2017; Toomes et al., 2020). Uneven sampling and reporting likely affect the reported total numbers of alien taxa, particularly in Africa, South America, Central Asia & the Pacific (Henriksen et al., 2024). In addition, articles not written in English and grey literature were more difficult for us to include in our analyses, resulting in a bias towards English sources.

Using comprehensive and well-curated taxon-specific databases (Table 2), we calculated the range of species numbers for mammals, birds, and vascular plants in the individual taxon-specific databases compared to the full database (Table 3). The differences in numbers are high with nearly 30% more species reported in the full database compared to the taxon-specific ones. The difference among databases is likely an effect of using different criteria for adding species by, for example, selecting information that is more or less robust. Thus, absolute numbers of species should be treated with caution and considered as estimates and often minimum numbers of true alien species levels.

Across taxonomic groups, vascular plants are by far the largest contributors to global alien species numbers, followed by insects and fishes (Table 3). For many taxonomic groups, all regions except Africa report similar numbers of established alien species. For instance, the numbers of vascular plant species reported for The Americas, Asia & the Pacific, and Europe & Central Asia are similar, while the number for Africa is much lower. Similar patterns are observed for alien bird, fish, and mammal species. By contrast, algae show a different pattern, with Europe & Central Asia harbouring the highest alien species numbers, followed by Asia & the Pacific, The Americas, and Africa. However, some of the observed patterns are certainly influenced by variation in survey intensity and availability of information around the world.

## (2) Biases and information availability

Patterns of the distribution of alien species are influenced by the uneven sampling of alien species occurrences across the globe and the uneven availability of information. For example, hotspots of alien species occurrences (i.e. areas of high
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Trends in numbers of established alien species for Intergovernmental Panel of Biodiversity and Ecosystem Services (IPBES) world regions. Panels by taxon show cumulative numbers (left panels, thick lines) and numbers of new alien species per five-year intervals (right panels, thin lines). Lines in right panels indicate smoothed trends calculated as running medians. Note that the range of the y-axes differs among panels.</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Numbers of established alien species per region. Species from all realms (marine, freshwater, and terrestrial) were considered if they could be assigned to one of the regions used in this study. Species only recorded for the open sea are not included in this map. Note that numbers may deviate from those reported in the text due to variation among data sources.</figcaption></figure>
alien species numbers relative to other regions; Dawson *et al.*, 2017) coincide with global hotspots of data availability and study sites (Martin, Blossey & Ellis, 2012; Meyer *et al.*, 2015), which influence our knowledge of species distributions (Hughes *et al.*, 2021). This conclusion is confirmed by the information provided here: mapping the number of available studies used to generate the underlying database of this study revealed that regions with the most information on alien species occurrences (Fig. 3) were also the hotspots of alien species occurrences (Fig. 2). Hence, knowledge of alien species occurrences is biased

towards well-sampled regions, such as Europe and North America, and taxonomic groups, such as vertebrates and plants, with most studies conducted in recent decades (Pyšek *et al.*, 2008; Jeschke *et al.*, 2012; Bellard & Jeschke, 2016). It remains unclear how much the distribution of alien species and documented hotspots are affected by spatial variation in research intensity and data availability, and how much represents the ‘true’ distributional pattern of alien species. Such biases need to be taken into consideration when assessing the trends and status of alien species (McGeoch *et al.*, 2023).

<table><caption>Table 3. Numbers of established alien species for the Intergovernmental Panel of Biodiversity and Ecosystem Services (IPBES) regions. The numbers are extracted from the SInAS database (see Section II) and may deviate from those reported in regional studies. The numbers should be considered as minimum values as the true level of invasion is likely higher. For mammals, birds, and vascular plants, ranges of values indicate variation among databases. Duplicated taxa for the same region and Totals were removed. SAR, Stramenopila, Alveolata, Rhizaria.</caption><thead><tr><th></th><th>Africa</th><th>The Americas</th><th>Asia & the Pacific</th><th>Europe & Central Asia</th><th>Totals</th></tr></thead><tbody><tr><td>Mammals</td><td>30–80</td><td>83–164</td><td>97–163</td><td>72–164</td><td>197–368</td></tr><tr><td>Birds</td><td>121–133</td><td>249–287</td><td>287–336</td><td>221–630</td><td>495–877</td></tr><tr><td>Fishes</td><td>187</td><td>803</td><td>633</td><td>469</td><td>1451</td></tr><tr><td>Reptiles</td><td>158</td><td>192</td><td>103</td><td>98</td><td>411</td></tr><tr><td>Amphibians</td><td>12</td><td>62</td><td>43</td><td>43</td><td>135</td></tr><tr><td>Insects</td><td>344</td><td>2636</td><td>2017</td><td>2747</td><td>6795</td></tr><tr><td>Arachnids</td><td>94</td><td>207</td><td>129</td><td>289</td><td>500</td></tr><tr><td>Molluscs</td><td>142</td><td>255</td><td>261</td><td>584</td><td>826</td></tr><tr><td>Crustaceans</td><td>125</td><td>248</td><td>158</td><td>563</td><td>813</td></tr><tr><td>Vascular plants</td><td>3109–4498</td><td>8005–9325</td><td>6141–9101</td><td>5146–8519</td><td>13,081–18,543</td></tr><tr><td>Algae</td><td>40</td><td>55</td><td>66</td><td>212</td><td>270</td></tr><tr><td>Bryophytes</td><td></td><td>61</td><td>44</td><td>37</td><td>118</td></tr><tr><td>Fungi</td><td>122</td><td>363</td><td>363</td><td>609</td><td>1149</td></tr><tr><td>SAR</td><td>22</td><td>150</td><td>103</td><td>373</td><td>534</td></tr><tr><td>Bacteria and protozoans</td><td>4</td><td>14</td><td>12</td><td>23</td><td>38</td></tr><tr><td>Totals</td><td>4510–5961</td><td>13,383–14,822</td><td>10,457–13,532</td><td>11,486–15,360</td><td>26,813–32,828</td></tr></tbody></table>

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Research intensity and data gaps for global alien species distribution records. Research intensity is indicated by the number of studies available for individual regions as listed in the database of this study. Islands are indicated by circles. Circle sizes increase with increasing numbers of studies.</figcaption></figure>
There are conspicuous areas of low research intensity (i.e. fewer studies), particularly over large parts of Africa, Asia, and Antarctica (Fig. 3). Some islands or archipelagos have been thoroughly studied, while information is scarce for many small and remote islands. In addition to regional biases, research intensity varies across taxonomic groups. Considerably more information is available on the distribution of alien vertebrates, particularly mammals and birds, and vascular plants than for other taxa. The reasons for these knowledge gaps include undersampling, underdeveloped taxonomies, incomplete knowledge about invasion status and distributions of alien species (Hughes et al., 2021; Carlton & Schwindt, 2024). Very little information is available for alien microorganisms or small invertebrates (e.g. Annelida, Porifera, or Nematoda) worldwide and recorded distributions often reflect the availability of studies rather than true species distributions. Moreover, even when such information is available, it is often highly incomplete.

## V. AFRICA

### (1) Bacteria and Protozoa

Among alien prokaryotes and protozoans, most records represent agents of infectious diseases either in humans or plants. *Vibrio cholerae* (cholera) is a famous example of alien Bacteria and one of the few prokaryotes with good distributional data. This species, originating from South India, was introduced to Africa in the early 19th century and is considered established in several African countries. As another example, the plant pathogen *Erwinia amylovora* (fire blight) was reported for Egypt in the 1960s and other mostly North African countries (Vanneste, 2008). No records of alien protozoans exist for Africa in the SInAS database.

### (2) SAR

Altogether, 22 species of SAR have been reported in the SInAS database (Table 4), including eight species from South Africa, five from Egypt, and three from Libya. A majority of these are aquatic species, such as Phaeophyceae (brown algae), Tubothalamea (Foraminifera), and Dinophyceae (dinoflagellates). The ciliate *Microfoliculina limnoriae* is reported as being alien for South Africa (Mead et al., 2011) and two species of the genus *Sargassum* (brown algae) were found in Sierra Leone (Norman et al., 2021). First records are often missing, but two of the earliest records of alien SAR in Africa stem from Libya with *Padina boryana* (brown alga), reported in 1974 (Zenetos et al., 2022) and the dinoflagellate *Dinophysis acuminata* in 1991 in South Africa (Mead et al., 2011). Several oomycete species, especially of the genus *Phytophthora*, have been recorded across Africa. For example, *P. infestans* (potato blight) was reported first in 1941 in Kenya, from where it spread to other African countries (Njoroge et al., 2019). Other species of that genus widespread in Africa are *P. cinnamomi* (root rot), *P. nicotianae* (black shank), and *P. palmivora* (coconut

budrot) (Barwell et al., 2020). *Plasmopara viticola*, the downy mildew of grapevine, was introduced from North America to other wine-producing regions of the world and recorded in South Africa for the first time in 1907 (Koopman et al., 2007).

### (3) Fungi

A comparatively low number of 122 alien fungal species is available for Africa according to the SInAS database, while true numbers are likely much higher (Table 4). One of the earliest reported introductions to Africa was *Armillaria mellea* (honey fungus) in South Africa (Cape Town region), which presumably was introduced in the early 17th century by European settlers (Coetzee et al., 2001). The plant pathogen *Claviceps africana* (ergot) was first reported in Kenya in 1924 and spread particularly in the 1980s, since when it has been recorded from several sub-Saharan countries from Ghana to Lesotho (CABI, 2009). *Batrachochytrium dendrobatidis* (chytrid fungus), likely originating from Asia, has caused amphibian declines worldwide, including in Africa (Scheele et al., 2019). The trend of recording new alien fungi increased until the 1980s, while new detections of alien fungal plant pathogens in Africa have been declining more recently relative to reported increases in other regions of the world (Waage et al., 2008). This is likely a consequence of low research effort rather than true decline, because new records of alien fungi are increasing worldwide (Seebens et al., 2017; Sandvik et al., 2019; Fuentes et al., 2020).

In South Africa, nine alien fungal species are known to infect native plants, while 23 host-specific fungi of alien plant species have likely been introduced together with their hosts (Wood, 2017). In addition, 11 alien saprotrophic species, and 61 species of alien fungi forming ectomycorrhizae have been reported (Wood, 2017). Furthermore, seven host-specific alien pathogens have been introduced for the biological control of invasive alien plants (Wood, 2017). *Ceratocystis fimbriata* (ceratocystis blight) was found as an alien plant pathogen in several countries of Central and Southern Africa (CABI, 2022).

Compared to other regions of the world, Africa, with 107 species, has the lowest number of known alien macrofungi (Monteiro et al., 2020), possibly because of lower research intensity. Of these, 40% of species belong to Agaricales, 29% to Boletales, and 13% to Russulales. The most widespread macrofungal species are *Pyrrhoderma noxium*, *Amanita muscaria* (fly agaric), *Pisolithus albus* (white dye-ball fungus), *Rhizopogon luteolus* (yellow false truffle), and *Suillus granulatus* (weeping bolete mushroom), recorded in eight or more countries. The highest numbers of alien macrofungi are reported for South Africa (65), Tanzania (25), Morocco (10), and Kenya (10).

### (4) Plantae

#### (a) Historical trends

The number of alien plant species in Africa has increased continually for centuries as reported for multiple African
<table><caption>Table 4. Numbers of established alien species for subregions of Africa. The numbers are extracted from the SInAS database (see Section II) and may deviate from those reported in regional studies. The numbers should be considered as minimum values as the true level of invasion is likely much higher. For mammals, birds, and vascular plants, ranges of values indicate variation among databases. Duplicated taxa for the same region and Totals were removed. SAR, Stramenopila, Alveolata, Rhizaria.</caption><thead><tr><th></th><th>Central Africa</th><th>East Africa and adjacent islands</th><th>North Africa</th><th>Southern Africa</th><th>West Africa</th><th>Totals</th></tr></thead><tbody><tr><td>Mammals</td><td>4–17</td><td>17–35</td><td>5–17</td><td>9–54</td><td>1–9</td><td>30–80</td></tr><tr><td>Birds</td><td>13–16</td><td>77–79</td><td>17–20</td><td>71–74</td><td>14–23</td><td>121–133</td></tr><tr><td>Fishes</td><td>26</td><td>56</td><td>130</td><td>46</td><td>17</td><td>187</td></tr><tr><td>Reptiles</td><td>2</td><td>33</td><td>8</td><td>124</td><td>9</td><td>158</td></tr><tr><td>Amphibians</td><td></td><td>5</td><td>2</td><td>2</td><td>5</td><td>12</td></tr><tr><td>Insects</td><td>33</td><td>143</td><td>71</td><td>227</td><td>48</td><td>344</td></tr><tr><td>Arachnids</td><td>9</td><td>29</td><td>10</td><td>70</td><td>11</td><td>94</td></tr><tr><td>Molluscs</td><td>2</td><td>11</td><td>75</td><td>67</td><td>7</td><td>142</td></tr><tr><td>Crustaceans</td><td>1</td><td>11</td><td>82</td><td>47</td><td>3</td><td>125</td></tr><tr><td>Vascular plants</td><td>880–1071</td><td>1738–2570</td><td>485–1162</td><td>1754–2292</td><td>645–818</td><td>3109–4498</td></tr><tr><td>Algae</td><td>1</td><td>4</td><td>30</td><td>7</td><td>1</td><td>40</td></tr><tr><td>Bryophytes</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Fungi</td><td>19</td><td>44</td><td>18</td><td>82</td><td>9</td><td>122</td></tr><tr><td>SAR</td><td>2</td><td>1</td><td>12</td><td>8</td><td></td><td>22</td></tr><tr><td>Bacteria and protozoans</td><td>1</td><td>2</td><td>1</td><td>2</td><td>1</td><td>4</td></tr><tr><td>Totals</td><td>993–1200</td><td>2171–3023</td><td>946–1638</td><td>2516–3102</td><td>771–961</td><td>4510–5961</td></tr></tbody></table>

countries (Henderson, 2006; Maroyi, 2012; Senan et al., 2012; Brundu & Camarda, 2013; Shaltout et al., 2016). Southern Africa experienced a steady increase in numbers of alien plant species during the entire 19th century, which was the most rapid rise of all African regions; this trend appeared to slow down only towards the end of that century (Fig. 4). By contrast, alien plant numbers in East Africa showed a marked acceleration starting in the final quarter of the 20th century and have not yet slowed down, whereas in North Africa, the numbers of alien plants increased slowly but steadily towards the end of the 19th century. No readily apparent trends could be detected for West Africa. However, trends are certainly affected by data availability and research intensity (Pyšek et al., 2008, 2020; Richardson et al., 2022).

### (b) Current status

For most of Africa, very little information is available on the status of alien plants. Although lists of alien plant species have been published for Algeria, Angola, Eswatini, Chad, Ghana, Lesotho, Madagascar, Namibia, and Zimbabwe (many in the last decade; references in Richardson et al., 2022), only for South Africa is detailed information of the status of species available – with 759 alien plant species (Richardson et al., 2020). The limited data for the rest of the continent summarised in Richardson et al. (2022) suggest that seven other countries harbour over 300 alien plant species: Congo (522), Ethiopia (421), Morocco (410), Mozambique (396), Benin (333), Algeria (328), and Eswatini (315). South Africa also has the highest number of invasive alien species (374). Species that are widely distributed over large parts of the continent include *Lantana camara* (lantana),

*Tithonia diversifolia* (Mexican sunflower), *Pontederia crassipes* (water hyacinth), *Chromolaena odorata* (Siam weed), *Leucaena leucocephala* (leucaena), *Prosopis juliflora* (mesquite), and *Parthenium hysterophorus* (parthenium weed) (Richardson et al., 2022).

Many tree species used in forestry and agroforestry, especially from the genera *Acacia*, *Eucalyptus*, and *Pinus*, have been introduced throughout Africa, and some shrubs and trees, such as *Acacia melanoxylon* (Australian blackwood), *Broussonetia papyrifera* (paper mulberry), and *Callandra houstoniana* (callandra), are well established in many parts of the continent (Richardson et al., 2022). Australian *Acacia* species are actively promoted for agroforestry in several parts of the continent (Richardson, Binggeli & Botella, 2023) and areas at higher altitudes have been heavily invaded by *A. melanoxylon*, *A. mearnsii* (black wattle), *Pinus patula* (Mexican weeping pine), and *P. radiata* (radiata pine). Pines are particularly invasive in the Southwestern mountains of South Africa and *Acacia* species invade many ecosystems in the country (Holmes et al., 2005). *Prosopis juliflora* (mesquite) is common in semi-arid and arid areas of Southern and Eastern Africa. Other tree and shrub invaders with impacts include several *Rubus* (bramble) species and *Biancaea decapetala* (Mysore thorn). *Azadirachta indica* (neem tree), *Lantana camara* (lantana), and *Leucaena leucocephala* (leucaena) are abundant invaders along the coastline of much of Africa, preferring hot and humid conditions. *Chromolaena odorata* (Siam weed) is now common in many countries in Central and Southern Africa, being abundant in open savanna grasslands, woodlands, riparian zones, forest gaps, and edges (Richardson et al., 2022).

La Réunion is estimated to have over 2000 alien plant species, with more than 100 of these classified as invasive, e.g. *Leucaena leucocephala* (leucaena), *Hiptage benghalensis* (hiptage), and *Ulex europaeus* (gorse) (Baret et al., 2006; Soubeyran
<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Trends in numbers of established alien species for Africa. Panels by taxon show cumulative numbers (left panels, thick lines) and numbers of new alien species per five-year intervals (right panels, thin lines). The actual numbers of alien species occurrences are underestimated due to a lack of data. Lines in right panels indicate smoothed trends calculated as running medians. Note that presented numbers may deviate from those reported in the text due to variation among data sources.</figcaption></figure>

et al., 2015). On Socotra, 88 alien plants have been recorded (Senan et al., 2012). A total of 28 major alien aquatic plants has been recorded in African waters, 16 of which are alien to the whole of Africa, and 12 are native to other parts of the continent (Howard & Chege, 2007; Sghaier et al., 2011). A recent review records the existence of 19 alien freshwater plants only in South Africa, mainly introduced through trade and hitchhiking via boating and angling (Hill et al., 2020). In South Africa, the most important alien freshwater macrophyte remains *Pontederia crassipes* (water hyacinth). The marine alien seagrass *Halophila stipulacea* is native to the East African coast, yet is invasive along the Mediterranean coast from Egypt to Tunisia, an early introduction through the Suez Canal (Winters et al., 2020).

(5) **Animalia**

(a) *Historical trends*

The number of alien animal species has increased continu-
ously for all taxonomic groups in all African regions
(Fig. 4), similar to the observed global patterns (Fig. 1).

European acclimatisation societies were very active, particu-
larly in South Africa, and introduced many plant, bird, and
mammal species to ‘improve’ landscape aesthetics to con-
form with European preferences around the mid-1800s
(Osborne, 2000; van Wilgen et al. 2020). Several islands off
the African coasts, such as St. Helena, Mauritius, and La
Réunion, played important strategic roles for international
shipping and ocean crossings during colonial times
(Cheke & Hume, 2008), and consequently have many early
records of species introductions (Fuller & Boivin, 2009). In
São Tomé and Príncipe, species introductions by Europeans
began in the 1470s with 14 alien mammal species established
on São Tomé and 12 on Príncipe (Dutton, 1994). In the
20th century, increasing global trade accelerated alien species
introductions across Africa and, combined with the advent of
the game-farming industry and ecotourism, resulted in a strik-
ing rise in introductions of alien vertebrates and invertebrates
(Picker & Griffiths, 2017; Measey, Hui & Somers, 2020; van
Wilgen et al., 2020; Muñoz-Mas et al., 2023).

For mammals, birds, and insects, documented increases
tended to be higher in East Africa and adjacent islands,
mostly because of many early species introductions on the islands of Mauritius and La Réunion that were not recorded elsewhere. For fishes and crustaceans, particularly high increases of new alien species were recorded in North Africa after 1869 (Fig. 4) when the Suez Canal opened and initiated the Erythraean invasion of marine species (Galil, 2023), a trend that continues until today (Fig. 4). In South Africa, the overall rate of introductions of alien freshwater animals accelerated sharply after 1880 and generally increased over time, with unintentional introductions of invertebrates playing a relevant role (Weyl et al., 2020). Only freshwater fish introductions decreased significantly after the 1950s, probably due to new legislation regulating such introductions, which decreased demand for new angling species (Faulkner, Robertson & Wilson, 2020). In general, the number of invertebrate introductions to South Africa rose over time (Faulkner et al., 2020), this pattern being reported for freshwater (Weyl et al., 2020), terrestrial (Janion-Scheepers & Griffiths, 2020), and marine invertebrate introductions (Robinson, Peters & Brooker, 2020). The observed rise in alien species seems affected by increased sampling intensity: while only four marine alien species were reported before 1900 (Robinson et al., 2020), several other species were likely introduced much earlier (Carlton & Schwindt, 2024).

### (b) Current status

The African continent currently harbours 44 alien mammals (Biancolini et al., 2021), although there is a range of 30–80 reported alien mammals depending on the source (Table 4). These alien species are mainly concentrated along the Western Mediterranean coast, South Africa, and Madagascar. Alien mammals can be found on each group of the 28 island groups in the Western Indian Ocean, with an average richness of five species per island group (Russell et al., 2016). There are 12 invasive alien mammal species on La Réunion and six of them on the nearby Îles Éparses (Russell & Le Corre, 2009). Most alien bird species in Africa are found in the far south of the continent, although *Corvus splendens* (house crow) is distributed from Sudan to South Africa along the East coast. The islands of East Africa are important hubs of alien reptiles and amphibians globally: Mauritius and La Réunion are inhabited by 17 and 15 alien species, respectively (Kraus, 2009; Capinha et al., 2017; Telford, Channing & Measey, 2019).

Although insects have the highest species richness in general, their proportional representation in Table 4 is still less than expected (344 alien insects out of at least 4510 alien species; Table 4), likely due to under-reporting of alien insects in the African continent. South Africa is the only African country with a comprehensive list of alien insect species, comprising 300 out of 571 alien animal species in South Africa (Picker & Griffiths, 2017). Most alien arthropods (including insects) in Africa are terrestrial, with the Hemiptera comprising the largest fraction, followed by the Coleoptera (Janion-Scheepers & Griffiths, 2020). Most insects are introduced accidentally and the Hemiptera are known to

be common contaminants of plants that are shipped internationally for propagation in agriculture, forestry or horticulture (Liebhold et al., 2012). Some of the most impactful insect invaders, which are widely distributed in Africa, include *Spodoptera frugiperda* (fall armyworm) and *Tuta absoluta* (tomato leafminer), both of which have severe impacts on food production in many African countries (Day et al., 2017; Rwomushana et al., 2019).

East Africa and its adjacent islands have the second highest numbers of alien fishes, probably because of introductions in the many lakes of the Rift Valley area, including the three largest, Lakes Victoria, Tanganyika, and Malawi (Craig, 1992; Pitcher & Hart, 1995), and Lake Naivasha (Kenya), where the rates of introductions have increased steadily since the 1950s (Gherardi et al., 2011). Twenty-one alien freshwater fishes have been established in South Africa (Ellender & Weyl, 2014; Weyl et al., 2020), while 16 alien fish species have been introduced in Central Africa (Brooks, Allen & Darwall, 2011) (26 alien fishes have been reported in total, Table 4). In Madagascar, one quarter of the freshwater fish fauna consists of alien species, with 26 alien species present (Šimková et al., 2019). On La Réunion, six species of fish and one decapod crustacean, *Macrobrachium rosenbergii* (giant freshwater prawn) were introduced by 2002, but only four were established by then (Keith, 2002).

Five freshwater alien crayfish have established populations in the wild, of which three have spread widely across Africa: *Procambarus clarkii* (red swamp crayfish), *P. virginalis* (marbled crayfish), and *Cherax quadricarinatus* (redclaw crayfish) (Madzivanzira et al., 2021). Seventy-seven alien freshwater animals, with numbers largely dominated by fishes, molluscs, and crustaceans, are currently established in South Africa, most of which were intentionally introduced (Picker & Griffiths, 2017; Weyl et al., 2020). Many species of alien molluscs have been recorded in African fresh waters, with 14 species of gastropods reported by 2011, some of which were released for the biological control of the intermediate hosts of schistosomiasis (Appleton & Brackenbury, 1998; Appleton, 2003). Only a few alien freshwater bivalves have been recorded in African waters, such as *Corbicula fluminea* (Asian clam) and *Sinanodonta woodiana* (Chinese pond mussel), both probably related to fish stocking (Darwall et al., 2011; Clavero et al., 2012; Mabrouki & Taybi, 2022). Among alien freshwater jellyfish, the cnidarian *Craspedacusta sowerbii* (peach blossom jellyfish) has been recorded in South Africa and Morocco (Oualid et al., 2019; Weyl et al., 2020).

Information about marine alien species is mostly limited to certain coasts, mainly in South Africa and North Africa: high numbers of marine alien species had been reported by 2020 along Mediterranean and Red Sea coasts of Northern African countries, such as Morocco (24), Algeria (41), Tunisia (166), Libya (77), and Egypt (266) (Galanidi et al., 2023). Numbers have since increased to 39 species in Moroccan Mediterranean waters (Mghili et al., 2024). Erythraean species are clearly expanding their distribution westwards along the African coast, with over 60% of alien species recorded in Tunisian waters considered to have been introduced through
the Suez Canal (Ounifi-Ben Amor et al., 2015). A strong positive relationship between the time elapsed since Erythraean species' first record in the Mediterranean and its westward spread calls attention to a considerable invasion debt advancing westwards (Galil et al., 2021). Along the South African coast, which includes two large marine ecosystems, the Agulhas current in the East and the Benguela current in the West (Mead et al., 2011; Robinson et al., 2020), 95 alien marine species have been reported, of which 56 are considered invasive. A variety of taxa are represented, from small protists (e.g. *Mirofolliculina limnoriae*) and dinoflagellates (e.g. *Alexandrium minutum*) to the most conspicuous macroalgae, molluscs, crustaceans, bryozoans, and tunicates. The most common taxa are ascidians (e.g. *Ciona robusta* and *Botryllus schlosseri*; Peters, Sink & Robinson, 2017). Populations of *Diadumene leucolena* (sea anemone) have been recorded from Senegal (Glon et al., 2020).

# VI. THE AMERICAS

## (1) Bacteria and Protozoa

Among Bacteria, 14 alien Proteobacteria have been reported in the SInAS database for The Americas. A well-known case is the introduction of *Vibrio cholerae*, causing cholera in humans, which has been introduced repeatedly and to different regions of The Americas (Colwell, 1996; Louis et al., 2003). The introduction of *V. cholerae* through ballast water was considered the cause of a severe cholera outbreak in South America (Colwell, 1996) but it has also been detected in the Chesapeake Bay, USA (Louis et al., 2003). The introduction of *Yersinia pestis*, the agent of bubonic plague, which has been reported from multiple countries in

South America, is another example. Agents of infectious diseases, such as the variola virus (smallpox virus; now eradicated globally), *Measles morbillivirus* (measles virus), Bacteria causing typhus, and *Vibrio cholerae* (cholera), were introduced through European colonisation mostly accidentally, although in some cases diseases were intentionally introduced to decimate populations of Native Americans (Oldstone, 2020). Other alien prokaryotes represent plant pathogens, including different species of the genus *Xanthomonas* (*X. campestris* and *X. axonopodis*), which have been reported from various countries throughout The Americas. Among protozoans, three species have been recorded as alien in Canada, USA, and Mexico, namely *Haplosporidium nelsoni*, *H. costale*, and *Glugea hertwigi* (Mills et al., 1993; Pederson et al., 2017; Simpson et al., 2018). These species are pathogens of oysters and fish.

## (2) SAR

For The Americas, a total of 150 species of SAR are included in the SInAS database (Table 5). One of the earliest records is the oomycete *Phytophthora cinnamomi* from 1850 in the USA (Aukema et al., 2010). This species is considered as one of the most devastating plant pathogens globally, with a widespread distribution and a broad host range of around 5000 known plant hosts (Hardham & Blackman, 2018). Several other species of the same genus have been reported. For example, the sudden oak death pathogen, *P. ramorum*, has infected ecologically, economically, and culturally important genera in North America including *Quercus* and *Notholithocarpus* spp. (McPherson et al., 2005). *Phytophthora lateralis* caused declines in the Port-Orford-Cedar, endemic to the Eastern USA, with first reports in natural stands from the 1950s (Jung et al., 2018).

<table><caption>Table 5. Numbers of established alien species for subregions of The Americas. The numbers are extracted from the SInAS database (see Section II) and may deviate from those reported in regional studies. The numbers should be considered as minimum values as the true level of invasion is likely much higher. For mammals, birds, and vascular plants, ranges of values indicate variation among databases. Duplicated taxa for the same region and Totals were removed. SAR, Stramenopila, Alveolata, Rhizaria.</caption><thead><tr><th></th><th>Caribbean</th><th>Mesoamerica</th><th>North America</th><th>South America</th><th>Totals</th></tr></thead><tbody><tr><td>Mammals</td><td>35–62</td><td>8–34</td><td>49–95</td><td>25–77</td><td>83–164</td></tr><tr><td>Birds</td><td>110–113</td><td>29–41</td><td>210–211</td><td>53–114</td><td>249–287</td></tr><tr><td>Fishes</td><td>91</td><td>226</td><td>619</td><td>144</td><td>803</td></tr><tr><td>Reptiles</td><td>60</td><td>60</td><td>121</td><td>56</td><td>192</td></tr><tr><td>Amphibians</td><td>20</td><td>8</td><td>41</td><td>16</td><td>62</td></tr><tr><td>Insects</td><td>153</td><td>163</td><td>2116</td><td>640</td><td>2636</td></tr><tr><td>Arachnids</td><td>33</td><td>36</td><td>168</td><td>76</td><td>207</td></tr><tr><td>Molluscs</td><td>26</td><td>60</td><td>212</td><td>68</td><td>255</td></tr><tr><td>Crustaceans</td><td>10</td><td>64</td><td>173</td><td>79</td><td>248</td></tr><tr><td>Vascular plants</td><td>1402–1761</td><td>1600–2242</td><td>6571–7424</td><td>2492–3099</td><td>8005–9325</td></tr><tr><td>Algae</td><td>2</td><td>14</td><td>32</td><td>21</td><td>55</td></tr><tr><td>Bryophytes</td><td></td><td></td><td>40</td><td>32</td><td>61</td></tr><tr><td>Fungi</td><td>17</td><td>15</td><td>174</td><td>219</td><td>363</td></tr><tr><td>SAR</td><td>4</td><td>93</td><td>40</td><td>34</td><td>150</td></tr><tr><td>Bacteria and protozoans</td><td>1</td><td>4</td><td>6</td><td>5</td><td>14</td></tr><tr><td>Totals</td><td>1964–2353</td><td>2380–3060</td><td>10,572–11,472</td><td>3960–4680</td><td>13,383–14,822</td></tr></tbody></table>
Other widespread SAR are marine species, such as the brown algae *Sargassum muticum* (wireweed), *S. fluitans*, and *Undaria pinnatifida*, or the dinoflagellate *Gymnodinium catenatum* (Williams, 2007; Simpson *et al.*, 2018). A high number of 94 alien SAR was reported in Mexico according to the GRIIS database (González Martínez *et al.*, 2020), followed by the USA (Simpson *et al.*, 2018), and Argentina (Zalba *et al.*, 2021). For a few countries comprehensive assessments of alien species have been conducted, which also list SAR. For example, five marine SAR have been reported for Chile (Fuentes *et al.*, 2020) and 12 for the USA (Simpson *et al.*, 2018), most of them being members of Ochrophyta (brown algae).

### (3) Fungi

While it is difficult to trace its initial introduction date, one early introduction was black stem rust of wheat (*Puccinia graminis*) with an epidemic in the USA in 1878. A colony in Rio Grande do Sul, Brazil, reported yield losses due to stem rust already in the 1700s (CABI, 2021). Earlier introductions of other fungal species likely occurred unnoticed. First reports of a cacao disease matching the infestation by *Moniliophthora roreri* (frosty pod rot of cacao) appeared in 1817 in Colombia, which is outside its presumed native range (Bailey *et al.*, 2018). Two important forest pathogens are *Cro-narium ribicola* (white pine blister rust), first reported in the USA in 1906 (Geils, Hummer & Hunt, 2010), and *Cryphonec-tria parasitica* (chestnut blight), first reported in the USA in 1904 (Rigling & Prospero, 2018), both having caused signifi-cant impacts to native biota. *Cryphonec-tria parasitica* has rendered *Castanea dentata* (American chestnut) functionally extinct in North America (Dutech *et al.*, 2012). *Ophiostoma ulmi* (Dutch elm disease), introduced to North America in 1928, and followed by the more aggressive *O. novo-ulmi*, severely impacted *Ulmus americana* (American elm) in the USA and Canada (Copeland *et al.*, 2023). In South America, *Hemileia vastatrix* (coffee leaf rust) was successfully intercepted in 1903 in Puerto Rico, but established in Brazil in the 1970s, marking the onset of the third great pandemic of coffee rust globally (McCook, 2006). First records of alien fungi in Chile were documented beginning in the early 20th century and have shown a continuous increase until the present (Fuentes *et al.*, 2020).

Altogether, 363 alien fungi are reported in the SInAS database for The Americas (Table 5). The USA has the high-est number of alien fungi (136), followed by Chile (81), Brazil (80), and Argentina (65) (see also Simpson *et al.*, 2018; Fuen-tes *et al.*, 2020). Two widespread and well-known alien fungal pathogens in The Americas are *Batrachochytrium dendrobatidis* (chytrid fungus), which has caused massive population declines and extinctions in amphibian species since the 1960s (Fisher, Garner & Walker, 2009), and *Claviceps africana* (ergot).

The Americas harbour at least 199 alien macrofungi species, with approximately 36% belonging to the order Agaricales, 32% to Boletales and 11% to Russulales (Monteiro

*et al.*, 2020). Some of the most widely distributed are ectomy-corrhizal fungi, such as *Suillus luteus*, *Rhizopogon roseolus*, and *Suillus granulatus* (weeping bolete), which seemed to play a key role in pine invasions of the Southern Hemisphere (Policelli *et al.*, 2019). South American countries with high numbers of known alien fungi include Brazil (75), Argentina (60), and Chile (40) (Monteiro *et al.*, 2020). In the remaining IPBES subregions, higher numbers of alien macrofungi were recorded in the USA (50), Canada, and Mexico (seven each).

### (4) Plantae

#### (a) Historical trends

The first explorers of the eastern coast of The Americas intro-duced *Lolium perenne* (perennial ryegrass), and *Trifolium repens* (clover) among others; these alien species were fully estab-lished prior to colonisation by Europeans (Crosby, 1986). English colonisation of the North American east coast in the 17th century led to deforestation and the establishment of farms where many European species were cultivated for ornamental [e.g. *Taraxacum officinale* (dandelion)] or medicinal [*Verbascum thapsus* (common mullein)] purposes, directly sown in pastures [*Poa pratensis* (smooth meadow-grass)], or brought in as weeds [*Capsella bursa-pastoris* (shepherd's purse)] (Crosby, 1986), or in ship's solid ballast [*Phragmites australis* (common reed)] (Saltonstall, 2002). Over the last two centu-ries, North America has had the most rapid cumulative rate of increase of alien plant species, accelerating at the end of the 19th century (Fig. 5) (Lavoie *et al.*, 2012; Pyšek *et al.*, 2019), with macrophytes such as *P. australis* invading across the continent in less than 200 years (Meyerson *et al.*, 2025). South America exhibited a slower cumulative increase, likely due to lower research intensity relative to North America (Frehse *et al.*, 2016; Schwindt & Bortolus, 2017; Schwindt *et al.*, 2020). Nevertheless, as early as 1877, an impressive number of plant species were recorded as introduced in Argentina from Europe through the port of Buenos Aires, from where they rapidly dispersed up to the southernmost end of Patagonia (Berg, 1877; Bortolus & Schwindt, 2022). The rates of newly recorded alien plants began decreasing in the mid-20th century, par-ticularly in North America (Fig. 5). The magnitude of intro-ductions in the Caribbean and Mesoamerica was much lower, but showed increases similar to those observed in North and South America (Fuentes *et al.*, 2008; Ugarte, Fuentes & Klotz, 2010; Rojas-Sandoval & Acevedo-Rodríguez, 2015).

#### (b) Current status

North America has the highest number of recorded alien plant species in the world with at least 5958 established taxa (van Kleunen *et al.*, 2015; Pyšek *et al.*, 2017), while South America harbours 2667 alien plant species (Pyšek *et al.*, 2019). Globally, California is richest in alien vascular plants, with 1753 taxa recorded and Florida is another
<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Trends in numbers of established alien species for The Americas. Panels by taxon show cumulative numbers (left panels, thick lines) and number of new alien species per five-year intervals (right panels, thin lines). Numbers shown here underestimate the real extent of alien species occurrences due to a lack of data. Lines in right panels indicate smoothed trends calculated as running medians. Note that presented numbers may deviate from those reported in the text due to variation among data sources.</figcaption></figure>

invasion hotspot harbouring 1473 alien plants (Kartesz, 2015). According to Pyšek et al. (2017), countries in Mesoamerica also harbour many alien plants (Nicaragua: 671, Mexico: 519, Costa Rica: 280, Panama: 263), but due to their high native diversity, alien plants make up only 2.0–2.8% of the total floras, the exception being Nicaragua with 10.4% (Correa A., Galdames & De Stapf, 2004; Chacón & Saborío, 2012; Pyšek et al., 2017). Some regions in the Caribbean are heavily invaded by alien plants, both in terms of actual species numbers (Cuba: 542, Bahamas: 356) and the proportion of alien plants in the national floras (Bahamas 24%, Barbados 14%). Other countries in the Caribbean harbour 20–110 alien plants and contributions to national floras do not exceed 8% (Acevedo-Rodríguez & Strong, 2008; Kartesz, 2015; Pyšek et al., 2017).

Alien species are widespread on islands along both the Pacific and Atlantic coasts of The Americas, notably the Caribbean islands (Kairo et al., 2003; Van der Burg et al., 2012; Rojas-Sandoval & Acevedo-Rodríguez, 2015). As an example, parts of Caribbean island forests are

dominated by alien tree species (Chinea & Helmer, 2003; Brandeis et al., 2009; Helmer et al., 2012), some of which are shade tolerant and could permanently change forest species composition (Brown et al., 2006). In addition, several alien species invade forest plantations, livestock pastures, and abandoned agricultural fields causing both economic and environmental impacts. Such is the case for *Dichrostachys cinerea* (sickle bush), an alien species of African origin that occurs across almost 800,000 hectares in Cuba (Hernández, Lahmann & Pérez-Gil Salcido, 2002).

An example of a widespread marine plant is *Halophila stipulacea* (broadleaf seagrass; native to the Indian Ocean and Red Sea), which invaded the coasts of the Mediterranean Sea as early as 1895 (Winters et al., 2020), and then the Caribbean Sea by 2002 (Willette et al., 2014). Compared to the slow spread of this seagrass across the Mediterranean from Egypt to Tunisia (~120 years), it invaded coasts from Puerto Rico to Venezuela in just 15 years (Winters et al., 2020), becoming the world's first circumglobal marine alien angiosperm. The seagrass *Zostera japonica* (dwarf eelgrass) was
introduced to the Pacific northwest in the mid-1900s likely *via* oyster aquaculture and has since spread and negatively impacted native *Zostera marina* (eelgrass) and ecosystem processes (Shafer, Kaldy & Gaeckle, 2014).

The Galapagos Archipelago harbours an estimated 1700 alien species with *Rubus niveus* (Mysore raspberry) being among the most common (Toral-Granda *et al.*, 2017), posing a serious threat to the foundation species of native forest, endemic daisy tree *Scalesia pedunculata* (Riegl *et al.*, 2023) and, together with *Cestrum auriculatum* and *Tradescantia fluminensis*, to the entire forest (Jäger *et al.*, 2024). Between the 1980s and 1990s, the number of alien plants has nearly doubled on the Galapagos Islands, reaching nearly 900 species (Torres & Mena, 2018). A study of the residence time and human-mediated propagule pressure of plants suggests that this archipelago is still in an early stage of plant invasions, due to the booming tourism industry and increasing human population size (Trueman *et al.*, 2010).

(5) Animalia

(a) *Historical trends*

The number of alien animals in The Americas has continu-
ously increased across all taxonomic groups, especially post-
1850, and across all subregions (Fig. 5). Particularly sharp
increases were observed for North America, followed by
South America, while alien birds steeply increased in the
Caribbean. The first introductions of alien mammals date
to pre-Columbian times in the Caribbean islands,
e.g. *Didelphis marsupialis* (common opossum) and *Dasyprocta
leporina* (agouti) (Long, 2003; Giovas, LeFebvre &
Fitzpatrick, 2012; Biancolini *et al.*, 2021). The number of
alien species introductions began surging in the 15th century
with European colonisation, peaked in the 20th century with
many introduced game species, and more recently, *via* the pet
trade (Long, 2003; Biancolini *et al.*, 2021). Since 1900, rates
of increase have declined in a few cases, particularly for
mammals and fishes (Fig. 5). The number of alien amphib-
ians and reptiles in The Americas has increased since the
1950s with new alien species introductions *via* the pet trade
projected to remain steady or accelerate (Kraus, 2009; Pow-
ell *et al.*, 2011; Stringham & Lockwood, 2018; Lockwood
*et al.*, 2019; Perella & Behm, 2020).

Several studies indicate increasing numbers of alien insects
in North America (Mattson et al., 1994; Aukema et al., 2010;
Nealis et al., 2016). Some of the earliest alien insect introduc-
tions were ground-dwelling beetles, such as Carabidae, acci-
dentally transported from Europe with rocks used as ship
ballast (Lindroth, 1954). During the early 1900s, many insect
species, such as *Quadraspidiotus perniciosus* (San Jose scale), were
accidentally introduced with plants imported for agriculture
and horticulture (Liebhold & Griffin, 2016). In South Amer-
ica, the number of reported alien aquatic organisms is rapidly
increasing (e.g. Fuentes et al., 2020; Schwindt et al., 2020;
Vitule et al., 2021). The first alien aquatic species introduc-
tions occurred in the 1500s in conjunction with European

colonisation. The number of introductions remained low in
the 1800s and 1900s, but increased thereafter (Vitule
et al., 2021). Beginning in the 2000s, both the number of
records and the number of studies on alien organisms
increased continuously (e.g. Frehse et al., 2016; Vitule
et al., 2021), with no sign of slowing down, both in terms of
alien species numbers and new spatiotemporal records
(e.g. Vitule et al., 2021).

For marine alien species, seminal studies have highlighted
rising numbers in American waters (Cohen & Carlton, 1998;
Carlton & Eldrege, 2015; Carlton & Schwindt, 2024). For
example, in temperate coastal waters of The Americas, new
data show an increase in the total number of alien species
detected, while the rate of new detections of alien
species has stabilised (Bailey et al., 2020). Following a dis-
tinct intensification of research efforts on marine alien spe-
cies in South America between 1997 and 2014
(Schwindt & Bortolus, 2017), the number of introduced spe-
cies increased by 160% between 2009 and 2019 in Brazil
(Teixeira & Creed, 2020). In Argentina and Uruguay,
marine alien species numbers increased by a factor of 4.5
between 2001 and 2019, with one new species estimated
to arrive every 178 days (Schwindt et al., 2020).

(b) Current status

The Americas host 96 species of alien mammals with partic-
ularly high species numbers on the East coast of North Amer-
ica, Alaskan islands, Southern USA, the Caribbean
Archipelago, Patagonia, and Falkland Islands (Malvinas)
(Biancolini et al., 2021). The number of reported alien mam-
mals ranges from 83 to 164, depending on the source
(Table 5). One of the most widespread alien mammals in
The Americas is *Urva europunctata* (small Indian mongoose)
which has established on many islands in the Caribbean
(Hays & Conant, 2007; Louppe et al., 2020; Biancolini
et al., 2021). North America is particularly rich in alien bird
species, notably in Florida and California where several
alien parrot species have established populations (Dyer,
Redding & Blackburn, 2017b). The Americas have the high-
est number of alien reptiles and amphibians in the world with
several hotspots in the USA (Kraus, 2009; Krysko et al., 2011,
2016; Capinha et al., 2017), including Florida, California,
and Puerto Rico (Meshaka, 2011; Powell et al., 2011;
Perella & Behm, 2020). Other Caribbean islands, such as
Cuba and the Bahamas, are also important global alien
amphibian hotspots (Kraus, 2009; Knapp et al., 2011; Powell
et al., 2011; Borroto-Páez et al., 2015). In South America,
Brazil has the highest number of alien amphibian and
reptile species (136), of which at least seven have established
wild populations (Kraus, 2009; Fonseca, Both &
Cechin, 2019). Globally, North America has the most alien
insect species (Liebhold et al., 2016) with the largest concen-
tration of species in Northeastern North America (Liebhold
et al., 2013). Even though there are more described species
of Coleoptera than any other order, there are slightly more
alien Hemiptera species established in North America
(Yamanaka et al., 2015; Liebhold et al., 2016) and Chile (López et al., 2023). The over-representation of the Hemi-ptera is most likely the result of the ease with which they are accidentally introduced with imported plants.

The most recent studies of freshwater fishes in South America indicate that more than 75 alien species have been translocated between different basins within South America (Bezerra et al., 2019; Vitule et al., 2019) and more than 80 alien fish species from other regions of the world (Vitule et al., 2019, 2021; Doria et al., 2021). A famous example of an alien aquatic invertebrate is *Linnoperna fortunei* (golden mussel) (Schwindt & Bortolus, 2017). North America has a long history of aquatic species introductions, particularly for fish, such as *Salmo trutta* (brown trout) or *Cyprinus carpio* (common carp) (Moyle, 1986; Courtenay & Meffe, 1989; Fuller, Nico & Williams, 1999), as well as many crustaceans and molluscs, such as *Bythotrephes longimanus* (spiny waterflea), *Dreissena* spp. (zebra and quagga mussels), and *Corbicula* spp. (basket clams). The Laurentian Great Lakes have been invaded by nearly 190 alien species (Ricciardi, 2006; Ricciardi & MacIsaac, 2022), more than any other freshwater ecosystem in the world. These species include several invasive alien animals of Ponto-Caspian origin, mostly introduced through ballast water (Ricciardi & MacIsaac, 2000; Ricciardi, 2001, 2006; Vanderploeg et al., 2002). Many species native to small regions of The Americas, such as *Faxonius rusticus* (rusty crayfish), *Gambusia* spp. (mosquitofishes), and salmonids, such as *Oncorhynchus mykiss* (rainbow trout), have been widely introduced throughout The Americas and elsewhere (Marr et al., 2013; Schwindt et al., 2018; Muñoz-Mas et al., 2023). A spectacular case is the North American beaver (*Castor canadensis*), which was introduced to the Tierra del Fuego Archipelago in 1946 and negatively impacts *Nothofagus* forests; subsequently, abandoned beaver ponds become hot-spots for the spread of alien pasture plants (Skewes et al., 2006).

Studies of marine alien species across The Americas are geographically and taxonomically patchy. Spatial and temporal surveys are scarce, even in well-studied regions, such as the USA, making it difficult to draw general conclusions (Bailey et al., 2020). The first comprehensive assessment was for the continental coasts of the USA, finding 298 marine alien species (Ruiz et al., 2000), but by 2006, 257 marine alien species were identified in California alone (Ruiz et al., 2011). In South America, the Southwestern Atlantic region is the most-investigated area for marine invasive alien species, with fish and molluscs represented in the largest numbers of studies, species, and spatiotemporal occurrence records (Frehse et al., 2016; Schwindt & Bortolus, 2017; Bezerra et al., 2019). Brazil has the highest number with 138 marine alien species (Teixeira & Creed, 2020), followed by Argentina and Uruguay with 129 alien species in total (Schwindt et al., 2020). On the Pacific coast, Chile reported 51 marine alien species (Castilla & Neill, 2009; Villaseñor-Parada, Pauchard & Macaya, 2017) and Colombia four (Gracia et al., 2011). As with essentially all coasts worldwide, the numbers of introductions are distinctly under-estimated

due to an absence of historical baselines and a lack of research (Schwindt & Bortolus, 2017; Carlton & Schwindt, 2024). For example, the number of marine alien species in Chile is likely to be much higher than the reported 51 (Stowhas Salinas et al., 2023).

## VII. ASIA & THE PACIFIC

### (1) Bacteria and Protozoa

Little is known about alien prokaryotes with very few reports in Asia & the Pacific. Chen, Sun & Zhan (2017b) reported three Proteobacteria in China, *Pseudomonas syringae*, *Acidovorax avenae*, and *Xanthomonas oryzae* (leaf blight), which are known for their role as plant pathogens, and one marine cyanobacterium *Trichodesmium erythraeum*. The proteobacterium and plant pathogen *Xanthomonas axonopodis* has been recorded across multiple Pacific Islands from Australia to the Hawaiian Islands (EPPO, 2016; Nahrung & Carnegie, 2020). *Erwinia amylovora* (fire blight) has been recorded for multiple countries mostly on mainland Asia (Vanneste, 2008).

### (2) SAR

The SInAS database includes 103 taxa of SAR for Asia & the Pacific (Table 6). While the earliest record is *Phytophthora infestans* (potato blight) from 1900 (APASD, 2024), the number of records showed a strong increase starting in the 1970s and continuing until today. Soilborne plant pathogens like *P. cinnamomi*, which originates from Southeast Asia (Hardham & Blackman, 2018; Thakur et al., 2019), and *P. ramorum* (sudden oak death) are also widely distributed and occur on several hosts in most countries in Asia (Davison, 2022; Garbelotto & Frankel, 2020) including several Australian islands (Auld & Hutton, 2004; Pickering, Bear & Hill, 2007), Hawaii (Davison, 2022), Fiji, Samoa, Tuvalu, and New Zealand (Campbell, 2010; Thaman, 2011; Thaman & O'Brien, 2011). Other frequently recorded species are the brown alga *Undaria pinnatifida* and the dinoflagellate *Alexandrium minutum*. A large number of the taxa reported in the SInAS database are marine (96 species), including species of the genus *Alexandrium*, *Sargassum*, and *Rhizosolenia*. For China alone, 60 species of SAR (in all realms) have been reported with the majority being Ochrophyta (31 species), such as five species of the genus *Rhizosolenia*, followed by Myzozoa (21) (Xu et al., 2012; Chen et al., 2017b). For Australia, 13 species have been recorded in the SInAS database, of which six were identified as forest pests (Nahrung & Carnegie, 2020).

### (3) Fungi

For Asia & the Pacific, the highest numbers of alien fungi are reported in New Zealand (180 species) and Australia (132) according to the SInAS database. However, the database *Biota of New Zealand* lists more than 2000 alien fungi just for New Zealand, indicating the huge discrepancy among
<table><caption>Table 6. Numbers of established alien species for subregions of Asia & the Pacific. The numbers are extracted from the SInAS database (see Section II) and may deviate from those reported in regional studies. The numbers should be considered as minimum values as the true level of invasion is likely much higher. For mammals, birds, and vascular plants ranges of values indicate variation among databases. Duplicated taxa for the same region and Totals were removed. SAR, Stramenopila, Alveolata, Rhizaria.</caption><thead><tr><th></th><th>Northeast Asia</th><th>Oceania</th><th>South Asia</th><th>Southeast Asia</th><th>Western Asia</th><th>Totals</th></tr></thead><tbody><tr><td>Mammals</td><td>28–53</td><td>50–105</td><td>12–28</td><td>38–54</td><td>5–20</td><td>97–163</td></tr><tr><td>Birds</td><td>119–129</td><td>169–175</td><td>29–38</td><td>84–85</td><td>84–139</td><td>287–336</td></tr><tr><td>Fishes</td><td>287</td><td>95</td><td>90</td><td>296</td><td>125</td><td>633</td></tr><tr><td>Reptiles</td><td>41</td><td>41</td><td>7</td><td>35</td><td>13</td><td>103</td></tr><tr><td>Amphibians</td><td>24</td><td>13</td><td>4</td><td>12</td><td>1</td><td>43</td></tr><tr><td>Insects</td><td>607</td><td>1521</td><td>111</td><td>89</td><td>101</td><td>2017</td></tr><tr><td>Arachnids</td><td>67</td><td>83</td><td>13</td><td>18</td><td>6</td><td>129</td></tr><tr><td>Molluscs</td><td>81</td><td>119</td><td>15</td><td>24</td><td>89</td><td>261</td></tr><tr><td>Crustaceans</td><td>43</td><td>75</td><td>12</td><td>19</td><td>63</td><td>158</td></tr><tr><td>Vascular plants</td><td>2219–2454</td><td>4631–6747</td><td>1055–3142</td><td>1313–1598</td><td>271–562</td><td>6141–9101</td></tr><tr><td>Algae</td><td>5</td><td>37</td><td>4</td><td>7</td><td>27</td><td>66</td></tr><tr><td>Bryophytes</td><td>3</td><td>43</td><td></td><td></td><td></td><td>44</td></tr><tr><td>Fungi</td><td>59</td><td>303</td><td>17</td><td>20</td><td>1</td><td>363</td></tr><tr><td>SAR</td><td>59</td><td>31</td><td>6</td><td>7</td><td>20</td><td>103</td></tr><tr><td>Bacteria and protozoans</td><td>7</td><td>4</td><td>3</td><td>2</td><td>4</td><td>12</td></tr><tr><td>Totals</td><td>3649–3919</td><td>7215–9392</td><td>1378–3490</td><td>1964–2266</td><td>810–1171</td><td>10,457–13,532</td></tr></tbody></table>

databases (Manaaki Whenua – Landcare Research, 2024). One of the earliest reports was *Hemileia vastatrix* (coffee leaf rust), which caused an epidemic in Southern India and Sri Lanka (Ceylon) in 1869 and spread further across coffee-producing regions in the Indian Ocean and Pacific (McCook, 2006). Data from China indicate that of the 27 known alien fungi, only two new additions were reported after 2000 (Xu & Qiang, 2018). Fifteen alien fungal pathogens were intercepted by plant quarantine in India (Akhtar et al., 2019, 2021; Dubey et al., 2021) between 2015 and 2020. Another significant plant pathogen is *Ceratocystis lukuohia*, the causal agent of Rapid ‘Ohi’a Death (ROD), reported since 2010 and causing high mortality in Hawaii’s endemic keystone tree species *Metrosideros polymorpha* (Fortini et al., 2019).

As for other regions in the Southern Hemisphere, the extensive cultivation of eucalypts and pines led to the co-invasion of a suite of associated fungi (Vellinga, Wolfe & Pringle, 2009). Ectomycorrhizal fungal communities of *Pinus contorta* in New Zealand were dominated by alien fungi, highlighting co-invasion as an important process (Dickie et al., 2010). In tropical Asia, pathogens of alien eucalypts, such as species of the genera *Mycosphaerella*, *Teratosphaeria*, and *Aulographina* eucalypti and *Cryphonectria cubensis*, were apparently co-introduced from Australia (Sankaran & Hussain, 2019). Ectomycorrhizal fungi of eucalypts, such as *Laccaria fraterna*, *Pisolithus albus*, and *P. arrhizus*, are alien species introduced into the region. The myrtle rust, *Austropuccinia psidii* (originally from South America), has been recorded in China, Indonesia, Japan, New Zealand, Australia, Singapore (Carnegie & Giblin, 2022), and New Caledonia (Soewarto et al., 2018). Other alien fungi recorded from the region include *Ceratocystis fimbriata sensu lato* (wilt of several hosts), *Melampsora medusae* (leaf rust of poplars), and *Puccinia horiana* (white rust) (Akhtar et al., 2019; CABI, 2022; EPPO, 2024). Twenty-seven invasive alien fungal pathogens were recorded from China (Xu & Qiang, 2018), 21 from India (Government of India, 2005; Akhtar et al., 2019, 2021; Dubey et al., 2021), 30 from the Maldives (Shafia & Saleem, 2003), and 15 from the Lao People’s Democratic Republic (Nhoybouakong & Khamphouke, 2003). Of the 42 powdery mildew species (*Erysiphales*) found in Australia, all are classified as introduced, mainly since the European colonisation of the continent (Kiss et al., 2020). However, it is clear from studies by Fisher et al. (2020) that several new alien fungi may have been introduced to the region from across the globe and the numbers are grossly underestimated. In addition, limited historical data on fungi hinder the identification of native and alien status with any degree of certainty.

A comparatively high number of alien macrofungi have been reported for Asia & the Pacific, which harbours at least 235 alien species (Monteiro et al., 2020). The majority belong to the order Agaricales (54%), followed by Boletales (21%), and Russulales (10%). The most widespread alien macrofungus is *Pyrrhoderma noxium*. Available data indicate that the highest numbers of alien macrofungi are known in New Zealand (170 species) followed by Australia (40 species).

## (4) Plantae

### (a) Historical trends

First records of alien plant species in Asia & the Pacific date back more than 1000 years (Wijesundara, 2010) and consistent increases in the number of alien species have been recorded for several Asian and Pacific countries
(Wijesundara, 2010; Wu et al., 2010; Lazkov & Sultanova, 2011; Jaryan et al., 2013; Shrestha, 2016; Vinogradov & Kupriyanov, 2016; Chen et al., 2017a; Pant et al., 2021; Cáceres-Polgrossi et al., 2023). The strongest increase in the cumulative number of alien plant species is recorded for Oceania, including Australia, New Zealand, and the Pacific Islands (Fig. 6). Introduction rates peaked around 1900, followed by a decline, and re-acceleration in the mid-20th century (Fig. 6). The trends for other Asian regions are similar to that for Oceania but have markedly lower absolute numbers of alien species per time period.

(b) Current status

Regions in Asia & the Pacific include global hotspots of alien plant species (Dawson *et al.*, 2017), and this holds true for islands in Oceania (Moser *et al.*, 2018; Essl *et al.*, 2019). Examples include New Zealand with 1798 alien plants (Brandt *et al.*, 2021), Tahiti (1346), and Guam (833; Raulerson, 2006). Australian states harbour from 1186 established species in Western Australia to 1584 in New

South Wales, corresponding to 12–25% of the total plant diversity in these states (Randall, 2002; Walsh & Stajsic, 2007; Pyšek et al., 2017). Australasia has had the most rapid accumulation of numbers of alien plants with expanding colonisation, and the Pacific islands show the steepest increase of all global regions using this measure (van Kleunen et al., 2015). The most widespread alien species on the Pacific Islands include *Euphorbia hirta* (garden spurge), *Cenchrus echinatus* (southern sandbur), and *Phyllanthus amarus* (Jamaica weed). In Australia and New Zealand, it is *Sonchus oleraceus* (common sowthistle), *Solanum americanum* (American black nightshade), and *Chenopodium murale* (nettle-leaf goosefoot) (Pyšek et al., 2017).

Global hotspots also occur in other Asian regions; in South Asia, India (471 alien plants; Inderjit et al., 2018), the Philippines (628; Pelser, Barcelona & Nickrent, 2011), and Indonesia (503; SEAMEO BIOTROP, 2003) are invasion hotspots. In Northeast Asia, Japan is richest in alien plants (1311) and numbers from China range from 861 to 933 (Hao & Ma, 2023). The most recent inventory based on 236 studies documented the presence of 241 invasive alien

<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Trends in numbers of established alien species for Asia & the Pacific. Panels by taxon show cumulative numbers (left panels, thick lines) and number of new alien species per five-year intervals (right panels, thin lines). Numbers shown here underestimate the actual extent of alien species occurrences due to a lack of data. Lines in right panels indicate smoothed trends calculated as running medians. Note that numbers presented may deviate from those reported in the text due to variation among data sources.</figcaption></figure>
plant species across seven countries of South Asia, with the highest invasive species richness in India (185) followed by Bhutan (53), Sri Lanka (45), Bangladesh (39), Nepal (30), Pakistan (29) and the lowest in Maldives (15) (Gulzar et al., 2024). Western Asia is comparatively species poor in alien plants (Pyšek et al., 2017). The most widespread alien plants in South and Southeast Asia include *Chromolaena odorata* (siam weed), *Lantana camara* (lantana), *Leucaena leucocephala* (leucaena), and *Prosopis juliflora* (mesquite) (Sankaran & Suresh, 2013). The Hawaiian Islands are a global hotspot of plant invasions with 1466 alien plant species, and numbers for individual main islands within the archipelago range from 398 to 963 alien species (Imada, 2019). Alien plants are a serious issue in forests of many Asian Pacific islands, such as Tahiti (Meyer & Florence, 1996), Fiji (Lenz et al., 2022; Forey et al., 2023), Lord Howe Island (Auld & Hutton, 2004), and Carnac Island (Abbott, Marchant & Cranfield, 2000). The most problematic species in urban areas of the islands include *Mikania micrantha* and *Spathodea campanulata* (Lowry et al., 2020).

(5) Animalia

(a) Historical trends

Before European colonisation, alien mammals in Southeast Asia were introduced *via* ancient exchanges between the Indonesian Archipelago, Papua New Guinea, and Australia with numerous prehistoric introductions, such as *Phalanger orientalis* (northern common cuscus), *Sus celebensis* (Sulawesi pig), and *Dendrolagus matschiei* (Matschie's tree-kangaroo) (Heinsohn, 2003; Long, 2003; Biancolini et al., 2021). Since 1500, the numbers of alien animal species increased continuously for all taxonomic groups and all subregions of Asia & the Pacific (Fig. 6). The steepest increases were observed in Oceania, particularly during the 19th century for all considered animal groups, except for fishes, mostly because of European colonisation. In other subregions, steep increases were mostly observed after 1950 for insects (Huang, Haack & Zhang, 2011; Yamanaka et al., 2015), gastropods (Barker, 1999; Roll et al., 2009), amphibians and reptiles (Lee et al., 2019), and marine alien species of different groups (Hewitt et al., 2004; Bailey et al., 2020). During the 19th century, acclimatisation societies sought to 'improve' local fauna by introducing many aesthetically pleasing and game species to Australia and New Zealand (Lever, 1992; Simberloff & Rejmánek, 2011; Pipek, Pyšek & Blackburn, 2015; Pipek, Blackburn & Pyšek, 2019). The Asia & the Pacific region has also experienced a growing number of alien bird, reptile, and amphibian introductions, a trend likely to continue in the future (Kraus, 2009; Keppel et al., 2014; Chapple et al., 2016; Lee et al., 2019; Pili et al., 2020; Toomes et al., 2020).

The numbers of alien freshwater species grew slowly in Asia & the Pacific until the 19th century (Fig. 6) after which they increased rapidly (Yuma, Hosoya & Nagata, 1998; Tan et al., 2020; Muñoz-Mas et al., 2023). During the 20th century, aquaculture was the main pathway for freshwater

fish species introductions (Xiong et al., 2015; Saba et al., 2020). In addition, mosquitofishes (*Gambusia* spp.) and guppy (*Poecilia reticulata*) were widely introduced for the control of mosquitoes and disease while several other freshwater fish species were introduced for ornamental purposes (Yuma et al., 1998; Tan et al., 2020). The number of ornamental freshwater fish rapidly increased towards the end of the 20th century and the ornamental trade is now the main pathway of introduction (Yuma et al., 1998; Goren & Ortal, 1999; Saba et al., 2020).

(b) Current status

Asia & the Pacific is the region with the highest number of alien mammals in the world (130 species) (Biancolini et al., 2021), with numbers ranging from 97 to 163 depending on the data source (Table 6). Areas with high numbers of alien mammals are Japan, the Indonesian archipelago, Australia, New Zealand, and the Pacific islands. For example, *Oryctolagus cuniculus* (European rabbit) is a prominent invasive alien species in Australia (Kirkpatrick, Page & Massam, 2008), and *Trichosurus vulpecula* (brushtail possum) was introduced to New Zealand in 1858 for the domestic fur and meat trade (Gormley et al., 2012; Forsyth et al., 2018). Mammals have been widely introduced on islands in Asia & the Pacific (Russell et al., 2017), with synanthropic species, such as mice, rats, rabbits, pigs, goats, cats, and foxes, causing devastating impacts on insular ecosystems (Russell & Kueffer, 2019). The number of alien bird species in Asia & the Pacific is among the highest within IPBES regions with particularly high numbers recorded for Oceania, followed by Southeast Asia and Northeast Asia (Table 6). More than 100 alien birds are found in tropical Asia, which is likely a consequence of the intensive bird trade in this region (Corlett et al., 2020). Particularly high numbers of alien birds were recorded in Australia, New Zealand, Taiwan, and Japan (Dyer et al., 2017a). Asia & the Pacific harbours two of the best-known examples of alien reptiles and amphibians, namely *Boiga irregularis* (brown tree snake) in Guam and *Rhinella marina* (cane toad) in Australia and other Pacific islands (Lever, 2003; Zug, 2013; Rogers et al., 2017; Engeman, Shiels & Clark, 2018; Shine, 2018). Invertebrates outnumber vertebrate species, and many examples of alien invertebrate introductions are known. Hawaii is a classic example of an archipelago heavily invaded by many species' groups, being among the three regions with most records of alien species in the world (Dawson et al., 2017), including 3000 arthropods (Nishida, 2002). In Guam, but also on Christmas Island, an *Anoplolepis gracilipes* (yellow crazy ant) invasion was assisted by the invasive *Tachardiaephagus tachardiae* (yellow lac scale insect) (O'Dowd, Green & Lake, 2003; Reaser et al., 2007). Other typical examples are gastropod invasions on many Polynesian islands, such as *Lissachatina fulica* (giant African land snail) and *Euglandina rosea* (Tsatsia & Jackson, 2022), with disastrous consequences for the endemic gastropod fauna (Gerlach et al., 2021).
The number of alien freshwater fishes is highest in China (61) (Luo et al., 2019), followed by Singapore (42) (Tan et al., 2020), the Philippines (39) (Casal et al., 2007), and Japan (23) (Yuma et al., 1998). Most of the alien fishes were introduced for aquaculture, while the proportion of introduced ornamental fishes is much lower (Casal et al., 2007; Luo et al., 2019; Muñoz-Mas et al., 2023). The national status of alien freshwater invertebrates was only reported in China and Singapore, including 13 alien invertebrates in China (Chen et al., 2017b) and 14 alien molluscs in Singapore (Ng et al., 2016); these figures include only the most conspicuous invertebrates and distinctly underestimate invertebrate introductions. As with fishes, most known invertebrates were introduced via aquaculture and ornamental pathways (Ng et al., 2016; Chen et al., 2017b). On Pacific islands, about 59 freshwater snails, 38 of them cryptogenic, have been introduced (Cowie, 2001). In Pearl Harbour (Hawaii) alone, 191 inland water species of eight phyla but mostly insects, crustaceans, molluscs, and annelids were identified by 1997–1998 (Englund, 2002).

A regional assessment of marine alien species across Asia & the Pacific is lacking, and, as in many other marine regions, the number of actual introductions is sorely underestimated (Carlton & Schwindt, 2024). Large-scale studies report 73 alien species for the Central Indo-Pacific, a region subject to intensive maritime traffic importing marine alien species from around the world for more than 500 years (Carlton & Schwindt, 2024). For the Northwest Pacific, 208 species are reported and 368 for the Northeast Pacific (Lee & Reusser, 2012; Kestrup, Smith & Therriault, 2015). Several marine studies exist for individual countries, such as Japan (42 marine alien species) (Iwasaki, 2006), Republic of Korea (41) (Lutaenko et al., 2013), and Russia (66) (Zvyagintsev et al., 2011). For the South Pacific Ocean, the highest research effort has been around Australia and New Zealand. In Port Phillip Bay (Australia), 100 marine alien species were reported (Hewitt et al., 2004), while 214 alien species were reported for New Zealand (Therriault et al., 2018). The knowledge of marine alien species of the Pacific Island countries and territories is scattered and dispersed in diverse publications. Surveys in Pago Pago Harbour (American Samoa) recognised 17 marine alien species (Coles et al., 2003), 40 alien species were detected from Guam (Paulay et al., 2002), and 11 alien species in Malakal harbour, Palau (Campbell, Hewitt & Miles, 2016). We emphasise that few, if any, of these numbers approach reality.

## VIII. EUROPE & CENTRAL ASIA

### (1) Bacteria and Protozoa

With 55 species of alien Bacteria (Magliozzi et al., 2022), Europe is the IPBES region with the highest number of species in this group. The main reason for the comparatively high number of species is the availability of a report of alien

Bacteria and viruses (Magliozzi et al., 2022), which is unique among IPBES regions. According to this report, the vast majority of alien Bacteria in Europe have been introduced from North America (64%), followed by South America (16%). Highest numbers of alien Bacteria were found in Italy, France, and the Netherlands. However, for a large number of species (56%) the invasion status, and thus the native range, could not be identified. Among prokaryotes, the majority of species are Proteobacteria, such as *Erwinia amylovora* (fire blight), and several species of the genera *Xanthomonas* and *Pseudomonas*. The second largest group is Cyanobacteria including the freshwater species *Cylindrospermopsis raciborskii*, *Sphaerospermopsis aphanizomenoides*, and *Raphidiopsis mediterranea*. For Central Asia, *E. amylovora* has been reported for Armenia (Vanneste, 2008). Eight species of alien protozoans have been reported for Europe & Central Asia with *Bonamia ostreae* being the most widespread species. This species is a pathogen of shellfish and was first reported in 1987 in Ireland (Pederson et al., 2017). Other species are the marine algae *Poropila dubia*, *Haplosporidium armoricanum*, and *Haplosporidium nelsoni*, the latter two being oyster pathogens.

### (2) SAR

The number of alien SAR has been rising continuously until recently with a peak in the early 2000s according to the SInAS database. Within the past 20 years, five downy mildew pathogens with the potential to cause significant losses have been introduced into Europe (Thines, 2011; Gilardi et al., 2013; Voglmayr, Montes-Borrego & Landa, 2014; Görg et al., 2017; Thines et al., 2020) with seed or latently infected plants. In total, 373 alien species of SAR have been reported for Europe & Central Asia (Table 7). The majority belong to Ochrophyta (140 species), followed by Myzozoa (87), Foraminifera (70) and Oomycota (59). Alien species widespread throughout the IPBES region include *Aphanomyces astaci* (crayfish plague), *Colpomenia peregrina* (oyster thief), *Prorocentrum cordatum* (dinoflagellate), and *Sargassum muticum* (brown alga). The highest numbers of alien SAR have been reported from France (72 species), followed by Ukraine (61), Turkey (58), and Germany (38) (see also Çinar et al., 2005; Rabitsch & Nehring, 2017). For several regions, comprehensive lists of alien species have been published, which also include SAR; for example, Austria (43 oomycete species; Voglmayr et al., 2023), France (36 oomycete species; Desprez-Loustau et al., 2010), Norway (18 species; Sandvik et al., 2020), Switzerland (36 oomycete species; Beenken & Senn-Irlet, 2016), the UK (13 species; Roy et al., 2012), and the Mediterranean Sea (85 species, Galanidi et al., 2023). The majority of SAR are marine. Europe has a well-documented history of invasions for certain alien pathogenic oomycetes, such as *A. astaci* (crayfish plague; Mrugała et al., 2015), *Phytophthora infestans* that triggered the Irish Great Famine in the 1840s (potato blight; Yoshida et al., 2013), *Phytophthora x alni* (alder dieback; Jung et al., 2018), and *Plasmopara viticola* (grapevine downy mildew), which was introduced in 1878 (Gessler, Pertot & Perazzolli, 2011). One of the few
<table><caption>Table 7. Numbers of established alien species for subregions of Europe & Central Asia. The numbers are extracted from the SInAS database (see Section II) and may deviate from those reported in regional studies. The numbers should be considered as minimum values as the true level of invasion is likely much higher. Empty cells may reflect a lack of research. For mammals, birds, and vascular plants ranges of values indicate variation among databases. Duplicated taxa for the same region and Totals were removed.</caption><thead><tr><th></th><th>Central and Western Europe</th><th>Central Asia</th><th>Eastern Europe</th><th>Totals</th></tr></thead><tbody><tr><td>Mammals</td><td>64–133</td><td>5–23</td><td>24–80</td><td>72–164</td></tr><tr><td>Birds</td><td>218–627</td><td>4–5</td><td>20–24</td><td>221–630</td></tr><tr><td>Fishes</td><td>423</td><td>51</td><td>119</td><td>469</td></tr><tr><td>Reptiles</td><td>94</td><td></td><td>6</td><td>98</td></tr><tr><td>Amphibians</td><td>42</td><td>2</td><td>5</td><td>43</td></tr><tr><td>Insects</td><td>2698</td><td>28</td><td>213</td><td>2747</td></tr><tr><td>Arachnids</td><td>289</td><td>2</td><td>6</td><td>289</td></tr><tr><td>Molluscs</td><td>557</td><td>4</td><td>75</td><td>584</td></tr><tr><td>Crustaceans</td><td>508</td><td>11</td><td>120</td><td>563</td></tr><tr><td>Vascular plants</td><td>4498–7896</td><td>134–361</td><td>1950–2400</td><td>5146–8519</td></tr><tr><td>Algae</td><td>210</td><td></td><td>5</td><td>212</td></tr><tr><td>Bryophytes</td><td>37</td><td></td><td>1</td><td>37</td></tr><tr><td>Fungi</td><td>594</td><td>3</td><td>28</td><td>609</td></tr><tr><td>SAR</td><td>332</td><td></td><td>79</td><td>373</td></tr><tr><td>Bacteria and protozoans</td><td>22</td><td></td><td>2</td><td>23</td></tr><tr><td>Totals</td><td>10,586–14,462</td><td>244–490</td><td>2653–3163</td><td>11,486–15,360</td></tr></tbody></table>

records of alien SAR from Central Asia is from Georgia with a collection of marine species (Editorial Board of AquaNIS, 2024).

### (3) Fungi

In Europe & Central Asia, 609 alien fungi have been reported (Table 7). Nearly all records are from Europe with only a few from Central Asia: five alien fungi in Georgia, two in Kazakhstan, and one in Azerbaijan and Uzbekistan. *Ophiostoma novo-ulmi* (Dutch elm disease) has been reported from all of these countries (Brasier, 1991; Brasier & Kirk, 2001). In addition to the impact of *B. dendrobatidis* on many amphibians, *Batrachochytrium salamandrivorans* causing declines in *Salamandra salamandra* (salamander) populations has been recorded recently in Germany, Belgium, Spain, and the Netherlands and is likely to invade other areas, such as North America (Scheele et al., 2019; Castro Monzon et al., 2022).

Earliest first records of alien fungi date back to a report of the mediterranean *Clathrus ruber* (latticed stinkhorn) in 1752 in Germany (Monteiro et al., 2020) and the North American *Ustilago maydis* (corn smut) in 1760 in France (Kreisel & Scholler, 1994). *Cronartium ribicola* (white pine blister rust) originates from Eastern Asia and was recorded first in the mid-19th century in Eastern Europe from where it spread to other European countries and North America (Richardson et al., 2009). The SInAS database reports a continuous increase in the number of new alien fungi species with a first peak of a mean of seven new alien fungi annually around 1900 and the highest numbers today (10 annually). Using dried reference-collection samples, Gross et al. (2021) demonstrated that three species of *Erysiphe* could be linked to the incidence of powdery mildew in oaks, a disease that emerged in Europe at the beginning of the 20th century. One century earlier, in 1845, powdery mildew of grapevine

(*Erysiphe necator*) was introduced from North America to England, a story analogous to that of the fungus-like grapevine downy mildew (Gadoury et al., 2012).

The highest numbers of alien fungi have been published for Austria (323; Voglmayr et al., 2023), Switzerland (247; Beenken & Senn-Irlet, 2016), Slovenia (216; de Groot et al., 2020), and France (191; Desprez-Loustau et al., 2010); mostly a result of high research efforts or national assessments. Those studies show steep increases in the number of newly recorded alien fungi particularly since 1900. A similar increase until today was found in an assessment of 79 alien fungi in Norway (Sandvik et al., 2019). For the UK, 157 alien fungi were recorded and the number of new alien fungi did not show a clear trend since 1970 with *ca.* 15–20 new species per five-year interval (Jones & Baker, 2007). The highest numbers of invasive forest pathogenic fungi are reported from the central part of Europe (France, Italy, and Switzerland; Santini et al., 2013); most are native to the Northern Hemisphere, but about one-third are of unknown origin (Desprez-Loustau, 2009). Some of the most widespread are *Hymenoscyphus fraxineus* (ash dieback) and *Ophiostoma novo-ulmi* (Dutch elm disease), which caused severe diebacks in ash and elm, respectively, throughout Europe. The number of alien powdery mildews (Erysiphales) in Europe is rather high (Desprez-Loustau et al., 2010; Beenken & Senn-Irlet, 2016; Voglmayr et al., 2023) and may reflect responses to climate change in a group adapted for long-distance aerial spore dispersal (Heluta et al., 2009).

### (4) Plantae

#### (a) Historical trends

Several studies have reported long-term increasing trends of alien plants for individual countries, such as Estonia
(Ööpik *et al.*, 2008), Albania (Barina *et al.*, 2014), Italy (Celesti-Grapow *et al.*, 2009), Turkey (Çinar *et al.*, 2005), Slovakia (Medvecká *et al.*, 2012), Poland (Tokarska-Guzik, 2005), Czech Republic (Pyšek *et al.*, 2012a), Ukraine (Protopopova & Shevera, 2014), Iceland (Wasowicz, Przedpelska-Wasowicz & Kristinsson, 2013), and Portugal (Almeida & Freitas, 2012). The reported temporal patterns are very similar across regions with a distinct acceleration of new plant records either during the early (e.g. Pyšek *et al.*, 2012a) or late 19th century (e.g. Wasowicz *et al.*, 2013). The first comprehensive study of alien plant invasions across European countries showed a slow increase of new introductions until 1800 and a marked acceleration thereafter (Lambdon *et al.*, 2008). Whereas before 1800 the majority of alien plants were of European origin, this changed during the 19th century with the large majority of alien plants now being of non-European origin (Lambdon *et al.*, 2008). In addition to cumulative numbers, the rate of new alien plant records increased from 1800 until today as well (Fig. 7), reaching around 20 new alien species records annually in Europe (Seebens *et al.*, 2017). Information about alien plant introductions in Central Asia is scarce (Lazkov & Sultanova, 2011) but it seems likely that trends resemble those of neighbouring regions in Europe and other Asian regions, such as Nepal (Shrestha, 2016) or Siberia (Vinogradov & Kupriyanov, 2016), albeit at different levels.

Similarly, numbers of alien bryophytes have increased continuously, starting with the first observed species introduced to Europe, *Lumularia cruciata*, in 1828 (Essl & Lambdon, 2009). First records for alien ferns indicate similar trends although the available data are too scant to reveal robust trends (Jones *et al.*, 2019). The introduction of alien aquatic plants increased after 1950, with the trade in ornamentals being the main pathway, followed by cultivation and contaminants of commodities; the former two exhibited similar rates in different European areas, while contaminants of commodities were mostly recorded in Southern Europe (Nunes *et al.*, 2015). The number of alien aquatic plant species is still relatively low in European freshwaters but sharply increasing and has e.g. doubled in ~30 years in Germany (Hussner *et al.*, 2010).

### (b) Current status

The most recent and most comprehensive survey reports 4139 alien vascular plants in Europe & Central Asia (van Kleunen *et al.*, 2019). According to this survey, the highest numbers have been recorded in England (1379), Sweden (874), Scotland (861), Wales (835), France (716), and Norway (595) (Pyšek *et al.*, 2017), indicating that the Northern part of the continent, particularly the UK and Scandinavia, is heavily invaded by alien plants (Lambdon *et al.*, 2008). Only a few regions in Eastern Europe harbour comparably high numbers of alien species, such as the European part of Russia (649), Ukraine (626), and Bulgaria (593). In some countries, such as the UK, Sweden, and Norway, alien

species make up 32–47% of the total flora (Pyšek *et al.*, 2017). Central Asia is generally less invaded by alien plants, with country floras in this region harbouring 50–70 alien species, corresponding to a 1.9–4.5% contribution to the total plant diversity (Pyšek *et al.*, 2017). In some cases, this paucity reflects environmental conditions that are unfavourable for invasion. In Mongolia, a recent study recorded 154 taxa of alien plants, of which 33 are established and 121 casual; the low number of established plants is attributed to harsh conditions and historic isolation of this country (Vanjil *et al.*, 2024). In Uzbekistan, intensive long-term research yielded 242 alien plant species of which 211 are established, and 36 of these are invasive (Makhkamov *et al.*, 2024). However, generally low alien species numbers in Central Asia can also be due to low research effort. Thirty-five alien species have become established in more than 30 regions of Europe, representing at least half of this continent's territory, the most widespread being *Erigeron canadensis* (Pyšek *et al.*, 2017). Some of the best-studied plant invasions in Europe include the terrestrial herbs *Heracleum mantegazzianum* (giant hogweed) (Pyšek *et al.*, 2017; Shackleton *et al.*, 2020), *Fallopia* sp. div. (knotweed) (Bailey & Conolly, 2000), *Carpobrotus edulis* (Hottentot-fig) (Novoa *et al.*, 2013), *Senecio inaequidens* (narrow-leaf ragwort) (Heger & Boehmer, 2005), the aquatic herbs *Elodea canadensis* (American duckweed), and the trees *Robinia pseudoacacia* (black locust), *Acer negundo* (ash-leaf maple), and *Acacia dealbata* (silver wattle) (Nentwig *et al.*, 2018).

There are 210 alien plants recorded in European freshwaters, mostly originating from North America and Asia (Nunes *et al.*, 2015). *Elodea canadensis* is the most widely distributed alien aquatic plant in Europe, occurring in 41 European countries, followed by *Azolla filiculoides* (water fern) (25), and *Vallisneria spiralis* (tape grass) (22). Some of the established aquatic species are widespread and have increasing negative impacts across Europe, such as *Elodea* spp. (waterweeds), *Hydrocotyle ranunculoides* (floating pennywort), and *Myriophyllum aquaticum* (parrot feather watermilfoil) (Hussner, 2012).

Among alien bryophytes, 32 identified alien species in Europe comprise 21 mosses, 11 liverworts but no hornworts. The countries with the most alien bryophytes are the UK (14 species) and Ireland (six species). Overall, countries and regions with a humid and cool climate are most invaded, whereas countries with drier and warmer climates are poor in alien bryophytes (Essl & Lambdon, 2009). *Campylopus introflexus* (present in 21 countries) and *Orthodontium lineare* (15 countries) are the most widely distributed alien bryophytes in Europe (Essl *et al.*, 2014).

Galanidi *et al.* (2023) list 110 marine alien plants from the Mediterranean Sea. The major putative pathway of introductions to the Northwestern Basin and the Adriatic Sea is shellfish transfer, and to the Eastern and Central Mediterranean, the Suez Canal. Well-known and widespread seaweeds are *Caulerpa taxifolia* (killer alga), *C. cylindracea* (grapecauserpa), *Codium fragile* (dead man's finger), the invasive strain of *Asparagopsis taxiformis*, and *Womersleyella setacea*. *Halophila stipulacea* (broadleaf seagrass), the sole alien seagrass species in
<figure><img src="image_9.png" /><figcaption>**Fig. 7.** Trends in numbers of established alien species in Europe & Central Asia. Panels by taxon show cumulative numbers (left panels, thick lines) and number of new alien species per five-year intervals (right panels, thin lines). Numbers shown here underestimate the actual extent of alien species occurrences due to a lack of data. Lines in right panels indicate smoothed trends calculated as running medians. Note that numbers presented may deviate from those reported in the text due to variation among data sources.</figcaption></figure>

the Mediterranean, was first recorded in 1894, a couple of decades after the opening of the Suez Canal (Fritsch, 1895). It has since spread across the Mediterranean. In the 2000s, it spread to the Caribbean (Winters et al., 2020).

(5) **Animalia**

(a) *Historical trends*

The number of alien animal species in Europe & Central Asia is increasing across various groups, including vertebrates (Rabitsch & Nehring, 2017), insects (Roques et al., 2016), molluscs (Peltanová et al., 2012), and freshwater animals (Nunes et al., 2015; Muñoz-Mas & García-Berthou, 2020), especially for Central and Western Europe (Fig. 7). The number of alien mammals introduced to European countries accelerated particularly in the late 19th century and continues to increase (Genovesi et al., 2012). The rates of new records of alien species remained low over the last 200 years, but rose sharply in recent decades for birds and invertebrates

(Seebens et al., 2017). Amphibians, reptiles, and mammals share a similar pattern: historical events and trade routes around the Mediterranean Basin have resulted in some of the oldest known introductions of vertebrates in the world. The observed increases in alien species numbers will likely continue (Seebens et al., 2017, 2021a), and the pet trade is expected to contribute more species in the near and medium future (Pleguezuelos, 2002; Mateo, Ayres & López-Jurado, 2011).

Introductions of alien freshwater fishes increased after the mid-19th century due to the activities of acclimatisation societies, mainly for angling (Gherardi et al., 2009), and again after World War II due to more intensive trade, openings of major inland canals and waterways, and the intensification of aquaculture (Goren & Ortal, 1999; Gherardi et al., 2009; Nunes et al., 2015). In Central and Northern Europe, interconnected canals and waterways were the main pathways of introduction for aquatic invertebrates, while introductions of vertebrates were mainly by releases and escapes linked to
aquaculture and pet and aquarium trades. A slight decrease in introduction rates was reported for recent decades on the Iberian Peninsula (Muñoz-Mas & García-Berthou, 2020). Three North American crayfish were intentionally introduced following the decimation of native crayfish populations: *Faxonius limosus* (spiny-cheek crayfish) in 1890s, *Pacifastacus leniusculus* (signal crayfish) in 1960s, and *Procambarus clarkii* (red swamp crayfish) in the 1970s. These species are ubiquitous and currently spreading in Central and Eastern Europe (Soto *et al.*, 2023). At least five North American species (genera *Faxonius* and *Procambarus*) have been subsequently documented. The fastest spreading crayfish is *Procambarus virginalis* (marbled crayfish), a parthenogenetic species previously related to *P. fallax*. Its fast growth, high fecundity, frequent spawning, short embryogenesis and environmental plasticity has enabled it to establish self-sustaining populations from Estonia to Israel within two decades of its first population in the wild (Kouba, Petrusek & Kozák, 2014; Aluma *et al.*, 2023; Carneiro, Galil & Lyko, 2023).

Across European Seas, the number of recorded alien species has increased continuously until today (Galil *et al.*, 2014; Zenetos *et al.*, 2022). In the North Sea, the annual introduction rate has more than doubled, from 2.9 species per year in 1950–1999 to 4.3 between 2000 and 2014, and 7 between 2015 and 2022 (ICES, 2022), whereas along the coast of Israel their number has tripled between 1970 (138 alien animal species) and 2020 (432 alien animal species), nearly 6 species per year (Galil *et al.*, 2021).

### (b) Current status

Currently, 85 alien mammals are known to be established in Europe & Central Asia, particularly in Central Western Europe, numerous Mediterranean islands, the British Isles, Italy, Scandinavia, Eastern Europe, and European Russia (Biancolini *et al.*, 2021). *Ondatra zibethicus* (muskrat), *Nyctereutes procyonoides* (raccoon dog), and *Neovison vison* (American mink) are among the most widespread (Genovesi *et al.*, 2012; Biancolini *et al.*, 2021; Tedeschi *et al.*, 2022). Many islands in European Seas have been invaded by large numbers of alien mammals for centuries (Bonesi & Palazon, 2007; Ruffino *et al.*, 2009; Chainho *et al.*, 2015; Capizzi, 2020). Europe has also been a hotspot of alien bird introductions for centuries, with highest numbers recorded in the UK, Spain, and Portugal (Dyer *et al.*, 2017a). Although European-Unionwide import bans on caged birds, established after the bird flu epidemic of 2005, have greatly restricted the bird trade with other continents (Reino *et al.*, 2017), there is still an extensive within-Europe trade of captive-bred birds that can potentially lead to new introductions. Europe hosts several global hotspots of alien amphibians and reptiles: the Balearic Islands, mainland Spain, Italy, France, and the UK (Ficetola *et al.*, 2007; Kark *et al.*, 2009; Kraus, 2009; Mateo *et al.*, 2011; Capinha *et al.*, 2017). Fewer alien reptiles and amphibians have been reported in Central Asian countries than in Europe (Kraus, 2009; Capinha *et al.*, 2017). Numbers of alien insects have steadily increased in Europe

but this accumulation lagged behind other regions, such as The Americas prior to 1900 (Figs 5 and 7). Historical patterns of insect species spread with Europe have been influenced by changes in governmental alliances and associated changes in trade (Roques *et al.*, 2016).

There have been 534 alien animal freshwater species recorded in Europe & Central Asia, with the Iberian Peninsula, France, Italy, the UK, and Germany harbouring the highest numbers (Nunes *et al.*, 2015). The most numerous known introduced organisms are fishes, arriving through stocking, aquaculture, or pet and aquarium trades, followed by crustaceans and molluscs, introduced mainly *via* ornamental trade and through canals and waterways (Nunes *et al.*, 2015). Fish, such as *Cyprinus carpio* (common carp), *Sander lucioperca* (pike perch), *Silurus glanis* (European catfish), or Ponto-Caspian gobies, have now established in much of European fresh waters (Leprieur *et al.*, 2008) and threaten native fish faunas with a high level of endemism (Clavero & García-Berthou, 2006). Data from Central Asia are scarce. At least, 31 alien freshwater fishes have been recorded in Uzbekistan (Yuldashov, 2018). Many freshwater and brackish water invertebrates, such as *Dreissena polymorpha* (zebra mussel), *D. bugensis* (quagga mussel), and many amphipods, such as *Dikerogammarus villosus* (killer shrimp), reached Europe from their Ponto-Caspian native range through human-made canals built for transport (Bij de Vaate *et al.*, 2002). Other Asian crustaceans were introduced through rice cultivation, which brought at least 13 ostracods to Mediterranean wetlands (Bisquert-Ribes *et al.*, 2023). The Chinese pond mussel (*Sinanodonta woodiana*) has been introduced through the introduction of fish, which were infected by the parasitic larvae of the mussel (Douda *et al.*, 2025).

The European Union (EU) has encouraged its member states to assess the status of marine alien species as part of the Marine Strategy Framework Directive (MSFD). Yet, there is no scientifically validated current inventory of marine alien animals recorded across European coastal and adjacent waters, and data are currently scattered among national and regional reports and databases. Several studies are available for individual countries, reporting numbers of marine alien animal species off, for example, Finland (27; Outinen *et al.*, 2024), Denmark (55; Jensen *et al.*, 2023), Republic of Ireland (65; Gittenberger *et al.*, 2023a), the Netherlands (133; Gittenberger *et al.*, 2023b), and the Atlantic coast of Spain (89; Png-Gonzalez *et al.*, 2023). The number of marine alien animal species recorded in the Mediterranean Sea (where only 8 of 22 countries are EU member states) is 810 (of a total of 1006 alien species) (Galanidi *et al.*, 2023), of which about 75% have been introduced through the Suez Canal (Galil *et al.*, 2021). Examples of marine invasions are the expansion of *Lagocephalus sceleratus* (silver-cheeked toad-fish) throughout the Mediterranean, an avid predator introduced through the Suez Canal (Ulman *et al.*, 2024), the spread of *Neogobius melanostomus* (round goby), a Ponto-Caspian introduction into the Baltic Sea (Kornis, Mercado-Silva & Vander Zanden, 2012), and the introduction of the
comb jellyfish *Mnemiopsis leidyi* (sea walnut) into the Black Sea (Oguz, Fach & Salihoglu, 2008).

## IX. ANTARCTICA

Antarctica has been much less affected by alien species than other regions, for several reasons: Antarctica is difficult to access, anthropogenic pressures have been low so far (Bennett, 2015; McGeoch et al., 2015; Galera et al., 2018), and its inhospitable environments, such as low nutrient soils, freezing temperatures, and high ultraviolet (UV) levels, do not favour establishment of alien species. However, climate change and increased human activities through tourism and research are enhancing introductions (Bender, Crosbie & Lynch, 2016; Duffy et al., 2017; Bartlett, Convey & Hayward, 2020; Chwedorzewska, Korczak-Abshire & Znój, 2020). Plants (seeds, fragments, and other propagules) and invertebrates, such as springtails, were introduced on clothing and personal equipment of tourists and national Antarctic programme ship and aircraft personnel, as well as associated with packing materials (Chown et al., 2012; Huiskes et al., 2014), vehicles (Hughes et al., 2010), and fresh food imports (Hughes et al., 2011). In 11 years of surveillance (2006–2017) at the Scott Base in the Ross Sea region of continental Antarctica, 68 invertebrate species (16 of which are known to be invasive elsewhere globally, including in some instances the broader Antarctic region) were intercepted on food (60%), clothing and equipment (11%), aircraft and cargo (11%), and packaging material (11%) (Newman et al., 2018). During 2007–2008, more than 20 alien lichens and fungi were intercepted in Antarctica in packaging, foodstuffs, and timber (Osyczka, 2010; Osyczka et al., 2012). Seeds of eight alien plant species were reported in the topsoil of Fildes Peninsula, King George Island (Antarctica), in areas intensively frequented by humans (Fuentes-Lillo et al., 2017).

Terrestrial alien plants in the Antarctic are predominantly herbs, mostly introduced unintentionally with soils or imported fodder for domestic animals (Frenot et al., 2005; Chwedorzewska et al., 2015). Alien plants have been introduced on several occasions since the 1950s: for example, *Poa pratensis* (smooth meadow-grass) was introduced unintentionally during tree transplantation experiments in the 1950s and eradicated in 2015 (Pertierra et al., 2017). Altogether, 15 alien species are known to occur in Antarctica: one plant, *Poa annua* (annual bluegrass), and 14 invertebrates [seven Collembola, four Arachnida, two Insecta (Diptera), one Annelida], most of which are found in the Antarctic Peninsula region (Hughes, Cowan & Wilmotte, 2015; Baird et al., 2019; Enríquez et al., 2019, Hughes et al., 2020). This could be due to several factors. The Antarctic Peninsula is the area closest to another continent (South America), it is the least climatically extreme region of Antarctica (and has also experienced a rapid rise in temperatures since the 1950s due to climate change) and has the largest

concentration of human activity (due to research teams and tourism) resulting in a relatively high propagule pressure (Hughes et al., 2020). On the sub-Antarctic islands, which circle the continent, at least 108 alien plants, 72 terrestrial invertebrates, and 16 vertebrates are reported (Frenot et al., 2005).

Alien vertebrates with established populations are reported only for sub-Antarctic islands. Environmental conditions on the continent of Antarctica itself are too extreme unless the species can live synanthropically: some mammals, such as rats and mice, were unintentionally introduced as early as the 18th century, while others, such as ungulates, cats, rabbits, and salmonids, were intentionally introduced beginning in the early 20th century (Frenot et al., 2005; Lecomte et al., 2013). Alien invertebrates, such as the springtail *Hypogastrura viatica*, were reported from the 1940s onwards in the Antarctic (Hack, 1949; Hughes et al., 2015).

To date, five marine alien invertebrate species have been found (plus one cryptogenic seaweed species); these were free-living specimens with no known established populations (McCarthy et al., 2019; Cárdenas et al., 2020). Marine alien species were likely introduced by vessels (three by hull fouling, one by ballast water), with the first recorded alien species (a bryozoan) dating back to 1960, followed in 1986 by a crab, and in 1996 by a tunicate and a hydroid; the most recent introduction (a mollusc) was recorded in 2019, although it is likely that this species has subsequently disappeared (McCarthy et al., 2019; Cárdenas et al., 2020). There is no evidence that any of these species are established in Antarctica (McCarthy et al., 2019).

The number of alien species is expected to increase in the future due to climate change and increasing human pressure, but reported numbers are also expected to be higher due to the greater research effort, as noted by the growing number of relevant publications (Hughes & Pertierra, 2016; Duffy et al., 2017; Ricciardi et al., 2017; Chan et al., 2019; Chwedorzewska et al., 2020). A recent horizon scan for future potentially invasive alien species in the Antarctic Peninsula underlined the main threat posed by marine invertebrates that can be unintentionally transported in ballast waters and on ship hulls (McCarthy et al., 2019; Hughes et al., 2020). The threat could be even greater considering the cruise ship volume from the Northern Hemisphere to Antarctica that may increase the probability of introduction.

## X. FUTURE TRENDS OF BIOLOGICAL INVASIONS

Many studies have been conducted to explore the potential future developments of the distribution and accumulation of alien species at various geographic scales. Across eight continental regions, alien species numbers for seven major taxonomic groups are projected to increase on average by 36% until 2050 under a business-as-usual scenario (Seebens et al., 2021a). Taxon-specific studies at global or regional scales about future invasion potential have been conducted
for different individual species or taxonomic groups, such as ants and termites (Chen, 2008; Buczkowski & Bertelsmeier, 2017), beetles (Berzitis et al., 2014; Wang et al., 2017), flies (Hill et al., 2016; Ryan et al., 2019), other insects (Hill, Gallardo & Terblanche, 2017; Lu et al., 2020), amphibians (Ilhlow et al., 2016; Forti et al., 2017), fish (Liu et al., 2019; Dong et al., 2020), and mammals (Louppe et al., 2019, 2020; Biancolini et al., 2024), mostly projecting an increase in range sizes. Species distribution models for 100 alien species with severe impacts (as assessed by the IUCN) found a decreased potential for future global distribution of mammals, birds, fishes, reptiles, and amphibians, but an increase in distributions of aquatic and terrestrial invertebrates due to region-specific projected changes in climate (Bellard et al., 2013).

Species-specific studies about future distributions of alien microorganisms are available only for a few species and the majority deal with just one species or multiple species from one genus (e.g. *Phytophthora*; Scott et al., 2019). In general, invasion potentials are projected to be higher than currently observed, both in terms of numbers of alien fungi present (Bebber et al., 2019; Barwell et al., 2021) and of occupied range (Kriticos et al., 2013; Feldmeier et al., 2016). For crop pests including herbivorous arthropods, pathogenic microbes and viruses, numbers within regions are projected to be higher than observed levels (Bebber et al., 2019). Hotspots of pest invasion are in Central America, Europe, East Asia, and Australia (Bebber, 2015). Global plant pathogen studies project an increase in potentially suitable areas, especially towards higher latitudes (Burgess et al., 2017; Avila et al., 2019). Crop pests are projected to shift poleward under climate change and increased human activities (Fisher et al., 2012; Bebber, Ramotowski & Gurr, 2013) and under currently observed trends, the main crop-producing countries will be saturated with crop pathogens by 2050 (Bebber et al., 2014).

Future hotspots for alien plant invasions have been identified in Europe, South America, North America, Southwest China, and New Zealand as well as the coast of West Africa and the Southern coast of Asia (Wan, Wang & Yu, 2016), while potential hotspots for cacti emerge in the Mediterranean, tropical savanna regions, and xeric shrubland biomes (Masocha & Dube, 2018). Most projections of future distributions of alien plant species identified expanding ranges (Adhikari, Tiwari & Barik, 2015; Wan et al., 2016; Dullinger et al., 2017), although for some species ranges are projected to shrink (Bellard et al., 2013). Studies for the USA and Europe suggest that most current invasion hotspots will remain spatially stable, but potential alien species numbers will increase by 64–102% (Allen & Bradley, 2016). For Europe, increases of alien plant numbers were projected for northern parts, while southern parts may experience declining numbers (Chytrý et al., 2012). South American countries, such as Brazil, Mexico, and Argentina, are expected to face comparatively strong increases in numbers of alien plant species based on global trade dynamics and climate change (Seebens et al., 2015), which may invert the current status of North America as more invaded by plants than South America (Pyšek et al., 2019).

Studies of alien animals often found projected expansions of current distributions. For example, for birds *Corvus splendens* (house crow) and *Acridotheres tristis* (common myna), the current distributions indicate a large potential to spread to new areas (Nyári, Ryall & Peterson, 2006; Magory Cohen et al., 2019). Similarly, mammals, such as *Sus scrofa* (feral pig), *Urva auropunctata* (small Indian mongoose), and *Procyon lotor* (raccoon), often have a large potential for further spread worldwide (Lewis et al., 2017; Louppe et al., 2019, 2020; Biancolini et al., 2024). For insects, several studies investigated the invasion potential of agricultural pest species (Kroschel et al., 2013; Kriticos et al., 2017; Marchioro & Krechemer, 2018), with the *Spodoptera frugiperda* (fall armyworm) (Early et al., 2018) and *Drosophila suzukii* (spotted wing drosophila) being prominent examples (dos Santos et al., 2017), and all studies found a high risk of invasion beyond the current realised distribution. The ranges of many problematic invertebrate invaders other than insects have also been projected to expand, such as the harmful freshwater bivalves *Dreissena* (zebra and quagga mussels), *Linnoperna* (golden mussel), and *Corbicula* (basket clams) (Gama et al., 2016; Petsch et al., 2021). In the marine realm, a study of 19 ascidian species found a large invasion potential especially at higher latitudes (Lins et al., 2018). Projections of planktonic and benthic species, as well as algae, suggest that under climate change scenarios the probability of establishment of alien species will increase at higher latitude (Seebens et al., 2016; Goldsmit et al., 2020).

In summary, the suite of studies available for projections of future dynamics of alien species suggests that overall ranges of alien species are expected to increase in most cases although with large variation due to a continuous introduction of new individuals and an expansion of ranges. In addition, ranges are expected to shift poleward as a consequence of global warming (Walther et al., 2009). However, projections of future dynamics of alien and invasive alien species are severely limited by (i) data availability of past and current distributions of species, (ii) limits in understanding of causal relationships between species occurrences, environmental changes, drivers of biological invasions, and the roles of dispersal, biotic interactions, and impacts caused by invasive alien species, (iii) lack of models to project future dynamics of biological invasions robustly, and (iv) the lack of scenarios covering a range of plausible future dynamics of drivers, which would allow exploring future trends under different scenarios. While models and scenarios can still be developed further, closing data gaps, particularly of historic distributions, is very difficult and even impossible in many cases.

## XI. WAYS FORWARD TO IMPROVE ASSESSMENTS ON BIOLOGICAL INVASIONS

Although we aimed to provide a balanced global overview for both regions and taxonomic coverage of species, we acknowledge that a complete global coverage across all
regions and taxa remains elusive due to many data and knowledge gaps. The true extent of biological invasions is vastly underestimated, as large numbers of species are probably not yet identified as established in many regions. No checklist is likely to be complete, but particularly large gaps in data exist for most African countries and parts of Asia; for invertebrates, fungi, SAR, and Bacteria; and for freshwater and marine species compared to terrestrial species.

To close the information gaps, we call for funding agencies, administrations, and scientists to collaborate to address the following key challenges (Fig. 8).

(1) *Checklists of alien species are lacking for many taxonomic groups and regions.* Checklists (i.e. species lists for individual regions) of alien species are missing for many taxonomic groups, particularly invertebrates, fungi, SAR, and prokaryotes, and for many countries in Africa and Asia. Developing such checklists is essential to enable more comprehensive and robust assessments of biological invasion trends.

(2) *Existing checklists of alien species are often incomplete and outdated.* We found substantial differences in alien species occurrences among neighbouring countries and across taxonomic groups. Such variations may arise from varying survey efforts and accessibility of data of alien species rather than actual occurrences. Because the spread of alien species is highly dynamic, maintaining up-to-date occurrence lists requires substantial investment in regular monitoring, taxonomic expertise, and continuous maintenance of databases and standards.

(3) *Available alien species lists lack standardisation.* Studies and reports often use different terms, definitions, concepts, taxonomies, data collection, and sampling methods making comparisons across regions and taxa challenging, particularly for distinguishing invasion status (i.e. introduced, established, and invasive). Such distinctions are often not specified, or if they are, definitions are often lacking (Wilson *et al.*, 2020). Comparisons and assessments of biological invasions need

international standards in monitoring and reporting (Latombe *et al.*, 2017; Packer *et al.*, 2017; Meyerson *et al.*, 2022).

(4) *Data are often difficult to access or are inaccessible.* Information about alien species occurrences is often stored in formats that are inaccessible or challenging to access, such as appendices of articles or tables in pdf documents (Crall *et al.*, 2006). Integration of standardised data into open databases or data portals, such as GBIF or the Ocean Biodiversity Information System (OBIS), would enable researchers and stakeholders to access information in a standardised way and to conduct tailored biodiversity assessments. Recording and storing data should follow standard and published protocols to make science, decision-making, and the assessment of biodiversity comprehensive, transparent, interoperable, and reproducible, which ultimately increases trust in results and decisions (e.g. De Pooter *et al.*, 2017; Groom *et al.*, 2017; Roy *et al.*, 2018; Haider *et al.*, 2022).

(5) *Coarse spatial resolutions of occurrences impede assessments of distribution and spread.* Although checklists provide the basis for analysing status and trends of alien species, their spatial resolution is too coarse to enable a robust assessment of distributions and spread of alien species. This is particularly problematic for large countries, but introduces challenges for monitoring species even within smaller countries. Further, checklists are often restricted to administrative units, such as countries, although information at biogeographic (e.g. biomes, islands) or ecological (e.g. habitat types) levels would allow improved assessments. Accurate assessments of biological invasions across spatial scales require data at finer spatial resolutions that are ideally geo-referenced. Only very few countries, such as the UK or Czech Republic, have updated information on alien species occurrences in the form of raster mapping (Preston, Pearman & Dines, 2002; Pyšek *et al.*, 2022).

(6) *Addressing the challenge of biological invasions requires international collaboration.* Greater international collaboration is

<figure><img src="image_10.png" /><figcaption>**Fig. 8.** Six recommendations to improve the assessment of the status of the distribution of alien species. GBIF, Global Biodiversity Information Facility; OBIS, Ocean Biodiversity Information System.</figcaption></figure>
needed to build more robust global networks for monitoring, data sharing, and technology transfer (Packer et al., 2017; Nuñez et al., 2021; Kuebbing et al., 2022; Meyerson et al., 2022; Soubeyrand et al., 2024). While several research networks, database repositories, intergovernmental and international organisations, and international agreements related to biological invasions do exist (reviewed in Meyerson et al., 2022), additional coordination and collaboration are needed, particularly because individual countries often lack the capacities to respond appropriately to biological invasions (Early et al., 2016; Pyšek et al., 2020). Engaging in discussions based on a genuine interchange that recognises and incorporates differences in knowledge, values, perspectives, and interests across political boundaries is critical (Courchamp et al., 2017) to increase understanding of biological invasions and support data acquisition.

Thoroughly assessing the trends and status of biodiversity requires deep knowledge about nature and the ecosystems supporting biodiversity. Although information about nature is accumulating at an unprecedented pace, major knowledge gaps persist, particularly for inconspicuous organisms, such as invertebrates, fungi, SAR, and Bacteria, and less-accessible systems, such as marine ecosystems and inland waters, and in geographic areas such as Central Africa, Central Asia, Antarctica, and several remote islands. Our current knowledge of species interactions with their environment is inadequate to understand comprehensively how species respond to environmental changes in order to build sufficient models to anticipate future biodiversity change under different scenarios of human development and climate change. Reducing these knowledge gaps is therefore key to improve policies that can safeguard nature and move societies towards sustainability. New technologies ranging from satellite products, automated sampling, citizen science and environmental DNA (eDNA) will help in gathering information and reducing gaps. Still, knowledge gaps will persist and must be considered explicitly when assessing status and trends of biological invasions.

## XII. CONCLUSIONS

1. Increasing amounts of data and new databases on various facets of biological invasions have become available recently. However, drawing robust conclusions from data also requires the assessment of data and knowledge biases and gaps by experts. To evaluate changes over time, such assessments and updates must be done regularly; this is essential for informing stakeholders and policymakers, and for guiding targeted management efforts.

2. Every region on Earth has experienced introductions of alien species. We have reports of introductions and establishments of alien species even from the most remote and inhospitable places, often in surprisingly large quantities.

3. The reported distribution of alien species worldwide is highly uneven. Many reports are from economically wealthy countries and far fewer are from countries of the Global South. It remains unclear how much collated distribution patterns reflect sampling effort rather than the 'true' distribution of alien species.

4. Numbers of alien species are increasing for all taxonomic groups and regions, and this trend is likely to continue.

5. Available information about numbers and distributions of alien species is often very incomplete. As a result, numbers and distributions shown in this review represent (sometimes severe) under-estimates. Although we can draw robust conclusions about the general trends of biological invasions for many regions and taxonomic groups, much more effort and data are needed to facilitate more thorough assessments of the status and trends for all taxonomic groups and regions.

6. We provide six recommendations to improve the situation of assessing status and trends of alien species. These cover priorities relating to data collation (including developing and applying standards), free and FAIR data provisioning, and concerted efforts across borders, among others.

## XIII. ACKNOWLEDGEMENTS

We are grateful to the IPBES, whose member states requested the production of the Invasive Alien Species Assessment, approved its summary for policymakers and accepted its chapters. We are indebted to the entire expert team and to the many contributing authors, to the members of the IPBES Bureau and Multidisciplinary Expert Panel who were part of the Assessment's management committee, and to the external reviewers who generously provided their expertise through the Assessment. The assistance, guidance, and leadership from the IPBES secretariat and the technical support unit have been invaluable. We thank our host organisations for their ongoing encouragement and support. H. S. acknowledges funding by the Deutsche Forschungs-gemeinschaft (DFG, German Research Foundation) (grant no. 521529463). P. P. and A. No. were supported by EXPRO grant no. 19-28807X (Czech Science Foundation) and long-term research development project RVO 67985939 (Czech Academy of Sciences). A. No. was also supported by the Ministry of Science, Innovation and Universities MCIN/AEI/10.13039/501100011033 and the FSE+ (Grant No. RYC2022-037905-I). L. A. M. was supported by The University of Rhode Island College of Environmental Life Sciences. D. M. R. acknowledges support from the project "Mobility 2019" (CZ.02.2.69/0.0/0.0/18_053/0017850), co-financed by the European Social Fund (ESF, Funder DOI-10.13039/501100004895) and the state budget of the Czech Republic through the Operational Programme Research, Development and Education, administered by the Ministry of Education, Youth and Sports of the Czech Republic, and the DSI-NRF Centre of Excellence for Invasion Biology. F. C. acknowledges funding by the CNRS
and the AXA Research Fund Chair of Biological Invasion at University of Paris Saclay. K. A. H. was supported by the UK Natural Environment Research Council (NERC) and contributes to the Scientific Committee on Antarctica Research (SCAR) Scientific Research Programme ‘Integrated Science to Inform Antarctic and Southern Ocean Conservation’ (Ant-ICON). C. H. was supported by the National Research Foundation of South Africa (NRF 89967). M. T. was supported by LOEWE in the framework of the Centre for Translational Biodiversity genomics (TBG) funded by the government of Hesse. P. G.-D. was supported by project CONTAIN under the Latin American Biodiversity Programme as part of the Newton Fund (NE/S011641/1). N. F. was supported by Fondecyt No 1181688. S. A. B. was supported by Fisheries and Oceans Canada and Transport Canada. A. P. was funded by the Chilean Agencia Nacional de Investigación y Desarrollo (ANID) ANID/BASAL FB210006, ANID ACT 210038 and Fondo Nacional de Desarrollo Científico y Tecnológico (Fondecyt 1231616). E. G.-B. was funded by the Spanish Ministry of Science, Innovation and Universities (MCIN/AEI/10.13039/501100011033) and the European Union (NextGenerationEU/PRTR) through projects PID2019-103936GB-C21, TED2021-129889B-I00, and RED2022-134338-T. E. M. was supported by FCT - Fundação para a Ciência e Tecnologia, I. P. by project reference UIDB/04004/2020 and DOI identifier 10.54499/UIDB/04004/2020. E. S. and A. B. were partially funded by CONICET (PIP 11220210100507). J. R. S. V. is very grateful to the Brazilian National Council for Scientific and Technological Development (CNPq) for research productivity scholarship (Process 310471/2023-0). E. T. acknowledges the support of the Project funded under the National Recovery and Resilience Plan (NRRP), Mission 4 Component 2 Investment 1.4 - Call for tender No. 3138 of 16 December 2021, rectified by Decree no. 3175 of 18 December 2021 of the Italian Ministry of University and Research funded by the European Union - Next-GenerationEU; Award Number: Project code CNS 00000033, Concession Decree No. 1034 of 17 June 2022 adopted by the Italian Ministry of University and Research, CUP J83C22000870007 and B833C22002910001, Project title ‘National Biodiversity Future Center - NBFC’. Open Access funding enabled and organized by Projekt DEAL.

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