ecologists are challenged to integrate all these abi-
otic and biotic influences in their management de-
cisions. However, there is relatively little known
about how to influence the composition and devel-
opment of soil communities (Kardolet al. 2008).
Community ecologists face the enormous challenge
of integrating all of these various disciplines to
improve current concepts and to develop novel
ones, and to facilitate environmental planning and
conservation decision-making.
7.4 Biological invasions
7.4.1 Community-related hypotheses that explain biological invasions
Biological invasions of exotic species are causing
major problems worldwide, because of their dis-
proportional abundance, negative effects on local
biodiversity and alterations of ecosystem processes
(Williamson 1996). There are many overviews and
reviews that discuss biological invasions in depth.
Here, I will focus on biological invasions in relation
to aboveground and belowground community in-
teractions. Community ecology theory would pre-
dict that the disproportional abundance of exotic
plants is caused by altered bottom-up and top-
down interactions in the novel environment. Unoc-
cupied niches can provide exotic species with abun-
dant resources, although some studies argue that
many invasive species occupy the same niches as
the native species (Scheffer and Van Nes 2006).
Alternatively, invasive exotic species may have
novel traits, such as the capacity to fix nitrogen
(Vitouseket al. 1987) or novel chemical properties
that do not have a coevolutionary history with
other organisms in the new range (Callaway and
Ridenour 2004).
Prominent hypotheses proposed to explain
biological invasions from a community perspective
are biotic resistance (BR) and enemy release (ER)
(Keane and Crawley 2002). BR was proposed by
Elton (1958), who concluded that species-rich com-
munities are invaded less by exotic species than
species-poor communities. There is evidence for
and against biotic resistance (Stohlgrenet al. 1999).
Certainly, much more work is needed to under-
stand when biotic resistance can prevent invasions.
For example, when species-rich plant communities
prevent the establishment of invaders, how do
these plant communities initially get and maintain
their high species richness? Moreover, species-rich
plant communities not only provide more potential
competitors of exotic plant species. They also may
harbour more natural enemies, which increases the
chance that there are local enemies suitable to at-
tack invading exotic plants. Therefore, the issue of
biotic resistance, which is crucial to predict the
long-term development of invasions, needs more
multidimensional and multitrophic approaches
than have been taken thus far.
The concept of enemy release is supported by
examples of successful control of invasive exotic
weeds by introduced biological control agents.
Other studies that support the ER hypothesis have
compared introduced and native plant species in
their amount of natural enemy species. For exam-
ple, in a review including more than 300 introduced
plant species, Mitchell and Power (2003) showed
that non-native species had fewer aboveground
pathogen and virus species than comparable native
plant species. The exotic plants also had fewer
pathogens in their new than in their native range.
Exotic plant species that were actively dispersed by
humans, for example crops and ornamental plants,
had more pathogen and virus species than other
exotic species. It is not clear why this is; possible
usage enhances the chance of exposure to potential
enemies, or used exotic species did not pass the
selection processes as strongly as non-used exotic
species. Numbers of enemy species of course are
not indicative of enemy effects, but these results at
least show that exotic plant species are less exposed
to aboveground pathogen and virus species than
natives (Mitchell and Power 2003). There are simi-
lar studies on exposure of exotic plants to above-
ground insects. They show that exotic invaders
have less specialist feeders, but that they still can
have generalists (Jobinet al. 1996; Memmottet al.
2000).
All of above-mentioned examples of enemy re-
lease and biotic resistance concern release from
aboveground enemies. Klironomos (2002) showed
that five exotic plants in an old field in Canada exert
neutral soil feedback, suggesting that these exotic
plants may have become released from natural90 APPLICATIONS