Introduction Patterns - Alien Reptiles and Amphibians - page 50

Fig. 2.30 Distribution of the numbers of introductions among small islands, large islands, and
continents. Bars are the total sum of introductions (solid bars) and sum of introductions lead-
ing to successful establishment (open bars), with establishment counted only once per jurisdic-
tion. Numerical values are percentages of introductions resulting in successful establishment
of populations. Differences in establishment rate are highly significant (G = 388.377, DF = 2,
p = 4.62e −85 )
Relative pathway importance also varies among these three categories of recipient
landmasses. Introductions to continents have been dominated by the pet-trade
pathway, and those to large islands by the cargo pathway (Fig. 2.31). In contrast,
those to small islands have involved a more even distribution of pathways, with
cargo and pet-trade pathways predominating, but with biocontrol, food, and nursery
trade pathways exhibiting greater importance than seen for continents (Fig. 2.31).
Some of these differences are less obvious if one considers only successful intro-
ductions. In that case, the pet-trade pathway is still of predominant importance for
continental situations, but successful introductions to both large and small islands
have resulted from a more even distribution of pathways (Fig. 2.32). In this case,
the cargo pathway still leads to the largest number of successful introductions on
small islands, but both the cargo and “intentional” pathways have resulted in the
highest numbers of established populations on large islands.
The largest number of introductions have involved species originating from
North America, with lesser numbers originating from Asia, Europe, and Africa
(Fig. 2.33). However, if the immensely popular Trachemys scripta is excluded from
these numbers, the predominance of North America declines to a value of 1,330,
only somewhat greater than that for Asia. As seen earlier, successful introductions
are less frequent (Fig. 2.33). In this respect, species originating from insular regions
appear to have resulted in more establishments than those from continental regions
(Fig. 2.34). This pattern could result for different reasons. First, it may be an artifact
that these insular species have most often been moved to other islands whose
habitats are similar enough to promote a high probability of establishment.