Residence time determines the distribution of alien plants - Invasive Plants: Ecological and Agricultural Aspects - page 74

archaeophytes respond like neophytes to some variables (climate, seasonal
development of crop) and, alternatively, like native species to other variables
(increasing agricultural intensification through time, human population densi-
ty). This indicates that the identity of crop and its introduction history are
important factors determining the current distribution of archaeophytic weeds
and acting in concert with the residence time of associated weeds.
Finally, a question may be raised: what is the effect of residence time at
spatial scales other than those considered in the present paper? For individual
invading species residence time is usually not known at local scale. An excep-
tion is the study of Müllerová et al. (Institute of Borany, Pr º honice, Czech
Republic; unpublished observation), who documented, by using historical aer-
ial photographs, 40 years of invasion by Heracleum mantegazzianum in the
Czech Republic. Knowing when this species appeared in particular localities
allowed quantification of the relative importance of the rate of spread and res-
idence time for the outcome of invasion. Both residence time and the rate of
spread significantly contributed to the resulting size of invading populations,
and the direct effect of the residence time was four times smaller than that of
the rate of invasion. However, since the residence time affected the rate of
invasion (which was faster in sites where the invasion started later), it exerted
an indirect effect on the size of invading populations. Consequently, the total
effect of the residence time was about the same as that of the rate of spread
(Müllerová et al., unpublished observation).
This is different from the results of path analysis performed on a number of
naturalized species of the Czech flora discussed above (Tab. 4) where the rate
of spread turned out to be more important than residence time. Both analyses
can be only compared with the awareness of the differences in data in mind.
Nevertheless, the higher relative importance of residence time found in the
study on H. mantegazzianum might reflect that 1) the rate of spread is closely
associated with a species' invasion potential and H. mantegazzianum is one of
the most successful European invaders [59]. It is likely that other, less com-
petitive alien species would be more limited by local constraints which would
accordingly increase the average importance of the rate of spread on behalf of
the residence time. 2) For H. mantegazzianum , the rate of invasion was direct-
ly measured from aerial photographs capturing the area invaded at particular
time intervals which is a more exact measure of the rate of spread than esti-
mation from cumulative increase in distribution over time [30, 32].
Unfortunately, data that would make it possible to compare the patterns at dif-
ferent scales and among species are not available.
Lack of data is a major obstacle to disentangling the intriguing phenomenon
of residence time in plant invasions. High quality data sets, using the same
classification criteria of the species' invasion status [35] and based on detailed
historical information are unfortunately rare. To proceed further, more data are
needed to allow for analyses that would make it possible to relate the effect of
residence time to environmental characteristics and local conditions of invad-
ed regions.