Figure 1.
Location of the studied cities in Central Europe.
Figure 2.
Sample-based rarefaction curves showing an increase in the cumulative number of native and alien land-snail species recorded in 32 Central European cities with increasing number of cities sampled.
Dashed lines indicate 95% confidence intervals.
Figure 3.
Variation in numbers of native (left) and alien (right) land-snail species among the studied habitat types.
Different letters at the top indicate significant differences between habitats based on generalized estimating equations with a Poisson error structure: X42 = 186.4, p << 0.001 for native and X42 = 86.2, p << 0.001 for alien species; individual categories of habitats significantly differed among each other at p < 0.02. The central line of each box refers to the median value, box height to the interquartile range, whiskers to the non-outlier range (i.e. 1.5 times the interquartile range at each side), and small circles to outliers.
Figure 4.
Numbers of positive (homogenization) and negative (diversification) values of the homogenization index resulting from pairwise comparisons among 32 cities (left), and variation in values of the homogenization index among the studied habitat types, showing the homogenization (hom) effect of alien species on species composition similarity.
Differences between Jaccard similarities based on all and native species were tested using a paired Wilcoxon test (***, p < 0.001). For explanation of box-and-whisker plots see Figure 3.
Figure 5.
Numbers of positive (homogenization) and negative (diversification) values of the homogenization index resulting from pairwise comparisons among 32 cities and calculated separately for seven types of urban habitats.
Only those plots that harboured four or more species were used, which implies a different number of comparisons for each habitat type: square (4% of all 496 possible pairwise comparisons were considered), boulevard (56%), residential area with open building pattern (100%), residential area with compact building pattern (100%), park (94%), early-successional site (30%), mid-successional site (94%).
Figure 6.
Variation in the values of homogenization index among the studied habitat types showing the diversification (div) or homogenization (hom) effect of alien species on species composition similarity.
Differences between Jaccard similarities based on all and native species were tested using paired Wilcoxon test; significance: **, p < 0.01; ***, p < 0.001; n.s., not significant. div = diversification, hom = homogenization. For numbers of pairwise comparisons for each habitat type see Figure 5.