Critical review of the literature on key invasive alien freshwater plants in Europe with special focus on their impact on the invaded ecosystems

441 Critical review of the literature on key invasive alien freshwater plants in Europe with special focus on their impact on the invaded ecosystems Dario Di Lernia1, Lorenzo Pinzani1,2 , Simona Ceschin1,2 1 Department of Science – University of Roma Tre, Viale G. Marconi, 446, 00146 - Rome, Italy 2 NBFC, National Biodiversity Future Center, 90133 - Palermo, Italy Corresponding authors: Dario Di Lernia ([email protected]); Simona Ceschin ([email protected]) Copyright: © Dario Di Lernia 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). Review Article Abstract Biological invasions of alien aquatic plants are a major threat to conservation of freshwater habitats, as well as a socio-economic problem. Introduced primarily by human activities, these alien plants compete with native species, reduce local biodiversity and alter structure and function of the aquatic ecosystems. This review examines the most relevant scientific literature on the major invasive alien aquatic plants (IAAPs) found in Europe (Alternanthera philoxeroides, Azolla filiculoides, Cabomba caroliniana, Egeria densa, Elodea canadensis, E. nuttallii, Gymnocoronis spilanthoides, Hydrilla verticillata, Hydrocotyle ranunculoides, Lagarosiphon major, Lemna minuta, Ludwigia grandiflora, L. hexapetala, L. peploides subsp. montevidensis, Myriophyllum aquaticum, M. heterophyllum, Pistia stratiotes, Pontederia crassipes, Salvinia molesta), with a special focus on impacts exerted by these species on invaded freshwater ecosystems. It includes both qualitative and quantitative analyses and evaluates the temporal trends of the scientific contributions considering the impact and management of these species in Europe and worldwide. Despite a recent increase in contributions on these IAAPs, the knowledge on their impacts in Europe remains mainly concentrated on a few species and rather fragmented and deficient on others. In fact, evident inequalities emerge among these IAAPs in both the number of dedicated contributions and in the treatment of each species, as some of them are currently excluded from the list of IAS of Union Concern. Moreover, the European level research on these IAAPs turned out to be scarcely influenced by the “listing” effect (i.e. inclusion of IAAPs in the Union list), showing little or no increase in the number of studies on impact or management. The studies found on impacts document that the selected IAAPs largely exert environmental impacts on invaded ecosystems by altering both the abiotic (water chemical and physical factors) and biotic (plant and animal communities) components. The impact mechanisms of these species vary (chemical, physical, structural, competitive, toxicity) and were classified according to the EICAT (Environmental Impact Classification for Alien Taxa) protocol. Overall, the review reveals significant gaps in knowledge about the environmental impacts of most of these IAAPs in Europe, despite some being included in the list of IAS of Union concern. To address these gaps and protect European freshwater ecosystems from biological invasions, more field studies supported by laboratory investigations are needed, followed by effective management interventions. In addition, it is considered necessary that impactful alien species with a wide distribution in Europe, but which are currently excluded from the EU list, be included as soon as possible. This would allow for coordinated management practices at the European level, which are essential for their containment. Key words: Bibliographic overview, biological invasions, ecosystem impact, Europe, exotic plants, freshwater ecosystems, invasive macrophytes, non-native plants, vascular plants Academic editor: Pedro Anastácio Received: 8 January 2025 Accepted: 23 May 2025 Published: 7 October 2025 Citation: Di Lernia D, Pinzani L, Ceschin S (2025) Critical review of the literature on key invasive alien freshwater plants in Europe with special focus on their impact on the invaded ecosystems. In: Anastácio P, Ribeiro F, Chainho P (Eds) Invasions in Aquatic Systems. NeoBiota 102: 441–472. https://doi.org/10.3897/ neobiota.102.146280 NeoBiota 102: 441–472 (2025) DOI: 10.3897/neobiota.102.146280 Advancing research on alien species and biological invasions A peer-reviewed open-access journal NeoBiota 442 NeoBiota 102: 441–472 (2025), DOI: 10.3897/neobiota.102.146280 Dario Di Lernia et al.: Review of European invasive alien aquatic plants Introduction Freshwater ecosystems cover 0.8% of the Earth’s surface and contain just 0.01% of its water (Dudgeon et al. 2006). Nevertheless, these are significant for their high biodiversity and numerous ecosystem services that they provide, primarily benefiting humans (Strayer and Dudgeon 2010; Hoppenreijs et al. 2024). Dynamic and vulnerable, freshwater ecosystems face frequent disturbances from anthropogenic pressures, such as hydrological and morphological alterations, water over-exploitation and pollution, leading to significant environmental instability. Such instability makes them highly susceptible to biological invasions, that is the spread of invasive alien species (IAS) outside their home range (Dudgeon et al. 2006; Anufriieva and Shadrin 2018; Lazzaro et al. 2020). Invasive alien plants typically thrive in altered environments due to their competitiveness, wide ecology and high reproductive potential (Perrings et al. 2000; Mazza et al. 2014). They outcompete native species, reducing local biodiversity (Pyšek et al. 2012; Viciani et al. 2020), and altering the structure and functionality of invaded ecosystems, and ultimately compromising their conservation status (Maes 2013; Seebens et al. 2017). In Europe, freshwater ecosystems are among the most heavily invaded by alien plants (Lazzaro et al. 2020), resulting in severe ecological and socio-economic consequences (Vilà et al. 2010; Hussner 2012). These plant invasions are often associated with the production of high biomass, which translates into various environmental impacts, such as changes in water chemistry and physical properties of the invaded ecosystem, alterations in the composition and biodiversity of native plant and animal communities, modifications in the structure of the food webs (Hussner et al. 2017). Furthermore, these invasions can cause socio-economic impacts, such as hindering navigation, aquaculture, and recreational activities, as well as creating unhealthy conditions for local plant and animal communities and human health (Stiers et al. 2011; Hussner 2012; Ceschin et al. 2020a). Macêdo et al. (2024) recently reported that between 1975 and 2020, the total cost to the global economy for the overall management of alien aquatic plants has exceeded 32 billion dollars. This highlights not only the importance of the problem, but also its relevant economic impact on a global scale. In response to the significant increase in biological invasions in Europe over the last decades, in 2014 the European Community approved the European Union (EU) Regulation 1143/2014/EC on IAS (IAS Regulation) that establishes a coordinated set of actions to prevent, control and mitigate the impact of the IAS. In 2016, the European Community drew up a list of IAS of Union concern which serves to direct research and management efforts (European Union 2016). With the latest update (European Union 2022), the list includes 41 alien plants, among which 13 are strictly freshwater species, including Alternanthera philoxeroides (Mart.) Griseb., Cabomba caroliniana A.Gray, Pontederia crassipes Mart. (= Eichhornia crassipes (Mart.) Solms), Elodea nuttallii (Planch.) H.St.John, Gymnocoronis spilanthoides (D.Don ex Hook. and Arn.) DC., Hydrocotyle ranunculoides L.f., Lagarosiphon major (Ridl.) Moss, Ludwigia grandiflora (Michx.) Greuter and Burdet, L. peploides (Kunth) P.H.Raven subsp. montevidensis (Spreng.) P.H.Raven, Myriophyllum aquaticum (Vell.) Verdc., M. heterophyllum Michx., Pistia stratiotes L. and Salvinia molesta D.S.Mitch. Species included in this list cannot be either released into the environment or traded or cultivated in any European Community country (European Union 443 NeoBiota 102: 441–472 (2025), DOI: 10.3897/neobiota.102.146280 Dario Di Lernia et al.: Review of European invasive alien aquatic plants 2014). However, this European list excludes some widespread aquatic plants considered as invasive neophytes in several European countries, such as Azolla filiculoides Lam., Egeria densa Planch., Elodea canadensis Michx., Hydrilla verticillata (L.f.) Royle, Lemna minuta Kunth and Ludwigia hexapetala (Hook. and Arn.) Zardini, H.Y.Gu and P.H.Raven (Ceschin et al. 2018a; Galasso et al. 2018; Magliozzi et al. 2020; Arianoutsou et al. 2023; Oficialdegui et al. 2023; Pelella et al. 2023a). In this background, the present investigation is aimed to (i) collect and critically review the relevant scientific literature on the invasive alien aquatic plants (IAAPs) occurring in Europe, and (ii) assess the current state of knowledge regarding their impact on invaded European freshwater ecosystems, with special focus on environmental impact. This review may highlight any gaps in knowledge on the environmental impacts exerted by these species in Europe. In addition, it may also provide a basis for directing future investigations into those IAAPs that are understudied. Methods IAAPs selection Bibliographic research focused on both IAAPs that are included in the European list of IAS of Union concern (European Union 2016, and subsequent updates), and some alien aquatic plants that, although outside this list, are widespread in European freshwater ecosystems based on distribution data extracted from Global Biodiversity Information Facility (GBIF 2023). In particular, the following 19 IAAPs were selected: Alternanthera philoxeroides, Cabomba caroliniana, Elodea nuttallii, Gymnocoronis spilanthoides, Hydrocotyle ranunculoides, Lagarosiphon major, Ludwigia grandiflora, L. peploides subsp. montevidensis (thereafter L. peploides), Myriophyllum aquaticum, M. heterophyllum, Pistia stratiotes, Pontederia crassipes, Salvinia molesta (IAAPs in EU list), and Azolla filiculoides, Egeria densa, Elodea canadensis, Hydrilla verticillata, Lemna minuta, Ludwigia hexapetala (IAAPs out EU list) (Table 1). Bibliographic research Bibliographic data sources Bibliographic research on the selected IAAPs was conducted using the online database Web of Science (WOS) (Clarivate Analytics 2023), considering worldwide distributed scientific contributions. In the research string, the keywords included for each species both scientific names (including all synonyms) and common English names, both taken from GBIF (2023), as well as the following terms: “alien*”, “allochthonous”, exotic*”, “introduc*”, “IAS”, “invas*”, “non-indigenous plant”, “non-indigenous species”, “non-native plant”, “non-native species” and “impact*”, “damage*”, “reduc*”, “extinct*”, “allelopathic”, “inhibit*”, “alter*”, “control”, “compet*”. The contributions were selected based on the following criteria: (i) accessible on the web, (ii) published in international peer-reviewed journals indexed in WOS, (iii) written in English, (iv) published up to November 2023, and (v) based on field and/or laboratory studies, excluding forecasting or simulation studies. 444 NeoBiota 102: 441–472 (2025), DOI: 10.3897/neobiota.102.146280 Dario Di Lernia et al.: Review of European invasive alien aquatic plants The collected contributions for each species were grouped based on the main topic addressed (thereafter macrotopic). Ten main macrotopics were identified: • “impact” macrotopic: contributions addressing the impact exerted by the selected IAAPs on the invaded ecosystem; • “management” macrotopic: contributions analyzing the various methods tested to manage the IAAPs; • “distribution” macrotopic: contributions with data, floristic records or distribution maps of the species considered; • “ecology”, “biology”, “physiology” and “genetics” macrotopics: contributions related to these aspects, respectively; • “uses” macrotopic: contributions exploring potential uses of the species (e.g., as bioenergy source, biofood, bioindicator, phytoremediation agent); • “general” and “other”: in “general” were included the more generalist contributions that reference three or more macrotopics without a specific focus on any one in particular, whereas in “other” the contributions with topics different from those mentioned above. Data collection For each scientific contribution collected, the following information was extracted, creating a digital database in Excel (Excel vers. 2409, Microsoft Corporation 365): • scientific name of the aquatic plant considered as alien in that contribution (2023); • year of publication; • main macrotopic addressed. Contributions referring to two different but equally addressed macrotopics were attributed to both, resulting in being counted twice in the final count of contributions considered. Specifically with regard to contributions on the “impact” and “management” macrotopics, the following information was also specified: Area of study – Study area within or outside Europe Type of study – Laboratory study (indoor, microcosms, greenhouse) – Field study (outdoor, mesocosms, nature) Type of impact – Socio-economic impact on society and local economy (e.g., public health issues, waterbody landscape alteration, aesthetic degradation, limitations on aquaculture, fishing, tourism, and high management costs) – Environmental impact (on abiotic and/or biotic component of the ecosystem) Impact environmental mechanism1 – On abiotic component (chemical, physical) 1 Categorization of mechanisms of environmental impact following the classification proposed by the IUCN (International Union for Conservation of Nature) in the EICAT (Environmental Impact Classification for Alien Taxa) protocol (Hawkins et al. 2015). 445 NeoBiota 102: 441–472 (2025), DOI: 10.3897/neobiota.102.146280 Dario Di Lernia et al.: Review of European invasive alien aquatic plants – On native biotic component (competition, direct physical disturbance, hybridization, structural, toxicity) Management methods – Prevention – Control method (chemical, physical, biological) Data analysis The scientific contributions resulting from the bibliographic research were analyzed according to points indicated in the section “Data collection”. The results of these analyses were shown in graphs and tables created using Microsoft Excel vers. 2409 (Microsoft Corporation 365). The total number of scientific contributions produced up to November 2023, and the average percentage of contributions falling within each of the 10 macrotopics, were calculated both considering all species together that for each IAAPs. For the macrotopic on impact, the percentage of contributions addressing different types of impact (socio-economic, environmental) was also calculated. In addition, to assess the temporal trend of the scientific production concerning this macrotopic on a global and European scale, the year of publication of each contribution was considered within five 8-year intervals (1984–1991, 1992–1999, 2000–2007, 2008–2015, 2016–2023), with the first interval determined based on the dates of the earliest contributions found. The last time interval considered includes the year 2016, which marks the publication date of the first list of IAS of Union concern. A similar temporal analysis was conducted for contributions related to the macrotopic on management of the selected IAAPs to verify any consistency in the temporal trend between the number of contributions published on the impact of these species and those dedicated to their management. In addition, a correlation analysis was carried out using R software (R Core Team 2023) to assess the relationship between the number of European countries invaded by IAAPs and the scientific contributions on the impact found on globally for each species. Pearson’s correlation coefficient was calculated using the cor. test function, obtaining both correlation value (r) and statistical significance (p value). Results Bibliographic analysis Based on the bibliographic analysis of the 19 IAAPs, 1918 scientific contributions published up to November 2023 in international peer-reviewed journals and indexed in WOS were collected. The analysis identified ecological and management studies as the most investigated macrotopics, representing 27.1% and 20.7% of the contributions, respectively. Secondarily, there were studies on the distribution (11.5%), uses (11.2%) and impact (10.5%) of these alien species. Each of the remaining macrotopics accounted for less than 7% of the total scientific contributions collected (Fig. 1). The in-depth analysis of the contributions referring to impacts exerted by the 19 IAAPs showed that 77% of the studies have a worldwide distribution, while the remaining 24% refer specifically to studies carried out in European countries. Of the worldwide contributions on impact, 81.2% (181) focus on the environmental impact, while the remaining 18.8% (42) on socio-economic impact. Among the studies on environmental impact, 70.5% (155) document impacts on biotic components (e.g., 446 NeoBiota 102: 441–472 (2025), DOI: 10.3897/neobiota.102.146280 Dario Di Lernia et al.: Review of European invasive alien aquatic plants microalgae, macrophytes, macroinvertebrates, aquatic birds) of the invaded aquatic ecosystems, while 29.5% (65) reveals impacts on abiotic components, such as physical and chemical factors (e.g., temperature, light, pH, dissolved oxygen, nutrients). 64.6% (128) of the contributions on environmental impact referred to field studies, while 35.4% (70) to indoor experiments performed in the laboratory or greenhouses. Over time, scientific production regarding the impact of the selected IAAPs showed a significant upward trend. Worldwide contributions increased exponentially ranging from the earliest time interval analyzed (1984–1991) to the latest (2016–2023), when the highest percentage of contributions (63.4%, 123 contributions) was recorded (Fig. 2a). A similar trend can be observed when considering only contributions on the impact studies produced in Europe. Indeed, in the last period, also coinciding with the inclusion of some of the IAAPs considered in the list of Union concern, most of the contributions produced on impact are concentrated. Similarly, contributions on the management of these species produced at both global and European level, have also grown progressively over time, with most studies (49.4%, 167) concentrated in the most recent time interval (Fig. 2b). Bibliographic analysis for each IAAPs Out of the 1918 scientific contributions collected in this review, about 64% are concentrated on 4 of the 19 selected IAAPs. In particular, P. crassipes (22.9%, 440 contributions), A. philoxeroides (16.3%, 312), E. densa (14.1%, 270) and H. verticillata (10.5%, 201) were proven to be the IAAPs most studied globally. In Figure 1. Percentages of scientific contributions on the selected IAAPs in relation to the 10 macrotopics considered. Figure 2. Temporal trends in the number of scientific contributions related to the impact (a) and management (b) of the selected IAAPs. 447 NeoBiota 102: 441–472 (2025), DOI: 10.3897/neobiota.102.146280 Dario Di Lernia et al.: Review of European invasive alien aquatic plants contrast, C. caroliniana (2%, 38), L. peploides (1.8%, 34), L. minuta (1.4%, 27), L. grandiflora (1.2%, 23), H. ranunculoides (1%, 19), M. heterophyllum (0.8%, 16) and G. spilanthoides (0.3%, 5) were the least studied ones (Fig. 3). Generally, the most represented macrotopics for most of the species refer to ecological and management studies, except for L. minuta, which showed a proportionally high number of contributions on the impact (Table 1). Correlating the number of European countries invaded from each of the considered IAAPs (index of species spread in Europe) with the total number of studFigure 3. Total number of scientific contributions found for each of the 19 selected IAAPs. Table 1. For each IAAPs, it is indicated whether the alien species is included (in, grey colour) or not (out, white colour) in the list of IAS of Union concern, the total number (n) and percentage (%) of scientific contributions related to the different macrotopics. In bold, contribution number ≥ 20. UE list IAAP Impact Biology Distribution Ecology Genetics Management Physiology Uses General Other in Alternanthera philoxeroides 36 (11.0%) 65 (19.8%) 24 (7.3%) 113 (34.5%) 18 (5.5%) 32 (9.8%) 22 (6.7%) 16 (4.9%) 2 (0.6%) - out Azolla filiculoides 4 (9.5%) 3 (7.1%) 7 (16.7%) 5 (11.9%) -15 (35.7%) -8 (19.0%) - - in Cabomba caroliniana 4 (9.8%) 1 (2.4%) 7 (17.1%) 8 (19.5%) 2 (4.9%) 10 (24.4%) 6 (14.6%) -3 (7.3%) - out Egeria densa 20 (7.1%) 7 (2.5%) 22 (7.8%) 106 (37.5%) 8 (2.8%) 32 (11.3%) 47 (16.6%) 25 (8.8%) 1 (0.4%) 15 (5.3%) out Elodea canadensis6 (6.1%) 2 (2.0%) 19 (19.2%) 49 (49.5%) 4 (4.0%) 9 (9.1%) 4 (4.0%) 3 (3.0%) 2 (2.0%) 1 (1.0%) in Elodea nuttallii 10 (13.0%) 5 (6.5%) 12 (15.6%) 33 (42.9%) 1 (1.3%) 12 (15.6%) 2 (2.6%) 2 (2.6%) - - in Gymnocoronis spilanthoides - - - 4 (80.0%) - - - - - 1 (20.0%) out Hydrilla verticillata 31 (14.9%) 9 (4.3%) 17 (8.2%) 54 (26.0%) 7 (3.4%) 57 (27.4%) 10 (4.8%) 12 (5.8%) 3 (1.4%) 8 (3.8%) in Hydrocotyle ranunculoides3 (15.0%) 1 (5.0%) 4 (20.0%) 4 (20.0%) 1 (5.0%) 5 (25.0%) -1 (5.0%) 1 (5.0%) - in Lagarosiphon major 7 (13.7%) 1 (2.0%) 5 (9.8%) 17 (33.3%) 1 (2.0%) 18 (35.3%) 1 (2.0%) -1 (2.0%) - out Lemna minuta 9 (31.0%) 2 (6.9%) 5 (17.2%) 8 (27.6%) 2 (6.9%) 1 (3.4%) 1 (3.4%) 1 (3.4%) - - in Ludwigia grandiflora 3 (13.0%) 1 (4.3%) 2 (8.7%) 6 (26.1%) 1 (4.3%) 6 (26.1%) 3 (13.0%) - - 1 (4.3%) out Ludwigia hexapetala 8 (15.4%) 7 (13.5%) 5 (9.6%) 16 (30.8%) -10 (19.2%) 4 (7.7%) 1 (1.9%) 1 (1.9%) - in Ludwigia peploides 1 (2.9%) 4 (11.4%) 6 (17.1%) 12 (34.3%) -5 (14.3%) 4 (11.4%) 2 (5.7%) 1 (2.9%) - in Myriophyllum aquaticum 4 (5.4%) 6 (8.1%) 6 (8.1%) 26 (35.1%) -18 (24.3%) 5 (6.8%) 4 (5.4%) 5 (6.8%) - in Myriophyllum heterophyllum 4 (25.0%) -2 (12.5%) 5 (31.3%) 1 (6.3%) 3 (18.8%) 1 (6.3%) - - - in Pistia stratiotes 3 (3.0%) 7 (7.0%) 12 (12.0%) 19 (19.0%) 2 (2.0%) 32 (32.0%) 2 (2.0%) 20 (20.0%) 2 (2.0%) 1 (1.0%) in Pontederia crassipes 54 (11.3%) 9 (1.9%) 72 (15.1%) 59 (12.4%) 1 (0.2%) 125 (26.3%) 17 (3.6%) 123 (25.8%) 12 (2.5%) 4 (0.8%) in Salvinia molesta 7 (9.1%) 7 (9.1%) 7 (9.1%) 8 (10.4%) 2 (2.6%) 31 (40.3%) -11 (14.3%) 2 (2.6%) 2 (2.6%) 448 NeoBiota 102: 441–472 (2025), DOI: 10.3897/neobiota.102.146280 Dario Di Lernia et al.: Review of European invasive alien aquatic plants ies carried out in the world on their impact, no correlation between the two variables is emerged (r = - 0.15, p > 0.05, Pearson correlation coefficient) since the most widespread species in Europe are not always those whose impact was most studied (Table 2). In fact, some of the alien aquatic species that are common in Europe (n ≥ 12, high number of European countries invaded) prove to be little studied in the world (e.g. Elodea canadensis, Azolla filiculoides, Myriophyllum aquaticum, Pistia stratiotes); on the other hand, some species that do not show wide European distribution (n ≤ 11, medium-low number of European countries invaded), have had many studies on their impact (e.g., Alternanthera philoxeroides, Egeria densa, Pontederia crassipes). The average number of contributions on the impact of IAAPs included and not-included in the list of the IAS of Union concern is 10.5 and 13.0 globally, and 2.3 and 4.0 for Europe, respectively (Fig. 4). The IAAPs included in the list result averagely less studied than those not-included, both at the global and European level, except for P. crassipes and A. philoxeroides, which far exceed these averages when considering global studies. By comparing the number of European studies on the impact and management of the IAAPs included in the Union list and the year in which each of these was included in this list (Table 3), it emerged that in most species, the number of studies on impact increased little after these were listed (minimal listing effect). In some cases, a lower number of studies was also recorded after inclusion in the list (E. nuttallii, L. grandiflora, L. peploides, P. stratiotes, S. molesta), or even no Table 2. Number of European countries invaded by each selected IAAPs and total number of scientific contributions extracted from the literature concerning their impact on invaded ecosystem. Distribution data of each IAAP within were obtained from GBIF (2023). Acronyms of European countries: Austria (AT), Belgium (BE), Bulgaria (BG), Switzerland (CH), Czech Republic (CZ), Germany (DE), Denmark (DK), Spain (ES), Finland (FI), France (FR), Greece (GR), Croatia (HR), Hungary (HU), Ireland (IE), Italy (IT), Lithuania (LT), Latvia (LV), Netherlands (NL), Poland (PL), Portugal (PT), Romania (RO), Sweden (SE), Slovakia (SK), United Kingdom (UK). IAAP Invaded EU countries countries (n) impact contributions (n) Alternanthera philoxeroides ES, FR, IT, NL, PT 5 36 Azolla filiculoides AT, BE, CZ, DE, DK, ES, FR, GR, HU, IE, IT, NL, PL, PT, RO, SE, UK 17 4 Cabomba caroliniana BE, CH, DE, DK, FR, HU, NL, PL, RO, SE, UK 11 4 Egeria densa BE, CZ, DE, ES, FR, IE, IT, NL, PT, UK 10 20 Elodea canadensis AT, BE, BG, CH, DE, DK, ES, FI, FR, HU, IE, IT, NL, PL, PT, RO, SE, UK 18 6 Elodea nuttallii AT, BE, BG, CH, DE, DK, ES, FI, FR, HU, IE, IT, LT, NL, PL, RO, SE, UK 18 10 Gymnocoronis spilanthoides HU, IT, NL, SE 4 - Hydrilla verticillata AT, CH, DE, ES, FR, IE, IT, LT, LV, PL, UK 11 31 Hydrocotyle ranunculoides BE, CH, DE, DK, ES, FR, HU, IE, IT, NL, UK 11 3 Lagarosiphon major AT, BE, CH, DE, FR, IE, IT, NL, PT, UK 10 7 Lemna minuta AT, BE, CH, DE, ES, FR, IE, IT, NL, PL, PT, RO, UK 13 9 Ludwigia grandiflora AT, BE, CH, ES, FR, HU, IE, IT, NL, PT, UK 11 3 Ludwigia hexapetala BE, ES, FR, HU, IT, PT, UK 7 8 Ludwigia peploides BE, ES, FR, IT, NL, PT 6 1 Myriophyllum aquaticum AT, BE, CH, DE, ES, FR, IE, IT, NL, PT, RO, UK 12 4 Myriophyllum heterophyllum AT, BE, CH, DE, ES, FR, HU, NL, SE, UK 10 4 Pistia stratiotes BE, CH, DE, ES, FR, HR, IT, NL, PL, SE, SK, UK 12 3 Pontederia crassipes BE, DE, ES, FR, HU, IT, NL, PL, PT, UK 10 54 Salvinia molesta AT, BE, CH, DE, ES, FR, HU, IT, NL, PT 10 7 449 NeoBiota 102: 441–472 (2025), DOI: 10.3897/neobiota.102.146280 Dario Di Lernia et al.: Review of European invasive alien aquatic plants Figure 4. Number of scientific contributions on the impact of the IAAPs included and not-included in the list of the IAS of Union concern. The black horizontal lines mark the average number of contributions for each IAAP group. Table 3. For each species included in the list of IAS of Union concern, the year of its inclusion and the number of scientific European contributions on its impacts and management were reported. Species are listed according to the year of their inclusion in the EU list. IAAP year of inclusion in EU list EU impact contributions EU management contributions before list after list before list after list Cabomba caroliniana 2016 - 1 - 1 Hydrocotyle ranunculoides 2016 1 1 1 2 Lagarosiphon major 2016 1 4 5 6 Ludwigia grandiflora 2016 2 1 1 2 Ludwigia peploides 2016 1 - 1 - Myriophyllum aquaticum 2016 1 2 1 2 Pontederia crassipes 2016 1 3 - 3 Alternanthera philoxeroides 2017 - - - 1 Elodea nuttallii 2017 5 3 3 7 Myriophyllum heterophyllum 2017 - 1 - - Gymnocoronis spilanthoides 2019 - - - - Salvinia molesta 2019 1 - - - Pistia stratiotes 2022 1 - - - contribution was dedicated either before or after (A. philoxeroides, G. spilanthoides) (no listing effect). As regards the contributions relating to the management of the IAAPs included in the list, it appears that in most cases, the number of contributions increased slightly after these species were included in the list (minimal listing effect). However, also in this case, there are species that were not investigated either before or after their inclusion in the list (G. spilanthoides, M. heterophyllum, P. stratiotes, S. molesta) (no listing effect). The 19 considered IAAPs were grouped into nine groups based on systematic affinity or morpho-structural similarity. A detailed description of the selected IAAPs, taking into account the main scientific contributions about them, is given below. 456 NeoBiota 102: 441–472 (2025), DOI: 10.3897/neobiota.102.146280 Dario Di Lernia et al.: Review of European invasive alien aquatic plants (Cook and Lüönd 1982). Despite being recognized as highly invasive in various European countries (Table 2), only L. major and E. nuttallii are listed as IAS of Union concern (European Union 2016, 2017). With 201 and 270 contributions, E. densa and H. verticillata are among the most studied IAAPs. However, the other species of this group have also received considerable attention, particularly on their ecological aspects (Fig. 3). Contributions on management are numerous, especially for L. major (35.3%) and H. verticillata (27.4%). Conversely, E. densa and E. canadensis have few studies on this topic (10%) (Table 1). The management methods documented for these species include mainly chemical and physical control. As for chemical control, the use of synthetic herbicides was documented, such as Diquat for E. densa in Brazil and Connecticut (Martins et al. 2008; Bugbee et al. 2020), Flumioxazin for E. canadensis in New Zealand (Hofstra et al. 2021) and Endothal and Florpyrauxifen-benzyl for H. verticillata in United States (Ortiz et al. 2022). The use of Diquat and Endothal is prohibited in Europe (European Commission 2025). Physical control was used especially against E. nuttallii by adding biodegradable dyes in water or applying shading cover to reduce light availability and inhibit plant growth (Hoffmann et al. 2013; Zefferman 2014). In addition, cases of biocontrol were documented, such as the use of the Asian fly Hydrellia pakistanae Deonier against H. verticillata in Florida (Wheeler and Center 2001) and H. lagarosiphon Deeming against L. major in Ireland (Mangan and Baars 2023). Despite the substantial number of contributions found for this group of species, the studies analyzing their impact prove to be relatively few (Table 1). The most documented impact mechanisms for these species are both chemical and physical, as it occurred in some invaded waterbodies of Ireland, Norway and France (Mjelde et al. 2012; Kelly et al. 2015; Ribaudo et al. 2018), and competitive with native plants, as observed for H. verticillata in South America, E. densa in California and E. canadensis in Norway (Santos et al. 2011; Mjelde et al. 2012; Silveira et al. 2018). In addition, some studies document that E. densa has a negative impact on native plants in China (Dai et al. 2023), while E. nuttallii and L. major affect native European plants by releasing allelopathic substances that inhibit their growth (Erhard and Gross 2006; Cuthbert et al. 2020). Gymnocoronis spilanthoides (Asteraceae – emergent rhizophytes) Gymnocoronis spilanthoides, native to South America (Tippery et al. 2014), was introduced as an ornamental aquarium plant in East Asia, Australia, New Zealand and Europe. In Europe, although it occurs in few countries (Table 2), it is considered invasive (Lukács et al. 2014; Ardenghi et al. 2016) and was included in the list of IAS of Union concern (European Union 2019). Bibliographic research revealed very few studies on this species, counting only five published contributions mainly focused on its ecology (Table 1; Fig. 3). No contributions related to the impact or management of this species was found. Discussion Critical literature analysis The bibliographic research on the main European IAAPs showed that the scientific knowledge gained worldwide on these species is unequal. It is focused only on certain species (P. crassipes, A. philoxeroides, E. densa, H. verticillata) and very scarce 457 NeoBiota 102: 441–472 (2025), DOI: 10.3897/neobiota.102.146280 Dario Di Lernia et al.: Review of European invasive alien aquatic plants and fragmented on others (G. spilanthoides, H. ranunculoides, L. minuta, M. heterophyllum, L. grandiflora, L. peploides). Such knowledge inequality is a phenomenon already documented in literature and can be attributed to a combination of factors. As highlighted by Pyšek et al. (2008), geographical, socio-economic and taxonomic biases can play a crucial role in driving knowledge on invasion ecology. Species that invade regions of greater socio-economic interest, such as Europe or USA, and those belonging to taxa that are better known or perceived as more impactful, generally tend to receive more attention. This may explain why species, such as P. crassipes or S. molesta, which are known to be impactful, are the target species of numerous studies globally, while others, such as G. spilanthoides, which are less widespread in relevant regions, or perhaps perceived as less problematic, remain relatively ignored. Furthermore, Hulme et al. (2013) pointed out that factors, such as the availability of research resources and the perception of an economic impact as a result of the invasion of an alien species, contribute to widening these disparities. Species with evident economic or ecological damage tend to catalyze scientific interest, explaining the observed differences in the knowledge between the considered IAAPs. This difference highlights on the one hand the need for more balanced research on these IAAPs and on the other hand the risk of underestimating the impact of less studied taxa. This bibliographic research shows that most scientific contributions produced on the investigated IAAPs up to 2023 have primarily addressed ecological aspects and, secondarily, management strategies, often neglecting important topics, such as the impact exerted by these alien species on the invaded ecosystem. Indeed, the scientific production of studies analyzing the environmental impacts of these IAAPs turned out to be limited in numbers, often qualitative and deficient in field data. What is surprising is that for many of the IAAPs included in the list of Union concern (G. spilanthoides, H. ranunculoides, L. peploides, L. grandiflora, P. stratiotes, C. caroliniana, M. heterophyllum, L. major, E. nuttallii), there are only a few field studies evaluating the real extent and severity of their impacts on invaded ecosystem (Table 1). It is worth noting the importance of field studies that allow for collecting real data that are capable of showing what actually occurs in natural conditions, although sometimes the complex abiotic and biotic interactions may create confusion in the interpretation of the data itself. For this reason, laboratory studies, which however suffer from certain limitations, such as short time frames, reduced volumes and lower ecosystem complexity (Carpenter 1996; Schindler 1998), by reducing the variables to be analyzed, allow to focus only on what is experimentally important to evaluate. It is therefore evident that, in order to obtain a complete understanding of a phenomenon being investigated, it would be necessary to perform field studies combined with laboratory investigations. Another important aspect emerging from this investigation is that for some of the IAAPs considered, such as E. canadensis, L. minuta, A. filiculoides, L. hexapetala and E. densa, despite various authors document their invasiveness and environmental impact in several European countries, they are not included in the list of IAS of Union concern. Consequently, in Europe, these species might evade any type of controls of their commercialization, on free use, or of release into the environment. The reason why these IAAPs are not yet included in this EU list might only be a matter of time. Indeed, the scientific documentation on their impacts in Europe may still be under evaluation by the European Commission, which may not allow for regulated and coordinated European actions against these 458 NeoBiota 102: 441–472 (2025), DOI: 10.3897/neobiota.102.146280 Dario Di Lernia et al.: Review of European invasive alien aquatic plants species to date. However, some of these IAAPs, such as A. filiculoides, E. canadensis and E. densa, are included in legally regulated national lists (e.g., Spanish Catalogue of IAS by Spanish Ministry of Agriculture, Food and Environment 2013 - Royal Decree 630/2013). The promulgation of these national catalogues allows the member states of the European Community to manage IAS that pose a threat to conservation of the national territory, even if not prioritized at European level. Understanding how (mechanism) and how much (severity) an IAS exerts on impact is fundamental to accurately assessing whether that species, despite being locally widespread, poses a real threat to the conservation of native biodiversity and integrity of the ecosystem invaded. Should significant evidence emerge regarding the danger of a species, it becomes imperative to promptly contain its spread. This involves promoting prevention campaigns, as well as monitoring, field study, and eradication activities, to prevent ecosystem damage and management costs from becoming unsustainable. For example, with reference to the latter aspect, a study conducted in Spain showed that the management of P. crassipes alone cost approximately 55 million dollars between 1997 and 2019 (Angulo et al. 2021). Furthermore, an estimate by Cuthbert et al. (2021) on data published from 1971 to 2020 on IAAPs, revealed that the overall management of IAAPs cost the global economy more than 20 billion dollars. Temporal analysis of the collected literature The number of contributions on the impact of the considered IAAPs increased exponentially from 1980 to 2023, with a particular peak in recent years (2016– 2023) (Fig. 2a). This trend is aligned with both the increasing number of introductions of alien species outside their native ranges (Seebens et al. 2017), and the awareness of researchers of the serious impacts that invasions of these species can entail on ecosystem conservation. Based on these considerations, and limiting the analysis to the studies carried out in Europe on the IAAPs included in the list of IAS of Union concern (from 2016 onwards), one would have expected that the number of contributions on the impact and management on these species would increase once they were included in the list; in fact, the inclusion of an alien species in this list should entail the acquisition of the status of potential environmental threat, such that investigations, monitoring and control activities should be intensified. Instead, contrary to what was expected, the results showed that scientific production in Europe on the impact and management of these species was not much affected by the “listing” effect; indeed, for most of the species, after being included into the EU list, only a slight increase was recorded (minimal “listing” effect) or even a decrease or absence (no “listing” effect) of European studies about (Table 3). Based only on European studies found on the impact of the selected IAAPs, and distinguishing between included and not-included species in the EU list, it emerged that the first group was on average less studied than the second (Fig. 4). This underlines once again that a species, even if included in the EU list, has not necessarily been the target of greater scientific interest in Europe, although the opposite would have been expected. Furthermore, the higher average number of contributions focused on the non-listed IAAPs, whose environmental impact was often documented by scientific evidence, should suggest a future consideration of these species in the list updates of IAS of Union concern. 459 NeoBiota 102: 441–472 (2025), DOI: 10.3897/neobiota.102.146280 Dario Di Lernia et al.: Review of European invasive alien aquatic plants Analysis of IAAPs impact mechanisms The investigated IAAPs were found to affect both abiotic and biotic component of invaded aquatic ecosystems. Impacts on abiotic component occur when these plants alter physical (physical impact) and chemical (chemical impact) properties of the ecosystem invaded, consequently affecting indirectly the composition and diversity of the native plant and animal communities. For instance, it is well documented that free-floating alien hydrophytes, such as A. filiculoides, L. minuta, S. molesta and P. stratiotes, form dense populations on the water surface and thus modify water physically (e.g., light, temperature) and chemically (e.g., dissolved oxygen, pH, nutrients), thus indirectly compromising the survival of native plant and animal communities (e.g., Pinero-Rodríguez et al. 2019, 2021; Wahl et al. 2021c; Lv et al. 2022). The mechanisms of impact were found to mainly result from interspecific competition for light and nutrients with native plants, often resulting in the dominance of invasive species over native ones (e.g., Ceschin et al. 2016, 2020b; Paolacci et al. 2018; Jaklič et al. 2020; Wahl et al. 2021b). Rooted alien hydrophytes, such as E. canadensis, E. nuttallii and P. crassipes, can produce dense and monospecific populations that replace native plant communities, reducing local plant diversity and simplifying plant community structure (Bubíková et al. 2021; Lahon et al. 2023). It was also demonstrated that extensive populations of H. ranunculoides, M. aquaticum and S. molesta lead to a structural simplification of the plant communities with serious consequences on macroinvertebrate communities, manifested by the decline of the most sensitive taxa and the survival of the most adaptable ones (Stiers and Triest 2017; Wahl et al. 2021c). Another impact mechanism that was documented is linked to the phytotoxicity showed by some IAAPs that are able to produce and release allelopathic substances inhibiting the seed germination and/or growth of native aquatic plants. For example, L. hexapetala releases glycosidic substances (quercitrin) which inhibit vegetative growth of the native Utricularia australis (Pelella et al. 2023b). Similarly, A. philoxeroides releases allelochemicals affecting negatively the seed germinability of the native Phragmites australis (Cav.) Trin. ex Steud. and the vitality of the soil microbiota (Ge et al. 2018; Liu et al. 2020). Hybridization is another detected impact mechanism, whereby alien species cross with native species, producing hybrids that are more invasive than parental alien species. An example is M. heterophyllum, which hybridizes in several areas of the USA with the native congeners M. hippuroides Nutt. ex Torr. and A. Gray and M. laxum Schuttl. ex Chapm. (Thum et al. 2011). Conclusions This study found that the spread of IAAPs in Europe poses a significant threat to the conservation of European biodiversity and freshwater ecosystems. Despite growing awareness of the severity of the negative effect exerted by these species in invaded habitats, research focused on their environmental impacts and the ecological traits upon which their competitiveness and invasiveness are founded, remains limited to only a few invasive alien species. In such context, this investigation highlighted both the different treatment between IAAPs (listed and not-listed species in the Union list) and the inequalities that exist in the knowledge of their impact and management. In particular, some species have hardly been investigated at either the European or global level. Therefore, it is evident that there is a need to fill this knowledge gap and develop management practices that can contain the invasion of these 460 NeoBiota 102: 441–472 (2025), DOI: 10.3897/neobiota.102.146280 Dario Di Lernia et al.: Review of European invasive alien aquatic plants alien plants in Europe. Increasing field studies, possibly supplemented by laboratory tests, becomes essential to provide reliable data on the impact of these species. Such an approach, based on real data rather than simulations or potential impacts, will be able to contribute to a more effective management of the considered IAAPs, as well as improving the robustness of environmental conservation policies. In this regard, it is hoped that the species that are currently not listed but were found to be impactful and widely distributed in Europe (e.g., E. densa, H. verticillata, L. minuta, E. canadensis, L. hexapetala), are included in the list of IAS of Union interest as soon as possible. This will support consistent management practices at the European level, a crucial requirement for protecting the integrity of native communities and ensuring the proper sustainability and functionality of freshwater ecosystems in Europe. Acknowledgements Thanks are due to the anonymous reviewers, whose suggestions and comments helped to improve this paper. The authors acknowledge the support of NBFC to Department of Science-University of Roma Tre, funded by the Italian Ministry of University and Research, PNRR, Missione 4 Componente 2, “Dalla Ricerca all’Impresa”, Investimento 1.4, Project CN00000033. Additional information Conflict of interest The authors have declared that no competing interests exist. Ethical statement No ethical statement was reported. Funding No funding was reported. Author contributions Conceptualization: SC, DDL. Data curation: DDL, SC. Formal analysis: DDL. Investigation: LP, DDL, SC. Resources: SC. Supervision: SC. Validation: SC. Visualization: DDL, SC. 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