1. Introduction
From the Neolithic Age to the era of colonisation and geographic exploration, the introduction of non-native plants from various regions has greatly benefited human life, especially in areas like food supply and healthcare [1]. Botanical gardens have played a pivotal role in cultural and economic development by introducing a wide range of alien plant taxa of agronomic, medicinal, ornamental, and scientific value [2,3]. However, most of these species have escaped the gardens’ fences [4,5]. As a result, botanical gardens have been significant contributors to the introduction and spread of several invasive alien taxa [6,7]. In fact, these gardens are associated with the initial introduction, early cultivation, and local spread of about half of the most problematic invasive plant species listed by the International Union for Conservation of Nature (IUCN) in global biodiversity hotspots [8,9]. Additionally, it is known that more than 93% of exotic plants naturalised worldwide have been cultivated in botanical gardens [10].
The establishment of botanical gardens has been associated with the global emergence of new alien plant species, which refers to the first recorded appearance of an alien species anywhere in the world [11]. Research exploring the global increase in alien plant species has identified botanical gardens as a significant contributor [12]. However, the quantitative evaluation of the impact that botanical gardens have on regional plant introductions remains underexplored [8].
Botanical gardens can play a crucial role in mitigating the harmful effects of plant invasions by collecting and sharing data on taxa that spread from cultivation early in the invasion process, effectively serving as sentinels of plant invasion [13]. Sharing information gathered by botanical gardens about plants escaping cultivation across a broad region is essential for addressing knowledge gaps and enabling accurate assessments of recently introduced ornamental plant species in Sardinia and Italy. By reporting and disseminating this data, we can facilitate the timely identification of taxa that are just beginning to spread.
The Hortus Botanicus Karalitanus (hereafter, HBK), is one of the largest historical botanical gardens in Italy. Established in 1866 by Patrizio Gennari, it was originally designed as an acclimatisation arboretum for tropical plants and as an experimental garden to introduce potential plant species and cultivars in Sardinia [14,15,16]. During the same period, other Mediterranean botanical gardens, such as Naples, Palermo, and Catania [17], as well as Barcelona, Valencia, and Algiers, were created with the same purposes and functions, while a first acclimatisation of species from the Americas typically occurred in the Canary Islands and Malaga [18].
The garden spans five hectares and is situated in the city centre of Cagliari, within the Palabanda Valley. Despite its urban location, it maintains a strong ecological connection to the rest of the Palabanda Valley and the Cagliari hills, some of which are home to natural habitats of EU conservation interest (Figure 1). Currently, the garden features various living collections, including succulent plants from xeric tropical habitats (142 taxa), a tropical and subtropical arboretum (~128 taxa), the “roccaglie della biodiversità” (rocky places of biodiversity, 47 taxa), gymnosperms (27 taxa), the palmetum (26 taxa), the sage garden (28 taxa), ferns and their allies (22 taxa), and a rose garden (~45 cultivars) [16,19].
Most of these taxa have not developed a capacity for naturalisation, remaining confined to flowerbeds or designated garden spaces for cultivation. However, some taxa have become spontaneous and, in certain cases, have either naturalised or become invasive, as has occurred in other botanical gardens [20,21]. While the intentional introduction of ornamental species is recognised as a major pathway for plant invasions worldwide [22], many species may be unintentionally introduced as seed contaminants or through nearby public and private gardens, as well as urban green spaces. These serve as potential accidental entry points of invasive species into the botanical garden.
Figure 1Satellite view of the study area within the red line (Hortus Botanicus Karalitanus, HBK). Base image obtained from Google Earth [23], and modified by the author.
[Figure omitted. See PDF]
This research explores the presence of alien plant species in one of Italy’s oldest botanical gardens and the role that the invasive component of these species has played in the biodiversity of Sardinia. The study aimed to create an inventory of alien vascular flora at the HBK and analyse which traits of these taxa serve or have served as reliable indicators of potential invasiveness and spread beyond its boundaries. The analysis covers various factors, including geographic origin, life form, introduction period (archaeophyte or neophyte), current status (casual, naturalised, or invasive) both locally and within the broader Italian flora, introduction pathways, and predominant reproductive strategies in relation to their spread within the HBK. Additionally, the study examines the component of the inventory reported in natural and anthropogenic habitats of Sardinia, with a focus on invasive species and their historical introduction pathways based on their first records on the Island, in Cagliari, and in the HBK.
The ultimate goal of this study is to provide comprehensive data to prevent the use of invasive species and to develop priorities supporting biodiversity conservation projects and public ornamental green space management. These inventories can assist stakeholders in making informed decisions regarding the use of alien ornamental species, helping to prevent biological invasions, particularly in Mediterranean natural habitats, with special attention to those most sensitive to invasive species.
2. Materials and Methods
The current inventory is based on data collected through field investigations conducted between 2015 and 2024, supplemented by a review of bibliographic sources and herbarium revision. In this study, not all taxa of the HBK floristic heritage were included (data are not available), but we focussed exclusively on HBK vascular plants that exhibited some degree of spontaneity—classified as casual, naturalised, or invasive—according to the invasion status framework proposed by Richardson et al. [24], Pyšek et al. [25], and Richardson et al. [26], which distinguishes casual plants (not self-sustaining and reliant on repeated introductions), naturalised plants (self-sustaining for at least 10 years without human intervention), and invasive plants (a subset of naturalised plants capable of producing abundant reproductive offspring and spreading rapidly over large areas).
The alien status data were observed at the local level and compared with the reference status for regional (SA) and Italian (IT) flora, according to Galasso et al. [27] and subsequent updates. We distinguished between archaeophytes and neophytes, based on their introduction before or after 1492/1500 A.D., respectively. Additionally, information about cryptogenic plants, sensu [28], was included.
The taxonomic treatment of taxa follows the last checklists of the Italian vascular native and alien flora [27,29] and the information gathered in websites such as Portal to the Flora of Italy [30], IPNI [31], and Med-Checklist [32]. To identify the species, we consulted national and international flora (e.g., [33,34,35,36,37,38,39,40]), herbarium samples in Herbarium CAG of the University of Cagliari, and online databases [41,42,43].
Concerning plant families, we followed the Angiosperm Phylogeny Group IV [44] and Pteridophyte Phylogeny Group I [45]. We expressed the life forms according to Raunkiaer’s classification system [46], using the abbreviations reported in Pignatti et al. [39] and IPFI [47]. The geographic origin of alien plants was based on POWO [48] and IPFI [47] or the relative literature. We also classified the taxa according to their intentional or unintentional anthropogenic introduction in the HBK, by subdividing them into four category pathways: ornamental; culture; weed (seed contaminant); accidental (hitchhikers). Regarding reproductive behaviour, the data observed in the field during the investigations were reported.
The taxa in the inventory were linked to the habitats they most commonly and naturally occupy in Sardinia. The habitat types were classified according to Puddu et al. [49] and subsequent updates: (1) agricultural; (2) synanthropic; (3) coastal; (4) matorral; (5) woodland; (6) wetland (including riparian habitats according to Mayoral et al. [50].
The historically reported plants were considered based on the works of Gennari [14], Cavara [15], Pontecorvo [16], Lallai [19], Cao and Mameli-Calvino [51], and studies on individual species, as listed in Appendix A, Table A1.
3. Results
The HBK vascular alien flora inventory (Table S1) numbers 146 taxa belonging to 109 genera and 55 families, of which 45% are naturalised (66 taxa), 43% are casual (62 taxa), and 12% are invasive (18 taxa).
As a whole, the most represented families are: Asparagaceae (14 taxa), Poaceae (nine taxa), and Solanaceae (eight taxa), followed by Crassulaceae and Asteraceae (seven taxa each), Fabaceae (six taxa), Araceae, Moraceae, Oxalidaceae, and Papaveraceae (five taxa each), (Figure 2), whereas the most represented genera are Agave (nine taxa), Kalanchoë, Oxalis and Papaver (five taxa each), Euphorbia, Erigeron, Opuntia, and Ficus (three taxa each).
The families with the highest number of invasives are Asteraceae (four taxa), followed by Poaceae with two taxa, while, among genera, Erigeron is represented by more than one taxon (two taxa).
Most of these taxa are neophytes (83%, 121 taxa), with 11% classified as archaeophytes (16 taxa), and 6% as cryptogenic species (nine taxa) (Figure 3). Among the 146 alien taxa at the HBK, many have previously been recorded as naturalised in both Italy (46 taxa) and Sardinia (41 taxa). Additionally, 21 taxa are casual alien in Italy and 37 are casual alien in Sardinia. Invasive taxa amount to 44 at the national level, and 31 in Sardinia. Conversely, 11 taxa are considered native to Italy, but absent in Sardinia. Furthermore, 24 taxa have not been yet reported for Italy, and 37 taxa remain unreported in Sardinia (Figure 4).
Among the 146 species in the HBK inventory, 34 were historically reported as cultivated within the HBK, of which 18 are currently present with naturalised status [Aeonium arboreum (cited as Sempervivum arboreum), Arundo donax, Brugmansia suaveolens (cited as Datura suaveolens), Convolvulus sabatius subsp. sabatius, Ficus microcarpa (cited as Ficus lucida), Glycyrrhiza glabra, Lycium boerhaviifolium (cited as Grabowskia boerhaviifolia), Mesembryanthemum cordifolium, Monstera deliciosa, Nicotiana glauca, Opuntia dejecta, O. ficus-indica, Papaver somniferum (cited as Papaver sativum), Parthenocissus quinquefolia, Searsia lancea (cited as Rhus viminalis), Schinus polygama (cited as Duvana dependens), Washingtonia filifera (cited as Pritchardia filifera), Ziziphus jujuba], and 16 with casual status [Abutilon theophrasti (cited as Sida abutilon), Agave americana, A. angustifolia (cited as A. jacquiniana), A. salmiana, Bosea yervamora, Ficus pumila (cited as Ficus stipulata), Furcraea selloana (cited as Fourcroya gigantea), Livistona chinensis (cited as Latania borbonica), Morus nigra, Passiflora caerulea, Phytolacca americana (cited as P. decandra), Pittosporum tobira, Ricinus communis, Zantedeschia aethiopica], and only two with invasive status [Vachellia karroo (cited as Acacia horrida) and Ailanthus altissima (cited as Ailanthus glandulosa)].
Life form analysis revealed a dominance of phanerophytes (63 taxa, 43%), followed by therophytes (32 taxa, 22%), geophytes (24 taxa, 17%), chamaephytes (13 taxa, 9%), and hemicryptophytes (12 taxa, 8%). Hydrophytes account for 1% (two taxa). An analysis of sub-life forms shows that scapose therophytes are the most represented (31 taxa), followed by scapose phanerophytes (21 taxa), rhizomatous geophytes (17 taxa), and liana phanerophytes (15 taxa) (Figure 5). Among invasive species, the most represented life forms are scapose therophytes (five taxa), followed by bulbous geophytes (three taxa), scapose phanerophytes, and liana phanerophytes (two taxa) (Figure 6).
In terms of chorology, the primary source of alien taxa for the HBK is the American component, comprising 55 taxa (38%), followed by the Asian component (19 taxa, 13%), the South African component (17 taxa, 12%), the Mediterranean sensu lato component (13 taxa, 9%), and the African component (11 taxa, 8%). Among the invasive species, most are of American (six taxa) and South African origin (four taxa) (Figure 7).
Regarding the introduction pathways, taxa cultivated as ornamental plants are by far the most common in the HBK alien flora, accounting for 102 taxa (70%). This is followed by hitchhikers, which account for 23 taxa (16%), and seed contaminant, which represents 18 taxa (12%). Three species (2%) fall into the “culture” category, referring to species introduced specifically for cultivation. Among the invasive taxa at the HBK, ornamental units and seed contaminants are the most numerous groups, each with seven taxa (39%), while hitchhikers account for four taxa (22%) (Figure 8). Notably, no invasive species have been identified among the taxa of cultural introduction.
Reproduction by seeds is the most common method employed by the alien flora at the HBK, utilized by 60 taxa (41%). This is followed closely by combined reproduction (57 taxa, 39%), while sole vegetative methods are less common (29 taxa, 20%). Among the invasive taxa, the predominant method is the combined seed/vegetative reproduction (10 taxa, 56%), followed by seed reproduction alone (six taxa, 33%) (Figure 9).
The presence of species from the HBK inventory reported in the main natural and anthropogenic habitats of Sardinia was also analysed. Among these, 25% of the taxa (37 taxa) have no recorded presence in any habitat in Sardinia, while 39% of them (56 taxa) are reported exclusively in anthropogenic habitats. Specifically, 33% of the taxa (48 taxa) are found in synanthropic habitats, and 6% (8 taxa) in agricultural habitats (Figure 10). Additionally, 36% of the taxa (53 taxa) are reported in natural habitats, with 12% (18 taxa) exclusively in wetlands and 8% (12 taxa) exclusively in coastal habitats. The remaining species are recorded in more than one habitat, both natural and anthropogenic, reaching a total of 25% (36 taxa) in wetland habitats and 11% (17 taxa) in coastal habitats (Figure 10).
Focussing on the invasive component of the HBK present in Sardinian habitats (Table 1), among the 48 species in the HBK inventory recorded in synanthropic habitats of Sardinia (SA, Table 1), 6 are invasive (INV SA, Table 1). Of these, three are invasive both in Sardinia and in the HBK (INV shared, Table 1), while the remaining three are invasive in the HBK (INV HBK, Table 1) but, although present in Sardinian habitats, do not have invasive status there. In Sardinian agricultural habitats (SA, Table 1), eight species from the HBK inventory are present, of which only one is invasive in these habitats (INV SA, Table 1) and none are invasive in the HBK (INV HBK, Table 1). In natural habitats, among the 36 species recorded in Sardinian wetland habitats (SA, Table 1), 18 are invasive (INV SA, Table 1). Of these, eight are also invasive in the HBK (INV shared, Table 1), and three are invasive only in the HBK (INV HBK, Table 1). In coastal habitats, out of the 17 species recorded (SA, Table 1), six are invasive in Sardinia (INV SA, Table 1), and one is invasive in the HBK (INV HBK, Table 1), but this last does not have invasive status in Sardinian habitats (INV shared, Table 1). In matorral habitats, two species are reported in Sardinian habitats, with one being invasive. In woodland habitats, no species from the HBK inventory are present. Finally, none of the 37 species absent from Sardinian habitats (SA, Table 1) but present in the HBK inventory are classified as invasive in the HBK.
The focus on invasive species and their historical introduction pathways was based on their first records on the Island, in Cagliari, and within the HBK (Table A1). Of the 18 invasive species, 15 have their ‘Primus inventor’ recorded outside the HBK, while only 1 species was first recorded within the HBK; for 2 species, no data are available. Seven species were reported for the first time in the HBK, although either as cultivated or with uncertain status; six species are recorded in the HBK for the first time in this study, and four were recorded in the HBK only after their first report in the Cagliari area outside the HBK.
4. Discussion
The presence of 146 spreading alien vascular plant taxa recorded within the Hortus Botanicus Karalitanus (HBK), 109 of which are already part of Sardinian alien flora—nearly 19% of the 559 alien taxa reported by Galasso et al. [27], and an additional 37 taxa not yet documented on the island—is concerning (28% of the inventory). This high number aligns with trends observed in other botanical gardens [7] and underscores the potential threat these reservoirs of alien plants pose to local flora [52,53]. Special attention has been given to the 18 local invasive taxa present in the garden, which is a significant figure. An effort has been made to trace the introduction history of these taxa into the HBK. Although currently confined to a limited area, these taxa could have multiple opportunities to escape from the botanical garden and spread beyond its boundaries, especially if they exhibit the invasive tendencies observed both locally and in other contexts.
Situated in the isolated Palabanda Valley, the HBK remains connected to several areas of significant natural interest, such as the hills of Cagliari [54,55,56]. For many species, it is challenging to determine whether their spread originated from the HBK or vice versa, as in the case of Aristolochia sempervirens, reported by Bocchieri and Mulas in 1982 (CAG) in the nearby Roman Amphitheatre. In fact, the authors had documented the species as spontaneous in Sardinia, in a different part of Cagliari, a year earlier [57]. In another case, Gennari [58] noted some of these invasive taxa as already historically spontaneous in the Palabanda Valley, such as Acanthus mollis and Oxalis pes-caprae. His work also recorded Lycium europaeum, a cryptogenic species no longer found in the valley. However, Lycium ferocissimum, an invasive species often mistaken for L. europaeum, is present in the HBK, likely spread by birds dispersing its fruits.
Among the taxa listed in the first historical guide to the HBK [14], Vachellia karroo is the only one showing invasive behaviour within the garden. While other species listed in the guide are invasive regionally or nationally, they have not had the same success as V. karroo. Of these, 18 are still present with naturalised status.
In Cavara [15], Ailanthus altissima was mentioned for the HKB as an acclimatised plant alongside V. karroo, although it was later declared adventitious for the HBK in 1928 [51]. Both of these invasive species were subsequently reported in other studies about areas adjacent to the botanical garden [54,59,60]. However, the introduction of A. altissima in the HBK is not clear, and, even if documented in Cavara [15], it is very likely that it arrived from Villa d’Orri, where it was cultivated starting from 1819 [61]. Its widespread diffusion not only in the HBK—where it is necessary to carry out containment interventions to resist its diffusion—but throughout the city has been favoured by its use as an ornamental tree.
Additional invasive species were recorded in the HBK during the first two decades of the 20th century, including Asparagus asparagoides, Mirabilis jalapa, and Nothoscordum borbonicum (cited as N. fragrans) [51]. This work also mentioned Commelina coelestis, which may have been mistaken for Commelina erecta, a species currently present and invasive in the HBK.
The largest number of taxa that became spontaneous (i.e., casual, naturalised, or invasive) in the HBK belongs to the Asparagaceae family, with 14 taxa, the most representative genus being Agave, with 9 taxa. However, the only invasive species from this family found in the botanical garden is Asparagus asparagoides, first reported as spontaneous and casual in Sardinia in 2009 [62]. The species is currently considered invasive in Italy only in Sardinia [27], but there are documented cases in which it has proven to be a serious threat to ecosystems and has competed with native species, with both biodiversity and economic impacts. [63]. The genus Agave, which includes several taxa considered invasive throughout the Mediterranean [64,65], shows a strong tendency towards naturalisation. However, its spread in the HBK appears to be controlled by gardeners. A similar tendency has been observed in the genus Opuntia (Cactaceae), with O. ficus-indica, O. dejecta, and O. elatior being the only three species found to be sub-spontaneous in the HBK. Notably, O. ficus-indica and O. elatior exhibit strong tendencies toward invasiveness in natural environments [66], with O. ficus-indica recognised as one of the most invasive species on Mediterranean islands [67,68].
Another group of naturalised species in the HBK belong to the Crassulaceae family (seven taxa), particularly represented by the genus Kalanchoë, with five naturalised species. One of these, the hybrid Kalanchoë × houghtonii, is considered invasive in natural environments in some Italian regions [69]. Although it is not yet recorded as invasive in Sardinia, it has recently been reported as naturalised [30].
The Poaceae family is also well represented in the HBK, with nine taxa, including two invasive species: Ehrharta erecta and Eleusine indica. This family also includes species that are invasive in Sardinian natural habitats, such as wetlands (Arundo donax and Paspalum distichum [50]) or in other Italian regions, as in the case of Dactyloctenium aegyptium [70] and Digitaria ciliaris [30]. However, these last two are naturalised and non-invasive within the HBK. The success of the Poaceae family is well documented, as they are resilient taxa with broad climatic tolerance, capable of thriving in wildfire subjected regions, as well as through drought and frost. They can adapt to both cold and hot, dry environments depending on their photosynthetic pathways (C3 vs. C4), and possess a highly competitive root system [71].
Among the Solanaceae family (eight taxa), the only invasive species in the HBK is Lycium ferocissimum. However, other species from this family, such as Datura stramonium, D. wrightii, and Nicotiana glauca, are known to be invasive in natural environments [27,49].
The Asteraceae family is also notable, with six taxa present in the HBK, of which four are invasive. Three species, Erigeron bonariensis, E. sumatrensis, and Symphyotrichum squamatum, likely entered the HBK accidentally, either as seed contaminants in soil or from nearby natural areas [59,60,72]. In contrast, there is no documented evidence of Senecio angulatus being introduced before 2015 [19], though it was likely brought earlier for ornamental purposes. In the Fabaceae family, six taxa are represented in the HBK, but only Vachellia karroo exhibits invasive behaviour in the HBK, as in natural habitats across Sardinia [30].
The status of species that exhibit invasive behaviour locally within the HBK (18 taxa) has been analysed, even though many of these species are not invasive in other contexts (seven are not invasive both in Sardinia and in Italy). Additionally, we examined species that are casual or naturalised within the HBK but are considered invasive in other natural settings. Among the 11 invasive taxa in the HBK that are also considered invasive at national (IT) and regional (SA) levels, the most notable are Ailanthus altissima and Oxalis pes-caprae, which are considered among the most invasive species in the Mediterranean and Europe [73,74,75,76]. These species likely accidentally spread into the HBK.
In addition to Asparagus asparagoides, Mirabilis jalapa, and Vachellia karroo, which were introduced for ornamental purposes [14,51] and are now widespread both inside and outside of the HBK [27,77], other taxa remain invasive only within the HBK. Most of them are currently of casual or naturalised status in Sardinia and Italy, including Nothoscordum borbonicum, Lemna valdiviana, Aristolochia sempervirens, Commelina erecta, Nephrolepis cordifolia, Ehrharta erecta, and Eleusine indica. These taxa deserve closer attention due to their behaviour within the HBK. Among them, E. erecta is recognised as one of the most invasive species worldwide [78], while Eleusine indica is invasive in other regions of Italy [27].
The only archaeophyte invasive in the HBK which is considered native in certain regions of Italy [29] is Acanthus mollis. However, in Sardinia, this species is invasive in wetlands and fallow lands, as observed in other Mediterranean areas [50].
The biological forms analysis of the species inventory revealed a notable dominance of phanerophytes (63 taxa out of 146), which aligns with the botanical garden original purpose as an acclimatisation arboretum [14,15]. However, when focussing on invasive taxa, therophytes and geophytes emerge as the prominent groups. In particular, scapose therophytes (five taxa) and bulbous geophytes (three taxa) have shown higher success. This can be attributed to their remarkable environmental adaptability, especially during dry periods, allowing them to thrive in marginal and uncultivated areas or as weeds within cultivated sections of the botanical garden. These species, often non-ornamental with broad natural ranges, have developed morphological and metabolic adaptations that enable them to complete their life cycle and establish soil seed banks, contributing to their persistence and spread. Among the hydrophytes, Lemna valdiviana is the only species exhibiting invasive tendencies. This species was likely introduced accidentally by birds, and its potential diffusion poses a significant risk to natural environments since it has similar behaviour to other invasive hydrophytes [79,80]. Its potential impact underlines the importance of monitoring hydrophytes closely to prevent their spread into vulnerable ecosystems.
From a chorological perspective, the American origin stands out as the most prevalent among the taxa present in the HBK, with 55 out of 146 taxa originating from the Americas. This trend is also reflected in the invasive component, where 6 taxa out of 18 invasive taxa are of American origin. This pattern is consistent with findings from inventories of alien flora in Sardinia and other Mediterranean regions [49,75]. As highlighted for other botanical gardens as well [7], that a larger number of alien species originated from American continent points to the need to improve biosecurity controls on existing relationships. In addition, four invasive species of South African origin are present in the HBK, a noteworthy but not unexpected observation. The similarities between the South African climate and the Mediterranean climate facilitates the adaptability and spread of these species, making them more competitive compared to taxa from regions with different climatic conditions [81]. The ability of South African species to thrive under Mediterranean conditions underscores the importance of monitoring and managing such taxa to prevent further invasions.
Ornamental species, as expected in a botanical garden, are most common among the alien flora at the HBK, with 102 out of 146 taxa falling under this category. However, only seven of these ornamental species (around 7% of the all ornamental) exhibit invasive tendencies. In contrast, species introduced accidentally, primarily through seed contamination (weeds), show a proportionally higher tendency towards invasiveness. Of the 18 weeds present, 7 (around 39% of all weeds) have become invasive in the HBK, highlighting the risk posed by unintended introductions.
Regarding the reproductive strategies, while seed reproduction is predominant among all spreading alien taxa of the HBK (60 out of 146, 41%), invasive species tend to exhibit a combination of both sexual and vegetative reproduction. This dual reproductive strategy appears to be a key factor in their invasiveness. Specifically, 10 out of 18 invasive species are able to use both forms of reproduction, which enhances their ability to spread and establish themselves. Among them are well-known invasive species in other parts of Sardinia and Italy, such as Acanthus mollis, Ailanthus altissima, Asparagus asparagoides, Lycium ferocissimum, Senecio angulatus, and Vachellia karroo. In addition, species like Aristolochia sempervirens, Commelina erecta, Lemna valdiviana, and Nothoscordum borbonicum, al-though not yet invasive on a broader scale, display concerning invasive tendencies and warrant closer attention for potential future spread.
The analysis of Sardinian habitats affected by species included in the HBK inventory reveals that, although anthropogenic habitats, such as synanthropic ones, host a greater number of alien species, natural habitats are more vulnerable to invasions, particularly wetlands. In fact, among the 11 species identified as invasive in both Sardinian habitats and the HBK, only three are primarily invasive in synanthropic habitats (Erigeron bonariensis, E. sumatrensis, and Senecio angulatus), whereas eight species are invasive in the HBK and also reported as invasive in wetlands (Acanthus mollis, Ailanthus altissima, Asparagus asparagoides, Lycium ferocissimum, Mirabilis jalapa, Oxalis pes-caprae, Symphyotrichum squamatum, Vachellia karroo), though not exclusively to these habitats. These environments are particularly susceptible to colonisation by ubiquitous taxa adapted to diverse conditions, which enable them to invade multiple habitats, as seen in these eight species. Several studies indicate that wetlands are among the most invaded ecosystems in the Mediterranean Basin [50,82] and are highly sensitive and threatened by habitat degradation. The loss of vegetation due to land reclamation and intensive land use further facilitates the introduction of exotic plants [50,83,84]. Regarding the remaining 7 species (“not shared”) among the 18 invasive species that locally exhibit invasive behaviour in the HBK (Nothoscordum borbonicum, Lemna valdiviana, Aristolochia sempervirens, Commelina erecta, Nephrolepis cordifolia, Ehrharta erecta, Eleusine indica), these are not yet present or invasive in Sardinian habitats. However, they should be included in a watchlist, following approaches to prevent their spread in nature as already done in other botanical gardens [20], as they could potentially threaten the biodiversity of sensitive habitats such as wetlands or other natural ecosystems.
Cross-referencing this information with the analysis of introduction history, it emerges that only four species may have been influenced by the botanical garden in their spread beyond its boundaries. The species whose first records are linked to the HBK—Ailanthus altissima, Asparagus asparagoides, Aristolochia sempervirens, and Vachellia karroo—are present in Sardinian habitats, but only three are also found in natural habitats, as Aristolochia sempervirens is reported exclusively in synanthropic habitats. Meanwhile, the remaining invasive species in the HBK were first reported before their introduction into the HBK, suggesting that, historically, the HBK played a less significant role in the spread of these invasive species into natural habitats than other botanical gardens [7].
However, botanical gardens play an important role in supporting the cultivation and distribution of alien plants used for ornamental purposes [10] that in most cases do not cause problems to the native flora. As in other botanical gardens, exotic species are valuable for both their historical significance and their role in conserving endangered species from other countries [85]. The HBK has also historically contributed to the establishment of ornamental greenery and to the introduction of some species that are part of the alien flora of Sardinia, as demonstrated by the results on plant introductions in the HBK, whether they originated from the HBK or vice versa. Anyway, it is undeniable that a component of invasive species has been unintentionally introduced into the HBK, both as seed contaminants (weeds) or through spreading from adjacent natural and semi-natural areas. In such cases, special attention should be given to improve soil sterilisation techniques, not only at a botanical garden level, but mostly in nurseries where plants are commercially traded. Monitoring species with invasive behaviour within the HBK, especially the emerging species not yet included in official lists, is crucial, as it can provide insights into potential future scenarios, particularly from the perspective of climate change [77]. Similarly, mapping the distribution of invasive species—as reported by Girmay et al. [53] and Lim et al. [86]—may support the removal of the most widespread species that threaten native flora and natural habitats, and even ornamental plants cultivated in the HBK’s collections [87]. Furthermore, immediate action is needed for the continuous monitoring and early intervention against species known for their invasiveness and included in important lists of European legislation (EU Regulation no. 1143/2014 [88]), as previously achieved in the recent past, with the removal of Pontederia crassipes from open water spaces and the red palm weevil (Rhynchophorus ferrugineus) biological control in the HBK. A comprehensive inventory can help prevent plant invasions by documenting and sharing data on taxa that spread from cultivation in early invasion stages, serving as early warning indicators [89]. This information can guide the development of regional codes of conduct aimed at preventing, controlling, and managing plant invasions across broader territories [90,91]. In conjunction with these measures, continuous monitoring and the implementation of communication and awareness projects [92] are essential for enhancing public knowledge. These efforts can engage citizens, public administrations, and sector operators, ultimately fostering better compliance with regulatory frameworks regarding biological invasions.
5. Conclusions
In this study, we present the first inventory of vascular alien plants in the Botanical Garden of Cagliari (HBK), based on data collected through field surveys exclusively focussed on plants exhibiting a degree of spontaneity as casual, naturalised, or invasive, and introduced both intentionally and unintentionally by humans. Most of these alien plants are phanerophytes, originate from various parts of the world, were introduced for ornamental purposes, and primarily reproduce by seed. Nonetheless, different traits characterise the invasive component of the HBK. Among these distinguishing traits are an annual life form (therophytes), introduction after the European landing on the Americas (resulting in a predominance of neophytes of American origin), unintentional introduction (accidental and as seed contaminants), a reproductive strategy combining both sexual and vegetative propagation, and a tendency to affect wetland areas when reaching natural habitats.
The results are encouraging because several of these plants are still absent from both the alien flora of Sardinia and Italy. However, this should not be a reason to lower alertness. Instead, greater prevention measures should be taken to ensure that these species do not escape the garden’s fences, particularly since they exhibit locally invasive behaviour. Historical data on the first records of these species in Sardinia and the HBK suggest that past escapes may have contributed to their spread in natural habitats, as seen in at least three invasive species: Ailanthus altissima, Asparagus asparagoides, and Vachellia karroo.
These inventories can assist stakeholders in making informed decisions regarding the use of alien ornamental species, helping to prevent biological invasions, particularly in Mediterranean natural habitats, with special attention to those most vulnerable to invasive species.
Conceptualisation, L.P., A.L., and G.B.; methodology, L.P., A.L., G.C., and G.B.; resources, L.P., A.L., G.C., F.M., G.I., and G.B.; data curation, L.P., A.L., G.C., F.M., and G.B.; writing—original draft preparation, L.P., A.L., G.C., and G.B.; writing—review and editing, L.P., A.L., G.C., F.M., G.I., and G.B.; supervision, G.B.; funding acquisition, G.B. All authors have read and agreed to the published version of the manuscript.
Not applicable.
Data are contained within the article and
We would like to thank all the staff of the HBK botanical garden for the support provided for all the research conducted during the period 2015–2024, and Roberta Lai from the Cagliari Herbarium for her assistance with herbarium research.
The authors declare no conflicts of interest.
Footnotes
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Figure 2 Number of taxa in the main families of the HBK alien vascular flora.
Figure 3 Number of taxa per plant status (CAS: casual; NAT: naturalised; INV: invasive), among neophytes (Neo), archaeophytes (Arch), and cryptogenic (Crypto) of the HBK alien vascular flora.
Figure 4 Number of taxa per plant status (NAT: naturalised; CAS: casual; INV: invasive; AUT: autochthonous; ABS: absent), among alien taxa already reported in Italy (IT), Sardinia (SA), and the HBK.
Figure 5 Number of taxa per life form (P: phanerophytes; NP: nanophanerophytes; T: therophytes; G: geophytes; Ch: chamaephytes; H: hemicryptophytes, I: hydrophytes) and life subform (scap: scapose, caesp: caespitose, lian: lianas, succ: succulent, rept: reptant, rhiz: rhizomatous, bulb: bulbous, frut: frutescent, suffr: suffrutescent, ros: rosulated, nat: natant, rad: radicant/rooting) of the HBK alien vascular flora.
Figure 6 Number of taxa per life form and life subform (P: phanerophytes; NP: nanophanerophytes; T: therophytes; G: geophytes; Ch: chamaephytes; H: hemicryptophytes, I: hydrophytes) and life subform (scap: scapose, caesp: caespitose, lian: lianas, succ: succulent, rept: reptant, rhiz: rhizomatous, bulb: bulbous, frut: frutescent, suffr: suffrutescent, ros: rosulated, nat: natant, rad: radicant/rooting) of the HBK invasive alien vascular flora.
Figure 7 Comparison of the number of taxa per geographic origin of invasive alien flora and total alien flora in the HBK.
Figure 8 Comparison of the number of taxa per introduction pathways of invasive alien flora and total alien taxa in the HBK.
Figure 9 Comparison of the number of taxa among reproductive methods of invasive alien taxa and total alien taxa in the HBK.
Figure 10 Percentage and number of taxa of the alien vascular flora of the HBK reported in the different habitat types of Sardinia.
Number of taxa of the alien vascular flora of the HBK reported in the different habitat types of Sardinia (SA), with a focus on invasive species (INV).
| Habitat | SA | INV (SA) | INV (HBK) | INV Shared HBK-SA | |
|---|---|---|---|---|---|
| anthropogenic | synanthropic | 48 | 6 | 6 | 3 |
| agricultural | 8 | 1 | 0 | 0 | |
| coastal | coastal | 12 | 5 | 0 | 0 |
| coastal/synanthropic | 3 | 0 | 1 | 0 | |
| coastal/matorral | 2 | 1 | 0 | 0 | |
| wetland | wetland | 18 | 7 | 4 | 2 |
| wetland/synanthropic | 14 | 10 | 5 | 5 | |
| wetland/agricultural | 3 | 0 | 1 | 0 | |
| wetland/coastal | 1 | 1 | 1 | 1 | |
| absent | 37 | 0 | 0 | 0 | |
| total | 146 | 31 | 18 | 11 | |
Supplementary Materials
The following supporting information can be downloaded at:
Appendix A. Inventory of HBK Invasive Alien Flora
Inventory of HBK invasive alien flora. ? = Doubtful report; (CAG) = specimens in Herbarium CAG (Cagliari); (CAG?) = Not recovered in Herbarium CAG (Cagliari); ND = Not determinable; (C) = Cultivated, not spontaneous; * in [
| No. | Family | Species Name | Primus Inventor | First Report in Cagliari | First Report in the HBK |
|---|---|---|---|---|---|
| 1 | Acanthaceae | Acanthus mollis L. | [ | [ | [ |
| 2 | Amaryllidaceae | Nothoscordum borbonicum Kunth | Ardenghi 2009 (FI, Herb Ardenghi) * | [ | [ |
| 3 | Araceae | Lemna valdiviana Phil. | Bacchetta and Mascia 2009 (CAG) ** | Bacchetta and Mascia 2009 (CAG) ** | Bacchetta and Mascia 2009 (CAG) ** |
| 4 | Aristolochiaceae | Aristolochia sempervirens L. | Bocchieri and Mulas 1981 (CAG) *** | Bocchieri and Mulas 1981 (CAG) *** | Ballero 1975 (CAG) **** |
| 5 | Asparagaceae | Asparagus asparagoides (L.) Druce | Bacchetta, Mascia and Podda 2009 (CAG) ** | Bacchetta, Mascia and Podda 2009 (CAG) ** | [ |
| 6 | Asteraceae | Erigeron bonariensis L. | [ | [ | Present work |
| 7 | Asteraceae | Erigeron sumatrensis Retz. | [ | [ | Present work |
| 8 | Asteraceae | Senecio angulatus Vahl | [ | [ | [ |
| 9 | Asteraceae | Symphyotrichum squamatum (Spreng.) G.L.Nesom | [ | [ | Present work |
| 10 | Commelinaceae | Commelina erecta L. | ND | ND | [ |
| 11 | Fabaceae | Vachellia karroo (Hayne) Banfi & Galasso | ND | [ | [ |
| 12 | Nephrolepidaceae | Nephrolepis cordifolia (L.) C.Presl | Bacchetta 2009 (CAG) ** | ND | Present work |
| 13 | Nyctaginaceae | Mirabilis jalapa L. | [ | [ | [ |
| 14 | Oxalidaceae | Oxalis pes-caprae L. | [ | [ | Mameli 1904 (CAG?) **** |
| 15 | Poaceae | Ehrharta erecta Lam. | Mulas 1988 (CAG) ***** | Zedda and Mascia 1978 (CAG?) **** | Zedda and Mascia 1978 (CAG?) **** |
| 16 | Poaceae | Eleusine indica (L.) Gaertn. | [ | [ | Present work |
| 17 | Simarumbaceae | Ailanthus altissima (Mill.) Swingle | [ | [ | [ |
| 18 | Solanaceae | Lycium ferocissimum Miers | [ | [ | Present work |
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; Lallai, Andrea 2
; Calvia Giacomo 3 ; Mascia, Francesco 4 ; Iiriti Gianluca 5 ; Bacchetta Gianluigi 1
1 Center for Conservation of Biodiversity (CCB), Department of Life and Environmental Sciences, University of Cagliari, Viale Sant’Ignazio da Laconi, 13, 09123 Cagliari, Italy; [email protected] (L.P.); [email protected] (G.B.)
2 Center for Conservation of Biodiversity (CCB), Department of Life and Environmental Sciences, University of Cagliari, Viale Sant’Ignazio da Laconi, 13, 09123 Cagliari, Italy; [email protected] (L.P.); [email protected] (G.B.), Hortus Botanicus Karalitanus (HBK), University of Cagliari, Viale Sant’Ignazio da Laconi 9–11, 09123 Cagliari, Italy; [email protected]
3 Faculty of Agricultural, Environmental and Food Sciences, Free University of Bozen-Bolzano, Piazza Università, 5, 39100 Bozen-Bolzano, Italy; [email protected]
4 Department of Life Sciences, University of Siena, Via P.A. Mattioli 4, 53100 Siena, Italy; [email protected]
5 Hortus Botanicus Karalitanus (HBK), University of Cagliari, Viale Sant’Ignazio da Laconi 9–11, 09123 Cagliari, Italy; [email protected]