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R.A. Hufbauer and M.E. Torchin
ent superiority, or an empty niche lead to strong invasion, the external environment
of the new range must be more favorable to the invasive species than
its native range.
A positive feedback between introduced species may enable them to
become invasive, a process termed invasional meltdown (Simberloff and Von
Holle 1999; Richardson et al. 2000). Often, mutualists are important in facilitating
invasional meltdown. Similarly to the enemy release hypothesis
described below, mutualists are likely to be lost during the process of invasion.
Some species rely upon mutualists for successful passage through crucial
stages of their life cycles. For example, obligatorily outcrossing plants that
rely upon pollination can not reproduce sexually without a pollinator. Other
species engage in mutualisms that are not obligate but that increase their fitness
in many situations, such as plants harboring mycorrhizal fungi. For
introduced species engaged in mutualisms to become invasive, their mutualists
must be replaced by mutualists from the novel range, or their mutualists
from the native range must be introduced to the novel range as well. Without
their mutualists, they may remain latent for years. For example, Pierce’s disease
(Xylella fastidiosa), a bacterial disease of many woody plants, did not
become invasive in North America until an efficient vector (glassy-winged
sharpshooter, Homalodisca coagulata) was introduced (Redak et al. 2004).
While the hypothesis that mutualisms facilitate invasions is well-supported, it
may not explain strong invasion unless other factors and interactions (in the
case of Pierce’s disease, the availability of many susceptible hosts) also come
into play.
The invasion process may “filter out” parasites (including specialized herbivores),
pathogens, or other natural enemies that occur in an invading host’s
native range through several mechanisms (Keane and Crawley 2002; Torchin
et al. 2003). The enemy release hypothesis (ERH) posits that this filtering
process releases introduced species from the top-down population regulation
exerted in the native range, enabling strong invasion. A key prediction of the
ERH is that introduced populations harbor fewer natural enemies compared
to populations within their original range (Williams 1954; Elton 1958). Additionally,
it is often postulated that there should be a corresponding shift to a
higher proportion of generalist enemies relative to the native range, since
generalists are more likely to shift to novel species. Another prediction of the
ERH is that introduced species may gain a competitive edge because they are
less likely to be affected by natural enemies than are native competitors (Elton
1958; Keane and Crawley 2002). These key predictions of the ERH are not
mutually exclusive, but the mechanisms may not occur simultaneously. Growing
evidence indicates that introduced species have fewer enemies than where
they are native (reviewed in Torchin and Mitchell 2004). Whether this loss of
enemies drives the unusual demographic expansion of some introduced
species remains equivocal (e.g., Lampo and Bayliss 1996; Beckstead and
Parker 2003; Reinhart et al. 2003).Additionally, the extent to which introduced