Agriculture Reference
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lyzed for recent invaders, to explore how species reaching the target areas at
different times over the last 2-4 centuries perform as current invaders. Second,
a question is raised whether the residence time still affects the current distri-
bution of species that were introduced millennia ago. The results are then dis-
cussed in the context of available literature dealing with temporal aspects of
plant invasion.
The data
Four data sets representing alien floras or their subsets and containing infor-
mation on the first record of each species in the area and some measure of their
present occurrence were collated: the Azores (38.00 N, 28.00 W; [28]), Czech
Republic (49.30 N, 17.00 W; [29, 30]), Hawaiian Islands (22.00 N, 160.00 W;
[31]), and New Zealand (41.00 S, 174.00 E; [32]). They were used to assess
the effect of minimum residence time (MRT) on the distribution and frequen-
cy of species that invaded in the last 2-4 centuries (see Tab. 1 for characteris-
tics of data and how the primary sources were standardized to provide compa-
rable information). In Europe, these species are termed neophytes and defined
as aliens that arrived after the year 1500 [33-35].
In addition, information on historical invaders introduced to a target region
between the beginning of the Neolithic up to the year c. 1500 (termed archaeo-
phytes, see [35] for definitions) was obtained for two regions: Czech Republic
[29] and Great Britain (54.00 N, 2.00 W; [36, 37]). In the New World, a con-
cept analogous to archaeophytes and neophytes is not being consistently used,
although early plant introductions are recognized, e.g., by Polynesians to
Pacific Islands [31, 38, 39]. These introductions are of minor importance in
terms of species number because the vast majority of modern invaders arrived
after the discovery of America [40]. In the Mediterranean, the concept of
archaeophytes is rather blurred as species that are archaeophytes in other parts
of Europe originated in the Mediterranean. Hence in the Azores, Hawaiian
Island and New Zealand, all alien species reported in the respective primary
sources were considered in analyses, with the exception of early plant intro-
ductions to Hawaiian Islands [31].
The present distribution of alien species in studied regions was expressed
using two measures: 1) the first measure (termed “range”) is related to geo-
graphical distribution, expressed by the number of occupied geographical units
(mapping squares, islands or regions, Tab. 1) and 2) the second measure
(termed “frequency”) is related to how frequent the species is in the region
regardless of how widespread it is in geographical terms. In original datasets,
frequency scales are based on qualitative assessment or estimates of the num-
ber of localities (Tab. 1). The number of herbarium specimens given for the
New Zealand data was also taken as a measure of frequency as it reflects the
number of localities (see [41] for discussion on biases associated with herbar-
ium specimens as sources of data).
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