Invasive Alien Plant Species in Black Sea Delta Protected Areas: Patterns, Impacts, and Management Recommendations

Abstract

Deltas are highly susceptible to biological invasions because of strong hydrological connectivity, frequent disturbance, and intense human use. Here, we synthesise coordinated monitoring observations and literature evidence on invasive alien plant species (IAS) recorded in four Black Sea riparian protected areas located across five countries, surveyed under the IASON/IASON+ initiatives (Danube Delta, Nestos Delta and Lake Vistonida, Kızılırmak Delta, Chorokhi Delta and Kolkheti National Park). Across the study sites, 17 IAS were documented, mainly represented by taxa native to North America and characterised by high propagule production and/or strong vegetative regeneration. Woody riparian invaders (e.g., Amorpha fruticosa, Robinia pseudoacacia, Acer negundo, Gleditsia triacanthos and Ailanthus altissima) exploited nutrient-rich floodplain soils and disturbances. In contrast, annual weeds (e.g., Ambrosia artemisiifolia, Sicyos angulatus and Xanthium orientale) remained associated with disturbed habitat edges. Aquatic dominance was confined to the Danube Delta, where Elodea nuttallii and Elodea canadensis formed dense submerged stands. Species were assigned to broad range expansion categories (slowly, moderately and rapidly spreading species) based on project observations and supporting records. We discuss shared invasion syndromes linked to reproductive and dispersal traits and outline management implications for Black Sea deltas, emphasising pathway prevention, early detection and rapid response for localised taxa, and sustained control combined with restoration for dominant invaders.

1. Introduction

Invasive alien species (IAS) have become a focus of scientists due to their negative impacts on ecosystems. Recent global assessments, including the IPBES 2023 Report, confirm that IAS rank among the main drivers of species extinction and among the top five direct drivers of biodiversity decline worldwide [1,2]. In response to these impacts, the European Union introduced Regulation 1143/2014 to prevent and manage the introduction and spread of these impacts. This Regulation aims to establish specific monitoring programmes, objectives, and management goals for IAS to conserve local biodiversity efficiently [3].

Invasive alien plant species have a strong capacity to displace and reduce native plant populations, facilitated by their strong competitive ability, often linked to release from natural enemies and reduced biotic resistance in the invaded range [4,5]. They compete with and displace native plants by monopolising resources such as space, light, moisture, nutrients, and pollinators that native plants require to grow [6,7]. The invasion can negatively affect both natural and agricultural landscapes, thereby increasing the costs of removing them. Economic analyses estimate that IAS cost the European economy at least €12 billion annually through impacts on agriculture, forestry, fisheries, and health [8]. Moreover, the total costs of IAS in Europe summed to €116.61 billion between 1960 and 2020, with the majority (60%) being damage-related and affecting multiple sectors [9]. Such impacts ultimately depend on the environmental constraints and ecological processes that shape species distributions.

Species’ geographic distribution is the outcome of the interaction between their niches and environmental conditions. Among these, climatic factors are well known to influence both species’ growth and their worldwide geographic distribution [10]. Ecological and biogeographical barriers, such as oceans and extensive mountain ranges, allowed ecosystems to evolve independently, and species in these ecosystems are adapted not only to local climates but also to one another, interacting in a delicate balance [11]. Moreover, climate change is expected to cause shifts in the current geographic distribution of species, as their optimal habitats will likely change as climate zones shift northwards and towards higher altitudes [12].

However, the expansion of plant and animal species beyond their natural distributions is a worldwide phenomenon. This is done either naturally, gradually, and slowly, or more rapidly, with human assistance [13]. These new alien species adapt well when introduced into sites with climates similar to their natural distribution [14]. In the past, humankind has greatly benefited from the introduction of alien species (e.g., potatoes and maize in Europe), and this trend is further enhanced by our globalised commerce [15]. As a result, rates of introduction and establishment of alien species are continuously rising. Among these species, many have been “naturalised” in their new environments and can now be found in several natural habitats [16,17]. Although many alien species face difficulties growing and reproducing outside their natural range, the new environments surprisingly favour others that are rapidly spreading. These species have become invasive, having multiple worrying ecological, economic and human health impacts [18]. Furthermore, climate change will likely increase the rate of invasive species establishment. Climate change and the forthcoming warming, altered precipitation, and extreme weather events are already shifting invasion fronts northward in Europe and globally, highlighting the strong interaction between climate change and invasion dynamics [19].

Deltas and other riparian ecosystems are highly vulnerable to IAS invasion. They are densely populated and support numerous economic activities worldwide, which increases both propagule pressure and management challenges [20]. Floodplain and deltas, such as the Danube (Romania and Ukraine), Po (Italy), and Rhine (The Netherlands), are regarded as hotspots of IAS establishment, where high connectivity, nutrient enrichment and frequent disturbance create ideal conditions for invasions [7]. Climate change is expected to exacerbate deltas’ vulnerability, with projections suggesting that up to 50% more delta surface area will become prone to flooding [20]. Such combined pressures highlight the urgent need to understand and manage IAS in delta ecosystems.

Early detection, as well as information on species’ distribution, habitat preferences, pathways of introduction and secondary spread, invasion dynamics, and ecological and socio-economic impacts, is fundamental for understanding the behaviour of invasive alien plant species [1,21,22]. Such integrated knowledge provides a scientific basis for assessing invasion risk and identifying priority areas for management [23]. Specifically, such information can support the development of effective, targeted management strategies in further preventing their spread, mitigating their impacts, and improving the control of invasive alien plant populations.

This study presents the results of the first coordinated monitoring of invasive alien plant species across four Black Sea delta ecosystems [Danube Delta (Romania and Ukraine), Nestos Delta and Lake Vistonida (Greece), Kızılırmak Delta (Türkiye), Chorokhi Delta and Kolkheti National Park (Georgia)]. Specifically, the study (i) documented the presence, distribution, and the habitat preferences of IAS across these deltas, (ii) identified the functional traits that enhance their establishment and spread, (iii) assessed their invasion dynamics and impacts, and (iv) evaluated the management challenges. Therefore, the paper provides a basis for improving IAS management strategies at both local and basin-wide scales.

2. Materials and Methods

2.1. Study Areas

2.1.1. Danube Delta (Romania and Ukraine)

The Danube Delta lies on the north-western Black Sea coast (Figure 1) and forms a highly connected mosaic of lakes, channels and islands at the river mouth. It is a transboundary delta shared by Romania and Ukraine, encompassing extensive wetlands and reedbeds of high conservation value within a UNESCO-designated biosphere reserve and a UNESCO World Heritage Site. The Danube Delta (45.15° N, 29.30° E) is located where the Danube River flows into the Black Sea, shared between Romania and Ukraine. It is one of Europe’s largest and best-preserved wetland, characterised by a complex mosaic of distributary channels, reed beds, floodplain forests, shallow lakes and marshes. The delta supports over 300 bird species and approximately 45 freshwater fish species. The total recorded biodiversity exceeds 5500 species of flora and fauna. It is designated as a Biosphere Reserve and a Ramsar site because its high habitat heterogeneity and hydrological connectivity create favourable conditions for native biodiversity.

In the Romanian Biosphere Reserve sector, the delta plain includes a fluvial sector and a younger maritime sector separated by former littoral cordons, generating strong gradients from freshwater reed marshes and shallow lakes to dune and coastal habitats. Recent vegetation syntheses for the Danube Delta report more than 1200 vascular plant species, 253 plant communities and 32 Natura 2000 habitat types, underlining the exceptional habitat heterogeneity of the reserve and the wide spectrum of niches available to both aquatic and riparian invaders [24]. The internal lake–channel network is also a key driver of aquatic vegetation structure, with lake typology and turbidity strongly shaping macrophyte assemblages [25].

For the present IAS synthesis, the Danube Delta should also be interpreted as a transboundary complex that includes a fluvial sector, a younger maritime sector, extensive reed marshes and shallow lakes, levees with floodplain forests, and locally disturbed settlement- or plantation-influenced sites. This environmental heterogeneity is directly relevant to invasion patterns because submerged IAS are concentrated in the connected channel-lake system. In contrast, woody and ruderal invaders are mainly associated with levees, riparian forests, embankments, dune margins, roadsides and other disturbed terrestrial habitats. At present, the evidence base used here is denser for the Romanian Biosphere Reserve sector than for the Ukrainian sector; accordingly, Danube Delta records are synthesised at whole-delta scale, but the spatial detail available is not uniform across the two national sectors [24,25,26].

2.1.2. Nestos Delta and Lake Vistonida (Greece)

The Nestos Delta (40.98° N, 24.73° E) is located in northeastern Greece (Figure 1), where the Nestos River discharges into the Thracian Sea. The area includes floodplain forests, coastal lagoons, freshwater ponds, riparian woodlands, and agricultural landscapes shaped by historical hydrological modifications. Despite past land-use transformations, it retains the most extensive riparian forest in Greece. The riparian forest of Nestos covered 127,000 ha in the early 20th century. However, significant deforestation began and converting forests to farmland. By 1946, after World War II, the forest had shrunk to 72,000 ha, though it remained the largest riparian forest in the southern Balkans [27]. In 1974, with Greece’s ratification of the Ramsar Convention, the uncontrolled destruction of the riparian forest ceased, and the area became a protected zone, later considered for inclusion in the Natura 2000 network. Today, the Nestos forest covers 5000 ha under special protection.

Dune plant communities characterise the coast of the Delta, whereas in the southern parts of the Delta, large areas are covered by halophytic plant communities. Around the marshes and canals, there are shrubs of Tamarix hampeana and Tamarix parviflora. Additionally, reed beds grow along the banks of the marshes, canals, and lagoons [28,29]. The riparian forest is categorised into hardwood stands (oak, ash, and elm) and softwood stands (mainly Populus alba). The delta also hosts non-zonal forests and diverse plant communities depending on local conditions, including sandy shore and dune vegetation.

Lake Vistonida (41.04° N, 25.12° E) is classified as a shallow lake. It covers an area of 42,400 ha and has an average depth of 2.5–3.5 m. It is one of the most beautiful wetlands in Greece, featuring both lagoon and lake characteristics. The lake exhibits a unique phenomenon in Greece: its northern part contains fresh water due to the inflow of three rivers. In contrast, its southern part exhibits variable salinity, as it receives seawater through communication channels with the Porto Lagos lagoon [30]. The lake’s vegetation consists of sand-loving, salt-tolerant plant communities, grasslands, reed beds, and shrublands [31], and all surrounding ecosystems are characterised by high biodiversity.

Recognised as one of the world’s most significant wetlands, Lake Vistonida is protected under the Ramsar Convention and attracts numerous birdwatchers and nature enthusiasts. Due to their ecological importance, the Nestos Delta and Lake Vistonida are designated as Special Protection Areas (SPA) for birds under EU legislation and form part of the Natura 2000 network.

2.1.3. Kızılırmak Delta (Türkiye)

The Kızılırmak Delta area (41.60° N, 35.90° E), located on Turkey’s Black Sea coast (Figure 1), is one of the most important and biggest deltaic areas in Türkiye. The delta area covers 56,000 ha, of which 11,600 ha consists of natural and semi-natural wetland areas. Specifically, a large part of the total area of the delta (11,580 ha) is open water surfaces, fresh and salt water swamps, wet meadows, and pastures, whereas on the coastal part of the delta, dunes reach 200–300 m in length and 7–8 m in height (2330 ha). Out of the 3100 ha that constitute the forest area of the delta, 1850 ha are flooded forests, whereas other broad-leaved tree species dominate the remaining 1250 ha. The combination of different habitats, such as the sea, rivers, lakes, reeds, swamps, meadows, pastures, forests, dunes, and agricultural areas, has given the delta a uniquely important level of biological diversity. Due to the delta’s important biological diversity, 22,000 ha were declared a Ramsar site in 1998. It was designated a habitat for waterfowl in 1994 [32,33].

2.1.4. Chorokhi Delta and Kolkheti National Park (Georgia)

The Chorokhi Delta (41.59° N, 41.57° E) is located in the most southwestern part of Georgia, in the Autonomous Republic of Adjara (Figure 1). It covers the territory from the Georgian–Turkish border (Sarpi) to the Koroli River and is approximately 8500 ha [34,35,36]. The city of Batumi is built on the delta territory. Today, the non-urban area of the Chorokhi Delta is approximately 3500 ha. The Chorokhi Delta is primarily composed of sediments deposited by the Chorokhi River. The Kakhaberi Plain, where the Chorokhi Delta is located, is mainly formed by the alluvium of the Chorokhi River. There are several small streams (stream) and artificial water catchment channels located in the Chorokhi Delta. The delta includes freshwater ponds, swamps, riparian forests, coastal dunes, and meadow grass vegetation.

On the banks of the Chorokhi River and the edges of the canal, there is a fragmented seaside lowland forest, where Alnus barbata is the main tree species. Small clusters of bushes of Hippophae rhamnoides, and more commonly, species of raspberry (Rubus spp.), occur in the delta. There are also many small ponds, in which Trapa spp., Nuphar lutea, Utricularia minor and Lemna minor occur. Around the ponds and at the edges of the canals, there are several wetland-loving plants [36,37]. Due to its high plant and habitat diversity, the area has been designated as part of the “Emerald Network” of protected areas [35]. Moreover, as it is an area which is very important for bird species, it constitutes a Special Protection Area for birds in Georgia, as well as an Important Bird and Biodiversity Area [35,38].

Kolkheti National park (42.14° N, 41.83° E) is situated in western Georgia along the eastern Black Sea coastline, within the historical Colchis region (Figure 1). Specifically, the Park is located in the central part of the Kolkheti Lowland, covering part of the water area adjacent to the Nabadi and Churia districts on the eastern coast of the Black Sea, as well as the Paliastomi Lake basin. The area of the national park is 44,308.5 ha, including 29,032.5 ha of land area, and 15,276 ha of marine water area between the confluences of the Rioni and Churia rivers. The national park includes the Katsoburi reserve, which covers 270.8 ha [39,40]. In 1996, Georgia joined the Ramsar Convention on Wetlands of International Importance, and thus the specific area was designated a Ramsar site. In 1999, the area was designated a National Park, and in 2021 it was granted UNESCO World Natural Heritage status.

The park includes extensive living percolating sphagnum peatlands, relict forests with peat cover, natural freshwater ponds, coastal dunes, lakes, and wetland meadows, forming one of the most important wetland systems in the Caucasus. Specifically, sphagnum peatlands are surrounded by relic Colchian peat forests, which are rich in species and home to several globally threatened woody species (e.g., Pterocarya fraxinifolia, Buxus colchica), Caucasian and Georgian red list species (e.g., Quercus hartwissiana) [41,42,43].

2.2. Data Sources and Field Observations

The information on invasive alien plant species was compiled from multiple complementary sources: (a) IASON/IASON+ projects’ field observations and partners’ datasets, (b) monitoring records and expert knowledge provided by local institutions working in the study areas (e.g., protected-area administrations, research institutes and environmental authorities), and (c) targeted searches of peer-reviewed and grey literature (reports, management plans and databases) documenting species occurrences, pathways and impacts. Field observations used to support the synthesis were based on systematic field activities focusing on inspections of riparian and human-modified habitats (e.g., riverbanks, levees, lake edges, roadsides, meadows, pastures, agricultural lands), and—where applicable—on boat-based observations in channels and shallow lakes of aquatic macrophytes. Field observations and surveys were conducted from 2020 to 2026 within the framework of the IASON (BSB1121) and IASON+ (BSB00174) projects. Surveys were carried out across the four study areas using field protocols to record invasive alien plant species. Additional information on IAS occurrences was obtained from monitoring records and expert knowledge provided by local institutions and experts working in the four study areas (e.g., protected-area administrations, research institutes/universities), as well as from literature sources, when available. At each site where IAS were present, geographical coordinates, habitat characteristics, reproductive strategies, spatial configuration (e.g., isolated individuals, scattered populations, or dense thickets), and observed ecological and economic/social impacts on native species, habitats, and human activities were recorded.

Following the fieldwork conducted, a list of all invasive alien species of the four study areas was compiled. Taxonomic names were checked and harmonised to the currently accepted nomenclature (Plants of the World Online: https://powo.science.kew.org/, accessed on 5 April 2026). For each invasive alien plant species recorded in the study areas, we summarised (a) the invaded habitats (riparian/terrestrial vs. aquatic channels and lakes), (b) the reproductive strategy, referring to sexual reproduction and/or clonal growth, and seeds’ viability and dispersal, (c) the possible pathways of introduction, as well as potential factors contributing to the secondary spread of the invasive alien species in the study areas, (d) the range expansion category within the studied delta ecosystems, and (e) any reported and/or observed ecological and socio-economic impacts together with locally applicable control measures.

The invaded habitats were described using (a) brief field-based habitat descriptions and (b) the EUNIS habitat classification system for terrestrial habitats [44] to ensure standardisation, transparency, and comparability between countries. The information about the reproductive strategy of the invasive alien plant species primarily was based on field observations, whereas information on seeds’ viability and dispersal when available, was obtained from GISD, CABI, DAISIE, and EPPO databases. The same databases together with field assessments made by experts were also used to present the ecological and economic/social impacts of the invasive alien plant species. To describe the pathways of introduction, the categories proposed by the Convention on Biological Diversity [45] were used.

The range expansion categories were defined using field observations of species dis-tribution patterns, local abundance, population structure, and evidence of spatial spread in at least one study area, as determined by surveys conducted from 2020 onwards. Three categories of species were defined based on their invasion dynamics: slowly spreading species, moderately spreading species, and rapidly spreading species. Specifically, slowly spreading species were those represented by isolated individuals or localised populations with limited spread; moderately spreading species had established populations with noticeable spread across suitable habitats in the study areas; rapidly spreading species formed extensive or dense populations with clear evidence of ongoing spread and colonisation across the study areas.

3. Results

Seventeen invasive alien plant species (IAS), in total, were recorded in the four deltas and riparian areas surveyed (

Table 1. Ecological characteristics of invasive alien plant species in the Black Sea Region (IASON+ project study area). Pathway categories follow the Convention on Biological Diversity pathway classification (1 = Release in nature; 2 = Escape from confinement; 3 = Transport–contaminant; 4 = Transport–stowaway; 5 = Corridor; 6 = Unaided secondary spread).

Scientific Name	Occurrence of IAS by Study Area	Native Region	Habitat	Reproductive Strategy	Possible Pathway of Introduction and Factors Affecting Secondary Spread
Amorpha fruticosa L.	Danube Delta Nestos Delta and Lake Vistonida Kızılırmak Delta Chorokhi Delta and Kolkheti National Park	North American	Riparian zones and wetlands, moist open woodlands.	Produces numerous seeds, but it is also able for clonal growth.	Categories of pathways: 2, 4, 6 Silviculture, road construction, water transport, floods.
Acer negundo L.	Danube Delta Nestos Delta and Lake Vistonida	North American	Riparian zones, Mediterranean riparian woodland, mesic grasslands, moist and urban areas.	Produces numerous seeds, but it is also able for clonal growth.	Categories of pathways: 2, 4, 6 Wind seed dispersal, water transport, through wildlife and along transportation corridors.
Ailanthus altissima (Mill.) Swingle	Danube Delta Nestos Delta and Lake Vistonida Kızılırmak Delta	East Asian (China)	Urban areas, roadsides, woodland edges, riparian zones, disturbed sites, shrublands, mesic and xeric woodlands.	Produces numerous seeds, but it is also able for clonal growth.	Categories of pathways: 2, 6 Wind seed dispersal, water transport.
Ambrosia artemisiifolia L.	Danube Delta Chorokhi Delta and Kolkheti National Park	North American	Pioneer species establishing after disturbance, in nutrient rich and slightly acidic soils.	Produces numerous seeds.	Categories of pathways: 2, 6 Commercial bird mixes, contaminated cereal seed, like sunflower and sorghum, cultivated in Ukraine as medicinal plant.
Broussonetia papyrifera (L.) L’Hér. ex Vent.	Nestos Delta	East Asian	Riparian zones, forest edges, disturbed sites, urban areas.	Both sexual (seed production) and vegetative reproduction (root suckers). Clonal growth is dominant in areas of invasion.	Categories of pathways: 1, 2, 6
Datura stramonium L.	Danube Delta Nestos Delta and Lake Vistonida	North and Central American	Disturbed habitats, agricultural fields, roadsides, riverbanks, urban wastelands.	Sexual reproduction and prolific seed production (thousands per plant). Seeds remain viable below ground for decades.	Categories of pathways: 3, 4, 6 Spreads through agricultural activities and soil movement.
Elodea canadensis Michx.	Danube Delta	North American	Freshwater lakes, ponds, slow-flowing rivers, canals, submerged habitats.	Vegetative reproduction via fragments (main). Sexual reproduction is rare in its introduced range.	Categories of pathways: 2, 5, 6 It is considered that spread in European freshwater systems occurs predominantly through vegetative fragments, dispersed by a combination of natural vectors such as water currents and animals, and anthropogenic vectors including boats, fishing gear, water management activities, and the disposal of aquatic plant material.
Elodea nuttallii (Planch.) H.St.John	Danube Delta	North American	Freshwater bodies (lakes, rivers, ponds, reservoirs, slow-flowing or still waters, shallow to deeper zones). Mesotrophic to eutrophic waters.	Almost entirely vegetative.	Categories of pathways: 2, 5, 6 Aquariums and ponds.
Erigeron canadensis L.	Danube Delta Nestos Delta	North American	Disturbed sites, agricultural fields, roadsides, urban areas, riverbanks. Rapidly spreading in agroecosystems and urban landscapes.	Sexual reproduction. Produces an extremely high number of seeds (up to >200,000 seeds per plant). Seeds are capable for long distance wind dispersal.	Categories of pathways: 3, 4, 5, 6 Secondary spread via grain trade, and transport corridors.
Gleditsia triacanthos L.	Danube Delta Nestos Delta and Lake Vistonida Chorokhi Delta and Kolkheti National Park	Central and North American	Riparian forests, floodplains, open woodlands, grassland edges, riverbanks, agroecosystems, disturbed and urban habitats.	Sexual reproduction via abundant seed production. Seeds dispersed mainly by livestock and waterways. It forms a permanent, long-lived seed bank.	Categories of pathways: 1, 2, 6 Used as windbreak, hedge and for fodder.
Phytolacca americana L.	Danube Delta Nestos Delta and Lake Vistonida	North American	Disturbed areas, field edges, clearings, roadsides, fence lines, waste places, forest edges.	Produces numerous seeds, which can live for up to 40 years in the soil.	Categories of pathways: 2, 6 Easily dispersed by birds.
Robinia pseudoacacia L.	Danube Delta Nestos Delta and Lake Vistonida Kızılırmak Delta	North American	Riverbanks, ruderal habitats, dry grasslands and rocky outcrops. Open, sunny, disturbed sites in well-drained soils.	Reproduces both by seed and vegetatively. Produces numerous seeds.	Categories of pathways: 1, 2, 6 Widely planted and naturalised in Europe.
Sicyos angulatus L.	Danube Delta Chorokhi Delta and Kolkheti National Park	North American	Sunny grasslands to shady forests. Prefers fertile, moist or even wet soils, such as floodplains, riverbanks, field edges, ditches and a variety of disturbed habitats.	Reproduces by seeds that can remain viable in soil seed bank for many years.	Categories of pathways: 2, 4, 6 Fruit dispersal in flowing water.
Solanum elaeagnifolium Cav.	Nestos Delta and Lake Vistonida	Central and South American	Disturbed areas, such as roadsides, field edges, wastelands, margins of agricultural land, canal banks.	Sexual reproduction and vegetative reproduction via root fragments, rhizomes, and dormant buds in the root system.	Categories of pathways: 3, 4, 6 Seeds often dispersed by water, animals or human vectors.
Solidago canadensis L.	Danube Delta Chorokhi Delta and Kolkheti National Park	North American	Disturbed sites, such as roadsides, abandoned fields, forest margins, grasslands, riparian margins.	Sexual reproduction and vegetative reproduction via rhizomes.	Categories of pathways: 2, 6 Wind-dispersed seeds.
Verbena brasiliensis Vell.	Chorokhi Delta and Kolkheti National Park	South American	Wetland/riparian zones, along canals and disturbed ground. Capable of growing in both wet and drier habitats, often found in old fields.	Sexual reproduction. It produces numerous seeds.	Categories of pathways: 2, 3, 6 Seeds can be dispersed by water, animals, or human vectors.
Xanthium orientale L.	Danube Delta Nestos Delta and Lake Vistonida	North, Central and South American	Wetland/riparian zones, along canals and disturbed ground. Capable of growing in both wet and drier habitats, often found in old fields.	Sexual reproduction. It produces numerous seeds.	Categories of pathways: 3, 4, 6 Seeds in spiny burs, dispersed easily via attachment to animals or humans. Burs also float and dispersed by water.

). The majority of the species originated from North, Central and South America, including Amorpha fruticosa, Acer negundo, Ambrosia artemisiifolia, Elodea canadensis, Elodea nuttallii, Phytolacca americana, Gleditsia triacanthos, Robinia pseudoacacia, Solidago canadensis, Solanum elaeagnifolium, Verbena brasiliensis and Xanthium orientale. A second group was of Asian origin, represented by Ailanthus altissima and Broussonetia papyrifera. Although these taxa belong to different taxonomic groups, they present similar functional traits that promote rapid establishment and spread in floodplain and riparian environments.

At the country level, the area richest in invasive alien plant species was the Danube Delta (14 species), followed by the Greek study area (Nestos Delta and Lake Vistonida: 11 species), the Georgian area (Chorokhi Delta and Kolkheti National Park: 6 species), and finally the Turkish area (Kızılırmak Delta) with 3 invasive alien plant species recorded. IAS richness across different chorological categories in each study area is shown in Figure 2. A clear variation in the geographical origin of invasive alien plant species is evident across the four study areas. North American taxa are dominant across all study areas, with the highest contribution recorded in the Danube Delta (13 species), followed by Nestos Delta and Lake Vistonida (8 species), Chorokhi Delta and Kolkheti National Park (5 species), and Kızılırmak Delta (2 species). The groups of the Central and South American plant taxa are only moderately represented in the alien floras of the Danube and Nestos Deltas. In contrast, the presence of such species is limited or absent in the other study areas. On the contrary, Asian species are present in all study areas—except the Georgian area—but their numbers are low.

Information on the distribution of the 17 invasive alien plant species recorded in the four study areas across the five countries of the IASON/IASON+ projects, together with an estimation of their invasiveness in non-occupied areas is presented below:

Amorpha fruticosa was the only species that was recorded in all four deltas and riparian areas. It was extremely abundant along riverbanks, flood channels, and wet or mesic grasslands, where it formed dense monospecific thickets that suppressed native flora. Its distribution in the four geographically distinct deltas and riparian areas demonstrates high ecological plasticity.

Acer negundo was recorded in the Nestos Delta, occurring as isolated or scattered individuals, a pattern plausibly linked to long-distance dispersal via its winged samaras. Its occurrence in the 91E0* habitat type (alluvial forests with Alnus glutinosa and Fraxinus excelsior), mainly in the understory but occasionally in the overstorey, suggests the potential to displace native tree species and progressively modify successional trajectories. A comparable concern is reported for the Danube Delta, where A. negundo is highlighted among invasive alien trees that restrict natural rejuvenation of 91E0* habitat and is therefore targeted by management and restoration planning. In the Romanian Danube Delta, Acer negundo is likewise included among alien woody species associated with disturbed riparian and plantation-influenced habitats, which supports concern about its future expansion in alluvial forests.

Ailanthus altissima was recorded in the Danube, Nestos and Kızılırmak Deltas. In the Kızılırmak Delta, it colonises the delta through anthropogenic conduits, including drainage canals, transport corridors, and degraded lands adjacent to human settlements. Along the Nestos River and around Vistonida Lake, it occurs in dry grasslands as scattered individuals, not yet extensively distributed. The species was also recorded in the Danube Delta, where, despite its restricted distribution from anthropogenic and plantation-affected habitats on the maritime delta, its potential spread is expected to increase.

Ambrosia artemisiifolia was recorded in Adjara/Chorokhi and the Danube Delta, where it was among the most frequent IAS, colonising ruderal sites and edges of agricultural areas. Its ability to rapidly colonise disturbed soils threatens both biodiversity and crop production.

Broussonetia papyrifera was found only along the Nestos River, where it occurs exclusively in the river’s western part. It has been recorded either as isolated individuals or in small groups in semi-arid environments, and it does not pose a serious threat at the moment.

Datura stramonium has been recorded along the Nestos River and Lake Vistonida. It typically occupies disturbed habitats (agricultural fields, roadsides, urban wastelands), produces large numbers of seeds, and can expand quickly where disturbance creates opportunities for establishment. In the Danube Delta, it has also been found in open or disturbed habitats (e.g., pastures, grasslands), supporting the view that similar disturbance-driven pathways can facilitate persistence and spread there too.

Elodea canadensis was recorded in the Danube Delta, where it occurs in slow-flowing channels, canals, and shallow lakes. It reproduces primarily vegetatively via stem fragments, enabling rapid spread by water currents and transport on boats and fishing/field equipment.

Elodea nuttallii was restricted to the Danube Delta, where it dominated aquatic habitats. It develops extensive mats in lakes, channels, and irrigation canals, displacing native aquatic vegetation. In the Danube Delta, Elodea nuttallii is treated as an established neophyte of connected lotic-lentic systems.

Erigeron canadensis was recorded in the Danube and Nestos Deltas. It occurs in species-poor communities, and in the Nestos Delta, its impacts are currently limited at the local scale but may increase as habitat structure and community composition change. The species is also included in floristic syntheses of the Danube Delta (often treated as Conyza canadensis), indicating that it can establish in deltas and potentially expand where open/disturbed habitats are available.

Gleditsia triacanthos was reported from a relatively small number of sites along Nestos River, but where present, it poses a notable threat to riparian forest and grassland habitats. As a fast-growing species, it can form dense (even monospecific) stands that outcompete native flora, altering habitats. The species is also recorded from the Danube Delta among alien woody plants surveyed across multiple habitat settings (e.g., banks, canals, localities, riparian forests), supporting concerns that similar riparian environments can be vulnerable to its establishment and spread. In Katsoburi (Georgia), it occurs primarily in wet habitats, such as along rivers, lakes, and drainage canals, although it is also present in comparatively drier sites. Its spatial distribution is distinctly mosaic. In certain locations, it forms dense, nearly impenetrable monospecific stands, while in others it penetrates alder-dominated forests, where Gleditsia triacanthos continues to actively expand and colonise new areas.

Phytolacca americana occurs in the Nestos Delta and around Lake Vistonida, where it is the most widespread IAS, usually occurring as isolated plants or small clusters in grasslands and forest understories. Despite its wide distribution, its ecological impact is comparatively limited compared with the impacts of other shrub and tree invaders. In the Danube Delta, it has also been recorded in recent field observations, demonstrating its ability to establish in coastal/dune–grassland mosaics and associated ruderal vegetation. However, its impacts in the Danube Delta are still limited.

Robinia pseudoacacia was recorded in the Nestos, Danube and Kızılırmak Deltas. In the Nestos Delta it was initially introduced to mitigate erosion and promote timber production and now is the second most widespread IAS in the area. Moreover, it is now spreading into dry and mesic grasslands and broadleaved woodlands, altering habitat conditions and reducing vital space for several native species. In the Danube Delta, it is likewise treated as an invasive alien tree in surveys of alien ligneous species, with reports of a substantial presence, especially in the fluvial delta, consistent with its capacity to spread in managed or disturbed floodplains. In the Kızılırmak Delta, Robinia pseudoacacia has been planted on reclaimed land to control erosion and has been used for ornamental purposes. This non-native species predominantly utilises anthropogenic landscapes—such as agricultural lands, hazelnut orchards, and transportation corridors—as primary entry points.

Sicyos angulatus was found in the Adjara/Chorokhi area, where it was recorded forming vigorous climbing populations in riparian corridors. It has been found to alter vegetation structure in riverine zones, demonstrating high invasiveness. The species has also been recorded in the Danube Delta, indicating that Danube Delta riparian and forest-edge habitats may be similarly exposed to structural impacts if populations expand.

Solanum elaeagnifolium was recorded in the Nestos Delta but was most common around Lake Vistonida, mostly in dry grasslands, where it usually forms dense populations.

Solidago canadensis was recorded only in Adjara/Chorokhi and in the Danube Delta. In the Kolkheti National Park and Chorokhi Delta, it is currently restricted to scattered patches along roadsides and disturbed sites and is not yet abundant locally. Still, it is expected to suppress native flora due to the dense stands that it can form. It was also recorded in the Danube Delta, indicating an established presence and suggesting that under suitable disturbance and propagule pressure, it could also become more influential in open delta habitats.

Verbena brasiliensis was restricted to Adjara/Chorokhi, where it occurred along riverbanks and in ruderal habitats. While rare in the surveyed delta, its invasiveness in other Mediterranean regions suggests it could expand under suitable conditions.

Xanthium orientale is only known from the Danube and the Nestos Deltas, as well as from Lake Vistonida, where it occurs around the lake margins. In the Danube Delta, it occupied disturbed areas and river margins. Known for its burs, which animals and humans disperse, it is considered a competitive agricultural weed and a substantial threat to native communities. In the Greek study area (Nestos Delta and Lake Vistonida), it was only rarely found; thus, it does not currently display high invasiveness.

In total, three broad categories of IAS can be identified according to their habitat preferences (Table 1): (a) riparian invaders, (b) grassland invaders, and (c) species occurring exclusively in freshwater habitats. Riparian invaders constitute a diverse group of species (Amorpha fruticosa, Acer negundo, Robinia pseudoacacia, Ailanthus altissima, Sicyos angulatus) that successfully exploit nutrient-rich, although periodically disturbed, floodplain soils. Grassland invaders (Gleditsia triacanthos, Phytolacca americana, Solidago canadensis, Verbena brasiliensis) were observed to thrive in relatively open habitats and, in some cases, in disturbed habitats (Table 1). At the same time, ruderal weeds (Ambrosia artemisiifolia, Solanum elaeagnifolium, Xanthium orientale) were confined exclusively to disturbed sites, edges of agricultural areas or temporary uncultivated fields. Elodea nuttallii and Elodea canadensis recorded only in the Danube Delta, were the only species that presented aquatic dominance.

The habitat types in which the 17 invasive alien plant species were recorded according to the EUNIS habitat classification are presented in the Supplementary Materials. Based on their species occurrence in the different habitat types, the recorded invasive alien plant species can be distinguished into habitat specialists, species with intermediate habitat breadth, and broad habitat generalists. Elodea canadensis and Elodea nuttallii exhibit the narrowest habitat association, being restricted to eutrophic aquatic habitats. On the contrary, Amorpha fruticosa, Robinia pseudoacacia, Ailanthus altissima, Gleditsia triacanthos, Acer negundo and Solanum elaeagnifolium show broader habitat distribution and can be considered broad habitat generalists within the study areas.

Various reproductive strategies were observed among the invasive alien plant species recorded in the deltas and riparian ecosystems of the study areas (Table 1). Specifically, Amorpha fruticosa, Acer negundo, Robinia pseudoacacia, and Ailanthus altissima exhibit a combination of prolific seed production and clonal regeneration through root suckers or stump resprouting, thereby having an advantage under disturbance. Similarly, Phytolacca americana can resprout from underground organs, whereas Solidago canadensis and Solanum elaeagnifolium have persistent seed banks, allowing them to withstand repeated disturbance. Solidago canadensis, in addition to reproducing by seed, can also propagate clonally by means of underground shoots (rhizomes). Vegetative fragments of Elodea nuttallii, an aquatic invader, can readily root, enabling rapid colonisation of channels and irrigation networks. On the other hand, Ambrosia artemisiifolia, Gleditsia triacanthos, Sicyos angulatus, Verbena brasiliensis and Xanthium orientale exhibit abundant seed production, dispersed by wind, water or animals, ensuring long-distance dispersal and colonisation.

The 17 invasive alien plant species recorded in the four delta areas are associated with multiple introduction pathways in the countries around the Black Sea, with each species linked to more than one category (Table 1). The most common pathway category was “escape from confinement”, mainly related to agriculture, horticulture, ornamental purposes, and to a lesser extent forestry, followed by the “transport–stowaway” category using various vectors. It is worth noting that once introduced, all these species are capable of unaided secondary spread.

The distribution of invasive alien plant species across the three range expansion categories is shown in

Table 2. Ecological and socio-economic impacts of IASON+ project species in the Black Sea region.

Scientific Name	Distribution Trend	Ecological Impact	Economic/Social Impact	Control Measures
Amorpha fruticosa L.	Rapidly spreading species	Negative. Dense thickets competing native flora, lower plant richness/diversity, alter invertebrate communities, modify riparian ecosystem processes. Nitrogen-fixing ability and reported allelopathy can shift community dynamics.	Nitrogen-fixing species, used as an ornamental, for stabilising slopes, for forage, constitutes good grazable material, and tolerant of defoliation, easy to resprout. Seeds are used as an oil source in the manufacture of glycerol.	Prevention is often the most effective strategy; mechanical control; chemical control.
Acer negundo L.	Moderately spreading species	Medium, decrease understory plant diversity.	Rapid growth and environmental tolerance, used as an ornamental along street pavements and shelterbelts, suitable for beekeeping, source of food and shelter, especially in the winter.	Mechanical control.
Ailanthus altissima (Mill.) Swingle	Slowly spreading species	Modifies communities and ecosystems, as it grows extremely rapidly and interferes with growth of native species.	Used as an ornamental, because it grows quickly, it can be trained into an attractive shape, and has attractive foliage and fruits. Tree-of-heaven is considered an important timber and fuelwood tree.	Chemical control; mechanical control; biological control.
Ambrosia artemisiifolia L.	Rapidly spreading species	It displaces native vegetation in its introduced range especially after a disturbance.	Seasonal aeroallergen in late summer to early fall, weed pests in agricultural crops. Its fruits can cause illness in livestock that ingests it. An essential oil of Ambrosia artemisiifolia acts as an antimicrobial, having antibacterial and antifungal compounds.	Chemical control; mechanical control; biological control.
Broussonetia papyrifera (L.) L’Hér. ex Vent.	Slowly spreading species	Alters forest structure and understory composition. Creates dense monospecific stands and suppresses native tree regeneration due to shading and aggressive clonal growth.	Its pollen is highly allergenic and causes respiratory problems.	Mechanical control; chemical control.
Datura stramonium L.	Rapidly spreading species in disturbed and agricultural areas	Competes strongly with crops, alters plant community composition in disturbed ecosystems. However, its impact in natural habitats is limited.	It reduces crop yields and increases weed-control costs. It is highly toxic to humans and livestock, as it causes poisoning.	Mechanical control (only before seed set; chemical control (in crops).
Elodea canadensis Michx.	Moderately spreading species	Dense submerged mats can reduce light and displace native macrophytes. It modifies habitat structure for aquatic fauna.	Impedes recreation and water management (navigation, fishing). Where dominant, it increases maintenance needs in canals and water bodies.	Prevention (avoid releases); mechanical removal/harvesting where feasible.
Elodea nuttallii (Planch.) H.St.John	Rapidly spreading species	It makes dense monospecific populations and colonises all of water bodies, cut off light, produce anoxic conditions and trap sediments in the system. Plant decomposition induces a secondary eutrophication toxic to many plants. Outcompete several native aquatic plants and provide a poor habitat for aquatic animals.	They interfere with recreation activities and increase the risk of adjacent land flooding. Floating Elodea mats block the entrance section of hydroelectric power plants.	Chemical control; mechanical control; biological control.
Erigeron canadensis L.	Rapidly spreading species in agroecosystems and urban landscapes	It dominates early successional communities suppressing native and crop species through competition. It presents limited impact on natural ecosystems.	Constitutes a major agricultural weed.	Integrated management required; mechanical control (before seed set); chemical control (increasingly difficult due to resistance).
Gleditsia triacanthos L.	Moderately spreading species	Forms dense stands, suppresses native vegetation through shading and competition. It alters grassland and riparian biodiversity.	Negative impacts on grazing lands due to thorny stems. It hinders livestock movement.	Mechanical control; chemical control; grazing management to reduce seed dispersal and seed bank persistence.
Phytolacca americana L.	Moderately spreading species	Low risk in biodiversity.	Toxic to livestock, pets, and humans. Used as ornamental.	Mechanical control.
Robinia pseudoacacia L.	Rapidly spreading species	Expands rapidly by root suckering and stump sprouting, and forms dense clones creating shaded islands from which most native plants are outcompeted. It modifies soil properties and favours the development of a nitrogen-demanding vegetation.	Used for erosion control and soil stabilisation. Wood is durable, used for fences and furniture. Used as ornamental tree in many urban areas.	Mechanical control; chemical control.
Sicyos angulatus L.	Rapidly spreading species	It forms dense mats that compete with native vegetation, especially in riparian and floodplain habitats. It reduces light under canopy and it hinders regeneration of trees/shrubs in riparian zones. It alters plant community composition and reduces plant diversity.	It can reduce crop yields as a weed.	Mechanical control; chemical control.
Solanum elaeagnifolium Cav.	Rapidly spreading species	Competes with native flora, especially in disturbed or marginal habitats, reducing species richness. Alters soil microbial/nematode communities.	Negatively affects crop yields. Serves as host for agricultural pests and pathogens. Toxic to livestock.	Mechanical control; chemical control.
Solidago canadensis L.	Moderately spreading species	Competes with native vegetation and reduces plant diversity and richness.	Contains bioactive compounds that are believed that have antioxidant, antimicrobial, and anticancer activities. Used as ornamental.	Mechanical control; chemical control.
Verbena brasiliensis Vell.	Moderately spreading species	Competes with native vegetation and reduces species richness. Alters native plant community structure, especially in riparian/disturbed habitats.	Used as ornamental	Mechanical control; chemical control.
Xanthium orientale L.	Moderately spreading species	It forms dense stands that shade out other plants and alter the microclimate of the area. This can lead to changes in soil moisture, nutrient availability, and the composition of the plant community, ultimately affecting ecosystem functioning.	Major weed of crops. Toxic to livestock. It contains medicinal substances, and is used in the traditional herbal medicine.	Mechanical control; chemical control.

. Specifically, the category of the rapidly spreading species, which are the most widely distributed, includes Amorpha fruticosa, Ambrosia artemisiifolia, Elodea nuttallii, Robinia pseudoacacia, Solanum elaeagnifolium etc. The category of the moderately spreading species includes the species Acer negundo, Phytolacca americana, Solidago canadensis and Verbena brasiliensis, whereas the category of the slowly spreading species includes Ailanthus altissima and Broussonetia papyrifera.

As shown in Figure 3, rapidly spreading species predominate (approximately 50% of the IAS fall into this category) in all four study areas. Together with moderately spreading species, they account for over 80% of the recorded species. The Kızılırmak Delta area is an exception, where rapidly and moderately spreading species together account for about 67% of the total IAS recorded there.

In the study areas, except for P. americana, all others negatively affect biodiversity, particularly plant diversity and richness, especially when forming dense stands (Table 2). However, the ecological impacts of the 17 specific IAS are not always negative and limited to plant diversity and richness; two species (Amorpha fruticosa and Robinia pseudoacacia) positively affect soil properties. Despite the ecological impacts of the 17 IAS in the study areas, the field observations and literature review revealed that eight species were associated solely with negative economic and/or social impacts (Broussonetia papyrifera, Datura stramonium, Elodea canadensis, Elodea nuttallii, Erigeron canadensis, Gleditsia triacanthos, Sicyos angulatus and Solanum elaeagnifolium), six solely with positive impacts (Amorpha fruticosa, Acer negundo, Ailanthus altissima, Robinia pseudoacacia, Solidago canadensis and Verbena brasiliensis), whereas three species (Ambrosia artemisiifolia, Phytolacca americana and Xanthium orientale) exhibited both beneficial and adverse economic and social effects.

The most commonly reported management measures for the 17 invasive alien plant species recorded across the four study areas involve mechanical and chemical control methods (Table 2). Removal using mechanical methods (such as cutting, uprooting, and mowing), as well as the application of herbicides, is frequently used to reduce population size and limit further spread, particularly in early-stage or localised IAS populations. However, species established in an area and become widely distributed can be difficult to eradicate due to their extensive seed banks and/or high dispersal ability, which cause reinvasion.

4. Discussion

4.1. Distribution Patterns of Invasive Alien Plant Species in Deltas Around Black Sea

Ecosystems in deltas are widely recognised as biodiversity hotspots and as providers of essential ecosystem services. They sustain fisheries, agriculture, grazing and recreation, while buffering floods and storing carbon. However, these systems are very fragile. Most large deltas worldwide are shrinking under the combined pressures of sediment reduction by dams, hydrological regulation and agricultural expansion [46,47]. Such changes reduce habitat heterogeneity and weaken ecological resilience.

Invasive alien plant species (IAS) represent a distinct and particularly strong pressure. Frequent disturbance, nutrient enrichment and hydrological connectivity make deltas especially vulnerable to invasion [48,49]. Moreover, unlike other forms of artificial pressure, these biological invasions cannot be easily reversed. Once established, IAS reinforce ecological change by altering succession in artificial habitats, modifying nutrient cycles and reshaping species interactions [1]. Our observations from the four Black Sea deltas and riparian areas indicate that IAS are no longer occasional elements of local flora but many of them have become part of the vegetation of riparian forests, aquatic channels, and grasslands. This confirms what has been observed in other European floodplains [1].

The available dataset indicates that some species are associated with a limited range of habitat types, whereas others show broader, more eurytopic habitat associations. Elodea canadensis and Elodea nuttallii were restricted to eutrophic aquatic habitats, suggesting a more specialised association with submerged freshwater environments. However, these aquatic invasive macrophytes can have strong local impacts by altering water flow, light availability, and oxygen conditions, and have a significant impact on native macrophyte communities [50].

On the contrary, Amorpha fruticosa, Robinia pseudoacacia, Ailanthus altissima, Gleditsia triacanthos, Acer negundo and Solanum elaeagnifolium were recorded in several habitat types and can be considered broad habitat generalists within the present dataset. Their presence in riparian habitats, grasslands, wetland margins and disturbed habitats suggests ecological plasticity and tolerance of variable environmental conditions. Such patterns are consistent with invasion theory, which recognises invasion success as the outcome of interactions among species traits, propagule availability, disturbance and habitat invasibility [51]. Many of the IAS recorded present high seed production, vegetative reproduction or both, and therefore their increased propagule availability improves the possibility of establishment, persistence and secondary spread, particularly in disturbed or resource-rich habitats [52].

Moreover, the frequent occurrence of several species in anthropogenic, agricultural and disturbed habitats also indicates the importance of disturbance in facilitating invasion. Such habitats often provide open microsites, increased light availability, reduced competition and temporary increases in resource availability [53]. Therefore, roads, agricultural margins, man-made vegetated areas and disturbed wetland edges may function as source habitats for further spread into natural or semi-natural habitats.

4.2. Reproductive Traits and Dispersal Mechanisms Driving Plant Invasions

Species traits associated with invasiveness have been a central theme in invasion ecology [54]. The success of an invasive species to dominate is based on its functional traits and ecological strategies [55]. The IAS recorded in the present study shared a restricted suite of functional traits that reinforce their success in floodplain environments. Despite belonging to diverse taxonomic groups, woody taxa such as Amorpha fruticosa, Robinia pseudoacacia, Acer negundo and Ailanthus altissima combine prolific seed production with vigorous clonal regeneration. It is well documented that the strongest determinant of invasiveness is the vegetative regeneration (suckering, resprouting, runners or rhizomes) [56]. This dual strategy makes them highly resilient to disturbance and challenging to remove once established. Núñez-Mir et al. [56] also point out that differences in generation time among species with vegetative regeneration play a significant role in species expansion (e.g., lianas are the most invasive growth form, followed by shrubs and then trees).

Seed dispersal distance can be an important variable for the species distribution [57]. The results of our study highlight the importance of long-distance dispersal, which enhances invasiveness by facilitating introductions as far as possible from the initial site. Birds have been identified globally as the most prevalent mode of dispersal among invasive trees and shrubs [58]. Similarly, seeds can be transported long distances down streams and rivers, or along coastal currents. The vast majority of invasive plants in our database that exhibit water-seed dispersal thrive along bodies of water. Aquatic species like Elodea nuttallii spread primarily via vegetative fragmentation, allowing fragments to colonise new habitats via hydrological connectivity. On the other hand, annual herbs (Ambrosia artemisiifolia, Sicyos angulatus, Xanthium orientale) rely on massive seed production and multiple dispersal vectors, including wind, water, animals and human activities [59,60]. Perennial herbs (Solidago canadensis, Verbena brasiliensis, Phytolacca americana) combine persistent seed banks with underground resprouting. Finally, Gleditsia triacanthos produces large pods dispersed by animals and tolerates periodic flooding [61].

4.3. Invasion Dynamics and Impacts of Alien Plant Species

Invasion dynamics are frequently described in terms of two distinct phases. The first is a lag phase, when alien plant species remain at low numbers or in scattered sites, and their presence appears stable. This may last for years or even decades before, for several reasons, passing to an expansion phase, in which their populations grow rapidly and spread across new habitats [62,63]. The 17 species recorded in this research exhibited contrasting invasion dynamics. Some, including Amorpha fruticosa, Robinia pseudoacacia, Elodea nuttallii, Ambrosia artemisiifolia, Sicyos angulatus, and Solanum elaeagnifolium, were clearly in a rapidly spreading phase. These taxa were already forming dense populations and appeared capable of dominating patches of habitat. Others, such as Solidago canadensis and Verbena brasiliensis, showed moderate expansion but remained primarily associated with disturbed grassland or corridor habitats. On the other hand, Acer negundo and X. orientale were found more broadly, though generally as scattered individuals or small groups rather than as dense stands. Finally, two species (Ailanthus altissima and Broussonetia papyrifera) were relatively rare.

Amorpha fruticosa stood apart from the others because it was found in every delta and in high abundance. On many sites, it has already formed dense, monospecific thickets along riverbanks, flood channels, and wet grasslands. The species has already reshaped large parts of the riparian vegetation due to its high plasticity [64,65,66,67]. This pattern shows that Amorpha fruticosa is now in a phase of broad expansion, causing serious problems where it occurs, although it is not yet listed in the Union Concern IAS list [68]. Therefore, it is a qualitatively different case from the other observed invaders, for which management options may still exist.

Other plant species, although widespread in some deltas, were absent or scarce in others. Robinia pseudoacacia, Elodea nuttallii, Ambrosia artemisiifolia, Sicyos angulatus, and Solanum elaeagnifolium belong to this group. These species were rapidly expanding in at least one delta but not across the whole region. They appear to have exited a local lag phase, yet their distribution remains uneven. Invasive species dominance is, therefore, not continuous across all deltas, as invaders may form patches or ‘islands’ of invasion forming a mosaic expansion [69], where environmental variation or propagule pressure allows populations to build rapidly in specific habitats while remaining rare in others [63,70].

A third group of species, Solidago canadensis, Verbena brasiliensis, Acer negundo, and Xanthium orientale, was found at several sites, but usually only as scattered individuals or in small groups of individuals. None of them dominated the vegetation. At present, their distribution suggests a lag phase or a slow shift out of it. Populations of these species seem to be gradually increasing, but without the sudden, explosive spread already seen in Amorpha fruticosa or Robinia pseudoacacia. Finally, Ailanthus altissima and Broussonetia papyrifera were rare and localised. They may be in lag phases, although this does not mean they are harmless. However, the species can remain in a lag phase for decades before suddenly spreading if conditions change or new genotypes are introduced. In that sense, they represent “sleeper species” whose potential impacts may only become visible in the future [63,70].

4.4. Management Challenges and Limitations

The invasion patterns observed in the four deltas are consistent with “invasion syndromes” defined as “a combination of pathways, alien species traits, and characteristics of the recipient ecosystem which collectively result in predictable dynamics and impacts, and that can be managed effectively using specific policy and management actions” [71]. Different taxa share functional traits that lead to successful establishment and spread [71,72]. Despite their taxonomic diversity, the recorded species share a limited set of strategies (prolific reproduction, clonal regeneration, and flexible dispersal) that enable them to exploit disturbances, nutrient enrichment, and connectivity typical of delta ecosystems.

Therefore, the challenge is whether established invasions in delta areas can be reversed. For dominant taxa such as Amorpha fruticosa, Robinia pseudoacacia and Elodea nuttallii, eradication seems unrealistic. Their regenerative capacity allows them to resprout after cutting or to spread further following disturbance [50,65,73]. In most cases, monitoring documents continued expansion rather than offering a chance for elimination. This raises the fundamental dilemma: how much investment of time and financial resources is justified to suppress invasions that may be irreversible, and, if not, what is the acceptable impact on biodiversity and agriculture.

Experience in Europe shows how hard it is to remove a completely dominant IAS once it is established. For example, in the United Kingdom, long-term mechanical and chemical efforts to remove Fallopia japonica were insufficient. However, this was the trigger for organising the first EU-approved biocontrol programme to eliminate the species from its distribution, with relative success [74]. Mechanical removal, chemical treatment, and grazing were successfully used to control the spread of Heracleum mantegazzianum. However, the control measures must be applied across multiple seasons because each plant can produce an enormous number of seeds that can sustain rapid recolonisation [75].

A wide range of management techniques has been tested for Elodea nuttallii and Elodea canadensis, but none achieve permanent eradication. Mechanical harvesting, cutting, and dredging are the most widely applied approaches, often using weed-cutting boats or dredges. While they temporarily reduce biomass, these methods usually promote rapid regrowth, as cutting produces fragments that readily regenerate and light availability increases in cleared patches [76].

While such measures seldom eliminate a species, they may keep populations below ecological harm thresholds. This pragmatic approach—managing rather than eradicating—is likely to be more realistic for many taxa in the Black Sea deltas. For these deltas, a differentiated management approach is required. Species still in a lag phase (Ailanthus altissima, Gleditsia triacanthos, Verbena brasiliensis) should be prioritised for eradication, while this remains feasible. For dominant invaders, such as Amorpha fruticosa and Robinia pseudoacacia, control measures combined with native vegetation restoration are probably more effective. Preventive measures, particularly stricter regulation of ornamental plant introductions and better control of dispersal pathways (e.g., shipping and agriculture), are likely to yield the greatest long-term benefits. Because river systems cross national borders, effective management requires transboundary cooperation.

However, approaches to IAS management are not applied uniformly across countries, due to differences in legislation, priorities, and available resources. Based on current knowledge, for the Danube Delta, management priority should be given to species that combine high dispersal capacity, strong ecological impact, and occurrence in highly connected wetland habitats. Among terrestrial and riparian IAS, Amorpha fruticosa should be considered one of the highest-priority species, because it is already widely established along channels, lake margins, reedbed edges and riparian forests, where it can form dense shrub layers and alter the structure and regeneration of native wetland and alluvial vegetation [77]. Among aquatic macrophytes, Elodea nuttallii deserves particular attention in eutrophic lakes, canals and slow-flowing waters, because it spreads efficiently by vegetative fragments, is highly competitive in nutrient-rich waters, and has already been reported from the Romanian Danube corridor and the Danube Delta [78,79].

Among the invasive alien plant species recorded in the Nestos Delta and Lake Vistonida area, Amorpha fruticosa, Robinia pseudoacacia and Solanum elaeagnifolium should be considered species of particularly high concern, as they combine high dispersal capacity, broad ecological tolerance and strong potential for ecological and socio-economic impacts. Therefore, these three taxa may be classified as high-risk species in terms of dispersal, establishment and ecological impact, and should be prioritised for systematic monitoring, early containment and long-term integrated management.

In the Chorokhi Delta, Ambrosia artemisiifolia and Sicyos angulatus are well adapted to the local soil and climatic conditions, and thus are rapidly spreading [36,80]. However, the invasive potential and management priority of these species vary according to habitat characteristics and distribution patterns within the study area. Due to their ecological plasticity and capacity for expansion, these species should be considered priority targets for continuous monitoring and control measures. Early detection and rapid management responses are essential to prevent further spread and potential impacts on native plant communities and wetlands. Within the administrative boundaries of Kolkheti National Park, Gleditsia triacanthos and Solidago canadensis should be prioritised in future monitoring and management programmes. The development of species-specific control and eradication strategies is necessary to reduce their further spread and minimise ecological impacts within sensitive wetland ecosystems. Contrary to all these areas, the IAS recorded in the Kızılırmak Delta do not currently constitute a serious threat to native biodiversity.

5. Conclusions

Our research in the four Black Sea deltas shows that invasive alien plant species (IAS) are already integrated into riparian forests, aquatic channels and grasslands. Several taxa, such as Amorpha fruticosa, Robinia pseudoacacia, and Elodea nuttallii, are beyond early establishment and in a rapidly spreading phase. They are already reshaping habitats through dominance and altered ecosystem functioning. However, others remain in localised or lag phases and may represent “sleeper species” with the potential for rapid future spread.

The observed invasion dynamics are consistent with established invasion syndromes (high reproductive output, clonal regeneration, and long-distance dispersal) facilitated by hydrological connectivity and human trade. These traits can explain IAS’s capacity to thrive in deltas, which are already under stress from land-use intensification and climate change.

Complete eradication of dominant invaders is rarely feasible. However, management experience in Europe suggests that long-term control can mitigate the impacts, especially when integrated with restoration of native vegetation and stricter prevention of propagule pathways. Early eradication of species still in lag phases, coupled with coordinated transboundary action, may yield the greatest long-term benefit.