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 Rejmanek,
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