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Plant invasions on small Mediterranean islands 1121

                                                                substratum varies considerably (i.e. volcanic, sedi-
                                                                mentary, metamorphic) and many of them display
                                                                marked topographic variations, with mountains, the
                                                                highest of which is found on the Island of Elba and
                                                                rises to 1,050 metres, and prevalently rocky shores.
                                                                They all belong to the Mediterranean climatic region
                                                                (Blasi et  al.  2014), although  there are some  varia-
                                                                tions between the islands in temperature and annual
                                                                rainfall along the north–south gradient, as their lati-
                                                                tude ranges from 35° 29′ N to 44° 2′ N.
                                                                   All the islands investigated have been subjected,
                                                                albeit to varying degrees of intensity, to the impact of
                                                                man since ancient times, mainly in the forms of forest
                                                                exploitation, wildfires, introduction of domestic
                                                                herbivores, agriculture and, more recently, tourism,
                                                                with the consequent expansion of settlements,
                                                                infrastructures and communication routes. Most
                                                                are  currently  permanently  inhabited, though  some
                                                                have been used exclusively as prisons, military areas
                                                                or  nature  reserves,  thereby  limiting  human  access.
                                                                Eight of these islands, i.e. Palmaria which lies off
                                                                the Cinque Terre coast, and Alicudi, Filicudi, Lipari,
                                                                Panarea, Salina, Stromboli and  Vulcano in the
                                                                Aeolian  archipelago  to  the  north  of  Sicily  (Figure
                                                                1), are included in the UNESCO World Heritage
                                                                sites on account of their landscape and natural
              Figure 1. Location of the 37 islands analysed. 1 Palmaria; Liguria.   peculiarities (WHC  2016). Further details on the
              2 Gorgona, 3 Capraia, 4 Elba, 5 Pianosa, 6 Montecristo, 7 Giglio,
              8 Giannutri;  Tuscany. 9 Zannone, 10 Palmarola, 11 Ponza,    islands’ characteristics can be found in Ricciardi
              12 Ventotene, 13 Santo Stefano; Lazio. 14 Procida, 15 Ischia,    (1998), Ricciardi et al. (2004), Domina and Mazzola
              16 Capri; Campania. 17 La Maddalena, 18 Caprera, 19 Asinara,   (2008, 2011), Pretto et al. (2010, 2012), and Lazzaro
              20 Isola Piana, 21 Tavolara, 22 Molara, 23 San Pietro; Sardinia.
              24 Ustica, 25 Stromboli, 26 Panarea, 27  Alicudi, 28 Filicudi,   et al. (2014a, 2016).
              29 Salina, 30 Lipari, 31 Vulcano, 32 Marettimo, 33 Levanzo,    The information on the non-native vascular flora
              34 Favignana, 35 Pantelleria, 36 Linosa, 37 Lampedusa; Sicily.
                                                                was compiled by a team of botanists from each of
                                                                the 6 Italian administrative regions to which the 37
                                                                islands belong (Liguria, Tuscany, Lazio, Campania,
              Study area and methods
                                                                Sardinia and Sicily; Figure 1), following a consoli-
              We analysed the same islands as those selected by   dated tradition in Italy of compiling floristic inven-
              Pretto et al. (2012) so as to be able to detect the most   tories by integrating data collected by local research
              recent changes in their introduced flora. The study   groups at the regional scale. The network largely in-
              area therefore included 37 small islands located   volved the same researchers as those who provided
              in the middle of the Mediterranean sea, off the   data on the flora of these islands during the previous
              Tyrrhenian Coast of Italy and around Sicily and   survey (Celesti-Grapow et  al.  2010b; Pretto et  al.
              Sardinia (Figure 1). As small islands were defined as   2012). To allow the data collected during this survey
              islands whose area covers less than 10,000 km  and   to be compared with those from the previous survey,
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              that have a population of less than 500,000 inhab-  we used the taxonomy scheme adopted in the lat-
              itants, Sicily and Sardinia were excluded from the   est general inventory of the non-native flora of Italy
              data-set of Pretto et al. (2012) on account of both   (Celesti-Grapow et al. 2009), although in some cases
              their size (25,707 km  and 24,090 km , respectively)   applying the most recent nomenclature (Euro+ Med
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              and population (5,074,261 and 1,658,138, respec-  2006).
              tively) (ISTAT  2016). Several other islands were    The information on the distribution and the
              also excluded from the data-set for specific reasons.   establishment status of each species on every island
              Sant’Antioco, for instance, the fourth largest island   was updated and re-assessed using the recently
              in Italy after Sicily, Sardinia and Elba, was excluded   published literature and new field surveys (Arrigoni
              because it is connected to Sardinia by a bridge. The   & Viegi 2011; Domina et al. 2012; Stinca & Motti
              size of the 37 islands varies from 0.31 to 224.20 km    2013; Stinca et  al.  2013,  2016; Ardenghi  et  al.
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              and their resident population varies from 0 to 56,105   2014; Lazzaro et  al.  2014a; Del Guacchio  2015;
              inhabitants (Pretto et  al.  2012). Their  lithological   Del  Guacchio  & Vallariello  2015; Camarda  et  al.
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