A global synthesis of naturalised and invasive plants in aquatic habitats
Abstract
Global databases have contributed to our understanding of alien, naturalised and invasive plant species distributions. Still, the role of species invasions in habitats, specifically in aquatic habitats, remains underexplored at the global scale. Accordingly, a comprehensive global synthesis of the status of plant invasions in aquatic habitats has been missing. Here, we focus on macroecological patterns of naturalised non-invasive and invasive plants in aquatic habitats using the recently built SynHab database. Amongst all the plant records compiled in SynHab, 592 are assigned to aquatic habitats, of which 183 are unique plant taxa (further termed 'species') belonging to 49 families. Of the total number of records, 462 refer to taxa with naturalised non-invasive occurrences and 130 to invasive occurrences. The species pool analysed here refers to 78 regions distributed across all botanical continents as defined by the World Geographical Scheme for Recording Plant Distributions. The number of naturalised non-invasive aquatic species is similar across different continents and biomes, but Tropical Asia had more and the Mediterranean zonobiome had fewer invasive species than expected. Tropical Asia, Temperate Asia and Africa have the highest proportions of naturalised species that have become invasive, while across continents, invasive proportions were highest for tropical and subtropical zonobiomes. New Zealand, Italy and California contained disproportionately more naturalised species than expected, given the area covered by aquatic habitat in those regions, whereas South Sudan, Papua New Guinea and Kyrgyzstan had disproportionately fewer species. In pairwise dissimilarity comparisons, all continents had distinct species compositions (from 0.73 to 0.92 of the Jaccard dissimilarity index) and so did zonobiomes (0.69 to 1.00). The high proportion of invasive species in Tropical Asia in comparison with terrestrial invasions in this region, indicates a greater susceptibility of warmer regions to aquatic plant invasions. This may be exacerbated by further naturalisations in the future, as data from temperate regions suggest a larger pool of available species.
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473 A global synthesis of naturalised and invasive plants in aquatic habitats Alessandra Kortz1, Martin Hejda1, Jan Čuda1, Zarah Pattison2, Josef Brůna3, Ana Novoa1,4 , Jan Pergl1, Pavel Pipek1,5 , Kateřina Štajerová1, Paulina Anastasiu6, Michael Ansong7, Margarita Arianoutsou8, Julie F. Barcelona9, Suneeta Bhatta1,5 , Farzaneh Bordbar10 , Israel Borokini11 , Laura Celesti-Grapow12 , Eduardo Chacón-Madrigal13,14 , Wayne Dawson15 , Dorjee16 , Franz Essl17 , Lilian Ferrufino-Acosta18 , Estrela Figueiredo19 , Rodolfo Flores20 , Guillaume Fried21 , Nicol Fuentes22 , Pablo Galán23 , Christian Gilli24 , Michael Glaser17 , José Ramón Grande Allende25 , Zigmantas Gudžinskas26 , Rachael Holmes27 , Philip E. Hulme28 , Inderjit29 , Eun Su Kang30 , Holger Kreft31 , Dan W. Krix32 , Ingolf Kühn33 , Omar Lopez34,35 , AnaLu MacVean36 , Trobjon Makhkamov37,38 , Elizabete Marchante39 , Hélia Marchante40 , Alfred Maroyi41 , Rachid Meddour42 , Pierre Meerts43,44 , Sharif A. Mukul45,46,47 , Brad R. Murray48 , Megan L. Murray48 , Daniel L. Nickrent49 , Prince E. Norman50 , Ali Omer17,51 , Annette Patzelt52 , Pieter B. Pelser9, Joan Pino53,54 , Marc Riera53 , Dagoberto Rodríguez Delcid55 , Julissa Rojas-Sandoval56 , Roser Rotchés-Ribalta53,57 , José Yader Sageth Ruiz-Cruz58,59 , Stepan Senator60 , Alexander N. Sennikov61 , Bharat Babu Shrestha62 , Gideon F. Smith19 , Sima Sohrabi63 , Barbara Tokarska-Guzik64 , Mark van Kleunen65,66,67 , Montserrat Vilà68,69 , Viktoria Wagner70 , Patrick Weigelt 31,71 , Marten Winter72 , Ayşe Yazlık73 , Elena Zykova74 , Petr Pyšek1,5 1 DepartmentofInvasionEcology,InstituteofBotany,CzechAcademyofSciences,Průhonice,CzechRepublic 2 Faculty of Natural Sciences, Biological and Environmental Sciences, University of Stirling, Stirling, Scotland, UK 3 DepartmentofGeoecology,InstituteofBotany,CzechAcademyofSciences,Průhonice,CzechRepublic 4 EstaciónExperimentaldeZonasÁridas,ConsejoSuperiordeInvestigacionesCientíficas(EEZA-CSIC),Almería,Spain 5 DepartmentofEcology,FacultyofScience,CharlesUniversity,Prague,CzechRepublic 6 FacultyofBiology&BotanicalGardenD.Brandza,UniversityofBucharest,Bucharest,Romania 7 DepartmentofSilvicultureandForestManagement,FacultyofRenewableNaturalResources,KwameNkrumahUniversityofScienceandTechnology,Kumasi,Ghana 8 Department of Ecology and Systematics, Faculty of Biology, National and Kapodistrian University of Athens, Athens, Greece 9 SchoolofBiologicalSciences,UniversityofCanterbury,Christchurch,NewZealand 10 UniversitéLibredeBruxelles,Herbarium,Brussels,Belgium 11 Department of Ecology, Montana State University, Bozeman, USA 12 DepartmentofEnvironmentalBiology,SapienzaUniversity,Rome,Italy 13 HerbarioNacional,MuseoNacionaldeCostaRica,SanJosé,CostaRica 14 CentrodeInvestigaciónenBiodiversidadyEcologíaTropical,UniversidaddeCostaRica,SanJosé,CostaRica 15 Department of Evolution, Ecology and Behaviour, Institute of Infection, Veterinary and Ecological Sciences, University of Liverpool, Liverpool, UK 16 NationalPlantProtectionCentre,DepartmentofAgriculture,Thimphu,Bhutan 17 DivisionofBioInvasions,GlobalChange&Macroecology,DepartmentofBotanyandBiodiversityResearch,UniversityofVienna,Vienna,Austria 18 HerbarioTEFH,EscueladeBiología,FacultaddeCiencias,UniversidadNacionalAutónomadeHonduras,Tegucigalpa,Honduras 19 RiaOlivierHerbarium,DepartmentofBotany,NelsonMandelaUniversity,Gqeberha,SouthAfrica 20 DepartamentodeBotánicayHerbarioPMA,FacultaddeCienciasNaturales,ExactasyTecnología,UniversidaddePanamá,Panamácity,Panama 21 PlantHealthLaboratory,ANSES,Montferrier-sur-Lez,France 22 DepartamentodeBotánica,FacultaddeCienciasNaturalesyOceanográficas,UniversidaddeConcepción,Concepción,Chile 23 LaLibertad,ElSalvador 24 DivisionofSystematicandEvolutionaryBotany,DepartmentofBotanyandBiodiversityResearch,UniversityofVienna,Vienna,Austria 25 Departamento de Botánica, Facultad de Ciencias, Universidad de Granada, Granada, Spain 26 LaboratoryofFloraandGeobotany,StateScientificResearchInstituteNatureResearchCentre,Vilnius,Lithuania Copyright: © Alessandra Kortz et al. This is an open access article distributed under terms of the Creative Commons Attribution License (Attribution 4.0 International – CC BY 4.0). Research Article NeoBiota 102: 473–494 (2025) DOI: 10.3897/neobiota.102.151156 Advancing research on alien species and biological invasions A peer-reviewed open-access journal NeoBiota
474 NeoBiota 102: 473–494 (2025), DOI: 10.3897/neobiota.102.151156 Alessandra Kortz et al.: A global synthesis of plant invasions in aquatic habitats 27 Department of Geography, Geology and the Environment, University of Leicester, Leicester, UK 28 TheCentreforOneBiosecurityResearch,AnalysisandSynthesis,DepartmentofPest-ManagementandConservation,LincolnUniversity,Canterbury, NewZealand 29 DepartmentofEnvironmentalStudies,CentreforEnvironmentalManagementofDegradedEcosystems(CEMDE),UniversityofDelhi,Delhi,India 30 KoreaNationalArboretum,Pocheon,SouthKorea 31 Department of Biodiversity, Macroecology & Biogeography, University of Göttingen, Göttingen, Germany 32 NewSouthWalesRuralFireService,Sydney,Australia 33 DepartmentofCommunityEcology,HelmholtzCentreforEnvironmentalResearch-UFZ,Halle,Germany 34 SmithsonianTropicalResearchInstitute,PanamaCity,Panama 35 Inter-AmericanInstituteforGlobalChangeResearch,PanamaCity,Panama 36 EnvironmentalHorticultureDepartment,YorkCollegeofPennsylvania,York,USA 37 DepartmentofBotanyandGenetics,NationalUniversityofUzbekistan,Tashkent,Uzbekistan 38 DepartmentofForestryandLandscapeDesign,TashkentStateAgrarianUniversity,Tashkent,Uzbekistan 39 CentreforFunctionalEcology–ScienceforPeople&thePlanet,AssociateLaboratoryTERRA,DepartmentofLifeSciences,UniversityofCoimbra, Coimbra,Portugal 40 ResearchCentreforNaturalResourcesEnvironmentandSociety(CERNAS),PolytechnicUniversityofCoimbra,CoimbraAgricultureSchool(ESAC), Coimbra,Portugal 41 DepartmentofBotany,UniversityofFortHare,Alice,SouthAfrica 42 DepartmentofAgronomicSciences,FSBSA,MouloudMammeriUniversity,TiziOuzou,Algeria 43 Botanic Garden Meise, Meise, Belgium 44 UniversitéLibredeBruxelles,Brussels,Belgium 45 DepartmentofEnvironmentandDevelopmentStudies,UnitedInternationalUniversity,Dhaka,Bangladesh 46 TropicalForestsandPeopleResearchCentre,UniversityoftheSunshineCoast,Maroochydore,Australia 47 Department of Earth and Environment, Florida International University, Miami, USA 48 SchoolofLifeSciences,UniversityofTechnologySydney,Ultimo,Australia 49 Plant Biology Section, School of Integrative Plant Science, College of Agriculture and Life Science, Cornell University, Ithaca, USA 50 SierraLeoneAgriculturalResearchInstitute(SLARI),Freetown,SierraLeone 51 Department of Forest Management, Faculty of Forestry, University of Khartoum, North Khartoum, Sudan 52 VegetationEcology,LandscapeArchitecture,UniversityofAppliedSciencesWeihenstephan,Freising,Germany 53 CentredeRecercaEcològicaiAplicacionsForestals(CREAF),Bellaterra,Spain 54 BABVE,UniversitatAutònomadeBarcelona,Bellaterra,Spain 55 AsociaciónJardínBotánicoLalaguna,HerbarioLAGU,LaLibertad,ElSalvador 56InstituteoftheEnvironment&DepartmentofGeography,Sustainability,Community,andUrbanStudies,UniversityofConnecticut,Storrs, Connecticut, USA 57 UniversitatdeVic-UniversitatCentraldeCatalunya,Vic,Spain 58 UniversidaddeElSalvador,FacultadMultidisciplinariaOriental,DepartamentodeCienciasNaturalesyMatemática,SeccióndeBiología,SanMiguel, El Salvador 59 Fundación Naturaleza, San Salvador, El Salvador 60 127273,Moscow,Russia 61 BotanicalMuseum,FinnishMuseumofNaturalHistory,UniversityofHelsinki,Helsinki,Finland 62 CentralDepartmentofBotany,TribhuvanUniversity,Kathmandu,Nepal 63 RiceResearchInstituteofIran(RRII),Rasht,Iran 64 InstituteofBiology,BiotechnologyandEnvironmentalProtection,FacultyofNaturalSciences,UniversityofSilesiainKatowice,Katowice,Poland 65 Ecology, Department of Biology, University of Konstanz, Konstanz, Germany 66 ZhejiangProvincialKeyLaboratoryofPlantEvolutionaryEcologyandConservation,TaizhouUniversity,Taizhou,China 67 ZhejiangKeyLaboratoryforRestorationofDamagedCoastalEcosystems,SchoolofLifeSciences,TaizhouUniversity,Taizhou,China 68 EstaciónBiológicadeDoñana,EBD-CSIC,ConsejoSuperiordeInvestigacionesCientíficas,Sevilla,Spain 69 Departamento de Biología Vegetal y Ecología, Universidad de Sevilla, Sevilla, Spain 70 DepartmentofBiologicalSciences,UniversityofAlberta,Edmonton,Canada 71DepartmentofEnvironmentalScience,RadboudInstituteforBiologicalandEnvironmentalSciences(RIBES),RadboudUniversity,Nijmegen, Netherlands 72 GermanCentreforIntegrativeBiodiversityResearch(iDiv)Halle-Jena-Leipzig,Leipzig,Germany 73 DepartmentofPlantProtection,FacultyofAgriculture,DüzceUniversity,Düzce,Turkiye 74 Novosibirsk,Russia Correspondingauthor:AlessandraKortz([email protected])
475 NeoBiota 102: 473–494 (2025), DOI: 10.3897/neobiota.102.151156 Alessandra Kortz et al.: A global synthesis of plant invasions in aquatic habitats Abstract Global databases have contributed to our understanding of alien, naturalised and invasive plant species distributions. Still, the role of species invasions in habitats, specifically in aquatic habitats, remains underexplored at the global scale. Accordingly, a comprehensive global synthesis of the status of plant invasions in aquatic habitats has been missing. Here, we focus on macroecological patterns of naturalised non-invasive and invasive plants in aquatic habitats using the recently built SynHab database. Amongst all the plant records compiled in SynHab, 592 are assigned to aquatic habitats, of which 183 are unique plant taxa (further termed ‘species’) belonging to 49 families. Of the total number of records, 462 refer to taxa with naturalised non-invasive occurrences and 130 to invasive occurrences. The species pool analysed here refers to 78 regions distributed across all botanical continents as defined by the World Geographical Scheme for Recording Plant Distributions. The number of naturalised non-invasive aquatic species is similar across different continents and biomes, but Tropical Asia had more and the Mediterranean zonobiome had fewer invasive species than expected. Tropical Asia, Temperate Asia and Africa have the highest proportions of naturalised species that have become invasive, while across continents, invasive proportions were highest for tropical and subtropical zonobiomes. New Zealand, Italy and California contained disproportionately more naturalised species than expected, given the area covered by aquatic habitat in those regions, whereas South Sudan, Papua New Guinea and Kyrgyzstan had disproportionately fewer species. In pairwise dissimilarity comparisons, all continents had distinct species compositions (from 0.73 to 0.92 of the Jaccard dissimilarity index) and so did zonobiomes (0.69 to 1.00). The high proportion of invasive species in Tropical Asia in comparison with terrestrial invasions in this region, indicates a greater susceptibility of warmer regions to aquatic plant invasions. This may be exacerbated by further naturalisations in the future, as data from temperate regions suggest a larger pool of available species. Key words: Macrophyte invasion, plant invasion patterns, SynHab database Introduction Aquatic habitats are particularly prone to invasion by alien species due to several factors, including natural disturbance regimes (e.g. river flow variation and lake water level fluctuations), high levels of connectivity and anthropogenic activities (e.g. damming for hydropower and water extraction for agriculture) (Richardson et al. 2007). High levels of connectivity, both natural and anthropogenic (such as raw water transfer schemes and canals), make monitoring and managing pathways of invasion in aquatic habitats difficult (Waine et al. 2025). Invasive aquatic plants (macrophytes) often act as autogenic engineers and primary producers (Ricciardi and MacIsaac 2010), i.e. they transform freshwater systems, change habitat structure (Valley and Bremigan 2002; Ceschin et al. 2020) and reduce water quality by altering flow and primary productivity (Perna and Burrows 2005; Gallardo et al. 2016; South et al. 2016), resulting in changes in local native diversity (Schooler et al. 2006). In addition, projected accumulations of naturalised alien plant species suggest faster increases in aquatic systems than in terrestrial habitats (Seebens et al. 2021), indicating that future impacts are likely to intensify. The severity of impacts caused by alien plants is magnified because aquatic habitats are extremely diverse relative to the area they occupy, compared to other habitats (Román-Palacios et al. 2022). The ecological impacts of invasive alien plant species and management efforts impose substantial economic costs. From 1975 to 2040, the documented global costs of invasive macrophytes are projected to reach US$ 32.8 billion (in 2017 dollars, Macêdo et al. 2024). However, this figure likely represents a significant underestimation due to data limitations and geographical gaps in current research Academic editor: Pedro Anastácio Received: 28 February 2025 Accepted: 8 July 2025 Published: 7 October 2025 Citation: Kortz A, Hejda M, Čuda J, Pattison Z, Brůna J, Novoa A, Pergl J, Pipek P, Štajerová K, Anastasiu P, Ansong M, Arianoutsou M, Barcelona JF, Bhatta S, Bordbar F, Borokini I, Celesti-Grapow L, ChacónMadrigal E, Dawson W, Dorjee, Essl F, Ferrufino-Acosta L, Figueiredo E, Flores R, Fried G, Fuentes N, Galán P, Gilli C, Glaser M, Grande Allende JR, Gudžinskas Z, Holmes R, Hulme PE, Inderjit, Kang ES, Kreft H, Krix DW, Kühn I, Lopez O, MacVean A, Makhkamov T, Marchante E, Marchante H, Maroyi A, Meddour R, Meerts P, Mukul SA, Murray BR, Murray ML, Nickrent DL, Norman PE, Omer A, Patzelt A, Pelser PB, Pino J, Riera M, Rodríguez Delcid D, Rojas-Sandoval J, Rotchés-Ribalta R, Ruiz-Cruz JYS, Senator S, Sennikov AN, Shrestha BB, Smith GF, Sohrabi S, Tokarska-Guzik B, van Kleunen M, Vilà M, Wagner V, Weigelt P, Winter M, Yazlık A, Zykova E, Pyšek P (2025) A global synthesis of naturalised and invasive plants in aquatic habitats. In: Anastácio P, Ribeiro F, Chainho P (Eds) Invasions in Aquatic Systems. NeoBiota 102: 473–494. https://doi.org/10.3897/ neobiota.102.151156
476 NeoBiota 102: 473–494 (2025), DOI: 10.3897/neobiota.102.151156 Alessandra Kortz et al.: A global synthesis of plant invasions in aquatic habitats (Macêdo et al. 2024), crucially concerning the underestimated costs of invasive plants (Novoa et al. 2021). This raises additional concerns about the extent to which the economic costs of invasive plants in aquatic habitats may be reliably captured. As such, updated knowledge of regional levels of invasion is crucial for a better understanding of impacts and cost estimates. The World’s 100 worst invaders list (Lowe et al. 2000) provides examples of the harmful effects that plant invasions can have on aquatic habitats. One of the globally most prominent invaders is water hyacinth (Pontederia crassipes Mart., synonym Eichhornia crassipes (Mart.) Solms), which is native to South America and has spread to over 70 countries (Gezie et al. 2018). Pontederia crassipes is a fast-growing floating macrophyte that forms dense mats on the water surface, alters habitat structure and disrupts the ecological functioning of aquatic ecosystems (Tobias et al. 2019). This species also has direct socioeconomic impacts, including reduced navigability and recreational quality, restricted water supply for agriculture due to damaged pipe systems and reduced access to fishing grounds and boating (Kateregga and Sterner 2009; Villamagna and Murphy 2010). Similarly, Azolla filiculoides Lam., often found alongside Pontederia crassipes and Pistia stratiotes L., is a small (< 25 mm) floating macrophyte that impacts physical and chemical water features, reduces the richness and biomass of macrophytes, alters zooplankton composition and reduces the survival of amphibian larvae (Pinero-Rodríguez et al. 2021). Lagarosiphon major (Ridl.) Moss is a submerged macrophyte (Howard-Williams and Davies 1988) that forms dense underwater beds, outcompeting native macrophytes in the range it has invaded (Martin et al. 2018). Specific inherent biological attributes (such as functional traits) determine the invasiveness of species and increase their potential to become invasive (Gioria et al. 2023) by interacting with native biota and the environment (Pyšek et al. 2020). Additionally, one of the most important factors explaining the success of plant invasions is propagule pressure (e.g. Von Holle and Simberloff 2005; Colautti et al. 2006; Cassey et al. 2018), which also applies to invasive macrophytes (Chadwell and Engelhardt 2008; Xie et al. 2013). High levels of connectivity can facilitate the spread of propagules, increasing dispersal distances for invasive plants within and between waterbodies in aquatic habitats (Richardson et al. 2007; Leuven et al. 2009). For example, natural flood events accelerate the dispersal process by facilitating the movement of propagules within catchments (Gurnell et al. 2008; Čuda et al. 2017). Climate-related changes to the flow regime further facilitate the spread of propagules by increasing the frequency of extreme floods (Pattison et al. 2017). In addition, dams create lentic habitats with slow-moving waters, which are preferred by Azolla, Pistia and Pontederia species. Given the expected increase in dam construction to mitigate climate-driven rainfall reductions, this may result in enhanced proliferation of these macrophytes. Raw water transfer schemes (water diversions) move large volumes of freshwater between catchments via complex infrastructure networks, facilitating the long-distance spread of invasive species (Waine et al. 2024a, b). Research on plant invasions in aquatic habitats is under-represented compared to terrestrial habitats, particularly in the Tropics (Evangelista et al. 2014; Havel et al. 2015; McKnight et al. 2017), with certain species or taxonomic groups dominating the literature (Stevenson et al. 2023). Here, we utilise the newly-developed SynHab database (https://www.synhab.com/the-project), which contains information on plant naturalisations and invasions in specific habitat types worldwide (Pyšek et al. 2022; Dawson et al. 2025), to achieve a more balanced perspective of
477 NeoBiota 102: 473–494 (2025), DOI: 10.3897/neobiota.102.151156 Alessandra Kortz et al.: A global synthesis of plant invasions in aquatic habitats the overall diversity of naturalised plants in aquatic habitats. We address this knowledge gap by asking the following questions: (i) Which continents and biogeographical regions (zonobiomes) have the highest levels of naturalisation and invasion of plants in aquatic habitats? (ii) Which aquatic naturalised non-invasive and invasive plants are most widely distributed? (iii) Does the vulnerability of aquatic habitats to naturalisations and invasions of plants differ in temperate vs. tropical regions? (iv) Which regions (regions or states) harbour more naturalised plant species in aquatic habitats than expected? (v) Which regions are more similar in terms of naturalised plant species composition in aquatic habitats? (vi) Which naturalised aquatic plant species are indicative of continents and zonobiomes? Materials and methods Habitat classification and macrophyte definition Habitat classification, adapted from Hejda et al. (2015) for the SynHab database, resulted in 14 categories (Pyšek et al. 2022). The aquatic habitat is a category defined as “waterbodies and streams with submerged and floating plant species”. Macrophytes can be defined in several ways, with some definitions including macroalgae (e.g. Chambers et al. 2008); however, here we restrict our focus to aquatic vascular plants. Species were assigned to the aquatic habitat if they exhibited growth in water as submerged or floating, including littoral species that can cope with long-term flooding, with roots anchored below the water level. Some species, such as Iris pseudacorus L., Eleocharis palustris (L.) Roem. & Schult., Juncus effusus L. and Jacobaea aquatica (Hill) G.Gaertn., B.Mey. & Scherb., can grow in the littoral zones of freshwater or brackish waterbodies, as well as in mud. In such cases, they were assigned to both aquatic and wetland habitats in SynHab, of which only the former is dealt with in the present paper. Species inhabiting mud, but intolerant to prolonged flooding, such as Lysimachia nummularia L. were excluded. Dimorphic species, such as Alternanthera philoxeroides (Mart.) Griseb., exhibiting mud-dwelling and littoral forms, were also affiliated with both aquatic habitats and wetlands. In such cases, species were assigned to the aquatic habitat if the habitat descriptions explicitly confirmed growth in a submerged environment of freshwater lakes, rivers, running waters, coastal basins or streams. Species that grow along rivers, but not in the water were not included. We thus included also aquatic vascular plants that are not exclusively aquatic, but that are important invaders in the aquatic habitat. Data acquisition The data on habitat affiliations for the SynHab database were extracted from literature sources (regional checklists of alien floras), where the verbal description of habitats allowed unequivocal assignment of a given species (see Suppl. material 1: table S1 for the complete references). If published sources lacked information on habitat affiliations, we invited the authors of original publications to collaborate and provide habitat affiliations for SynHab. The SynHab project leaders (PP, MH, AK) assessed the received information to harmonise the habitat classification with other datasets and then the final habitat affiliations were agreed upon in collaboration with data providers. In total, 78 regions (countries or states) from all continents were included (Suppl. material 1: table S1).
478 NeoBiota 102: 473–494 (2025), DOI: 10.3897/neobiota.102.151156 Alessandra Kortz et al.: A global synthesis of plant invasions in aquatic habitats Each plant species was assigned the invasion status as ‘naturalised’ or ‘invasive’ in the region, following the definitions of Richardson et al. (2000) and Blackburn et al. (2011) ca; note that the species’ invasion status can vary across different regions. Invasion status was assigned, based on information in the GloNAF (Global Naturalized Alien Flora) database (van Kleunen et al. 2015, 2019; Pyšek et al. 2017; Davis et al. 2025) or inferred from the description in the original source or provided by data contributors for their regions. In the context of the analyses described below, the term ‘naturalised’ refers to all naturalised taxa and is composed of two separate subsets: ‘naturalised non-invasive’ and ‘invasive.’ Each taxon name, as given in the original source, was standardised according to The World Checklist of Vascular Plants (WCVP) database (https://powo.science.kew.org/about-wcvp; Govaerts et al. 2021) using the rWCVP package for R (Brown et al. 2023). Accepted names of the species were used in the analyses to enable species comparisons amongst regions. Four taxa were identified at the subspecies level, but were merged with the corresponding species level for analyses. Habitat area To quantify the area of aquatic habitats within each region, gridded global land-cover data, with a spatial resolution of 300 m, were acquired from Defourny et al. (2023). These data comprised 37 land-cover classes, conforming to the United Nations Land Cover Classification System (UN-LCCS) (Di Gregorio and Jansen 2005). We converted the year 2019 raster to a shapefile and intersected it with SynHab regions for further analysis. We excluded permanent snow and ice classes, which resulted in 35 classes included in the analyses. Water surface area of classes was summed for use in analyses. Biogeographical variables: continents and zonobiomes Each of the 78 regions for which data on naturalised plant species in aquatic habitats were available was assigned to a continent and a state or country (levels 1 and 4, respectively, of the TDWG World Geographic Scheme for Recording Plant Distributions; Brummitt 2001): Northern America (data available for n = 13 regions); Southern America, n = 11; Africa, n = 16; Europe, n = 16; Temperate Asia, n = 9; Tropical Asia, n = 11; and Australasia, n = 2. Only one record was sampled in the Pacific, which was not considered in the analyses. Further, regions were assigned to one of the following zonobiomes: I. Tropical (equatorial), n = 18 regions; II. Tropical (savannah), n = 15; III. Subtropical (arid), n = 7; IV. Mediterranean, n = 8; VI. Temperate (nemoral), n = 17 regions; VII. Arid temperate (continental), n = 4; VIII. Cold temperate (boreal), n = 1; and n = 8 regions were assigned as “multiple” zonobiomes (Chile, China, Flores, Faial and Santa Maria, Azores, Florida, New South Wales, Russia Kostroma, Russia Middle Volga and Russia Novosibirsk based on Walter and Breckle (1991). While some regions, categorised as ‘multiple’, may encompass the V. Warm Temperate Zonobiome, no regions were exclusively assigned to it. Statistical analyses To test whether naturalisation and invasion in aquatic habitats differ amongst continents and zonobiomes, we used generalised linear models (GLM) with Poisson distribution. Using standardized residuals from generalised linear
479 NeoBiota 102: 473–494 (2025), DOI: 10.3897/neobiota.102.151156 Alessandra Kortz et al.: A global synthesis of plant invasions in aquatic habitats models (GLMs), we determined if observed species richness was significantly higher or lower than expected across zonobiomes and continents. Statistical significance was assessed by comparing these residuals to critical values corresponding to P < 0.05 (± 1.96), P < 0.01 (± 2.58) and P < 0.001 (± 3.29). Positive values indicate higher and negative values indicate lower naturalised species richness than expected by chance. The species-area relationship was calculated by fitting a linear model to the number of all naturalised species recorded and the area of aquatic habitat in each region on a log-log scale. To compare the dissimilarity of the regions in terms of their naturalised species composition in aquatic habitats, we calculated the Jaccard dissimilarity index using the “betapart” package (Baselga and Orme 2012; Baselga et al. 2023). To identify which species were significant indicators of each continent and zonobiome, we used the “multipatt” function of the indicator value analysis (IndVal) in the R package “indicspecies” (Dufrêne and Legendre 1997; De Cáceres and Legendre 2009), calculated considering groups of regions within each continent or zonobiome. Complementary to the species composition analysis, the indicator value analysis determines groups of indicator species by measuring their association with each group (continent or zonobiome) or combination of groups and it does not explicitly consider climate, local environment or introduction history. All statistical analyses were performed in R version 4.4.1 (R Core Team 2024). Results Naturalised and invasive species numbers Of the 585 records in aquatic habitats, 458 referred to naturalised, non-invasive occurrences and 127 to invasive occurrences. The same species can be naturalised, non-invasive in one region and invasive in another; thus, the focus was on the number of occurrences rather than the number of species. In total, we recorded 183 naturalised plant taxa (further referred to as ‘species’ for simplicity) belonging to 46 families and 88 genera; of these 183, there were 52 classified as invasive in at least one region. The 10 families with the largest number of species were Hydrocharitaceae, Plantaginaceae, Cyperaceae, Poaceae, Araceae, Alismataceae, Onagraceae, Nymphaeaceae, Pontederiaceae and Salviniaceae. Naturalised aquatic species occurred in 78 regions on all continents (Suppl. material 1: table S1). Of the sampled regions, 34 (i.e. 43.6%) contained information on invasive species. New Zealand had the largest number of naturalised species (n = 37), followed by California and Italy (n = 29 each), France (n = 27), Florida (n = 23), Chile and Virginia (n = 20). Species recorded as naturalised in at least one region were native to Temperate Asia (n = 94), Northern America (n = 87), Tropical Asia (n = 81), Africa (n = 79), Southern America (n = 77), Europe (n = 58), Australasia (n = 45) and the Pacific (n = 21). The most widespread invader, occurring in 55.7% of the sampled regions (n = 43), was Pontederia crassipes (this species was considered invasive in 15 of the 43 regions), followed by Pistia stratiotes and Elodea canadensis Michx., Azolla filiculoides, Elodea densa (Planch.) Casp. and Myriophyllum aquaticum (Vell.) Verdc., all occurring in at least 25% of the sampled regions (Table 1). In contrast, 105 species (56.6% of all sampled species) were found in only one region.
480 NeoBiota 102: 473–494 (2025), DOI: 10.3897/neobiota.102.151156 Alessandra Kortz et al.: A global synthesis of plant invasions in aquatic habitats Biogeographical patterns The numbers of naturalised aquatic species (considering naturalised non-invasive and invasive species together) per continent and zonobiome were not significantly different from the values expected by chance. However, a different pattern was evident for invasive species. Aquatic habitats in Tropical Asia harboured more invasive species than expected; the opposite was found for the Mediterranean zonobiome, where invasive aquatic species were under-represented (Table 2). Using the proportion of naturalised species that have become invasive as a measure, Tropical Asia, Africa and Tropical Asia ranked the highest amongst continents, with 58.6%, 42.3% and 40.7%, respectively (Fig. 1). Tropical zonobiomes, both Savannah (44.4%) and Equatorial (43.5%) and the Subtropical Arid zonobiome (38.7%) had the greatest proportions of naturalised species recorded as invasive (Fig. 1). No species were sampled in the Warm Temperate zonobiome. A continental-scale comparison of naturalised species richness in aquatic versus terrestrial habitats, using a combined dataset of all naturalised species, demonstrated that in Africa and Temperate Asia, the observed proportion of aquatic naturalised species was significantly lower than expected by chance (Table 3). For zonobiomes, the aquatic habitats in the Mediterranean harboured disproportionally more and Warm Temperate fewer (none) naturalised species than terrestrial habitats in this region (Table 3, Fig. 2). For invasive species, the number of aquatic compared to terrestrial was higher than expected by chance in Tropical Asia and Northern America and lower in Temperate Asia (Table 3). Regarding zonobiomes, the representation Table 1. The top 21 aquatic species recorded in the largest number of SynHab regions presented by invasion status. These represent 11.3% of species recorded and 50.3% of all records. Am-S = Southern America, Am-N = Northern America, EU = Europe, AUS = Australasia, AS-temp = Temperate Asia, AS-trop = Tropical Asia, AF = Africa; I = number of records as invasive, N = number of records as naturalised non-invasive. Species I N Total Continent of origin Pontederia crassipes 15 28 43 Am-S Pistia stratiotes 7 16 23 AF, Am-N, Am-S Elodea canadensis 7 15 22 Am-N Azolla filiculoides 7 14 21 Am-N, Am-S, Antarctica Elodea densa 5 15 20 Am-S Myriophyllum aquaticum 6 14 20 Am-S Nasturtium officinale 2 17 19 AF, AS-temp, AS-trop, EU Salvinia molesta 10 7 17 Am-S Hydrilla verticillata 5 11 16 EU, AF, AS-temp, AS-trop, AUS Alternanthera philoxeroides 6 7 13 Am-S Potamogeton crispus 1 9 10 EU, AF, AS-temp, AS-trop, AUS Myriophyllum spicatum 3 7 10 EU, AF, AS-temp, AS-trop, Am-N Pontederia vaginalis 1 8 9 AS-temp, AS-trop, AUS Elodea nuttallii 2 6 8 Am-N Lemna minuta 2 7 9 Am-N, Am-S Veronica anagallis-aquatica 8 8 AF, AS-temp, AS-trop, EU Nymphoides peltata 1 6 7 AF, AS-temp, AS-trop, EU Acorus calamus 7 7 AS-temp, AS-trop, Am-N Typha angustifolia 1 5 6 EU, AF, AS-temp, AS-trop, Am-N Nelumbo nucifera 6 6 EU, AS-temp, AS-trop, AUS Sagittaria latifolia 6 6 Am-N, Am-S
481 NeoBiota 102: 473–494 (2025), DOI: 10.3897/neobiota.102.151156 Alessandra Kortz et al.: A global synthesis of plant invasions in aquatic habitats Figure 1. Proportion of invasive species amongst naturalised species in continents (a) and in zonobiomes (b). Africa Temperate Asia Tropical Asia Australasia Europe Northern America Southern America 0 1 2 3 4 0102030 Naturalised aquatics (%) a Boreal Continental Mediterranean Multiple Nemoral Subtropical Arid Tropical Equatorial Tropical Savanna Warm Temperate 0 1 2 3 4 0102 03 0 Average naturalised terrestrials (%) b Figure 2. Naturalisation in aquatic habitat compared to terrestrial habitats (mean for all other habitats) by continent (a) and zonobiome (b). The blue line shows the theoretical linear increase in mean naturalisation in both aquatic habitat and terrestrial habitats, connecting zero and mean naturalisation in each habitat (marked by a blue point). Table 2. Number of naturalised non-invasive and invasive aquatic species in continents and zonobiomes. Light orange cells indicate more and blue fewer alien species than expected by chance based on the generalised linear models. Significance indicated by asterisks (* P < 0.05). Continent Naturalised non-invasive Invasive Africa 20 11 Temperate Asia 18 11 Tropical Asia 21 17* Australasia 39 6 Europe 67 14 Northern America 61 15 Southern America 45 10 Zonobiome Boreal 3 0 Continental 8 1 Mediterranean 69 10* Multiple 50 18 Nemoral 79 19 Subtropical Arid 22 12 Tropical Equatorial 33 17 Tropical Savannah 27 16 Warm Temperate 0 0
488 NeoBiota 102: 473–494 (2025), DOI: 10.3897/neobiota.102.151156 Alessandra Kortz et al.: A global synthesis of plant invasions in aquatic habitats Pavel Pipek https://orcid.org/0000-0003-1116-1013 Kateřina Štajerová https://orcid.org/0000-0001-7824-1793 Paulina Anastasiu https://orcid.org/0000-0001-6355-2126 Michael Ansong https://orcid.org/0000-0003-3811-3230 Margarita Arianoutsou https://orcid.org/0000-0002-6743-9240 Julie F. Barcelona https://orcid.org/0000-0001-5087-8637 Suneeta Bhatta https://orcid.org/0000-0002-2993-7193 Farzaneh Bordbar https://orcid.org/0000-0002-3042-8329 Israel Borokini https://orcid.org/0000-0002-1258-7932 Laura Celesti-Grapow https://orcid.org/0000-0002-9533-6919 Eduardo Chacón-Madrigal https://orcid.org/0000-0002-8328-5456 Wayne Dawson https://orcid.org/0000-0003-3402-0774 Dorjee https://orcid.org/0000-0001-6866-9278 Franz Essl https://orcid.org/0000-0001-8253-2112 Lilian Ferrufino-Acosta https://orcid.org/0000-0002-2065-9174 Estrela Figueiredo https://orcid.org/0000-0002-8511-8213 Rodolfo Flores https://orcid.org/0000-0002-7911-9228 Guillaume Fried https://orcid.org/0000-0002-3653-195X Nicol Fuentes https://orcid.org/0000-0002-3773-9832 Pablo Galán https://orcid.org/0000-0002-7037-7853 Christian Gilli https://orcid.org/0000-0002-6679-4654 Michael Glaser https://orcid.org/0000-0002-4695-6150 José Ramón Grande Allende https://orcid.org/0000-0002-7066-0608 Zigmantas Gudžinskas https://orcid.org/0000-0001-6230-5924 Rachael Holmes https://orcid.org/0000-0002-6045-8705 Philip E. Hulme https://orcid.org/0000-0001-5712-0474 Inderjit https://orcid.org/0000-0002-4142-1392 Eun Su Kang https://orcid.org/0000-0003-3499-0869 Holger Kreft https://orcid.org/0000-0003-4471-8236 Dan W. Krix https://orcid.org/0000-0002-0733-1254 Ingolf Kühn https://orcid.org/0000-0003-1691-8249 Omar Lopez https://orcid.org/0000-0003-4953-2123 AnaLu MacVean https://orcid.org/0000-0001-7256-8453 Trobjon Makhkamov https://orcid.org/0000-0003-2667-7960 Elizabete Marchante https://orcid.org/0000-0003-1303-7489 Hélia Marchante https://orcid.org/0000-0002-3247-5663 Alfred Maroyi https://orcid.org/0000-0001-7965-3415 Rachid Meddour https://orcid.org/0000-0003-2936-2470 Pierre Meerts https://orcid.org/0000-0003-4215-027X Sharif A. Mukul https://orcid.org/0000-0001-6955-2469 Brad R. Murray https://orcid.org/0000-0002-4734-5976 Megan L. Murray https://orcid.org/0000-0002-0417-4337 Daniel L. Nickrent https://orcid.org/0000-0001-8519-0517 Prince E. Norman https://orcid.org/0000-0002-0150-8610 Ali Omer https://orcid.org/0000-0001-5687-3386 Annette Patzelt https://orcid.org/0000-0003-3510-4582 Pieter B. Pelser https://orcid.org/0000-0002-6990-1419 Joan Pino https://orcid.org/0000-0003-0939-7502 Marc Riera https://orcid.org/0000-0002-3860-6046
489 NeoBiota 102: 473–494 (2025), DOI: 10.3897/neobiota.102.151156 Alessandra Kortz et al.: A global synthesis of plant invasions in aquatic habitats Dagoberto Rodríguez Delcid https://orcid.org/0000-0002-0688-4615 Julissa Rojas-Sandoval https://orcid.org/0000-0001-6620-4741 Roser Rotchés-Ribalta https://orcid.org/0000-0003-4311-5863 José Yader Sageth Ruiz-Cruz https://orcid.org/0000-0003-4252-0488 Stepan Senator https://orcid.org/0000-0003-1932-2475 Alexander N. Sennikov https://orcid.org/0000-0001-6664-7657 Bharat Babu Shrestha https://orcid.org/0000-0002-9457-2637 Gideon F. Smith https://orcid.org/0000-0002-5417-9208 Sima Sohrabi https://orcid.org/0000-0002-0775-8362 Barbara Tokarska-Guzik https://orcid.org/0000-0002-4058-1220 Mark van Kleunen https://orcid.org/0000-0002-2861-3701 Montserrat Vilà https://orcid.org/0000-0003-3171-8261 Viktoria Wagner https://orcid.org/0000-0002-2665-9888 Patrick Weigelt https://orcid.org/0000-0002-2485-3708 Marten Winter https://orcid.org/0000-0002-9593-7300 Ayşe Yazlık https://orcid.org/0000-0001-7059-0761 Elena Zykova https://orcid.org/0000-0002-1847-5835 Petr Pyšek https://orcid.org/0000-0001-8500-442X Data availability All of the data that support the findings of this study are available in the main text or Supplementary Information. References Adebayo A, Briski E, Kalaci O, Hernandez M, Ghabooli S, Beric B, Chan F, Zhan A, Fifield E, Leadley T, MacIsaac H (2011) Water hyacinth (Eichhornia crassipes) and water lettuce (Pistia stratiotes) in the Great Lakes: Playing with fire? Aquatic Invasions 6: 91–96. https://doi.org/10.3391/ ai.2011.6.1.11 Baselga A, Orme CDL (2012) betapart: An R package for the study of beta diversity. Methods in Ecology and Evolution 3: 808–812. https://doi.org/10.1111/j.2041-210X.2012.00224.x Baselga A, Orme D, Villeger S, De Bortoli J, Leprieur F, Logez M, Martinez-Santalla S, Martin-Devasa R, Gomez-Rodriguez C, Crujeiras RM, Henriques-Silva R (2023) betapart: partitioning beta diversity into turnover and nestedness components. R package version 1.6. https:// CRAN.R-project.org/package=betapart Bellard C, Thuiller W, Leroy B, Genovesi P, Bakkenes M, Courchamp F (2013) Will climate change promote future invasions? Global Change Biology 19: 3740–3748. https://doi.org/10.1111/gcb.12344 Blackburn TM, Pyšek P, Bacher S, Carlton JT, Duncan RP, Jarošík V, Wilson JRU, Richardson DM (2011) A proposed unified framework for biological invasions. Trends in Ecology & Evolution 26: 333–339. https://doi.org/10.1016/j.tree.2011.03.023 Brown MJM, Walker BE, Black N, Govaerts RHA, Ondo I, Turner R, Nic Lughadha E (2023) rWCVP: A companion R package for the World Checklist of Vascular Plants. The New Phytologist 240: 1355–1365. https://doi.org/10.1111/nph.18919 Brummitt RK (2001) World geographic scheme for recording plant distributions. 2nd edn. Hunt Institute for Botanical Documentation, Carnegie Mellon University, Pittsburgh, 1–137. http:// rs.tdwg.org/wgsrpd/doc/data/ Cassey P, Delean S, Lockwood JL, Sadowski JS, Blackburn TM (2018) Dissecting the null model for biological invasions: A meta-analysis of the propagule pressure effect. PLOS Biology 16: e2005987. https://doi.org/10.1371/journal.pbio.2005987
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