soil-borne enemies. As soil feedback indicates a net
effect, it could be that the neutral effect was due to
less pathogenic, or to more symbiotic, activity, as
symbionts may overrule the effects of pathogens.
However, in the same study, native dominant
plants also had neutral or even positive feedback
(Klironomos 2002), so that the exotic plants could
have shown the same soil feedback as in their na-
tive range, when they were dominants. Enemy re-
lease includes two components. The first is that
exotic species escape from their native enemies
and the second is that the exotic species have less
enemy exposure in their new range. Therefore, such
studies need a comparative approach, including
both the native and non-native ranges (Hierro
et al. 2005).
Further evidence on ER from soil pathogens
stems fromPrunus serotina(black cherry), which is
invasive in Europe and native in the USA. In Eur-
ope, soil feedback was neutral, whereas, in the
USA, the soil feedback was negative (Reinhart
et al. 2003). In that example, there is still a possibility
that symbionts provided an overwhelmingly posi-
tive effect, so that the neutral soil feedback effect
could have been due to highly effective symbionts.
In a study on Kalahari savanna grasses, however,
fungi were isolated from a native grass (which ex-
pressed negative soil feedback) and from an exotic
invader (which had neutral soil feedback). The soil
fungi from the native species were pathogenic to
their own host, but not to the exotic plant, whereas
soil fungi from the exotic plant were not pathogenic
to the native and non-native plant species (van der
Puttenet al. 2007). The exotic species was not exam-
ined in its native habitat. Therefore, these results all
point to ER from soil pathogens; however, the ulti-
mate test, specifically including pathogen species
instead of treating soil feedback as a ‘black box’, is
still lacking.
7.4.2 Mount Everest or tip of the iceberg?
In spite of the enormous research effort focused on
biological invasions, most studies still are correla-
tive (Levineet al. 2003). Moreover, BR and ER have
many more dimensions than have been explored
thus far. Therefore, it seems as if we have hit the
tip of the iceberg, rather than getting a panoramic
view from the world’s highest peak. Here, I will
point out some new viewpoints and conclude that
biological invasions provide an enormous chal-
lenge to community ecology both from a funda-
mental and from an applied perspective.
A substantial amount of exotic plants contain
chemicals that are novel then native species of the
invaded range (Cappuccino and Arnason 2006).
These novel chemicals may influence plant–plant
interactions by allelopathic compounds (Callaway
and Ridenour 2004), or they may reduce the inocu-
lation potential of root symbionts as arbuscular
mycorrhizal fungi in invaded habitats (Stinson
et al. 2006). These views contrast with that of the
evolution of increased competitive ability (EICA)
(Blossey and No ̈tzold 1995), which assumes that
defences impose trade-offs with growth when
plants are exposed less to natural enemies. Howev-
er, the novel weapons hypothesis has, thus far, not
been widely tested for a range of plant species,
whereas EICA is contradicted in a number of stud-
ies (Wolfeet al. 2004). Tests of these hypotheses
usually lack the inclusion of negative controls,
which would be exotic species that do not become
invaders. These are the unseen majority of non-
native species, according to the 10s rule of William-
son (1996). The problem is, however, species that do
not become established will not be of use in ecolog-
ical studies. Therefore, it would be possible to in-
clude mild invaders in the studies and examine
their means of control. Nevertheless, it would still
be possible that these species have not yet reached
their stage of invasiveness, or that they are already
over the top of their invasiveness. Including such
‘false positives and negatives’ would enhance the
objectiveness of ecological studies on causes of in-
vasiveness.
In the debate on EICA and novel weapons, most
studies implicitly assume that plant defences are
mainly direct. However, indirect defence, through
recruiting the enemies of your enemies, may play
an important role in plant abundance. Loss of the
‘third trophic level’ has rarely, if ever, been consid-
ered in invasion studies. In contrast, it has been
argued that exotic plants may have easy access to
symbionts, such as pollinators or mycorrhizal fungi
(Richardsonet al. 2000). This hypothesis has not
been rigorously tested and the evidence for theAPPLICATIONS OF COMMUNITY ECOLOGY APPROACHES 91