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R.A. Hufbauer and M.E. Torchin
introduced ranges, examining how their population dynamics are influenced
by several types of interspecific interactions and the abiotic environment. The
biotic interactions they examined comprise predation, parasitism, mutualism,
and competition, making this the first theoretical integration of these key
groups.
Furthermore, Mitchell et al. (2006) highlight that phylogenetic relationships
between an invader and the members of the invaded community may
play a critical role in the outcome of an introduction. Introduced species
invading communities with close relatives should be more susceptible to enemies
in that community, and thus accumulate a broader suite of enemies compared
to invasions of communities in which relatives are absent (Strong et al.
1984; Mack 1996). However, they may also be more likely to gain mutualists
from their relatives. The comparative importance of enemies and mutualists,
and their relative differences in host specificity will determine whether invading
a community containing close relatives is a disadvantage for invaders. The
relatedness of members of the community may also be an indicator of suitability
of the abiotic environment, when related species have similar environmental
requirements.
Additional connections and synergies between hypotheses are possible,
and for the field to advance, those links should be clarified and formalized in
a predictive framework. We offer an initial attempt at such a framework
(Table 6.2), focusing on interactions among enemy release, biotic resistance,
and evolutionary change (including both evolution of increased competitive
ability, and hybridization). By formalizing the connections among hypotheses
to explain invasion, we can generate specific, testable predictions. These predictions
can guide research efforts, and resultant data can feed back into the
predictive framework.
For example, we suggest that genetic variation and changes associated with
introductions may interact directly with enemy release and biotic resistance
in at least two key ways. First, with a severe bottleneck in population size upon
introduction, a species will lose genetic variation, but is also more likely to
lose natural enemies (Torchin et al. 2002). Thus, adaptive evolution may be
most limited by lack of genetic variation when enemy release is likely to be
greatest, setting up a useful comparison between two potentially opposing
pathways for invasion success (Drake 2003; Table 6.2, hypothesis 4a). Additionally,
reductions in variation will define the context within which new
interactions with enemies develop, and may affect the ability of the invader to
defend against enemies.
A second way that genetic variation may interact with enemy release and
biotic resistance is through changes associated with hybridization. Often, it is
only in novel environments that hybrid and backcross offspring are fitter than
their parents. One key novel aspect of a new range may be the lack of enemies.
Thus, ways in which hybridization alters interactions with natural enemies
could be particularly important. One mechanism may be simply that