Human activities deeply altered the Earth biota, producing transformations that modified the ecological equilibria [ 1,2]. In the current globalization era, increasing transportation, communications, large scale trading promoted the transfer of a huge amount of plant species outside of their native geographical range, determining a constant erosion of existing biogeographic barriers [ 3,4,5]. Several alien species now represent a permanent component of the flora of the countries where have been introduced [ 6]. Aliens are today considered one of the main threats to the biodiversity conservation [ 7,8,9] because they are often able to modify the colonized habitats, where they reproduce and spread, altering the ecosystem services, with heavy consequences on ecological, economic and social order [ 10,11,12]. The occurrence of exotic species usually characterizes anthropized areas, especially those where human settlements, farming and industries are concentrated [ 13]. The destruction of natural environment, urbanization and the anthropic disturb ineluctably promote the propagation of exotic species [ 14,15], due to their ability to spread at the expense of native vegetation and substitute it thank to several adaptations [ 16,17,18,19]. Moreover, the human induced climate change is recognized as one of the factors responsible of the competitive advantage of alien species [ 20,21]. Although not all the alien species show the same invasive tendency, every new exotic taxon could negatively affect the native phytocoenoses, and anyway the long-term invasiveness of each taxon in a given area it is not always easy to foresee. The damage of alien species is usually wider in areas expressing highest naturality, such as native forests, national parks and wetlands, where defending biodiversity is crucial for conservation. The level of mixing of floras with different biogeographical origin in all continents is actually so high and uncontrolled, due to human activities, that physical barriers and even geographical distances among continents are virtually inexistent, and anyway unable to keep biogeographical regions well separated. That’s why some scientists believe that biosphere is today experiencing a sort of “Pangea effect”, under which unprecedented numbers of vascular plants with any biogeographic origin mix each other determining an increasing biotic homogenization [ 22], resulting in a dangerous global loss of floristic uniqueness [ 23] and in inedited combinations, some of which are reversible (hybrid ecosystems) and some irreversible (novel ecosystems) [ 24] whose medium and long term consequences on biodiversity and ecosystem services are completely unpredictable.

At European level, Italy is one of the countries with the highest number of exotic species, representing about 20% of national flora [ 25] due to an ancient history of trading among the Mediterranean and European countries, and even the massive industrialization of Northern regions [ 26,27]. Massive is also the voluntary introduction of species from North America and SE-Asia, such as Robinia pseudoacacia with forestry purpose, Ailanthus altissima and Artemisia annua respectively for fabric and medicine purposes, Prunus laurocerasus and Parthenocissus quinquefolia for ornamental use. Calabria as well, has a large contingent of exotic species as part of its vascular flora, showing increasing importance and interest demonstrated by the growing number of new records reported for this territory during recent years [ 28].

Aim of this research is to report new exotic species found in Calabria during the 2021-2023 investigations, so providing further information about the occurrence of exotic flora in Calabria and in Italy.

Calabria is the southernmost continental region of Italy (Figure 1A), surrounded E, W and S by seas (Ionian, Tyrrhenian and Messina Strait), and abruptly separated from the continental Italy by the Pollino massif, whose highest mountain top is Serra Dolcedorme (2,267 m a.s.l.). The complex orography – characterized by Calabrian Apennine mountain ranges (Pollino, Orsomarso, Sila, Catena Costiera, Serre and Aspromonte) quite close to coasts – and the extremely heterogeneous geology are at the basis of a wide climatic diversity and different plant communities [ 29] occurring in Calabria. The vascular flora native to the region counts about 2,700 taxa [ 30], part of which are the 60 strictly regional endemics and the 270 national endemics [ 31], the highest values recorded among Italian continental regions. According to Spampinato et al. [ 28] Calabria flora counts 382 alien taxa, 14% of its total vascular flora.

During 2021-2023, Catanzaro urban area, a few localities surrounding the administrative capital of Calabria, and few areas encompassed in the Sila National Park have been specifically surveyed for exotic flora (Figure 1B).

The surveyed stands and the specimens collected have been georeferenced using a GPS device (Garmin Montana 650t). Each plant has been collected and dried in at least two voucher copies applying the common herbarium techniques. Exsiccata have been stored in the herbarium of Department of Agraria, Mediterranean University of Reggio Calabria (code REGGIO, according to Thiers [ 32]). A voucher copy of each specimen is also stored in the Herbarium Capuano (Catanzaro).

Maps have been drawn using Quantum GIS software [ 33] and data have been tabled and analysed by Microsoft Excel software [ 34].

Nomenclature, taxa delimitation, and regional distribution are reported according to Galasso et al. [ 25] and the following updates synthetized by Spampinato et al. [ 28]. The specimens were identified according to Flora d’Italia [ 35,36], Flora of North America [ 37], Flora of China [ 38], Flora of Japan [ 39,40] plus additional literature. Life forms and native ranges have been provided according to Pignatti [ 35], Pignatti [ 36], Portal to the Flora of Italy [ 41] and the linked Acta Plantarum database [ 42]. The time of introduction of the recorded taxa has been added according to the Portal to the Flora of Italy [ 41].

Each taxon has been described in detail as a file containing the following information: accepted binomial, basionym and most relevant synonyms, plant family, period of introduction (whether neophyte or archeophyte), native range, life form, data record in Calabria or Italy, date of collection, discovery localities (municipality, administrative province), exsiccata with details on the location according to the information reported on the specimen label, invasiveness status according to Pyšek et al. [ 43], decimal degrees geographic coordinates (datum WGS84), altitude (m a.s.l.), habitat features, legit and determinavit, herbaria where the specimens are stored, and current distribution.

The survey found a total of 18 alien taxa new for the regional territory that now counts totally 400 alien vascular plants. The invasiveness status proposed for all the recorded taxa, according to Richardson et al. [ 44] and Pyšek et al. [ 43] is casual, except for Bidens subalternans, deserving the naturalized status. Several of the found species, in fact, occur in one or few stands, proving colonization processes to be in an early stage.

Each genus is represented, in this survey, by just one species. The most represented families are Asteraceae, with 3 genera (Bidens, Erigeron, Rudbeckia), and Rosaceae with 2 (Prunus, Pyracantha), while all other taxa are represented by just one genus.

The most common life form inside this contingent of alien taxa are scapose therophyte and scapose phanerophyte both counting 5 species (27.8%), followed by caespitose phanerophytes (4 species; 22.2%) (Table 1A).

By biogeographical point of view, most of the recorded taxa come from North American biogeographical region (6 species; 33.3%), followed by 4 species (22.2%) coming from the Asian biogeographical region, and as many coming from South America (Tab. 1B). According to available data all the recorded species can be considered neophytes.

All species have been found in synanthropic habitats, 8 of which (44.4%) at roadsides, while 2 species (11.1%) both inside flowerbeds and in uncultivated areas (Table 1C).

The recorded species have been found covering an altitude range of 3-1,286 m a.s.l..

The amount of taxa found, their biogeographical origins, the prevailing ruderal and synanthropic habitats confirm the trend of an increasing pressure of alien plant species at regional and national level, as respectively already proposed by Spampinato et al. [ 28] and Galasso et al. [ 25]. The occurrence of many therophytes in this survey probably testifies the high efficiency of this life form to develop and persist in disturbed habitats, while the high number of phanerophytes is probably due to the past introduction of alien trees in Europe for ornamental or forestry purposes that still continues sometimes nowadays. Many of these plants show specific adaptations to the vegetative reproduction which support surviving and spread along the time. A few peculiar synanthropic ecological niches, such as abandoned fields and urban areas, according to Wagner et al. [ 45], could sometimes represent ecological corridors that promote the spread of these species across natural and seminatural environments. Phanerophytes included in this survey, according to other European sources [ 46,47,48] have been confirmed occurring even outside of urban areas, despite most of the findings have been associated to urban or suburban contexts. Most of the reported species have been found in highly anthropized environments, due probably to the ability of alien species to settle on weak ecological niches where most of the native species could not develop permanent colonies. In fact, urban and suburban areas are well known alien species hot spots, also as a result of the amount of exotic species used as ornamentals in private and public spaces, such as parks and gardens, often without any preliminary consideration for the biogeographical origin and the invasive potential of the chosen taxa. Moreover, the wide altitudinal range, along with the diversity of colonized habitat, suggests the huge potential of these taxa to potentially invade many different ecosystems in the investigated territory and the surrounding regions. A high number of taxa coming from American continent confirms data provided by Pyšek et al. [ 49], according which almost 50% of European exotic flora (except alien species of European origin) are from Northern and Southern America. Consistently with Pyšek et al. [ 43], Pyšek et al. [ 49] and Follak et al. [ 50] our data confirms that human modified habitats (e.g. agriculture and industrial zones, park and gardens) host in Europe most of alien species, much more than grasslands and wooded areas.

During last years the investigation on bioinvasions attracted increasing interest by scholars, mainly because of negative effects on natural environment and ecosystem services [ 43,51]. Recent field survey has shown the increasing number of exotic species new not just to the regional territory, but even to Italy and Europe as well [ 52,53,54,55,56,57,5828,59,60]. The lacking knowledge about the Calabrian vascular exotic flora, according to Musarella et al. [ 61], was probably due to missing deep territory exploration. Calabria, according to Spampinato et al. [ 28], and also with the additions provided by this work, is a Mediterranean and European region where a high number of exotic plant species has been recorded. The timely identification of exotic species in a territory allows to understand and predict in advance the dynamics of colonization and expansion, so that management plans can be scheduled [ 62,63]. Control and eradication, almost always potentially useful to reduce the impact of some invasive aliens, rarely can be applied because of technical difficulties and, above all, reduced resources specifically dedicated to this goal [ 64,65], despite in the today scenario of climatic-environmental crisis, the amount of alien species is tendentially destinated to increase, further threating biodiversity and native ecosystems, and so representing one of the challenges requesting most commitment in the next future [ 66]. A kind of long term effective approach to the mitigation of effects of bioinvasions would be based on a mixed strategy, including actions such as (1) implementing in the school curricula lab based educative programs addressed to pupils belonging to different age classes, in order to provide them the best possible awareness about risks coming from the introduction of exotic organisms [ 29] (2) adopting specific legislative barriers (e.g. city green rules) discouraging (e.g. higher taxes) or forbidding the introduction of new exotic species [ 67], (3) introducing strict limitations to the cultivation of non-native species, especially in the next proximity of rural or natural areas, (4) avoiding the abandonment of areas potentially highly sensible to bioinvasions [ 68,69], (5) favoring the spontaneous recover of natural vegetation when possible, (6) recovering degraded habitats when technically (hybrid ecosystems) and financially possible implementing specific eradication plans, (7) mitigating the effect of bioinvasions when eradication is practically impossible (novel ecosystems), (8) monitoring trajectories taken by restoration processes, (9) adjusting trajectories according to realistic target ecosystems when necessary [ 24].