Cities act as hotspots for alien species (Moro and Castro 2015), and escape from cultivation – for example from common green spaces such as tree-lined avenues, parks, and private gardens – represents a major pathway for the spread of Invasive Alien Plant species (IAPs) in urban habitats (Pyšek et al. 2011). Planted alien species may overcome dispersal barriers that species with poor dispersal abilities would hardly pass through (Čeplová et al. 2017).

Urban environments contain a broad range of micro-habitats, shelters, and ecological niches, that allow many alien plant species to establish and grow (Gentili et al. 2024). Moreover, cities often have high levels of impervious surface coverage, leading to reduced vegetation and, consequently, less competition with native plants (Flores-Reyes et al. 2025). On the other hand, they may provide important ecosystem services such as green air quality improvement, climate regulation, nutrient recycling, and pollination, and they also play a cultural role by providing recreational opportunities for citizens resulting in improved well-being (Buchholz et al. 2015; Campagnaro et al. 2018; Mori et al. 2025). Successful invaders are typically generalist taxa that are often preadapted or highly plastic, enabling them to take root in a wide range of urban habitats (Potgieter et al. 2020).

IAPs may cause a decrease of native biodiversity in cities and directly affect citizens’ health and daily activities, contributing to issues such as allergies, higher fire risk, and damage to infrastructure and cultural heritage (Celesti-Grapow and Ricotta 2021; Montagnani et al. 2023).

IAPs may spread from urban to peri-urban, rural, and natural areas (Cadotte et al. 2017; Campagnaro et al. 2022), and they can significantly alter the structure, function, and productivity of natural ecosystems, with a consequent decline of biodiversity in native habitats (Vilà et al. 2011; Pyšek et al. 2012; Lazzaro et al. 2020).

In cities where the interface between urban, rural, and semi-natural areas is particularly narrow, the probability of IAPs spreading from urban environments into natural habitats increases.

Moreover, the restoration and expansion of green areas and infrastructures may enhance the biological permeability of cities, thereby facilitating the dispersal of alien species (Montagnani et al. 2026).

The compilation of inventories that include ecological information on urban green spaces provides valuable insights into ornamental species that are currently invasive or have a high invasion potential, thus enabling prevention measures and targeted interventions (Bartoli et al. 2021; Venturella et al. 2024; de Francesco et al. 2025; Di Gristina et al. 2025; Sarigu et al. 2025; Varricchione et al. 2026).

Identifying the most widespread or potential Invasive Alien Plants and their urban distribution patterns is therefore essential to address management strategies and reduce their impacts in urban and natural habitats (Štajerová et al. 2017; Dana et al. 2019).

For assessing alien plant distribution in urban habitats, the application of the EUNIS Habitat classification system (EEA 2025) may facilitate harmonized ecological assessments and comparative analyses across different urban contexts in Europe (Varricchione et al. 2024; D’Angeli et al. 2026).

A local, but also national and European, strategy is indeed required to the prioritization process and to mitigate the impact of IAPs (Brundu et al. 2020).

In this context, the present study aims to investigate the occurrence and distribution of a pool of 26 IAPs along the urbanization gradient and across urban EUNIS Habitats in a Mediterranean city of Southern Italy (Campobasso). Target species were selected by Montagnani et al. (2026) for the monitoring of alien plants across Italian cities, based on their invasive status (Galasso et al. 2024), their relevance in Italian urban contexts (Bartoli et al. 2021), and their documented severe impacts on native ecosystems, economy, and human health at both European and global scales (Nentwig et al. 2018).

Moreover, the findings seek to inform effective prevention strategies to mitigate the growing challenge of alien plant invasions.

Our results revealed the presence in the city of Campobasso of 14 of the target 26 urban IAPs, belonging to 11 taxonomic families. Asteraceae family accounts for 28.6% of the total IAPs. The majority of the registered IAPs come from temperate Asia and Northern America (Table 2).

All the recorded species are classified as invasive neophytes (N INV) at national level in Italy. However, only 4 (Ailanthus altissima, Helianthus tuberosus, Robinia pseudoacacia, and Senecio inaequidens) are considered N INV in Molise region (Portal to the Flora of Italy 2025), one species (Artemisia annua) is naturalized neophytes (N NAT), and two species (Acer negundo and Ligustrum lucidum) are casual neophytes (N CAS). For half of the recorded species, the invasion status in Molise region is not available (N/A) (Table 2).

Out of the 634 occurrences of target IAPs, the most frequently observed was Prunus laurocerasus (25%, primarily cultivated), followed by Senecio inaequidens (23%, entirely spontaneous), Robinia pseudoacacia (16%, mainly spontaneous), and Ailanthus altissima (11%, mainly spontaneous) (Table 2).

The species richness and occurrence of the investigated pool of IAPs varied along the urbanization gradient. Specifically, grid cells characterized by a high cover and patch number of artificial surfaces (HH and HM) showed significantly higher values of IAP richness (9 vs. 4 species on average, p-value = 0.03) and occurrences (67 vs. 16 on average, p-value = 0.04) compared to grid cells with low cover and patch number of artificial areas (MM and ML; Fig. 2A). Considering only the cultivated individuals/populations of IAPs, the most urbanized grid cell (HH) showed a significantly higher values of IAPs richness (7 vs. 2 on average, p-value = 0.03) and occurrences (36 vs. 5, p-value = 0.03) than the most natural grid cell type (ML; Fig. 2B). A similar trend also emerged when considering only spontaneous individuals/populations of IAPs. The most urbanized grid cell types (HH and HM) had higher spontaneous individuals/populations species richness (5 vs. 2 on average, p-value = 0.04) and occurrences (36 vs. 5 on average, p-value = 0.03) than the most natural grid cell type (ML; Fig. 2C).

With regard to the distribution of occurrences of the selected IAPs across EUNIS Habitats, 33.9% are associated with small-scale ornamental and domestic garden areas (V22), 20.5% with dry perennial anthropogenic herbaceous vegetation (V38), 20.3% with transport networks and other constructed hard-surface areas (J4), and 11.3% with large-scale ornamental garden areas (V21) (Suppl. material 2). Most occurrences in V21 and V22 involved cultivated individuals/populations, whereas those in V38 and J4 were predominantly spontaneous.

Slightly more than half of the total occurrences of target IAPs (334) consisted of spontaneously established individuals/populations belonging to 10 species. Among these, the majority (>80% of the occurrences) were accounted for Senecio inaequidens (44%), Robinia pseudoacacia (23.7%), and Ailanthus altissima (20.1%) (Fig. 3).

Senecio inaequidens was primarily recorded along the transport networks and other constructed hard-surface areas (J4), followed by the dry perennial anthropogenic herbaceous vegetation (V38). As an herbaceous species, it generally occurred in small areas, often as single individual or small individual groups (Fig. 4).

Robinia pseudoacacia mainly occurred in V38, where it occupied large areas and formed dense patches composed of numerous individuals/populations, including mature trees (Fig. 4). Although less common, Robinia pseudoacacia also occurred in J4, forming large patches along railway lines, as well as in ornamental and domestic garden areas (V22), where they were mostly represented by small, spontaneously established individuals derived from cultivated trees (Fig. 4).

Ailanthus altissima, by contrast, was more frequent in V38, where it mainly occupied large areas, typically forming small but dense patches. It also occurred in J4, in small areas, and was often represented by isolated or young individuals/populations growing in sidewalk cracks or along road edges (Fig. 4).

The findings yield novel insights into how a suite of IAPs is distributed along the urbanization gradient and across EUNIS habitat types in a Mediterranean city.

The target IAPs recorded in Campobasso are mainly woody perennial species, used as ornamental species (Varricchione et al. 2026), in the green furnishing of public and private spaces (Venturella et al. 2024). However, there are also herbaceous species introduced for ornamental and/or food purposes (Artemisia annua and Helianthus tuberosus) or arrived accidentally (such as Senecio inaequidens) (Vacchiano et al. 2013; Quaglini et al. 2025).

Among the target IAPs, the Asteraceae family prevails, in line with the overall composition of non-native flora in Italy (Galasso et al. 2024) and with what was observed in invaded natural habitats (Montecchiari et al. 2020; Viciani et al. 2020; Compagnone et al. 2024) and urban environments (Štajerová et al. 2017; Appalasamy et al. 2020; Ibáñez et al. 2023; Richardson et al. 2025). The native distribution range of the recorded IAPs corresponds to areas with a similar climate to that found in the study area (i.e., the temperate zones of North America, the Asian continent, and, to a lesser extent, the Mediterranean climate areas of South Africa), as recorded for other cities in Southern Europe (Ibáñez et al. 2023; García-Mozo 2024; Sarajlić et al. 2025) and globally (Li et al. 2025; Richardson et al. 2025).

This notable prevalence of American species may indicate a long-standing history of trade and interaction across multiple geographical regions over several centuries (Arianoutsou et al. 2021), as well as the widespread availability in Europe of habitats analogous to those occupied in their native range (Fristoe et al. 2021; Guarino et al. 2021).

For half of the recorded species, the invasion status in Molise region is currently not available (N/A) (Galasso et al. 2024; Portal to the Flora of Italy 2025). This paper helps to fill this knowledge gap, as for 4 IAPs here are the first documented observations for the region as spontaneously established individuals/populations (Portal to the Flora of Italy 2025). These are Buddleja davidii, Lonicera japonica, Parthenocissus spp., and Prunus laurocerasus, already reported in neighboring regions as spontaneous (Galasso et al. 2016; Portal to the Flora of Italy 2025).

Based on the analysis of the urbanization gradient, we observed that the most urbanized grid cells have higher richness and occurrences of target IAPs, as recorded in other cities (Celesti-Grapow et al. 2006; Schmidt et al. 2014; Kalusová et al. 2019; Jogan et al. 2022). Conversely, we observed that IAPs richness/occurrences were lower in most natural grid cells. These findings agree with the assumptions that the most urbanized areas are hotspots for the presence of IAPs (Botham et al. 2009; Kühn et al. 2017; Boscutti et al. 2022) and that the low native vegetation cover coupled with high impervious surface coverage enhance IAPs rooting (Santangelo et al. 2022; Flores-Reyes et al. 2025).

Among the target IAPs species spontaneously established, the highest occurrences were accounted by Senecio inaequidens (44%), followed by Robinia pseudoacacia (23%), and Ailanthus altissima (20.1%). These findings partially align with previous records from Milan, Turin, and Rome, where the most widespread and frequent spontaneous species were Ailanthus altissima, Sorghum halepense, Phytolacca americana, and Robinia pseudoacacia (Montagnani et al. 2026). As for S. inaequidens, Campobasso is currently the only Italian city in which it showed a high occurrence. Presumably, other Italian cities in hilly and mountainous areas are experiencing an invasion of this IAP, as its recently documented ability to adapt to progressively higher elevations makes Senecio inaequidens one of the most successful neophytes regarding the span of altitude (Bazzato et al. 2024). Further studies should be implemented for updating the distribution of this species in urban and natural habitats.

Regarding the distribution of target IAPs across EUNIS Habitats in the study area, individuals/populations found in cultivation are associated with small-scale ornamental and domestic garden areas (V22) and large-scale ornamental garden areas (V21) (Suppl. material 2). These public and private green spaces are, in fact, the urban areas that host many invasive alien ornamental species, as also observed in other Italian (Bartoli et al. 2021; Venturella et al. 2024; Di Gristina et al. 2025; Varricchione et al. 2026) and European cities (Jogan et al. 2022; Richardson et al. 2025; Sarajlić et al. 2025). This result highlights the importance of informing public decision-makers responsible for urban green spaces, as well as nursery and horticultural sector operators, about the ecological impacts these species may cause in natural environments (Hulme et al. 2018; Brundu et al. 2020).

In contrast, the pool of IAPs spontaneously established are mainly associated with dry perennial anthropogenic herbaceous vegetation (V38), found in abandoned areas within the city, and with transport networks and other constructed hard-surface areas (J4). These are the primary colonization sites for Invasive Alien Plants in urban environments, as they offer space, resources, and low native species cover (Appalasamy et al. 2020; Boscutti et al. 2022).

Analysing which EUNIS Habitats hosted the most abundant spontaneously growing IAPs (Senecio inaequidens, Robinia pseudoacacia, Ailanthus altissima), it can be noted that the most widespread one, the South African Senecio inaequidens, mainly occurred in J4, as also documented in other studies (Bornkamm 2002; Heger and Böhmer 2005; Lenzin et al. 2009). Roadside slopes and residual terrain abutting sidewalk pavement are the most frequently recorded habitats of this species in the study area, as observed in other cities (Ernst 1998; Blanchet et al. 2015; Kocián 2016; Quaglini et al. 2025). However, the species also grows in V38 (i.e., on abandoned areas where ruderal vegetation is established, dominated by perennial species such as Dittrichia viscosa, Scrophularia canina, and Hypericum perforatum (Di Pietro et al. 2017)).

The edges of roads and railway lines represent the ecological corridor that allows rapid dispersal of IAPs both in urban environments and in more natural areas (McDougall et al. 2018; Bhadouria et al. 2025; Brundu et al. 2025). In Central Italy, Senecio inaequidens occurrence has been documented up to 985 m a.s.l. and it is invasive in natural grasslands referable to the EUNIS habitats Thermophilous forest fringe of base-rich soils (R51), Trampled xeric grassland with annuals (V34), and Perennial rocky calcareous grassland of subatlantic-submediterranean Europe (R18) (Santoianni et al. 2024, 2025). The species was found at an elevation up to 2575 m a.s.l. in the Western Alps, confirming its pre-adaptation to mountain conditions (Digital Flora of Aosta Valley 2026). A key factor underlying the invasion success of Senecio inaequidens is its high seed dispersal capacity, which enables a rapid colonization of new areas. According to the EICAT (Environmental Impact Classification for Alien Taxa) framework (Vimercati et al. 2022), Senecio inaequidens is globally classified as having a moderate impact: it negatively affects native taxa but does not cause local extinctions (Quaglini et al. 2025). However, the species contains highly toxic pyrrolizidine alkaloids, making it hazardous to livestock and potentially other mammals, especially in regions where grazing pressure is high (Dimande et al. 2007; Monty et al. 2008). Moreover, climate change seems to favour its spread in natural habitats (Bazzato et al. 2024).

As regards Robinia pseudoacacia and Ailanthus altissima, they are mostly related to V38, corresponding to the vegetation stage with pioneer trees of secondary succession developing on dry abandoned urban or agricultural land or on land with man-made ground. This finding was also observed for Milan, Turin, and Rome (Montagnani et al. 2026), where, in response to the urbanization gradient, Ailanthus altissima was associated to highly disturbed central areas, while Robinia pseudoacacia was found more common in suburban contexts. A recent study analyzing the diversity of Campobasso urban woods and their distribution in relation to the disturbance indicator values (Midolo et al. 2023) and the Ecological Indicator Values for Europe (Dengler et al. 2023), highlighted that these IAPs only occasionally occurred and had low cover in the native urban forests dominated by Quercus cerris and Quercus frainetto (T19), which represent the potential natural vegetation of the area (Varricchione et al. 2024).

Robinia pseudoacacia forms wooded patches in degraded and vacant urban lots, and it does not seem to threaten urban habitats of conservation relevance. Furthermore, it has a fundamental role in allowing the natural greening of urban areas with degraded, nitrate-rich soil, where native tree species fail to take root (Jim 2013; Gavrilidis et al. 2023; Kato-Noguchi and Kato 2024). Robinia pseudoacacia is notable for its ability to thrive in both nutrient-rich and nutrient-poor soils. As a member of the Fabaceae family, it can overcome nitrogen limitations by forming a symbiotic relationship with rhizobia, allowing the tree to fix atmospheric nitrogen (Xiao et al. 2021). In addition, Robinia pseudoacacia produces allelochemicals that can suppress the germination, growth, or establishment of neighboring plant species, enhancing its competitive ability (Medina-Villar et al. 2017). This nitrogen-fixing capacity, combined with chemical defences against competitors and resistance to natural enemies, provides a significant advantage in nutrient-poor environments and likely enhances the species’ invasiveness, facilitating its successful colonization of challenging habitats (Carl et al. 2018; Kato-Noguchi and Kato 2024). These urban Robinia pseudoacacia wooded patches may contribute to some degree to regional biodiversity (Campagnaro et al. 2018) and can play an important ecological role in the local ecological network, especially for arthropod and vertebrate fauna (Buchholz et al. 2015; Mori et al. 2025). However, it should be pointed out that, along the riverbanks of the region, Robinia pseudoacacia is found to be highly invasive and causes a strong negative impact on riparian and lowland forests, with a reduction in the richness of native species and an increase in nitrophilous species in the understory (Varricchione et al. 2024). These findings were also documented in other Italian regions (Fogliata et al. 2021; Fanfarillo et al. 2023; Musarella et al. 2024).

Ailanthus altissima is also abundant in J4, colonizing side slopes along roads and railways. This habitat serves as a natural corridor facilitating the species’ dispersal and allowing swift movement both into and out of the city, as reported by Casella and Vurro (2013), Sitzia et al. (2016), and Motti et al. (2021).

Roads are also the gateway for this IAP towards protected areas, as documented by the research made by the Mountain Invasion Research Network (MIREN) (Haider et al. 2022; Santoianni et al. 2024) and the PREVALIEN research project (Lozano et al. 2023; Lozano et al. 2024; Marzialetti et al. 2025).

Ailanthus altissima is a species of Union relevance (European Commission 2019) and its ecological impact in the natural and semi-natural landscape has been documented at national level in several habitats of EU conservation interest (Lazzaro et al. 2020). It causes a decrease in the proportion of native plant species in the understory and an increase in ruderal plant species (Montecchiari et al. 2020; Brooks et al. 2021; Terzi et al. 2021).

Its occurrence in Central Apennine is high in areas where potential natural vegetation has been destroyed by humans, in afforestation, and where there is a recurrent anthropogenic disturbance that alters the soil (Santoianni et al. 2024). In the investigated urban context, it colonizes not only roadside slopes and areas with altered soil and landfills, but also steep rocky slopes and ancient historical sites, as documented in Rome (Celesti-Grapow and Ricotta 2021).

However, further investigations are needed to better understand its dynamics and ecology in Italian natural contexts. The participative science project AilantItaly was recently launched address to these issues (Compagnone et al. 2025).

For management and prevention purposes, the containment of Senecio inaequidens, Robinia pseudoacacia, and Ailanthus altis­sima, now widespread both in urban and rural landscapes, appears prohibitive in economic terms (Motti et al. 2021). In Italy, Senecio inaequidens, Robinia pseudoacacia, and Ailanthus altissima are also among the most frequent invaders of native plant communities in Natura 2000 sites (Lazzaro et al. 2020). Therefore, the eradication/containment efforts should focus on avoiding a further extensive spread in natural environments, and could be achieved in protected areas where these IAPs threaten the ecological integrity of habitats of conservation relevance (e.g., mountain grasslands (Senecio inaequidens), riparian and floodplain forests (Robinia pseudoacacia), and open oak forest (Ailanthus altissima)), as documented in several studies (e.g. Constán-Nava et al. 2010; Vacchiano et al. 2013; Sádlo et al. 2017; Nicolescu et al. 2020; Santoianni et al. 2024; Varricchione et al. 2024). As alien species cover decreases with native species cover (Loiola et al. 2018; Anibaba et al. 2023), enhancing the niche-filling of native vegetation in the semi-natural and natural environments could be the most effective and sustainable conservation action.

In the urban context, eradication/containment efforts should be addressed for the conservation of monumental or sacred sites, where several IAPs cause important damages (Celesti-Grapow and Ricotta 2021).

Still, it could be useful to invest in preventing the spread of “emerging” invasive or potentially invasive species that we have documented to be present at low frequencies in natural environments, but that could have the potential for expansion in the coming decades. In particular, on species that are currently casual neophytes in the study region, but which could become invasive in the coming years, as has already happened in other Italian regions (Lazzaro et al. 2020; Viciani et al. 2020; Portal to the Flora of Italy 2025). In our case, these species could be Acer negundo and Ligustrum lucidum. Moreover, we can consider the species that we documented as spontaneously growing in Campobasso, which are Buddleja davidii, Helianthus tuberosus, Lonicera japonica, Parthenocissus spp., and Prunus laurocerasus. In addition, there are species that are not yet found self-sown in Campobasso, but they have been documented as present in nature in the nearby regions, such as Quercus rubra, Paulownia tomentosa, and Trachycarpus fortunei.

In detail, Buddleja davidii, Helianthus tuberosus, Parthenocissus spp., Lonicera japonica, Acer negundo, and Quercus rubra could spread to riparian and riverbed habitats, as well as floodplains, as has been observed in other Italian and European areas (Saccone et al. 2013; Montaldi et al. 2024; Alessandrini et al. 2025; Bylak et al. 2025). These emerging invasive plants could show the capacity to rapidly establish and spread within disturbed riparian and anthropogenic environments, enabling them to track habitat instability and expand efficiently along river corridors. Their rapid growth and resource dominance may competitively suppress native vegetation, leading to reduced species richness and altered successional pathways. By reshaping vegetation structure and ground cover, they also have the potential to modify key ecosystem processes with cascading effects on native communities (Richardson et al. 2007; Catford et al. 2012; Deslippe and Veenendaal 2025). Moreover, projections under future climate scenarios indicate an increased likelihood of range expansion, suggesting that their ecological influence could intensify and extend across broader European regions. For example, Buddleja davidii can divert pollinators from native species, reducing their visitation rates and potentially impairing native plant reproduction and ecosystem stability (Bylak et al. 2025; Tourbez et al. 2025). As regards Ligustrum lucidum, Prunus laurocerasus, and Trachycarpus fortunei, climate changes can enhance the spread of these evergreen woody species in Mediterranean deciduous oak forests, determining a thermophilization and lauriphilization process of the flora, which is also being observed in other areas with a temperate climate (Rusterholz et al. 2018; Abrahamczyk et al. 2024; Iseli et al. 2025). By contrast, Paulownia tomentosa could colonize open stands of oak forests, soils rich in debris, and rocky habitats (Essl 2007).

The early warning for these taxa should be shared with key stakeholders such as nursery companies, urban architects, agronomists-forestry, citizens, schools, and technical offices of local authorities (Rainford et al. 2020; Venette et al. 2021). It is necessary to affect the production chain to avoid their propagation, sale, planting, and cultivation. Furthermore, alternative native species must be identified capable of providing the same growth and cultivation performance (Hulme et al. 2018; Clark and Crawford 2025), as well as aesthetic appreciation, to allow the replacement of invasive or potentially invasive neophytes (Russo et al. 2025).

One limitation of the present study is the pre-selection of a nationally defined list of 26 invasive alien species, which resulted in the exclusion of other invasive alien species occurring in the study area. This approach was adopted to ensure a standardized sampling framework and to keep sampling effort manageable, particularly in large urban areas such as Milan, Rome, and Turin (Montagnani et al. 2026). Despite this limitation, the study provides a first overview of the presence and spatial distribution of a subset of IAPs in the city of Campobasso, and lays the groundwork for future comparative analyses across the six Italian cities where data were collected simultaneously using the same methodology.

Results highlight that, even in inland and medium-small cities, the pool of investigated IAPs occurs and is either invasive or potentially invasive. The prevalence of occurrences in roadside habitats and urban abandoned areas, together with the close interface between urban, rural, and semi-natural environments, makes the landscape potentially permeable to the spread of these IAPs and may facilitate the outward dispersal of these species from the city toward areas of higher naturalness.

In the examined area, the South African Senecio inaequidens emerged as the most prevalent and spontaneously established IAP; this species can be harmful in hill and mountain pastures, especially in a region where livestock farming is still widespread and oriented toward high-quality dairy production.

These findings underscore the importance of preventing the spread of the studied IAPs into natural and semi-natural environments and into the surrounding of Natura 2000 sites, both near the city and across the region, through stakeholder engagement, periodic monitoring, and targeted containment or eradication measures aimed at safeguarding conservation-relevant habitats and grazed or mowed grasslands.

The use of the EUNIS classification can enhance the comparability of data on invaded urban and natural habitats, facilitating comparative analyses across biogeographical regions and cities of different sizes, thereby enabling the identification of common, and consequently more effective, guidelines for the monitoring, prevention, and management of IAPs.