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R.A. Hufbauer and M.E. Torchin
(1) differential parasitism in populations of a single species, both in its native
and introduced region (within-species, cross-regional comparison), (2) native
versus introduced species in the same region (between-species comparison of
ecologically analogous competitors), (3) introduced species that are not invasive
with introduced species that are invasive (between introduced species),
and (4) population growth rates and enemy abundance among populations
within a range (within species, within region) will help fully evaluate enemy
release and the role of enemies in biological invasions. When used in combination,
such comparisons will clarify the extent to which natural enemies
keep their host populations in check, and the consequences of release from
these natural enemies on population growth. To our knowledge, this joint
approach has not yet been employed.
6.3.2 Refining the Evolution of Increased Competitive Ability Hypothesis
Evidence for adaptive evolution in invaders supporting the EICA hypothesis,
particularly local adaptation to the new environment, is growing (e.g., Leger
and Rice 2003; Lee et al. 2003; Blair and Wolfe 2004; Maron et al. 2004), but is
not found in all cases (e.g., Willis et al. 1999; van Kleunen and Schmid 2003).
Similarly to cases dealing with enemy release, much of the conflict in the data
may be due to the specific comparisons that are made. For example, most of
the data come from common garden experiments with plants, largely comprised
of specimens that simply happened to be available for study, without
particular knowledge of the origin of an invasion (but see Maron et al. 2004).
Often, these are not comprehensive samples taken from across the native and
introduced ranges (but see Blair and Wolfe 2004). If common gardens include
inappropriate comparisons, then the data may not reflect the role of evolution
in invasions.
6.4 Recent Syntheses and Synergies Between Hypotheses
Efforts have been made to synthesize across hypotheses. Shea and Chesson
(2002) view biological invasions from the perspective of community ecology
theory. They discuss invasions in terms of niche opportunities that are determined
largely by resources, natural enemies, and the physical environment.
Top-down population regulation is exerted by both specialist and generalist
enemies, and for invaders, specialists may be relatively rare in the new range.
The availability of resources that are present in a habitat is governed by other
members of the community using those resources. Thus, Shea and Chesson
(2002) bring together aspects of the enemy release, biotic resistance, and
empty niche hypotheses.