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2566                                                                             F. Pretto et al.


          native species introduced into Italy since 1500 AD  Determinants of non-native species richness
          (Celesti-Grapow et al. 2009). According to their
          current establishment status, 49 taxa were classified  The number of total non-native and established non-
          as casuals, while the remaining 105 are established.  native species on each island was found to vary
          This last group of 105 established taxa contains only  greatly, ranging from zero to 56 species and from zero
          27 taxa that are currently invasive in at least one of the  to 45 species, respectively.
          37 islands, thus confirming that only a small subset of  The regression tree for total non-native species
          naturalized species have the capacity to spread over  richness indicated that the two most important
          considerable distances from the site in which they  predictor variables were related to human activities
          were introduced (Pys ˇek et al. 2004). The majority of  (Fig. 2a). The first split clearly separated the two
          the 154 non-native species recorded (99, 64.3 %) have  islands subjected to the highest degree of tourist
          been introduced as ornamental plants.           pressure (Capri and Ischia) according to the number of



             A




















             B


















          Fig. 2 a,b Regression tree models showing the determinants of  respectively; the cross-validated error was 0.41862 and 0.41862
          residuals between the number of non-native species (a) and the  respectively. Artificial areas = percentage of the area of
          number of established non-native species (b) and area on the 37  each island occupied by artificial surfaces; Closest island
          small Italian islands. Each node is characterized by the split  distance = distance from nearest island; Tourism = number
          variable and its split value, the response variable mean (Avg)  of ferries that reach each island on the same day at the height
          and the number of islands assigned to that node (N). The tree  of summer (15 July); Volcanic = percentage of volcanic
          explained 78.18 and 75.52 % of the variation in the response  substratum
          variable of the non-native and established non-native species,




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