The patterns discussed in this paper were derived from
numbers and proportions of
naturalized
species and little is
known about the rates at which introduced species become
naturalized. To study this, information on the total number of
aliens, including
casuals
, is needed. These two groups of species
can provide different results, as illustrated by Fig. 2. If the
number of naturalized species in floras is plotted against
latitude, there is a trend for high numbers of naturalized aliens
to occur in middle latitudes (Fig. 2a; see also Rejma
́
nek,
1996a). However, the naturalization rate, expressed as the
percentage of naturalized species among all aliens including
casuals, decreases with latitude, which explains 38% of
variation in the same data set (Fig. 2b). Further research on
naturalization rates, i.e. the proportion of all aliens that
successfully naturalize, for different taxa and on variation in
rates between taxa and regions, would provide us with better
understanding of the invasion process, species invasiveness and
community/region invasibility