Biology Reference
In-Depth Information
mention of all reported dates before 1850 (usually approximate, but sometimes
exact) that I could discover.
The database consists of records for 5,745 introductions, representing 675 taxa
and 2,141 record entries, where each “species x jurisdiction” combination counts
as a separate entry. Numerous entries in the database consist of >1 introduction of
a species to a particular jurisdiction. In these cases of multiple introduction, know-
ing that a species has become established tells us only that at least one of those
multiple introductions has been successful. It may be that more than one was suc-
cessful, but this is usually unknowable and unreported in the literature. Kolbe
et al.'s (2004) results using mitochondrial DNA to assess numbers of introductions
of
Anolis sagrei
to Florida illustrates one exception to this rule. Hence, for the
analyses that follow, measures of success rates necessarily can only consider counts
of jurisdictions to which species were successfully introduced and will serve as a
(probably slight) underestimate of true establishment success rates. Following this
approach, we find that these 5,745 introductions have resulted in 1,060 successfully
established populations involving 322 species.
Alien introductions of reptiles and amphibians have increased exponentially
since 1850 (Fig. 2.1), with a doubling time of 27.25 years. This growth curve is
described by the equation y = 43.6e
0.2532x
, and the fit of the data to this curve is
remarkably good (R
2
= 0.9978, Table 2.1), indicating that global growth in alien
introductions has increased surprisingly constantly through the past 150 years. The
dip at the end of the illustrated curve merely reflects the time lag involved in having
recent introductions reported in the literature, and it should not be interpreted as
indicating that introduction rates have recently declined. For example, in my earlier
analysis of subset of these data (Kraus, 2003c), the terminal dip in the cumulative
growth curve occurred in the 1990s, not the 2000s.
Fig. 2.1
Cumulative growth in global introductions of reptiles and amphibians
