Biology Reference
In-Depth Information
Fig. 2.19
Contrasting introduction rates between the period 1850-1979 (solid bars) vs. 1980-
2006 (open bars) for each major introduction pathway
different time periods (Fig. 2.19). It becomes clear at a glance that since 1980,
introduction rates for the cargo, nursery, and pet-trade pathways have all been
dramatically higher than seen for the prior 120 years, whereas the rate for the bio-
control pathway has declined just as dramatically, and the remaining pathways
have remained largely the same as their long-term averages. These results are
consistent with the variations seen in the exponential models for these pathways,
discussed above.
Geographic Variation
Rates of introduction and of successful establishment of alien reptiles and amphibians
vary geographically (Fig. 2.20). The large majority of all documented introductions
have been to Europe and North America, but successful introductions have been more
generally distributed, with the apparent rate of successful introduction varying con-
siderably among recipient regions (Fig. 2.21). This apparent difference is almost cer-
tainly a product of two effects, one artifactitious. First, unsuccessful introductions are
more likely to be reported in regions having many active scientists and interested nat-
uralists, making rates of successful establishment in such areas appear low compared
to regions receiving less scientific attention. And that is the pattern apparent in Fig.
2.21, with the lowest rates of successful establishment obtaining in Europe, North
America, and Australia. This is the artifactitious effect reflecting distribution of
interested parties to report failed introductions. Second, real regional differences in
establishment success probably do occur, independent of the reporting bias. This is
most strongly suggested by the three-fold difference in success rate between Europe
