Different letters at the top indicate significant differences between habitats based on generalized estimating equations with a Poisson error structure: X^{4}_{2} = 186.4, p << 0.001 for native and X^{4}_{2} = 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.