The ecology of biological invasions: past, present and future - Invasive Plants: Ecological and Agricultural Aspects - page 13

of the same host species, unfalteringly supported the ERH. However, evidence
from community studies, which compared the levels of attack between NIS
and native competitors, was much more equivocal. A number of alternate
explanations were proposed to help explain this apparent discrepancy: (1) the
'enemy of my enemy hypothesis' (EEH) occurs when a co-introduced enemy
does more damage to native competitors than to their co-evolved hosts because
of native host naiveté to introduced enemies (see also IFH, below); (2) the
'enemy inversion hypothesis' (EIH) describes a documented situation where a
co-introduced enemy may have actually increased the dispersal ability of its
host by complex interactions between the host, its natural enemy, and a native
species [83-85]; and (3) the 'increased susceptibility hypothesis' (ISH), which
predicts that genetic bottlenecks during the invasion process may render a host
species more susceptible to the effects of enemies, even though this may be
inflicted by a smaller number of species. Colautti et al. [82] also note that bio-
geographical studies may be biased by a failure to consider the confounding
effects of propagule pressure. In other words, many introduced species were
likely founded by a small subset of individuals from a restricted part of their
native range, and therefore a number of natural enemies had no chance of
introduction. Indeed, this seems to be the case for the European starling
Sturnus vulgaris , as the 'apparent' reduction in the number of enemy species,
calculated as a reduction in enemies between its native and introduced ranges,
was a gross overestimate of the 'realized' reduction, calculated after account-
ing for sub-sampling of the source region [85]. It is worth noting the difference
between the ERH, which predicts that invaders succeed because they escape
natural enemies, and the BRH, which predicts that invaders often fail to estab-
lish in the invaded range because of negative effects of enemies native to the
introduced range [55]. Indeed, all invaders likely lose enemies during the inva-
sion process, yet only a small proportion of these have appreciable ecological
or economic impacts.
Invasional facilitation hypothesis (IFH)
Contrary to the BRH and the ERH, the IFH predicts an increasing rate of inva-
sions through time [86]. The term 'invasional meltdown' was introduced by
Dan Simberloff and Betsy Von Holle at the University of Tennessee to describe
a process whereby facilitative interactions among NIS may increase invasion
rates [86]. Facilitative interactions among invaders have been demonstrated in
many systems [63, 87-89]. For example, invasion by the zebra mussel
Dreissena polymorpha in the Laurentian Great Lakes has drastically altered
the ecology of the lakes, including an increase in the diversity of macroinver-
tebrates, which use zebra mussel shells as refuge from predators [90]. The
invasion and subsequent proliferation of D. polymorpha in 1988 was closely
followed by invasions of three co-evolved species, which are also found in
high abundance: in 1994 by Echinogammarus ischnus , a macroinvertebrate