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R.A. Hufbauer and M.E. Torchin
hybridization may increase genetic variation in resource allocation, providing
variants that shunt resources away from defense (e.g., Floate et al. 1993; Fritz
et al. 2001). These variants may be strongly selected for in a novel environment
lacking enemies. Therefore, hybridization may promote the evolutionary
response predicted by the evolution of increased competitive ability
hypothesis. Similarly, hybridization may provide the variation necessary to
respond to selection imposed by changes in the competitive regime associated
with the new range (Table 6.2, hypothesis 8a).
Clearly, the variables that transform with introduction to a new range (e.g.,
enemies, competitors, mutualists, genetic variation) can all change at once,
and should be considered together. Figure 6.3 illustrates a simple conceptual
model for how invasiveness may vary with three of these variables.We predict
that in many cases, invasiveness will increase with release from enemies and
competitors, and with an increase in genetic variation. The speed and direction
of adaptive evolution will be influenced by extrinsic factors (e.g., enemies
and competitors) and intrinsic factors (e.g., genetic diversity). Thus, the
change depicted in invasiveness may be due to either ecological or evolutionary
processes, or a combination of both. Of course, exceptions to the general
trend shown in Fig. 6.3 are known to exist. For example, the Argentine ant
(Linepithema humile) has accrued an advantage with loss of genetic variation
(Tsutsui et al. 2000), although the change in natural enemies has yet to be evaluated
in this case.
Figure 6.3 can be used in two ways: knowing how enemies, competitors
and genetic diversity differ between the native and introduced range, one can
make initial predictions of relative invasiveness of species (e.g., whether R is
greater or less than 1).Alternatively, using a measure of invasiveness based on
the response ratio, we can make initial predictions of how these three variables
might have changed between the native and introduced range with
respect to the direction of change along the axes, and in some cases, the relative
magnitude of change. Testing such predictions should provide new
Fig. 6.3 Prediction of relative invasiveness
when combining three of the
variables likely to change with introductions
(see text for further information)