Biology Reference
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discounted or altogether ignored. And “minor” pathways can actually form a significant
percentage of all introductions for some taxonomic groups. Although the contribution
of minor pathways to the total introduction volume of heavily transported taxa is typi-
cally small (e.g., 6.3% for lizards, 3.2% for snakes, and 2.3% for turtles), they account
for an important component of total volume for frogs (13.1%) and crocodilians
(18.8%), and they actually account for the majority (59.5%) of salamander introduc-
tions. But that last observation is somewhat anomalous, with 38.8% of all salamander
introductions resulting from deliberate introductions for scientific research, and 83%
of these done by one individual in a “research” program of doubtful scientific rele-
vance. If these 39 introductions are excluded, the percentage of salamander introduc-
tions due to “minor” pathways is reduced to 40%. This is still a much larger number
than seen for other taxa, and it reflects the importance of bait use and the residual
research introductions in accounting for salamander dispersal by humans.
A related caveat applies to the taxonomic analyses. Even though I have demon-
strated which taxa predominate in herpetofaunal introductions (Figs. 2.2-2.5), it is
important to remember that not all taxa pose equal ecological or economic hazard.
So some species or higher taxa may be capable of generating damage dispropor-
tionate to their contribution to overall introduction volume. As just one example of
particular concern, snakes only rate as the fourth-most-frequently introduced taxon
of alien reptiles and amphibians (Fig. 2.3), comprising 11% of all herpetofaunal
introductions. Yet dangerously venomous or powerful snakes make up a disconcert-
ingly large portion (20%) of that total, a fact that increases the concern that might
be accorded that segment of herpetofaunal introductions. Successful naturalization
of such species has already occurred in Okinawa and Florida, and serious impacts
are anticipated to follow (see next chapter).
My prior analysis of an early subset of the current database (Kraus, 2003c) con-
cluded that the rate of successful establishment among introduced reptiles and
amphibians was much higher than expected from the so-called “tens rule”. This rule
postulates that approximately 10% of alien species imported to an area appear in the
wild (are “introduced”, as I have been using the term), 10% of introduced species
become naturalized, and 10% of naturalized species become invasive (Williamson
and Brown, 1986; Williamson and Fitter, 1996). Since the rule is statistical, the prob-
ability of successful transition from imported to introduced, introduced to established,
and established to invasive can vary from roughly 5-20% at each stage and still be
viewed as according with the rule (Williamson, 1996). I have no data to address the
first transition (from importation to introduction) because I have not gathered data on
contained importations, such as those for the pet trade, nor for the third transition
(from establishment to invasiveness) because most naturalized reptiles and amphibians
have not been investigated for invasiveness. Data presented here (Fig. 2.5), however,
make clear that the transition from introduction to establishment is higher for some
taxa than predicted by the tens rule. In particular, frogs and lizards appear to have
been more successful at naturalizing than predicted. And even salamanders and
snakes lie on the high end of the range acceptably compliable with the tens rule. The
same conclusion attended my earlier analysis, but the present conclusion is more
compelling because the denominators now include information on multiple introductions
