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inferred from the statistical regression relationships and how do they relate to
residence time? Three parameters of the regression can be used for compar-
isons. First, the slope of the regression line indicates how suitable the recipi-
ent region is for invasions; the higher the slope, the more distributed alien spe-
cies with the same MRT will be. Since all slopes were standardized (zero
mean, variance one; Tab. 1), the slopes obtained for different group of taxa and
regions are mutually comparable. Second, the proportion of explained varia-
tion (R 2 ) is another parameter and indicates how important MRT is in deter-
mining the outcome of invasions. R 2 parameters from linear regression models
with the same number of explanatory variables and the same transformation
can be directly compared with each other [48]. Whether an invading species
will be successful in a new region depends on a complex interplay of numer-
ous factors, that include traits of invading species, interaction with native biota,
constraints imposed by existing communities, environmental variables such as
climate and disturbances, but also chance and timing [19, 49-51]. The more
important MRT is relative to other factors, the higher the proportion of varia-
tion it explains. A non-significant effect of MRT would indicate the complete-
ly overwhelming effect of the other factors, hence when the species was intro-
duced would not affect its chances to become naturalized or invade. Third, the
intercept with y axis for MRT = 0 refers to the start of the spread; the higher it
is, the earlier the invasion started.
The data summarized in Table 1 allow for comparison with respect to the
invasion status (Pyˇek et al. [35]) and invaded region. First, it is plausible to
compare the subsets of naturalized and casual species within each data set as
the measures used to express the species' occurrence are the same. In the
Azores using the range as a measure (Fig. 2), naturalized species invaded ear-
lier than casuals, as indicated by a significant difference (F = 114.0; df = 1,
907; P < 0.001) between intercepts, and their range increased with MRT at a
faster rate than that of casuals since the slopes were significantly higher for nat-
uralized than casual species (F = 36.62; df = 1, 907; P < 0.001). Minimum ade-
quate model (MAM) describing this pattern explained 43.1% of variance
(F = 228.7; df = 3, 906; P < 0.001). Lower values of regression slopes for casu-
als compared to naturalized species reflect the fact that the latter generally
occupy a wider range. If frequency is used as a measure, the results are the
same, i.e., naturalized species invading earlier (F = 190.4; df = 1, 908;
P < 0.001) and increasing their frequency with increasing MRT faster
(F = 117.1; df = 1, 908; P < 0.001) than casuals (MAM: 29.7%; F = 192.1;
df = 2, 907; P < 0.001). However, unlike the former model, casuals do not
increase their frequency with MRT, as indicated by the slope from this rela-
tionship not being significantly different from zero (F = 1.34; df = 1, 907; NS).
Comparison of the two measures available for the Azores, i.e., the number of
occupied islands (as a proxy for range) and species frequencies, seems to indi-
cate that while in naturalized species both range and frequency increase with
MRT, for casual aliens this is true only for range but not for frequency. This is
in accordance with the characteristics of both species groups; casuals are often
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