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Table 4. Path and effect coefficients of the path model of the distribution range of naturalized aliens
in the Czech Republic (expressed as the number of phytogeographical mapping quadrats) as a func-
tion of MRT and rate of spread (data from [30]). Path coefficients a 1 , b 1 and b 2 represent direct effects;
a 1 is the regression slope for standardized variables rate of spread and MRT; b 1 and b 2 are standard-
ized regression slopes from multiple regression of range as a function of MRT and rate of spread.
Indirect effects are calculated as a product of path coefficients along the links between causal vari-
ables and the response variable through other causal variables. Effect coefficients are the sum of direct
and indirect effects.
Path coefficients:
a 1 , effect of MRT on the rate of spread (direct)
-0.63
b 1 , effect of the rate of spread on range (direct)
0.62
b 2 , effect of MRT on range (direct)
0.78
a 1 b 1 , effect of MRT on range (indirect)
-0.39
Effect coefficients:
b 2 +a 1 b 1 , MRT on range (total)
0.39
b 1 , rate of spread on range (total)
0.62
is in a seeming contradiction with the results provided by the minimal ade-
quate model using species traits, discussed above (Tab. 3). A comparison of
these two models indirectly indicates the importance of landscape features and
recipient communities [30]. In the minimal adequate model, including species
characteristics, some proportion of variation remains unexplained - a part that
can be related to environmental variables. Residence time therefore seems to
be more important than species traits on their own but if the rate of spread,
which can be viewed as a proxy for the complex effect of all factors related to
invasions, is included, those factors explain the distribution range of aliens bet-
ter than the time of their arrival.
Residence time not only represents another dimension of propagule pres-
sure [25] but also integrates culturally-determined processes [53]. With
increasing time since the first introduction, the probability also increases that
safe sites for establishment appear as a result of natural disturbances and
human-made changes in site conditions that both may facilitate invasions. For
example, Ailanthus altissima in central Europe started to spread vigorously
only after rubble sites appeared in destroyed cities after World War II. With
increasing time since the first introduction, the probability also increases that
the introduced species is propagated by various modes of secondary releases
by humans (e.g., deliberate planting or sowings in the wild) that may over-
bridge gaps between suitable, but not accessible sites [54]. It should be there-
fore borne in mind that the rate of spread also integrates spreading resulting
from ongoing human activities.
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