Biology Reference
In-Depth Information
Fig. 2.15 Cumulative growth in reptile and amphibian introductions by pathway. Biocontrol =
dark blue, cargo = red, food = yellow, nursery trade = blue, pet trade = purple, and “intentional”
= green
in importance in recent decades. Further, the nursery-trade pathway has increased
considerably in importance since the 1970s. These patterns can be explained by
looking at differences in the individual growth trajectories of each pathway. Each
pathway can be well modelled by exponential equations, as was apparent earlier
when examining growth in introduction rates for each major taxon (Table 2.1).
However, in this case, growth is not exponential for all pathways during the entire
time period considered here, and changes in pathway importance over the past 150
years can be explained by the amount of time that exponential growth occurred for each
pathway and the magnitude of the exponent involved in that growth (Table 2.4).
Examined in this light, several points are noteworthy. First is that exponential
growth can be halted. This is most evident for the biocontrol pathway, which enjoyed
exponential growth through the 1960s but has had its growth virtually terminated
since then. The food pathway may also be showing signs of decreasing growth since
the end of the 1980s, but it is probably too soon to be certain of this. Second, is that
the pathway of predominant importance in late 20th century introductions (pet trade)
is also that with the highest exponent and, hence, shortest doubling time (Fig. 2.15,
Table 2.4). In contrast, the only pathway still clearly growing exponentially whose
sum effect (to date) approximates that of the currently non-exponential pathways of
biocontrol and food is the nursery trade, which has had the longest doubling time
(Table 2.4). Third, is that pathway importance may stagnate for decades and then
change rapidly. As one example, the pet trade was a relatively negligible pathway
until the 1920s, at which point extremely rapid exponential growth set in. Prior to
that point, growth in the pet-trade pathway cannot be modelled by an exponential
equation; since that time, the number of introductions via that pathway has doubled
every 15.3 years. Similarly, although the nursery-trade pathway has the slowest
doubling time over the entire 150-year study period, there was a strong inflection in
rate during the 1970s, and, consequently, the equation describing growth in that path-
way's importance across that inflection point (Fig. 2.16) has a higher exponent and
much shorter doubling time (Table 2.4).
 
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