O R I G I N A L
A R T I C L E
Margarita Arianoutsou
1*
, Pinelopi Delipetrou
2
, Laura Celesti-Grapow
3
,
Corina Basnou
4
, Ioannis Bazos
5
, Yannis Kokkoris
1
, Carlo Blasi
3 and
Montserrat Vila`
6
1 Department of Ecology and Systematics,
Faculty of Biology, National and Kapodistrian
University of Athens, 15784 Greece,
2 Department of Botany, Faculty of Biology,
University of Athens, 15784 Greece,
3
Department of Plant Biology, Sapienza
University, Piazzale Aldo Moro 5, 00185 Rome,
Italy, 4 Center for Ecological Research and
Forestry Applications (CREAF), Universitat
Auto`noma de Barcelona, 08193 Bellaterra,
Spain, 5 Botanical Garden, Department of
Ecology and Systematics, Faculty of Biology,
National and Kapodistrian University of
Athens, 15784 Greece,
6
Estacio´n Biolo´gica de
Vespucio, s/n Isla de la Cartuja, E-41092
Sevilla, Spain
*Correspondence: Department of Ecology and
Systematics, Faculty of Biology, National and
Kapodistrian University of Athens, 15784
Greece.
E-mail: marianou@biol.uoa.gr
A B S T R A C T
To investigate alien plant species invasion levels in different habitats and alien species traits by comparing the naturalized flora in different areas of the same biogeographical region.
Spain, Italy, Greece and Cyprus.
Comparison of floristic composition, species traits and recipient habitats of naturalized alien neophytes across an east–west gradient comprising four countries in the European Mediterranean basin.
A total of 782 naturalized neophytes were recorded; only 30 species were present in all four countries. Although floristic similarity is low, the four alien floras share the same patterns of growth form (mostly herbs), life cycle (mostly perennials) and life form (mostly therophytes, hemicryptophytes and phanerophytes). The majority of the recipient habitats were artificial. Wetlands were the natural habitats, with the highest numbers of naturalized species. Floristic similarity analyses revealed: (1) the highest floristic similarity between Italy and
Spain, both of which were more similar to Greece than to Cyprus; (2) two groups of floristic similarity between habitat categories in each country (Greece–Cyprus and Italy–Spain); (3) a higher degree of homogenization in the plant assemblages in different habitats in Greece and Cyprus and a lower degree of homogenization in those in Italy and Spain; and (4) a higher degree of homogenization in artificial and natural fresh-water habitats than in the other natural habitats.
The floristic similarity of naturalized neophytes between the four countries is low, although the overall analysis indicates that the western group (Italy–Spain) is separated from the eastern group (Greece–Cyprus).
Similar patterns emerged regarding the life-history traits and recipient habitats.
The artificial habitats and the natural wet habitats are those that are invaded most and display the greatest homogenization in all four countries. Coastal habitats display a lower degree of homogenization but a high frequency of aliens. Dry shrubs and rocky habitats display a lower degree of homogenization and a low frequency of aliens.
Biological invasions, biotic homogenization, European Mediterranean countries, floristic similarity, invasibility, level of invasion, naturalized plants, plant traits.
I N T R O D U C T I O N
Biological invasions threaten the conservation of biodiversity and ecosystem integrity worldwide at various levels, owing to the erosion of gene pools, the local extinction of endemic species, and the alteration of habitat and ecosystem functions
(Hulme, 2007; Gaertner et al.
, 2009; Vila` et al.
, 2010). In order to mitigate the impact of biological invasions, it is important to identify both the traits that are common to the invasive species, if any, and the habitats most prone to invasion. Much

effort has been devoted to the quantification of the habitat level of invasion within individual countries. Some clear patterns emerge from these studies, the main one being that it is the anthropogenic habitats (those most affected by human activities) that exhibit the highest levels of plant invasion as a result of higher propagule pressure and higher disturbance levels (Chytry´ et al.
, 2005, 2008a). Another common pattern is that mesic habitats tend to host higher numbers of aliens, as these habitats have higher nutrient availability and are more disturbed (Stohlgren et al.
, 2003; Vila` et al.
, 2007). In order to assess the risk of habitat vulnerability to invasion, we need to identify the consistency of the levels of invasion between habitats across countries (Chytry´ et al.
, 2008b). Despite constituting the basis of invasion risk maps (Chytry´ et al.
,
2009), such information remains somewhat scarce (Sax et al.
,
2005). As a first step, comparisons between climatically similar countries would elucidate the role of human activities in promoting such invasions (Pauchard et al.
, 2004; Jime´ nez et al.
, 2008).
In Europe, the most recent comprehensive alien species database, provided by the DAISIE project (Delivering Alien
Invasive Species Inventories for Europe; http://www.europe- aliens.org), offers an effective means of analysing invasion levels across Europe (Pysˇek et al.
, 2009; Winter et al.
, 2009).
Lambdon et al. (2008b) have shown that the alien flora of
Europe may be divided into five major geographical assem- blages: a north-western assemblage, comprising Scandinavia and the UK; a west-central assemblage, extending from
Belgium and the Netherlands to Germany and Switzerland; a
Baltic assemblage; an east-central assemblage, comprising the remainder of central and eastern Europe; and a southern assemblage, covering all of Mediterranean Europe.
The Mediterranean Basin is recognized as a biodiversity hotspot because of the presence of exceptionally high numbers of rare and endemic plants, which account for as much as 50% of the species in some areas (Cowling et al.
, 1996; Blondel &
Me´ dail, 2009). Mediterranean habitats have been associated with human activities for thousands of years (Cody, 1986) and are being increasingly disturbed by urban and infrastructure development as the demands of tourism increase (Me´ dail &
Que´ zel, 1997). These land-use changes are accompanied by the introduction of alien plants mainly for ornamental and amenity purposes (Hulme, 2004; Lambdon et al.
, 2008a).
The invasibility of specific geographical regions in the
Mediterranean has been the focus of several studies (Vila` &
Mun et al.
,
2008a) that have compared the native and the alien floras of
Mediterranean islands. Floristic comparisons have also been made between the alien species in Mediterranean regions and those in central European regions (e.g. Chytry´ et al.
, 2009) and throughout Europe (Lambdon et al.
, 2008a).
In this work we compare, for the first time, the alien floras of four Mediterranean countries: Spain, Italy, Greece and
Cyprus. The countries were selected (1) to form an east–west gradient across the northern (European) Mediterranean Basin,
(2) to represent a wide range of Mediterranean ecosystems,
1812 and (3) on the basis of the reliability and comparability of the datasets compiled and harmonized through the DAISIE project
(see data sources section).
The aim of our study was to explore similarities in the taxonomy, species traits and invaded habitats between the naturalized alien species of these four countries in order to answer the following questions. (1) How similar is the composition of the naturalized flora? (2) What are the most common growth and life forms? (3) Which habitats display the highest invasion levels? (4) How homogeneous is the taxo- nomic composition across habitats both within a country and across countries?
MA T E R I A L S A N D M E T H O D S
Areas of study
Spain occupies 493,486 km
2 of the Iberian Peninsula. Its geography is characterized by two central plateaus crossed by three main rivers. These plateaus are surrounded by mountains and two main depressions in the north-east and in the south. It is a mountainous country, in which elevations over 1500 m a.s.l. are common and the highest peak rises to 3480 m a.s.l.
The climate in Spain consists of a heterogeneous mosaic consisting of three main climatic types: oceanic, mediterranean continental and mediterranean maritime (Ninyerola et al.
,
2005). Owing to its geographical, topographical, climatic and geological characteristics, Spain is characterized by a high degree of landscape diversity, as well as by natural ecosystems that are somewhat fragmented despite the presence of a large number of protected areas (Costa et al.
, 1990; Valladares et al.
,
2005). The population in Spain is currently concentrated in metropolitan areas (e.g. Madrid and Barcelona, with 5.6 and
4.6 million inhabitants, respectively) and coastal areas.
Italy covers an area of about 301,336 km
2
. The majority of the territory is covered by mountains (35%) or hills (42%).
The highest mountains reach an elevation of 4810 m a.s.l. in the Alps, which stretch across northern Italy. The peninsula is surrounded by various islands and archipelagos. Italy stands at the intersection of the Eurosiberian and Mediterranean biogeographical regions (Rivas-Martı´nez et al.
, 2001). This is reflected in a wide range of climates that range from temperate, in northern Italy and along the mountain ranges, to Mediterranean, along the coasts and on the islands (Blasi
& Michetti, 2007). The marked heterogeneity of the landscape determines the presence of 7634 species of vascular plants,
13.4% of which are non-native (Conti et al.
, 2005). Italy’s long history of human impact and its position at the centre of the Mediterranean has promoted the introduction and establishment of non-native plant species ever since ancient times. Italy has a population of 59.5 million people, with an average population density of 198 inhabitants per km
2
(ISTAT, 2001, 2008).
Greece occupies the southernmost part of the Balkan
Peninsula and covers an area of approximately 132,000 km
2
.
The insular part of the country occupies more than one-fifth of
Journal of Biogeography 37, 1811
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ª

its total area and comprises more than 2000 islands (only 227 of which are inhabited) (Strid & Tan, 1997). Eighty per cent of
Greece consists of mountains or hills, making it the third most mountainous country in Europe (Dax & Hovorka, 2005). The most extensive ecosystems are the Mediterranean maquis, phrygana and forests, which account for 40% of its overall area
(Arianoutsou & Diamantopoulos, 1985). Greece has the tenth longest coastline in the world (15,000 km total, 7300 of which are continental and 7700 on the islands). Greece has a population of about 11 million (2001 census data, National
Statistical Service of Greece, 2005). Approximately 90% of the population lives in the coastal region (population density is about 78 inhabitants per km 2 or 0.6 per km of coast). The population density in Greece is generally lower than in northern Europe (European Commission, 2005; United
Nations, 2005), but it is high in the Athens and Thessaloniki metropolitan areas, where half of the Greek population is concentrated.
Cyprus covers 9251 km
2
, with a coastline that is 778 km long. Its main topographical features are two mountain ranges,
Troodos in the southwest (1940 m a.s.l.) and Pentadaktylos in the north (1024 m a.s.l.), and Mesaoria, the sedimentary plain that formed between them. The bioclimate is mediterranean mesophytic to xerophytic oceanic, with zones ranging from thermo-mediterranean-semi-arid to supra-mediterranean-hu- mid. The largest continuums of natural vegetation occur in the mountain ranges, while the ecosystems in the lowlands and along the coasts are largely fragmented (Delipetrou et al.
,
2008). The population of Cyprus is slightly over a million people (110 inhabitants per km
2
), 70% of whom live in urban areas (Republic of Cyprus Statistical Service, 2009).
Data sources
The basic datasets used for the alien plants of Cyprus, Greece and
Italy (including all the islands within their political borders), and
Spain (including the Balearics but not the Canary Islands) were those compiled for the DAISIE database (Pysˇek et al.
, 2009, http://www.europe-aliens.org) and updated with more recent data for Greece (Arianoutsou et al.
, 2010) and Cyprus (P.
Delipetrou, M. Arianoutsou, K. Georghiou, unpublished data) and more recent national databases for Italy (Celesti-Grapow et al.
, 2009). In the DAISIE database, the presence of alien species was geographically assigned to NUTS 0 (Nomenclature of Territorial Units for Statistics – Statistical Regions of Europe, http://ec.europa.eu/eurostat/ramon/nuts) because a finer-scale resolution (e.g. UTM) was not available for most species and countries. Consequently, it was not possible to compare the island areas with the mainland areas. Only naturalized neo- phytes (plant species introduced after ad 1500), according to
Pysˇek et al. (2004), were used in the analysis. The Angiosperm
Phylogeny Group (APG; Stevens, 2001 onwards) system was adopted for classifying species into families.
The basic datasets of naturalized neophytes were scrutinized for taxonomical discrepancies and complemented with data concerning growth form (grass, herb, sub-shrub/shrub, tree
Journal of Biogeography 37, 1811
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ª and vine), life form and life cycle. Life forms were identified according to the system proposed by Raunkiaer (1934, 1937) basis of the following categories: chamaephytes, geophytes, hemicryptophytes, hydrophytes, phanerophytes and thero- phytes. As regards their life cycle, plants were classified as annual, biennial or perennial. The number of naturalized species was used as a measure of the ‘level of habitat invasion’ throughout the paper, following Lambdon et al. (2008b).
Species were assigned to the hosting habitat in the introduced country according to the EUNIS classification system, levels 1–3. The EUNIS Habitats Classification is a standard classification of European habitats developed by the
European Environment Agency (http://eunis.eea.europa.eu/ habitats.jsp). The EUNIS habitat codes were grouped into broad habitat categories (B, coastal habitats; C, inland waters; E, grasslands; F, forests; G, shrublands; H, rocks and screes; I, cultivations/plantations; J, constructed/inhab- ited) and divided into artificial (a) and natural/semi-natural
(n) habitats, and into dry (d) and wet (w) habitats (see
Appendix S1 in the Supporting Information). The term
‘artificial’ was used for anthropogenic habitats (e.g. archae- ological sites, EUNIS code X21). The term ‘semi-natural’ was used for habitats that have been profoundly altered by, or have evolved under, human influence, but sustain native species (e.g. regularly grazed heaths, EUNIS code F7.4). The term ‘disturbed’ was used to describe natural habitats that have been subject to human activities, such as gravel extraction, disposal of waste and trampling. Habitats depen- dent on any type of water body (including riparian habitats and water courses) were characterized as ‘wet’. The soil in wet habitats is typically inundated or permeated by water for at least a short period every year, the characteristic species having an Ellenberg indicator value of 7 or above (Ellenberg et al.
, 1992; Bo¨ hling et al.
, 2002).
Data analyses
Plant species were classified according to whether they were present exclusively in one country or shared by more than one country.
Differences in the patterns of life forms, growth forms and life cycles between the four countries were investigated by the nonparametric Kruskal–Wallis ANOVA by ranks test (3 degrees of freedom). The proportion of species per life history category (growth form, life form and life cycle) per country was the explanatory variable, while country was always the independent variable. Proportions, as opposed to raw num- bers, were used for the species data in the analysis to minimize the potential effect of large differences in species numbers between countries.
The species–area relationship for the naturalized species was investigated using the log–log and the semi-log model, and with or without square-root transformation of the number of species. The correlation between the number of naturalized species and native species was also tested. These analyses were
1813

performed using statistica v. 7 software (StatSoft Inc.,
2004).
Habitat data for each species were regrouped into: (1) 16 habitats-per-country categories corresponding to the four combinations of natural/artificial and dry/wet for the four countries, and (2) 28 habitats-per-country categories corre- sponding to the combinations of seven broad habitat groups
(B, C, E, FG, H, I, J, i.e. the eight broad habitat categories listed in Appendix S1, though with categories F and G united into category FG) for the four countries. Matrices of species presence/absence in each country and in each habitat-per- country category were generated. Cluster analysis of species in countries was performed by means of average linkage, using the Preston z similarity index. This index is not affected to any great extent by marked differences in the numbers of taxa (or in the areas) of the regions compared (Preston, 1962). The index ranges between 0 (highly similar regions) and 1
(completely different regions). primer 6.1.4 software (Clarke
& Gorley, 2006) was used for the similarity analysis of species in the habitat-per-country categories. A cluster analysis was performed first, followed by a non-metric multi-dimensional scaling (NMDS) ordination for the investigation of any spatial patterns of similarity.
R E S U L T S
Taxonomic richness and similarity
A total of 782 naturalized plant species were recorded in the four countries (Table 1; Appendix S2). These species belonged to 416 genera and 117 families. The taxonomic richness of naturalized species and genera was higher in Spain and Italy (species/genus ratios 1.6 and 1.8, respectively), and lower in Greece and Cyprus
(species/genus ratios 1.3 and 1.4, respectively).
The species–area relationship produced a high Pearson’s r
(0.75–0.76) in all cases; the correlation was not, however, significant at the 0.05 level and the regression analysis resulted in non-significant curves. Furthermore, the correlation be- tween the number of native species and the number of naturalized species in each country was positive, but not significant (Pearson’s r = 0.71).
At the species level, the number of shared species was markedly higher in Greece (75%) than in the other countries
(42–49%; Table 1). This discrepancy may be due to the relatively lower number of naturalized species reported in
Greece. The number of species common to Greece, Italy and
Spain (67) was higher than the number of species common to
Cyprus, Italy and Spain (41); Cyprus, Greece and Spain (33); or Cyprus, Greece and Italy (33). The number of taxa shared by two pairs of countries as well as their percentages are shown in Table 2. Only 30 of the 782 naturalized plant species were present in all four countries (Table 3).
Asteraceae and Poaceae were the families with the highest number of naturalized species in the four countries (17, 13, 54 and 55 species of the Asteraceae family and 13, 17, 50 and 55 species of the Poaceae family, respectively, in Cyprus, Greece,
Italy and Spain – see Appendix S2). Amaranthaceae and
Fabaceae yielded the next highest numbers of naturalized species (14, 16, 18, 19 for the Amaranthaceae and 16, 5, 16, 24 for the Fabaceae, respectively, in Cyprus, Greece, Italy and
Spain – see Appendix S2).
At the genus level, Amaranthus was the genus with the highest diversity in the four countries, with 9, 11, 11 and 8 species in
Cyprus, Greece, Italy and Spain, respectively. The second most diverse genus in the four countries was Conyza , with 3 species in each of Cyprus, Greece and Italy and 4 in Spain. Oenothera had the highest species number in the western part of the biogeo- graphical gradient, with 13 and 8 species in Italy and Spain, respectively. Similarly, Solanum had the highest overall species numbers, with, respectively, 10 and 11 species in Italy and Spain, though only 3 and 2 in Greece and Cyprus (Appendix S2).
The application of the Preston z index of similarity to the floristic data (presence/absence of species) for the four countries yielded high dissimilarity values between the four countries. Clustering separated Cyprus from the other three countries, and Greece from Italy and Spain (Fig. 1). The lowest dissimilarity value was between Italy and Spain.
Species life-history traits
Most naturalized species in the four countries were herbs (over
40%), followed by shrubs, trees and grasses (Table 4). There
Table 1 Total number of naturalized neophytes per country for each of the four Mediterranean countries studied (a) and totalled for all the countries studied (c); and number of shared naturalized neophytes (plants occurring in more than one of the four countries) per country (b).
(a) Total per country (b) Shared (c) Total
Species
Families
Genera
Species
Cyprus Greece Italy
51
100
127
47
92
125
96
262
463
Spain Cyprus Greece Italy Spain All
87 45 (88)* 46 (98) 79 (82) 78 (90) 117
243 65 (65) 80 (87) 154 (59) 153 (63) 416
398 63 (50) 94 (75) 198 (43) 193 (48) 782
Native species 1612
1
Area km
2
5855
2
6611
3
7920
4
9251 132,700 293,805 491,310
1
Hadjikyriakou (1997);
2
Strid & Tan (1997), Tan & Iatrou (2001);
3
Conti et al. (2005);
4
Bueno et al. (1995).
*Numbers in brackets are the percentages of shared families, genera and species.
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Table 2 Naturalized neophyte taxa shared by all pairs of the four
Mediterranean countries studied.
Number (%)* of common taxa
Cyprus Greece Italy Spain
Italy 49 (39)
Cyprus 127 (100) 33 (26) 49 (11)
Greece 33 (26) 125 (100) 81 (17)
50 (13)
75 (19)
81 (65) 463 (100) 170 (45)
Spain 50 (39) 75 (60) 179 (39) 398 (100)
*Numbers in brackets are percentages of common taxa in the total number of alien species of each country.
Table 3 Naturalized plant species shared by Spain, Italy, Greece and Cyprus.
Species Family
Agave americana
Ailanthus altissima
Amaranthus albus
Amaranthus blitoides
Amaranthus hybridus
Amaranthus retroflexus
Amaranthus viridis
Aptenia cordifolia
Asclepias fruticosa
Aster squamatus
Carpobrotus edulis
Chenopodium ambrosioides
Conyza bonariensis
Conyza canadensis
Conyza sumatrensis
Cuscuta campestris
Datura stramonium
Echinochloa colona
Eucalyptus camaldulensis
Mirabilis jalapa
Nicotiana glauca
Oxalis pes-caprae
Paspalum distichum
Phalaris canariensis
Phytolacca americana
Robinia pseudoacacia
Solanum elaeagnifolium
Tagetes minuta
Xanthium strumarium subsp. italicum
Xanthium spinosum
Agavaceae
Simaroubaceae
Amaranthaceae
Amaranthaceae
Amaranthaceae
Amaranthaceae
Amaranthaceae
Aizoaceae
Apocynaceae
Asteraceae
Aizoaceae
Amaranthaceae
Asteraceae
Asteraceae
Asteraceae
Convolvulaceae
Solanaceae
Poaceae
Myrtaceae
Nyctaginaceae
Solanaceae
Oxalidaceae
Poaceae
Poaceae
Phytolaccaceae
Fabaceae
Solanaceae
Asteraceae
Asteraceae
Asteraceae was no significant difference between the four countries in the growth form patterns of the naturalized species (a = 0.05, d.f. = 3)
As regards the life forms, therophytes were the most prominent group in the four countries (29–46%), followed by phanerophytes and hemicryptophytes (Table 5). Hydro- phytes were the least represented form, especially in Cyprus and Greece. There were no significant differences between the four countries in the life form patterns (a = 0.05, d.f. = 3).
Figure 1 Floristic similarity between the four Mediterranean
Basin countries studied on the basis of the naturalized plant species. SP, Spain; IT, Italy; GR, Greece; CY, Cyprus.
Table 4 Number of naturalized plant species (percentages in brackets) classified according to growth forms in each of the four countries of the Mediterranean Basin studied.
Number (%) of naturalized plant species
Growth form Cyprus Greece Italy Spain All*
Grass
Herb
13 (10) 17 (13) 48 (11) 60 (15) 96 (13)
55 (42) 75 (60) 253 (56) 196 (49) 402 (52)
Subshrub/Shrub 27 (21) 19 (15) 100 (22) 72 (18) 159 (21)
Tree 31 (24) 9 (7) 37 (8) 49 (12) 88 (11)
Vine 1 (1) 5 (4) 10 (2) 21 (5) 22 (3)
*Total number of naturalized plant species for all four countries.
Naturalized species in all four countries were prevalently perennial plants (50–70%) and secondarily annuals (Table 6), while there was no significant difference between the four countries in the life cycle patterns (a = 0.05, d.f. = 3).
Habitat level of invasion
The frequency of naturalized species was highest in artificial habitats (Table 7), particularly in road networks (Jro), culti- vated areas (I1, I2, IFG) and constructions in urban or rural areas (J, Jwa, Jwp). However, a significant number of naturalized species also occurred in natural habitats, especially those in coastal areas and inland waters (B, C). Indeed, wet habitats, from dune wetlands (B1) and saline marshes (B3) to riparian zone grasslands (C), were the natural habitats with the highest frequency of naturalized species.
Journal of Biogeography 37, 1811
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Table 5 Number of naturalized plant species (percentages in brackets) classified according to life form in each of the four Mediterranean countries studied.
Number (%) of naturalized plant species
Life form Cyprus Greece Italy Spain All*
Therophytes and Therophytes/Hemicryptophytes
Geophytes
Hemicryptophytes
Hydrophytes
Chamaephytes and Chamaephytes/Hemicryptophytes
Phanerophytes
45 (35)
12 (9)
12 (9)
0 (0)
11 (9)
47 (37)
58 (46)
16 (13)
17 (14)
3 (2)
6 (5)
25 (20)
129 (29)
48 (11)
91 (21)
29 (7)
17 (4)
125 (28)
115 (29)
38 (10)
82 (21)
25 (6)
34 (9)
104 (26)
223 (29)
76 (10)
155 (20)
42 (6)
51 (7)
211 (28)
*Total number of naturalized plant species for all four countries.
Table 6 Number of naturalized plant species (percentages in brackets) classified according to life cycle in each of the four
Mediterranean countries studied.
Number (%) of naturalized plant species
Life cycle Cyprus Greece Italy Spain All*
Annual 45 (35) 59 (47) 131 (29) 119 (29) 221 (29)
Annual/perennial and 0 (0) 1 (1) 3 (1) 1 (0) 9 (1) annual/biennial
Biennial 0 (0) 1 (1) 12 (3) 1 (0)
Biennial/perennial 0 (0) 1 (1) 1 (0) 0 (0) 4 (1)
Perennial
9 (1)
82 (65) 63 (50) 301 (67) 280 (70) 524 (68)
*Total number of naturalized plant species for all four countries.
The NMDS ordination performed on the similarity analysis in wet versus dry and natural versus artificial habitats-per- country categories revealed three main groups (Fig. 2). The two first (which appear at the top left side of Fig. 2) consist of species recorded in the habitat categories of Greece and
Cyprus. The third group (which appears in the lower part of
Fig. 2) is formed by the habitat categories of Italy and Spain.
The remaining entries do not form any coherent group. Not only did the habitat categories of Cyprus not display any clear pattern, but the observed similarity between habitats was much lower than that of Greece (the distance between the objects was higher). Within the second group, the dry habitats of Greece hosted more similar species than the wet habitats, regardless of whether they were natural or artificial. Although Greece and
Cyprus are located near each other in the ordination space, the similarity between them is low, as indicated by different contours in Fig. 2. An even lower similarity was found within the third group, which distinguished Italy and Spain. The grouping of dry habitat categories observed in Greece was also noticed in Italy, where artificial dry habitats were grouped at a
25% level of similarity with natural wet ones. None of the habitat categories of Spain was grouped with any other habitat category, the only exception being natural wet habitats, which were ordinated close to the cluster of the natural wet habitats in Italy. All artificial wet habitats were ordinated separately from the previous groups and apart from each other, which is indicative of the high dissimilarity of the plant species assemblages they host, both within the group itself and when compared with the other groups. The largest within-country distances, which indicate greater differences in species assem- blages within each category, were observed for Spain and Italy, especially for the former. By contrast, distances were much smaller for Greece and Cyprus for all the categories with the exception of the artificial wet habitats. This indicates a higher similarity of species assemblages within different habitat groups.
The same analysis was performed on habitats classified in broad vegetation categories. The results are shown in Fig. 3.
Four distinct and partially overlapping groups were clearly distinguished and placed in the centre of the ordination space.
Starting from the bottom, the habitat groups of Cyprus, with the exception of grasslands and rocks and screes, were clustered together at the 15% level of similarity. However, objects within this group, particularly coastal habitats and forests and shrubs, were placed some distance from one another. The second group included all the Greek habitat types except rocks and screes. This group was characterized by relatively small distances between objects, and overlapped with the first group (it included the constructed and cultivated habitats of Cyprus). The third group, which overlapped partially with the second and fourth groups, consisted of the constructed and cultivated habitats of Spain and Italy. The fourth group consisted of the cultivated, constructed and fresh-water habitats of Italy. Standing alone, and relatively far from the core clusters, were the rock and screes and coastal habitats of Spain and Italy, the grasslands of Italy, the fresh- water habitats of Spain, and the rocks and screes of Cyprus.
Overall, in the vegetation category habitat grouping, as in the natural/artificial-dry/wet grouping, the species assemblages across habitat groups were most homogeneous in Greece, fairly homogeneous in Cyprus, and less homogeneous in Italy and
Spain.
D I S C U S S I O N
The comparison of the naturalized floras of four countries in the Mediterranean Basin (Spain, Italy, Greece and Cyprus)
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Table 7 Number and percentages (in brackets) of naturalized plant species in each habitat group.
Number (%) of naturalized plant species
Habitat group
Cyprus
Dry
Greece
Wet Dry
Italy
Wet Dry
Spain
Wet Dry Wet
Natural habitats
B1
B2
B3
Total
Coastal sand and shingle
Coastal rocks
Coastal wetland
8 (6)
4 (3)
3 (2) 38 (18)
12 (6)
7 (3) 27 (9)
8 (3)
3 (2) 11 (5)
C
E
F1
F2
F3
F4
G1
Inland water bodies – fens, bogs
Grassland
Shrub – maquis
Shrub – garrigue
Shrub – phrygana
Shrub – riparian
Forest – riparian
41 (33) 50 (23)
1 (1) 9 (7) 12 (6) 13 (6) 5 (2)
7 (6)
10 (8)
2 (2)
G2 Forest deciduous
G3 Forest – broadleaved evergreen 3 (2)
G4 Forest – coniferous
H Rocks/screes
18 (14)
4 (3)
8 (4)
5 (2)
5 (2)
1 (1)
11 (9)
2 (1)
4 (2)
5 (2)
11 (5)
12 (6)
18 (8)
6 (2)
1 (0)
13 (5)
40 (11)
8 (2)
3 (1)
41 (11)
187 (65)
5 (1)
21 (5)
14 (4) 9 (2)
3 (1)
85 (23)
68 (18) 41 (14)
16 (4)
16 (4)
15 (4)
25 (7)
57 (46) 68 (54) 103 (48) 111 (52) 60 (21) 228 (79) 150 (39) 227 (61)
Artificial habitats
E1 Grassland (ruderal, disturbed)
I1 Cultivations
I2 Gardens/parks
IFG Woody cultivations
IG Tree plantation
J Constructed/inhabited
Jro Road networks
Jwa Walls
Jwp Waste places
4 (1) 26 (8) 22 (2) 1 (0) 163 (28)
68 (23)
47 (16)
42 (14)
9 (3)
39 (13) 13 (4) 36 (11) 11 (3) 541 (51) 40 (4) 22 (4)
69 (23)
5 (2)
5 (2)
62 (18)
33 (10)
34 (10)
68 (20)
35 (10)
29 (9)
167 (16)
121 (11)
8 (1)
22 (2)
136 (13)
51 (9)
27 (5)
2 (0)
131 (23)
16 (3)
138 (24)
Total 288 (96) 13 (4) 322 (96) 13 (4) 1017 (96) 41 (4) 550 (95) 28 (5)
Percentages are calculated on the basis of the total number of occurrences in natural or artificial habitats and in wet or dry habitats. A species may occur in more than one habitat group. Symbols in habitat groups correspond to the EUNIS classification system level 1.
Figure 2 Diagram of the two-dimensional non-metric multi-dimensional scaling ordi- nation performed on the naturalized plant species observed across the habitat categories identified in the four Mediterranean coun- tries studied, pooled into four groups: arti- ficial (a), natural (n), dry (d), wet (w). SP,
Spain; IT, Italy; GR, Greece; CY,
Cyprus. Contours indicate habitats that are similar at the 25% similarity level.
Journal of Biogeography 37, 1811
–1823
ª
1817

Figure 3 Diagram of the two-dimensional non-metric multi-dimensional scaling ordi- nation performed on the naturalized plant species observed across the habitat categories identified in the four Mediterranean coun- tries studied, according to the EUNIS classi- fication system level 1, pooled into the following groups: B, coastal habitats; C, inland waters; E, grasslands; FG, forests and shrublands; H, rocks and screes; I, arable land; J, constructions. SP, Spain; IT,
Italy; GR, Greece; CY, Cyprus. Contours indicate habitats that are similar at the 15% similarity level.
revealed six major trends: (1) taxonomic similarity between the four countries was low, the greatest similarity being observed between Spain and Italy, while Cyprus displayed the least similarity with the other countries; (2) the majority of the naturalized species were herbaceous perennials; (3) despite differences in the floristic composition, the life-history trait patterns of the naturalized neophytes were similar in all four countries; (4) artificial habitats were the most frequent recipients of aliens among all the habitat categories identified, while the wetlands were the most frequent recipients of aliens among the natural habitat categories; (5) the naturalized species distribution in habitat categories displayed a certain degree of homogeneity, which was, however, higher for Greece and Cyprus and lower for Italy and Spain; and (6) the east– west divide observed in the taxonomic similarity patterns also emerged when the alien plants were clustered according to the level of invasion of the recipient habitats.
As the aim of this study was to compare the trends of naturalized alien species in Mediterranean countries rather than the determinants of their numbers, any differences in alien species richness between the countries studied were not explored in detail. Other studies (Palmer, 2006; Lambdon et al.
, 2008b) have demonstrated that area affects alien species richness and exhibits a log–linear relationship, without, however, being the sole explanatory factor. The lack of a significant alien species–area relationship between the four countries in this study may be due to the small number of countries considered and the reported weaker effect of area, compared with other factors, on alien species richness
(Lambdon et al.
, 2008b). The apparently high number of alien species reported for Cyprus, which is a small country, may be due to the extensive plantations and species introductions to which Cyprus was subjected during the 19th century (Thir- good, 1987). By contrast, the relatively low number of aliens in
Greece may be due to the fact that this country is more mountainous than the other three countries and less urbanized
1818 than either Italy or Spain. Indeed, a negative correlation between the level of invasion at the regional scale and high elevation (e.g. Pauchard & Alaback, 2004; Becker et al.
, 2005;
Chytry´ et al.
, 2005, 2008a; Mallen-Cooper & Pickering, 2008;
Gasso´ et al.
, 2009; Marini et al.
, 2009) and a positive corre- lation between level of invasion and high degree of urbaniza- tion (e.g. Chytry´ et al.
, 2005, 2008a,b; Sa´ dlo et al.
, 2007; Gasso´ et al.
, 2009) are widely acknowledged trends in the literature.
Although the Mediterranean Basin has been proposed by
Lambdon et al. (2008b) as one biogeographical zone that may be considered quite distinct from the other European zones in view of the homogenization of the naturalized plant species within the latter (figure 7 in Lambdon et al.
, 2008b), it should be borne in mind that the Mediterranean countries are placed along an east–west gradient as well as a precipitation gradient.
Our study confirms this trend, with the western part being represented by Spain and Italy and the eastern part being represented by Greece and Cyprus.
Historical and geographical reasons may be called upon to explain our findings. The history of introduction, as opposed to the pathways of introduction, may explain why the similarity analysis for most traits paired Italy with Spain and separated Greece and Cyprus. Spain and Italy might share more species because of the similar origin of introduction.
During the Spanish domination of southern Italy (through most of the 17th century) and even later, many American
(especially South American) species were introduced into Italy through Spain owing to the intense trade between the two countries (Saccardo, 1909). By contrast, relations between Italy and Greece, especially those of a commercial nature, date back much further, the Ancient Greeks having introduced a number of species into Italy, especially when they colonized southern
Italy. These relations thus increased the floristic similarity between the two countries, although this similarity does not emerge in the current study because these species are regarded as native in Greece and as archaeophytes in Italy.
Journal of Biogeography 37, 1811
–1823
ª

Although the majority of the alien plant species of Greece and Cyprus are of American origin (Georgiadis, 1994;
Arianoutsou et al.
, 2007; Bazos et al.
, 2009), these countries also host a high percentage of alien species of Asiatic origin
( c . 17 and 20% in Greece and Cyprus, respectively), which might explain the considerable floristic similarity between the two countries. The fact that these two eastern European countries were under Turkish occupation for several centuries
(Greece from the 15th to 19th centuries, and Cyprus from the
16th to 19th centuries) greatly affected their economy and development, and possibly even species interchange. Species introductions for agriculture, forestry and amenity in Cyprus were also influenced by British rule, which ended in the mid-
20th century (Jenness, 1962; Thirgood, 1987). It is also known that cereals introduced into Greece for cultivation much earlier than this period originated mainly in Asia. Therefore historical movements in human migration and events that affected the origin of alien flora may explain the similarity patterns we observed. In addition, differences in the geography and climate of the countries studied may further explain any differences in the alien flora they host. Italy and Spain are larger countries that have a higher biogeographical heterogeneity than either
Greece or Cyprus; they also include areas with a continental and alpine bioclimate, which increase the diversity of the alien flora (e.g. Celesti-Grapow et al.
, 2010).
It has been suggested (Richardson & Pysˇek, 2006) that a high representation of a plant trait on a broad biogeographical scale is linked to the availability of species with the same trait in the pool of species entering a region, as opposed to this trait having an advantage over the native species. This appears to be true, at least for Greece and Cyprus. Phanerophytes and hemicryptophytes account for 25 and 17%, respectively, of the naturalized flora of Greece, while in the native flora phanero- phytes rarely exceed 10% and hemicryptophytes range from 15 to 65% depending on their elevation (Bazos, 2005). In Cyprus, the percentage of therophytes is similar in the naturalized
(45%) and the native flora (45%), whereas the percentage of phanerophytes is 40% higher in the naturalized flora. This finding is ascribable to the introduction of numerous alien trees and shrubs in the 19th century, both for forestry activities and as ornamentals (Madon, 1881; Thirgood, 1987). Although attributes such as growth form and life cycle have failed to explain invasion success (Lloret et al.
, 2005), introduced species may tend to be larger than native plants and consist above all of trees and shrubs, owing to fact that such plants are more frequently introduced intentionally (Crawley et al.
,
1996).
Differences between the life form spectra of the naturalized and native plant species life forms may underpin the specific introduction pathway of the aliens. The overrepresentation of therophytes in the naturalized flora may be due to the fact that many therophytes have been accidentally introduced as seed contaminants. Seed contaminants are often weeds, a high proportion of which are therophytes that invade ruderal and anthropic habitats. Preliminary data on introduction pathways for naturalized neophytes in Greece suggest that most annual
Journal of Biogeography 37, 1811
–1823
ª species are introduced unintentionally (75%), while a sub- stantial proportion are seed contaminants (25%). Although the majority of therophytes may not be seed contaminants, they do represent the largest group of seed contaminants. The prom- inence of therophytes in the naturalized flora of all four countries highlights the fact that the distinct local annual weed flora, often including species whose distribution is limited in these countries, may be a natural element that increases the risk of invasions (Blondel & Aronson, 1999). Evidence of this risk may be found in the low, though not insignificant, proportion of aliens in dry grasslands (group E), particularly in
Greece and Spain, and the high proportion of aliens in anthropogenic grasslands (group E1) in Spain (Table 7).
Wetlands are apparently subject to higher levels of invasion than areas that are subject to the Mediterranean summer drought, especially in Italy and Spain, where there are more and larger water bodies. The ratio between alien naturalized plant species found in wet natural habitats and those found in dry natural habitats is always higher than 1 in all the countries except Greece (1.4, 2.5 and 1.5 for Cyprus, Italy and Spain, respectively). This may be explained by the more abundant resources available in such environments, and hence the lower competition and lower environmental stress (due to the absence of drought), as well as by the fact that the majority of these habitats are highly disturbed, rivers being one example
(Planty-Tabacchi et al.
, 1996; Alpert et al.
, 2000; Stohlgren et al.
, 2003); indeed, river corridors have been suggested as major conduits for alien species (Foxcroft et al.
, 2006).
The alien plants that have established in coastal habitats and inland surface waters in Greece are reported to exceed by far the numbers of alien plants recorded in all other natural habitats (Arianoutsou et al.
, 2007, 2010). Similarly, the inspection of approximately 1300 releve´ es from Cyprus revealed a higher frequency of alien species in riparian forests, water fringe vegetation and inland halophytic communities (P.
Delipetrou, C.S. Christodoulou, T. Kyriakou, M. Andreou, unpublished data). Moreover, alien plants in Cyprus are more frequent in irrigated cultivations and host higher numbers of aliens (Georgiadis, 1994). All these findings suggest that in
Mediterranean regions, where water stress is a common feature of the climate, wetlands are more susceptible to invasions.
The level of invasion of habitats has often been viewed exclusively as an ecosystem attribute (e.g. Crawley, 1987;
Rejma´ nek, 1999; Chytry´ et al.
, 2008b). It is, however, becom- ing increasingly clear that geographical and, above all, socio- economic drivers play a major role in the habitat levels of invasion as they result in spatial differences in propagule pressure (Vila` & Pujadas, 2001; Gasso´ et al.
, 2009; Rodrı´guez-
Labajos et al.
, 2009). Our study suggests that historical factors may also influence the similarities and differences in the alien flora of the four Mediterranean Basin countries we studied.
Increased taxonomic similarity, which is often used as an aspect that equates most obviously with floristic homogeniza- tion (McKinney, 2004; Olden et al.
, 2004; Ku
Castro et al.
, 2007; Winter et al.
, 2009), is observed within the two pairs of countries: Spain–Italy and Greece–Cyprus. The
1819

lower level of floristic homogenization across the habitat groups of Spain and Italy may, at least in part, be explained by the wider range of biogeographical conditions in these two countries (the Continental and Alpine regions in Italy, and the
Atlantic and Alpine regions in Spain) if compared with Greece and Cyprus. Nevertheless, similar invasion patterns (higher numbers of aliens in disturbed habitats and lower numbers in extreme habitats, along with a higher frequency of neophytes in coastal, littoral and riverine habitats) have been observed in the Mediterranean, Oceanic and Subcontinental regions of
Europe, despite marked differences in the floristic composition
(Chytry´ et al.
, 2008b).
In conclusion, certain similarities do emerge in the patterns of habitat invasion in the four countries studied here, with the most frequent recipient habitats being characterized by a higher degree of human disturbance regime and greater water availability.
A C K N O W L E D G E M E N T S
Most of the data compilation and harmonization took place within the framework of the European DAISIE Project
(SSPI-CT-2003-511202) of the 6th EU Framework Program for Research, Technological Development and Demon- stration (DAISIE, 2009; http://www.europe-aliens.org). This study was partially funded by the Spanish Ministerio de
Ciencia e Innovacio´ n projects MONTES (CSD2008-00040) and RIXFUTUR (CGL2009-7515), and the Junta de Andalucı´a
RNM-4031 to M.V. The following people contributed to the preparation of the initial checklists for Greece and Cyprus: P.
Andriopoulos, A. Yannitsaros, A. Zikos, E. Papacharalambous and K. Georghiou. David Richardson, handling editor of the paper, is gratefully acknowledged for his comments on the final revision of the manuscript. We thank the two anonymous reviewers for their comments on an earlier version of this work, F. Pretto and E. Carli for their contribution to the preparation of the Italian dataset, D. Danielidis and S. Spathari for their statistical advice, and Lewis Baker for the English editing.
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S U P P O R T I N G I N F O R M A T I O N
Additional Supporting Information may be found in the online version of this article:
Appendix S1 EUNIS habitat classification.
Appendix S2 Full list of the naturalized alien plants of the four Mediterranean countries studied.
As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer-reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.
B I O S K E T C H
Margarita Arianoutsou has a PhD in plant ecology awarded by the Aristotelian University of Thessaloniki. Currently she directs the terrestrial ecology research group of the National and Kapodistrian University of Athens (http://uaeco.biol.uoa.gr). The members of the research team whose names are on the paper participated in compiling the alien plant database in the European DAISIE
(Delivering Alien Invasive Species Inventories for Europe) project (http://www.europe-aliens.org).
Author contributions: M.A. and M.V. conceived the idea; M.A. analysed the data and was responsible for writing the paper; M.V. contributed to writing the paper and provided the data for Spain. P.D. collected the data for Cyprus, compiled and harmonized the data set and contributed to writing the paper. L.C. and C.Bl. compiled the data for Italy and commented on the text. C.Ba. contributed to the data compilation for Spain. I.B. and Y.K. compiled the data for Greece, and both commented on the text.
Editor: David Richardson
Correction added after online publication: the volume number in the Vila` et al. 2010 reference was changed from ‘3’ to ‘8’ in May
2010.
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