Residence time determines the distribution of alien plants - Invasive Plants: Ecological and Agricultural Aspects - page 67

than casuals (F = 250.9; df = 1, 688; P < 0.001) and the number of localities
they occupy increased with MRT at a faster rate than that of casuals
(F = 250.9; df = 1, 688; P < 0.001). In total, 54.6% of variance is explained by
MAM (F = 275.0; df = 3, 687; P < 0.001). Unlike in the Azores, casual species
that are present for a long time have higher frequencies than those that arrived
early. As the measures of frequency are very similar for both regions (Tab. 1),
the difference does not seem to be an artefact of the way the data were record-
ed. It may be hypothesized that differences between both regions, namely in
propagule pressure, which is higher in a densely populated mainland region
with intensively managed landscape and developed industry [46], are respon-
sible for the observed difference. That might explain why casuals increased
their frequency with increasing MRT in the Czech Republic but not in the
Azores.
Second, two comparisons are possible among regions . Four data sets pro-
vide information on the range of naturalized taxa (Tab. 1), which significantly
increased with increasing MRT (MAM: 32.9%, F = 872.0; df 1, 1778;
P < 0.001). In the Azores, Czech Republic, Hawaii and New Zealand (the lat-
ter based on a subset of woody plants only), neither the start of invasion
(F < 0.01; df = 3; 1775; NS) nor the rate of increase in range with MRT
(F = 0.67; df = 3, 1778; NS) significantly differed among these regions.
Measures of frequency are available for complete alien floras (including
casual species) of the Azores and the Czech Republic (Tab. 1). Both regions
were invaded at the same time (F < 0.01; df = 1, 1598 P < 0.001) but the rate
of increase in frequency was significantly (F = 6.09; df = 1, 1599; P < 0.05)
higher for the Czech Republic than the Azores.
Invasion status
Residence time affects not only the range and frequency of an alien species but
also its invasion status, i.e., whether it persists as casual or becomes natural-
ized or invasive [42]. These two measures, distribution and status, are closely
related (naturalized and invasive species are usually distributed over a wider
range and exhibit higher frequencies than casuals) but not necessarily; many
alien species are naturalized only locally [29, 52] and some casuals may be
quite distributed, but still relying on repeated input of diaspores [35]. Table 2
shows the difference in the mean MRT between alien species classified with
respect to invasion status. The pattern is consistent for the three floras (Azores,
Czech Republic and New Zealand) and corresponds to that found for the
range/frequency. Casual species have significantly shorter mean MRT than
naturalized aliens in both the Azores and the Czech Republic, and within the
latter region, invasive species have a tendency for a longer MRT than those that
are classified as naturalized but not invasive (Richardson et al. [42] and Pyˇek
et al. [35]). The same holds for New Zealand, where the difference between
naturalized and invasive species appears significant (Tab. 2).