Biology Reference
In-Depth Information
Fig. 2.14
Relative success of each of the major introduction pathways in leading to successful
establishment of populations. Relative success is estimated as the count of all jurisdictions to
which a successful introduction via that pathway has occurred divided by the sum of all introduc-
tions via that pathway
(Fig. 2.14). Given that reasoning, it might be wondered that an accidental-transport
pathway such as the nursery trade could result in similarly high success rates. But
several factors likely contribute to the high rate of successful establishment for this
pathway. First, the pathway involves the wholesale transfer of favorable habitat for
the transported animals, greatly increasing their chances of surviving the move.
Second, it may be that, on average, greater numbers of animals are involved in
nursery shipments than in other forms of cargo because such goods are inhabited
by several species of reptiles and amphibians prior to processing for shipment else-
where. Third, nursery shipments are rather fragile, requiring their transport to be
done quickly. Reduced transport time likely increases survivability for stowaways.
Fourth, shipment conditions are benign because of the need to keep the plants alive.
Lastly, plant shipments are generally made between regions having similar cli-
mates, increasing the likelihood that the destination will prove as climatically favo-
rable to the hitch-hiking herpetofauna as was the origin. These seem the salient
differences between transport via the nursery and other cargo pathways and likely
explain why introductions via other forms of cargo meet with less than half the
success rate of nursery introductions (Fig. 2.14).
Lastly, it remains to examine how pathway importance has changed through
time. It turns out that these changes have been tremendously important. The “inten-
tional” pathway accounted for most alien reptile and amphibian introductions up
through the end of the 1950s (Fig. 2.15). Beginning in the 1950s, introductions via
the pet trade began to skyrocket and that pathway has remained the predominant
pathway of introduction since the 1960s. During this entire period, the cargo path-
way has been of great, but secondary, importance, overtaking “intentional” introductions
