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appropriateness of the models was checked by plotting standardized residuals
against fitted values, and by normal probability plots.
Path analysis [45] was used to explore the inter-relationship between the
species distribution, MRT, and the rate of spread. This enabled an assessment
of relative direct and indirect effects by which the MRT contributed to the cur-
rent distribution of alien species through the rate of spread. An appropriate
path model was suggested by the regression analysis of the species distribution
measures, MRT, and the rate of spread. To achieve a comparable influence in
absolute values, as with the minimal adequate models, each parameter was
standardized to have a zero mean and variance of one.
Distribution of recent invaders is largely determined by residence time
For the three regions where data on complete alien floras were available
(Azores, Czech Republic and Hawaiian Islands), there was a steady influx of
alien species over the last two centuries (Fig. 1). Fluctuations in the pattern
of increase in species numbers over time among regions usually reflect
changes in research intensity or publication of important floral works [31] but
sometimes can be interpreted by historical circumstances. That historical
events markedly influence the immigration of alien plants into a region was
Figure 1. Temporal pattern of invasion into the three regions analyzed in the present study. Cumulative
number of species reported up to a given year is shown for the Czech Republic (n = 691; time scale:
1750-2001, data: [27]), Azores (n = 910; 1589-2001; [26]) and Hawaii (n = 786; 1791-1985; [29]).
See Table 1 for details on data sets.
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