Introduction

Allochthonous species introductions are considered among the main environmental issues that challenge global ecological conservation (Simberloff et al. 2013). This process has dramatically increased in the last decades, showing worldwide exponential tendencies that reflect the acuteness of this environmental issue (Seebens et al. 2017). Vascular plant species are among the main groups of organisms that have received attention, since reliable historical and ongoing data allows us to understand species introduction dynamics. Equally, allochthonous plant introductions have increased worldwide (Pysek et al. 2017; Seebens et al. 2017), being the Mediterranean basin among the most affected regions (Médail 2017). As a result of increased human activities such as tourism and trading through the Mediterranean, extensive research has been conducted to define patterns and consequences of increased plant introductions (Arianoutsou et al. 2021).

Plant records have been the main source of information to approach plant introductions and disentangle the proportion of invasive plants and their traits (Bresch et al. 2013). Based on this information, main introduction pathways and plant traits have been defined at both regional and global scale (Chytrý et al. 2008; Celesti-Grapow et al. 2016). Special interest has also been given to affected habitats and how alien species develop (Hulme et al. 2008). For these cases, the effect of alien flora has been mainly focused on proper evaluation of alien species and their impact on native taxa (Hejda et al. 2009). Considering the impact of plant introductions, several studies have indicated the importance of maintaining updated checklists, since worldwide plant introductions have shown to be a continuous process (Sebeens et al. 2017). Based on this information, the development of updated lists of alien species has allowed detailed information which allows further management effort to approach alien plant regulation (Burgiel et al. 2006). Socioeconomic factors such as trading relations and changes in economic activities can influence plant introductions patterns and their trends. These changes in introduction trends comprise the intensity and quantity of species introductions according to their origin, source, entrance pathway and newly colonised habitat (Pretto et al. 2012; Van Kleuen et al. 2018). Improved identification tools, increased sampling effort and greater availability of information are changing the magnitude of alien plant invasions in the Mediterranean. In this sense, several recent updated checklists are revealing significatively higher numbers of plant alien species than formerly reported (Puddu et al. 2016; Uludag et al. 2017; Galasso et al. 2018; Aymerich and Sáez 2019; El-Beheiry et al. 2020; Meddour et al. 2020; Guarino et al. 2021; Spampinato et al. 2022). These studies evidence the need to compile information and define both the current state of alien flora but also the variation in numbers of species and overall traits such as invasiveness or origin along time (Aymerich and Sáez 2019).

The Balearic Islands is an archipelago located in the western Mediterranean known to be influenced by human activities for millennia (Burjachs et al. 2017). Considerable number of habitats have been traditionally influenced by farming and cattle raising, being inland habitats (forests and shrublands) and mountains the most affected. By this means, the Balearic landscape and its plant communities have been strongly modified, establishing in some cases new plant species assemblies (Llorens and Gil 2017). As a result, Balearic Flora has been characterised by woody xeric species adapted to some degree to grazing, and for some cases with high affinity to nitrogen rich environments (Llorens and Gil 2017; Capó et al. 2021). Some habitats, such as coastal and rupicolous, have remained less affected, since they are less productive, and small remnants of other natural habitats have remained in less accessible areas. Since the 1950s, socioeconomic change to the tourism economy has caused a steady increase of urbanisation and rural abandonment (Pons and Rullán 2014). Landscape has been further modified by a process of reforestation, but also by problems as herbivory increases due to feral cattle (Capó et al. 2021). Previous flora checklists have included allochthonous species, but the first formal checklist of alien plants was published in 2005 (Moragues and Rita 2005). An updated number of exotic species and descriptive analysis was subsequently published in 2010 (Podda et al. 2010). Since then, several studies have indicated the presence of numerous taxa in the Balearic Islands, but information has remained sparse or unpublished.

Considering the recent increase in alien species reported among Mediterranean regions, the present study aims to update the Balearic Alien Flora checklist. We also discuss the time and spatial variation of species introductions, and the invasiveness of the different taxa. Moreover, we aim to predict the composition, magnitude, and impact of alien species in the next years using these data, in order to develop management and conservation strategies towards pre-introduction and serve as a model for other insular Mediterranean insular systems.

Material and methods

Area of study

The studied area comprehends the Balearic archipelago, located in the Western-Mediterranean Basin, comprising four inhabited main islands, Majorca (3610 km2), Minorca (Biosphere reserve; 701 km2), Ibiza (541 km2) and Formentera (82 km2) (Fig. 1). Additionally, there are more than 146 islets of variable size uninhabited but with variable human influence. Among them the most relevant are the National Park of Cabrera (sub-archipelago located southern Majorca; 5,01 km2) and the protected islet of Dragonera (southwest of Majorca; 2,88 km2). Based on geographical proximity and historical and floristic differences, the Balearic Islands can be divided into the Gymnesic Islands (Majorca, Minorca, Dragonera and Cabrera) and the Pityusic Islands (Ibiza and Formentera). Geology is similar among islands with calcareous soils, excluding north Minorca which is partially composed of silicic substrates. Bioclimatic profile following Rivas-Martinez bioclimatic classification (Rivas-Martinez 1999; Guijarro 1986) is Mediterranean with humid ombrotypes between semiarid to humid, being Pityusic Islands and Southern Majorca drier (annual 350 mm) and, the highest humid conditions achieved in the Serra de Tramuntana mountains of northern Majorca (annual 1500 mm). Mean annual rainfall for most of the archipelago falls between 450 and 650 mm. Rainfall, as occurs in a Mediterranean climate, concentrates in Autumn and Spring, being summers especially hot and dry (Llorens and Gil 2017).

Fig. 1
figure 1

Map of the Balearic Islands. A general view of the Western Mediterranean has been included in the bottom right corner, where the Balearic Islands area has been indicated with a red rectangle. In the main map, specific enlarged views of Cabrera (bottom) and Dragonera (top left corner) has been included to ease visualisation. Google Maps has been used for base map (www.google.es/maps), using the Spatial Analysis Software QGIS (www.qgis.org)

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Landscape and predominant habitats can be found in Llorens and Gil (2017). Forest and shrublands are dominated by sclerophyllous and xerophytic vegetation. Under humid and subhumid (or water compensated conditions) Quercus ilex woods tend to prevail, but under drier conditions or human disturbed areas, these are replaced by more thermophilous forests and shrublands of Pinus halepensis, Olea europaea and Pistacia lentiscus, especially in the Pityusic Islands. Rivers are lacking, but torrent (transient rivers) and riparian habitats can be found, especially in the Serra de Tramuntana and Minorca. Wetlands are also present as saltmarshes, being s’Albufera d’Alcúdia (Majorca) the biggest of them. Coastal habitats are dominated by rocky coasts and coastal cliffs, where several endemic cushion-like species develop. Dune systems can also be found, mainly in Majorca, Minorca, and Formentera. Perennial and annual grasslands are also relevant, among which Ampelodesmos mauritanicus formations in the Majorca mountains stand out, prompted thanks to excessive grazing by goats and human mediated fires. Extensive historical agricultural landscapes and more recent excessive urbanization favour the presence of both synanthropic and weed vegetation.

Data sources and compilation

Old references of species growing in the Balearic Islands are known from 1670 to 1800 and even as old as 1348 (Payeras 2006). Many records are related to native and endemic taxa, being cultivated species also mentioned. However, the first unified flora for the Balearic Islands, with detailed information both for locality and its status (cultivated, subspontaneous or naturalised) was compiled by Cambessedess in 1827 (Payeras 2006; Carrió-Cabrer 2013). The present study merges information from several floras, starting with Cambessedess (1827), followed by: Barceló i Combis 1867; Barceló i Combis 1879; Marès 1880; Femenias 1904; Bianor 1917; Knoche 1921; Bonafè 1977–1980; Duvigneaud 1979; Pla et al. 1992. Taxa considered allochthonous were selected according to alien checklists described from the Balearic Islands (Moragues and Rita 2005), Iberian peninsula (Sans-Elorza et al. 2004; Blanca et al. 2011) and European continent (Greuter 1984). Besides these checklists, an extensive survey was also conducted on data published in online databases (ANTHOS 2023; BIOATLES 2023; GBIF 2023; ORCA 2023) and published technical documents and manuscripts. Species only known from cultivation, with doubtful presence (lack of physical or any other evidence), doubtful identification or lack of basic information (locality or recording date) were excluded. However, a list of doubtful species which deserve attention due to voucher presence or which are recorded from reliable sources (i.e. institutions) is offered as complementary data (Table S1) but excluded for further analysis. For each species, year of registration was also recorded in the database to assess time variation of species recording (year recorded in the voucher and when not indicated year of manuscript publication was assumed). The island where each species was first recorded was based on original information of the oldest record. General distribution among islands for each species was based on subsequent references indicating its presence, and personal observation. Nomenclature followed the accepted name in the World Checklist of Vascular Plants (WCVP, http://wcvp.science.kew.org/) (Govaerts et al. 2021).

Characterizations of species

For each species several general traits were assigned based on previous references to characterise the alien Balearic Flora. Habitat and introduction path were separated in different categories as indicated in Podda et al. (2010), and species assigned based on information provided in manuscripts and further field observations. Life form followed Raunkjaer system (Raunkjaer 1934) mainly extracted from Sans-Elorza et al. (2004) and alternatively from other sources when required. Life forms considered in this study were: Phanerophytes s.l. (nano-, meso-, mega-), lianas, chamaephytes, therophytes, geophytes, hemicryptophytes and hydrophytes. Biogeographical origin was assessed with a classification adapted from the biogeographical regions proposed by Udvardy and Udvardy (1975), Rivas-Martinez (1999) and Takhtajan (1986), considering the natural distribution indicated for the different taxa available in the Plant of the World online (POWO, http://www.plantsoftheworldonline.org), and other references (Sans-Elorza et al. 2004; Blanca et al. 2011). Since certain biogeographical origins are less represented and to ease analysis, a synthetic version is offered as follows for overall analysis (specific regions are indicated in the supplementary list):

African continent regions which are represented are: the “Cape” Region and “African other” which includes the remaining regions (Malgache, Sahel-Sudan, Guinea-Congo, Somali-Etiope, and the Saharo-Arabigan region). American Continent regions are divided into “Neotropical” (South and Central American) and “Neartic” (Central-North America). Asiatic species are divided into “Tropical Asiatic” and “Non Tropical Asiatic” origins. “Tropical Asiatic” origin includes the Indo-malayan (Indian and Indochina peninsula and Malaysia islands) and the paleotropical origins (species that naturally distribute in both tropical Africa and Asia). “Non-tropical Asia” is defined by East-Central Asia (covers mainly the known Sino-Japanese region and part of temperate nearby areas of central Asia). Regions form Oceania and the pacific are indicated as “Australasia” and include Australia, New Zealand, and the different nearby island systems. “Euroasiatic” region includes all species which naturally range from Europe to Asia and species which are exclusively from the European continent. “Irano-Turanian” region is assigned to species distributed from the Mediterranean to Iran, reaching in some cases the Turanian region (i.e., central western Asia near de Caucasus, currently Kazakhstan and related areas). “Mediterranean” region includes species native to the traditional view of the Mediterranean biogeographical region (The Mediterranean basin). “Macaronesia” includes taxa mainly from the Canary Islands and other archipelagos from the Atlantic Ocean, near Europe and Africa.

Invasiveness status was assigned based on information provided in citation references according to the classification proposal of Blackburn et al. (2011). Additionally, invasiveness was also assessed by categorising species according to Kornas (1990) classification, based mainly on information provided in Sans-Elorza et al. (2004) when possible or inferred from general descriptions and assigned accordingly. For general analysis, Blackburn system is divided into five categories: Alien (B2 and B3), Casual (C0, C1 and C2), Naturalised (C3), Established (D1 and D2) and Invasive (E).

Statistical analysis

The differential number of alien taxa for each biogeographical origin, Blackburn category, habitat, Introduction path, Kornas classification and Life form was assessed by using Generalised Linear models (GLM), with Poisson family for count data (link = ”log”), considering the island as a predictive variable. Model selection was carried out considering the Akaike Information Criterion (AIC; Bozdogan 1987), while model accuracy was established comparing with null models and calculating McFadden pseudoR2 when possible (Veall and Zimmermann 1996). The effect of each categorization on the number of taxa, as well as the potential interaction between each factor and the island, was evaluated using Analysis of Deviance (Zuur et al. 2009). Differences between each group were established using the Tukey Honest Significance Test (Abdi and Williams 2010). This method was also used to analyse the number of taxa in each Blackburn classification and was established considering the different classification criteria of taxa (biogeographical origin, Habitat, Life form…). A time series analysis was done to predict the potential cumulative number of alien taxa in our flora according to the different categorization parameters. To do so, Generalized linear models were carried out for each categorization variable (Life form, Habitat, Introduction path) considering the year and the interaction as predictive variables, and were used to carry out predictions from the present to year 2100, using the predict.glm function from the stats package (R Core Team 2023). Finally, the number of alien taxa on each island was analysed, considering the blackburn classification as a factor variable and the potential interaction between both variables. Moreover, a presence-absence matrix of taxa on each island was created, and euclidean distance among each island was calculated and plotted using the ggdendro package (De Vries et al. 2022). All statistical analysis were carried out using the statistical software R (R Core Team 2023) and the libraries tidyverse (Wickham et al. 2019), multcomp (Hothorn et al. 2016), ggpubr (Kassambara and Kassambara 2020) and pscl (Jackman et al. 2020).

Results

Checklist of alien plant species and distribution among islands

A checklist of the different taxa is available as supplementary material (Table S2). Overall, records of allochthonous species since 1827 until 2022, indicate that Alien Balearic Flora is composed by 624 taxa from 371 different genera and divided into 100 families, from which 10 can be regarded as the most represented including more than 50% of all alien species. Conversely half of the remaining species belong to 90 families from which 51 are represented by 1 or 2 taxa. The richest genera are Amaranthus (15), Opuntia (12) and Euphorbia (10). Additionally, 8 species have been also formally recorded but remain doubtful (Table S2).

Fig. 2
figure 2

Proportion of taxa for each categorization variable (Biogeographical origin, Blackburn invasiveness, Habitat, Introduction Path, Kornas classification and Life form) in each island (Ma = Mallorca, Mi = Menorca, Ib = Ibiza, Fo = Formentera, Ca = Cabrera, Da = Dragonera). Statistical significance of each variable and the interaction between each variable and the Island are indicated in italics below each title, where Df = Degrees of Freedom, LR = Likelihood Ratio and p = p-value. Below each subplot the model p-value and de R2 is indicated. Letters in each bar are used to indicate significant differences in the Tukey Honest significant test

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Alien species have been recorded mainly in the Gymnesic islands, followed by the pytiusic and lastly the uninhabited islands of Cabrera and Dragonera. First records mimic this same pattern being Majorca the islands with higher number of first recorded taxa and Dragonera the least. Dissimilarity between islands shows similar pattern grouping, but high differences between islands (high Euclidean distances). Taxa common to all islands are 12 (increased to 98 if uninhabited islands are excluded) while exclusive species from one island are represented by 269 species, being the island of Majorca (177) with higher exclusive records, and the remaining islands from more exclusive alien species to least Minorca (75), Ibiza (11) and Formentera (6), being null in Dragonera, and Cabrera.

Fig. 3
figure 3

Proportion of taxa for each grade of Invasiveness (Blackburn classification; Alien, Casual Introduced, Naturalised, Established and Full Invasive) in each island (Mallorca, Menorca, Ibiza, Formentera, and Cabrera). Statistical significance of the Blackburn classification and the island is indicated in italics below each title, as well as the interaction between both variables, where Df = Degrees of Freedom, LR = Likelihood Ratio and p = p-value. Below the main plot the model p-value and R2 is indicated. On the right side a similitude dendrogram between each island is indicated in terms of presence-absence species data

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Characterization alien flora (geographical origins, habitat, introduction pathway, and life forms)

Figure 2 indicates general alien flora characterisation. Main biogeographical origins are Mediterranean (133), American (Neotropical and Neartic, 123 and 71, respectively), Euroasiatic (64) and Cape (60). Among the remaining origins, old world non-tropical origins, such as Irano-Turanian (35), Non-Tropical Asia (36) and Tropical Asian (32) can be considered jointly as relevant origins. The remaining origins are regarded as minor sources of alien species being jointly 11.2% of taxa. Habitats where more taxa have been recorded or described to enter are human altered habitats, specifically synanthropic (270) and agricultural (174), which jointly represent more than 50% of alien taxa. The remaining habitats represent 28% of alien taxa, being woodlands the least represented (15). Introduction pathway is mainly ornamental, with over 50% of taxa entering due to their appeal for gardening and horticultural practices. The remaining 50% can be divided between unintentional introductions and introductions due to agricultural activities. Main Life forms are phanerophytes which represent 30% and increase to almost 40% of all alien taxa if lianas are added (7.21%). Therophytes are the second more important Life form with over 30% of alien flora displaying this form. The remaining Life forms are similar with over 10% each, excluding hydrophytes which are the least represented (~ 1%).

Species degree of naturalisation and invasiveness

Invasive status of alien flora of the Balearic Islands shows that 44% of all taxa can be regarded as casual, while 35% are jointly naturalised (22) and established (13), and 17% can be considered invasive. Kornas system concurs with Blackburn system showing similar results and describing most alien taxa as diaphytes ergasiophytes (212), followed by metaphytes epecophytes (193). Established and thriving under seminatural and natural habitats (metaphytes hemi and holoagriophytes) represent 28%. Among islands, invasive status remains lower and more similar to Balearic invasive status in larger islands of Majorca and Minorca, where higher number of taxa recorded can also be noted. Smaller islands as occurs especially with Formentera and Dragonera show both smaller number of recorded taxa and higher proportion of invasive related categories (Fig. 3).

Invasiveness varies with all traits, excluding taxonomic identity (Fig. 4). However, taxonomic identity shows higher proportion of invasives and naturalised taxa in families from warmer or/and more arid areas (Agavaceae, Cactaceae, Aizoaceae and Amaranthaceae) and important world widespread families (Asteraceae and Poaceae). Lower invasiveness can be observed for taxa related to families more similar to the Mediterranean native flora (Solanaceae, Brassicaceae, Lamiaceae, Rosaceae, Apiaceae). Some families are the source of both casual and invasive species (Crassulaceae, Euphorbiaceae, Oxalidaceae, Chenopodiaceae, Apocynaceae, Convolvulaceae and Fabaceae). Biogeographical origin mimics taxonomic identity invasiveness, with higher proportion of invasive species from warmer and drier regions (Cape, africa other and Australasia) in contrast to colder regions (Euroasiatic), while Mediterranean and similar climates show moderate invasiveness with higher tendency to naturalisation. Habitat shows more invasive or naturalised species related to natural or semi natural habitats, where also smaller numbers of taxa have been recorded. Human related habitats (Synanthropic and Agricultural) tend to display high numbers of aliens with overall low invasiveness. Invasiveness increases according to the introduction pathway with a higher proportion of invasive taxa for ornamental mediated introduction, followed by unintentional and agricultural.

Fig. 4
figure 4

Proportion of taxa for each grade of invasiveness (Blackburn classification: Alien, Casual Introduced, Naturalised, Established and Full Invasive, indicated with different colours) for each classification variable (Biogeographical origin, Habitat, Introduction path, Kornas classification, Life form and taxonomic family). Statistical significance of each classification variable is indicated in italics below each title, as well as the interaction between each variable and the grade of invasiveness, where Df = Degrees of Freedom, LR = Likelihood Ratio and p = p-value. Below each subplot the model p-value and R2 is indicated. For each bar the total number of taxa is indicated

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Time variation

Time variation of alien recording shows an exponential increase of alien species for the Balearic Flora, being slope of increase changed in the decade of 1950 and 1970 (Fig. 5). Considering time lapse, in 130 years of data (previous to the 70 s decade) alien taxa is represented by 276 species while in the subsequent 50 years 347 species have been recorded.

Fig. 5
figure 5

Observed trends and predicted scenario of alien taxa in the Balearic Islands. Observed and predicted data was segmented by each classification variable (Blackburn classification, Habitat, Biogeographical origin, Introduction path, Life form, Taxonomic family) and individual predictions were carried out until years 2100. Dots are used to indicate observed data, while dashed lines are used to indicate predicted data and coloured areas to indicate the prediction error (standard error). Statistical significance of each classification variable is indicated in italics below each title, as well as the interaction between each variable and time, where Df = Degrees of Freedom, F = F of Fisher, LR = Likelihood Ratio and p = p-value. Below each subplot the model p-value and R2 is indicated

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Time variation shows differential exponential increase of alien species according to their different traits being further supported by model predictions. Casual species tend to increase more than species regarded as more invasive. Naturalised taxa and fully invasive show similar slope, while established taxa grow with the smallest slope, displaying a steadier increase. Habitat variation shows an increase concomitant to general trends for synanthropic habitats, being coastal habitats also increased starting in the 2000. For both habitats acute increase is further predicted. The remaining habitats show trends and predictions of moderate and steady increase. Similar patterns can be observed for biogeographical origin with American and cape regions sharply increasing and moderately increasing for the remaining origins. Introduction path shows higher slopes mainly for ornamental followed by unintentional pathways, while agriculture shows modest increase. Taxonomic identity shows time trends favouring main families (Asteraceae, Fabaceae and Poaceae), and predictions show a sharp increase of Cactaceae family. Life form is mainly favoured historically for phanerophytes and therophytes, while predictions support further increase of phanerophytes and a sharp increase of lianas.

Discussion

Alien flora checklist and time variation

The present study reports 624 taxa considered allochthonous to the Balearic Islands (all included in a supplementary dataset), which represent a significant increase, almost doubling previous data reported for the archipelago (Moragues and Rita 2005; Podda et al. 2010). The number of taxa represents over a ~ 30% of all Balearic Flora (1500–1975 taxa, Sáez et al. 2013; Gil and Llorens 2017b). The number of alien species here reported rises to similar values as other continental islands of the western Mediterranean but exceeds considerably the proportion of alien taxa like Corsica (553; 17% of total flora; Puddu et al. 2016), Sardinia (598; 18%; Puddu et al. 2016) and Sicily (459; 14%; Raimondo et al. 2010; Domina 2021). These results are of special interest since Balearic archipelago surface (~ 5000 km2) is significantly smaller than the other western Mediterranean island systems (8722 to 25,711 km2) discarding surface-species relation as a factor for explaining Balearic increasing alien flora. At this surface scale of comparison and considering that all insular systems are well connected continental islands, alien flora is probably related to other homogenous factors rather than surface itself (Chiarucci et al. 2021). Instead, proportion of alien species in the Balearic Islands can be more closely related to data reported by other European regions, where human influence is higher (Verloove et al. 2006; Pysek et al. 2002). Aymerich and Sáez (2019) expose a similar phenomenon with the Alien Flora of Catalonia, indicating a higher number of recorded species than East-Mediterranean regions due to three key reasons (higher urban development, better recording, and biogeographical additions such as East-Mediterranean aliens). Balearic Alien Flora also similarly exceeds the number of taxa of less urbanized Mediterranean countries which range between 200 and 400 taxa as reported for East-Mediterranean like Greece (Arianoutsou et al. 2010a), Turkey (Uludağ et al. 2017) and Albania (Barina et al. 2014) but also West-Mediterranean countries like Algeria (Meddour et al. 2020). In this sense, increased alien floras in some Western Mediterranean regions are a general trend related to a combination of higher urban and disturbed habitats, increased transit and trading development, and better and earlier recording effort (Barina et al. 2014; Aymerich and Sáez 2019; Spampinato 2022).

Specifically for the Balearic Islands record variation with time and between-islands supports increased urban development and recording effort as the main causes for the exponential increase of alien taxa. However, other factors such as small island effect can be appreciated for the smallest islands such as Dragonera and Cabrera, where isolation combines with small surface in shaping alien flora variation as occurs in other Mediterranean archipelagos (Chiarucci et al. 2021). Chronological variation shows a low increase until the 1950 and a further second increase in the 1970. Early records are mainly related to monographs and isolated studies starting with Cambessedess (1827), Barceló i Combis (1867, 1879) and further additions of several authors (Ferrer; Bonafè; Bianor among others, Supplementary data). Between the 50 s and the 90 s the increase of alien flora can be related to the extensive contributions of Bonafè (1977–1980) which included many unrecorded old aliens. Strong tourism development, urbanisation, and population growth from the 50 s decade entailed the starting point of a dramatic landscape modification (Pons and Rullan 2014; Burjachs et al. 2017; Llorens and Gil 2017) and probable increase of alien taxa introductions (Podda et al. 2010; Meddour et al. 2020). Under any flora, exotic taxa are necessarily subjected to a time lag (invasive debt) between landscape and formal alien species recording (González-Moreno 2017). In term, species recording depends on sampling effort and plant identification tools which require infrastructure and resources (Verloove 2006; Celesti-Grapow et al. 2016). By this means, urban development starting in the 50 s can be argued as the main source for the many alien species recorded during the late 70 s and 90 s and following decades. This seems feasible since much scientific infrastructure and funding occurs in the late XX century with the Balearic Islands university aperture (1978) and the declaration of Minorca as a Biosphere reserve (1993). This infrastructure prompted frequent and detailed publications (i.e. Pla et al. 1992 flora, Moragues and Rita 2005; floristic notes and checklists included as Supplementary data) which also explained the higher data collected until now for the Gymnesic islands (Majorca and Minorca) in contrast to the Pityusic islands (Formentera and Ibiza).

Alien flora characterisation

Both time and spatial variation support urban development prompted by tourism as a main influencing factor driving Balearic Alien Flora. In this sense, the different traits show time variation patterns concomitant to tourism development, and spatial variations mainly between big populated and small uninhabited islands.

Taxonomic identity is diverse and covers a high spectrum of families. The 100 families reported in this study show high similarity compared to previous studies (Moragues and Rita 2005; Podda et al. 2010), and other Mediterranean (Aymerich and Sáez 2019) and European regions (Pysek et al. 2002; Lambdon et al. 2008). However, time variation reveals specific trends in recent times. Certain families preserve relevance maintaining a steady increase until our days (Brassicaceae, Solanaceae and Amaranthaceae), while for the three most relevant families (Asteraceae, Fabaceae and Poaceae), exponential increase patterns can be traced starting in the 50 s and further 70 s (Asteraceae). In addition, since the 90 s, new families have gained importance such as Cactaceae, and further projections support their increase. Higher rate of introductions has been linked to specific trade relations (Brunel et al. 2010) and relations conditioned by socio-economic context (Pretto et al. 2012) which in term have varied through historical periods (Arianoutsou et al. 2010b). Current biogeographical origin is in general agreement with previous studies (Moragues and Rita 2005; Podda et al. 2010), being only noticeable low variation for old-world related origins which probably associate to different assignment criteria. However, time variation trends allow us to distinguish two major groups: exponential growth since the 70-90 s for exotic origins such as Neotropical and Nearctic (American), and Cape regions, and steadier linear growth of closer biogeographical regions (Mediterranean, Irano-Turanian and Euroasiatic). This differentiation can be mainly explained by increased ornamental species originated from exotic regions, while Asian and Mediterranean aliens mainly relate to old introduction associated to agricultural (Meddour et al. 2020) and/or remain low due to lower climatic compatibility (e.g., Euroasiatic, Guarino et al. 2021).

Ornamental origin increase is a general trend for alien floras of developed countries both in the Mediterranean and other climates (Hulme et al. 2008; Pretto et al. 2012). Propagule pressure through bad management of gardening wastes is considered one of the most relevant paths (Cubino et al. 2015; Foxcroft et al. 2019). In contrast, agricultural abandonment has prompted a slower increase of agrarian related aliens. The abovementioned pathways hint that the Balearic economy has switched from a self-sustainable to a reliant economy. In this sense, increased human transit, but also higher trade for gardening (Cerrato et al. 2021) and food industry (Gervilla et al. 2019) are responsible for the unintentional pathway increase. These overall results are further supported by differential path introduction among islands, with uninhabited and smaller islands less influenced by ornamental mediated introduction. In this sense a higher proportion of ornamentals can be linked to higher urban and touristic development.

Among Life forms, phanerophytes and therophytes are the most common. Both are known to be dominant among alien floras of the Mediterranean, being phanerophytes (Arianoutsou et al. 2010a; Podda et al. 2010; Puddu et al. 2016) and therophytes (Uludag et al. 2017; El-Beheiry et al. 2020; Meddour et al. 2020) the main Life forms. Our results show that both forms have been and are predicted to be the major Life forms characterising alien flora of the Balearic Islands. However, trends support a turn from therophyte to phanerophyte predominance. Phanerophytes have been more related to direct introduction by human activities such as forestry and ornamental/gardening (Lloret 2004; Arianoutsou et al. 2010b; Čeplová et al. 2017). Since forestry is rare in the Balearic Islands, increasing use of ornamental trees and shrubs are the most likely explanation for the increase of phanerophytes. Therophytes remain high due to historical relevance, and their ongoing predicted increase probably relates to higher accidental introductions since contaminant weedy species tend to be annual (Arianoutsou et al. 2010a, b). Increase of lianoid species has been scarcely described, being only pointed out for Catalonia (Aymerich and Sáez 2019). Ornamental purposes probably also justify the accelerated increase and further prediction of lianas. The remaining Life forms display various trends which can be related to their various origins (both gardening and agricultural), while hydrophytes are a minority since suitable environments for their development are scarce as occurs for other Mediterranean regions (Podda et al. 2010, 2011).

In terms of habitats, as expected, our results support the widely held view that human-related habitats are more likely to receive exotic taxa. The increased record of alien species in human-related habitats is mainly explained by over-sampling, excessive nutrient and water availability, low competition, and strong propagule pressure. (Hulme et al. 2008; Chytrý et al. 2008; Aikio et al. 2012; Basnou et al. 2015; Celesti-Grapow et al. 2016; Guarino et al. 2021). The Balearic Islands have been strongly influenced since millennia and time variation of records assess both the strong historical agricultural influence and its replacement by the more recent urban development. Time trends show that alien flora is increasing in urban and coastal areas (where urban development mainly occurs), while agricultural and related riparian habitats (torrents near fields) are relevant due to old introduction but are expected to increase less. The remaining habitats are less relevant due to high resilience (shrubland and woodland) or scarce representation (wetlands).

Invasive status and variation among islands

Differences in the degree of invasiveness may be due to different criteria or to a different application of the definition of the invasiveness categories, as the assignment is often based on expert assessment. Instead, invasiveness based on specific data, such as number of records, can define better spatio-temporal invasive curves which offer a more reliable alternative to assess invasiveness (Buldrini et al. 2023). However, low number of records, unreliable data (records with unknow origin or of doubtful identification) and uneven sampling (both spatially and taxonomically) are limiting factors for most taxa. Our results followed Blackburn et al. (2011) being closely related to the system proposed by Richardson et al. (2000) which is used as a baseline for invasiveness assessment in many alien floras. By this means the following comparisons with other floras, and the overall analysis, requires taking into consideration that they are an approach, and that further data collection is required to ensure specific invasive assessment.

Results of invasive status show that half of all taxa are casual and over 17% invasive, being the remaining naturalised or fully established. Previous analysis by Podda et al. (2011) indicated that among all alien taxa, 14% can be considered invasive. Invasive species in other Mediterranean regions display variable number, being normally over 10–14% in the different regions of Italy (Galasso et al. 2018), Algeria (Meddour et al. 2020), Catalonia (Aymerich and Sáez 2019) or Greece (Arianoutsou et al. 2010a). Similar values can be found only for Corsica where recent alien checklists indicate 21% of invasive aliens (Puddu et al. 2016). Our results show that both the number of highly invasive and casual taxa can be explained mainly by the recent and ongoing recording effort during the last decades. A higher proportion of causal species has been associated with more urbanised and sampled regions (Puddu et al. 2016; Meddour et al. 2020). Temporal variation and differences between islands seem to support these sampling differences. More populated and sampled islands, such as Mallorca and Menorca, show a higher number of occasional taxa and a proportion of fully invasive taxa closer to the general data. In contrast, the less studied islands, and especially the smaller uninhabited islets (Cabrera and Dragonera) show a lower number of casual taxa and a higher proportion of totally invasive species. In this sense results concurs with other Mediterranean islets where aliens are scarce but invasive, since they relate to old introductions difficult to eradicate (Brundu 2013).

Taxonomic identity differs considerably from previous analyses (Podda et al. 2011), with the increase of Agavaceae, Iridaceae and Cactaceae among the major families. The latter are well represented families in the Mediterranean due to their succulence and clonal traits, which confer high ecological and physiological advantages in dry Mediterranean conditions (Podda et al. 2011). In contrast, aliens belonging to families shared with native flora (Brassicaceae, Solanaceae, Lamiaceae or Rosaceae) tend to naturalise. However, taxonomic identity can be considered overall unrelated since the high diversity of families does not allow to establish a general link. Instead, invasiveness has been related to climatic (Pinna et al. 2021) and soil similarity (Marco et al. 2010), being biogeographical origin a better predictor. The Mediterranean climate is known to display several abiotic constrictions, being aridity the main limitation for plant invasions (Thompson 2005). Many Central American (i.e., Opuntia spp. and Agave spp.) and Cape region (Aizoaceae and Iridaceae family) taxa are well adapted to more arid environments, which allows them to easily surpass the abiotic environmental constrictions given in the Mediterranean basin (Puddu et al. 2016). In contrast, species of tropical and temperate origins are less likely to thrive, while Mediterranean aliens are prone to naturalisation due to climatic and ecological similarity (Guarino et al. 2021). Moreover, this origin-invasiveness relation is probably related to the pathway of entrance. Invasiveness is higher if the passage is through ornamental purposes than if unintentionally or agriculturally mediated. Pysek et al. (2011) indicates that plants introduced unintentionally can in some cases become invasive, but normally remain less important compared to gardening introductions, as no trait selection (drought tolerance) and facilitation process (propagules pressure) occurs. Since many taxa used for gardening are originally from exotic regions and agricultural taxa are mainly from near-related areas, the link between invasiveness and biogeographical origin can also be understood by considering the pathway of entrance.

Life forms show an overall similar profile, excluding hydrophytes and therophytes. Lloret et al. (2004) indicates that fully invasive traits are more species specific and tend to be unrelated to Life forms. Moreover, it has been stated that there is not a definite profile for Mediterranean invasive species (Bresch et al. 2013), being invasive traits dependent on the recipient habitat (González-Moreno et al. 2013, 2017). Our data reflects that habitat and Life form may match invasiveness only for hydrophytes in wetlands, and therophytes in humanised habitats, being unrelated for the remaining. Invasiveness according to habitats is expected since much of the definition of an invasive species implies its success over native communities (Lambdon et al. 2008). For resilient habitats such as shrublands and woodlands, moderate invasiveness combines with low number of introductions (Fried et al. 2014). Considering that these habitats domain extensive areas, the combined results allow us to consider these environments as less overall affected (Arianoutsou et al. 2013; Puddu et al. 2016). In contrast, wetlands can be considered the most affected habitat since besides having the lowest number of species, more than half are considered fully invasive. As occurs in other Mediterranean regions, the Balearic Islands display low number/surface of aquatic habitats, and alien species introductions are scarce but intense when occurring (Mayoral et al. 2018). Similarly, our data shows that coastal habitats can also be regarded as vulnerable since urban development per se and high propagule pressure translate into higher alien introductions (Sáez et al. 2013; Giulio et al. 2020; Mayoral et al. 2021). Balearic coastal habitats display a profile of moderate invasiveness, resolving into a high invasive score if introduction rate is considered. Overall, our results support previous studies into establishing Coastal and Wetlands as the most vulnerable, while the remaining habitats show better resilience or less invasive threat.

Management implications

Data collected for the Balearic Islands alien records and their traits and temporal variation allow us to define the patterns of alien introduction in a strongly urbanised Mediterranean archipelago, serving as an example reference for other regions. These results allow us to give formal predictions for other Mediterranean and similar bioclimatic regions which are developing similar socioeconomic changes and are prone to mimic introduction patterns. In this sense, data described here can be used for pre-introduction management policies directed to avoid alien plant commercialization in those areas (including the Balearic Islands), both by preventing specific taxonomic identities (species, genera or families) or trait-related taxa (origin, Life form and pathway), offering predictions on trends that will require specific attention. In the case of the Balearic Islands, data collected allows the definition of a formal list on the basis of which further studies will be carried out on the specific effects of alien species on both the native habitat, and the general flora. The detailed grouping by invasiveness allows to also define pre-introduction management policies to avoid further entrance of similar taxa, and post-introduction policies since prohibition and priority management effort can be defined. In this sense a specific “Black list for the Balearic Islands” can be defined, and specific black, white and grey lists can be developed per island based on the current knowledge (Burgiel et al. 2006). Additionally, the updated list developed in this study allows a proper track on alien species numbers and invasive status which could change under further studies. Casual taxa here reported could display further invasive traits in the coming years with climate change, and our data can serve as a reference point to define long term invaders from strict casual species.