positive effect of arbuscular mycorrhizal fungi is
quite contrasting, whereas rigorous inoculation
studies are lacking (van der Puttenet al. 2007).
Some exotic plants may accumulate local viruses
and pathogens, thereby having an indirect negative
effect on native plant species (Fig. 7.3). For example,
exotic plants in Californian grasslands accumulate
local viruses without suffering from them. These
viruses spread into the surrounding native plant
communities and cause strong negative effects
(Malmstromet al. 2005). Similarly, the invasive
tropical shrubChromolaenain India accumulates
soil pathogens that have strong negative effects on
surrounding local plants (Manglaet al. 2008). Mod-
els show that such indirect effects could render
exotic plants invasive, even when these plants suf-
fer to some extent from mild biotic resistance
(Eppingaet al. 2006). Although these invasive spe-
cies make use of local enemies, their effects on
native communities could be quite similar to those
cases where novel weapons have been proposed to
cause invasiveness.
7.4.3 Conclusions and consequences for management
The role of community ecologists in preventing,
combating or mitigating effects of biological inva-
sions provides an enormous task for future re-
search. As soon as invasive species become
abundant, it is already often too late for their com-
plete eradication. Introducing biocontrol agents
may be helpful for some invasive species, but the
risk that biocontrol agents may switch to feeding on
native plants cannot always be excluded. More-
over, the long-term effectiveness of biocontrol
agents could be limited. In a comparison of biocon-
trol studies, Burdon and Marshall (1981) concluded
that biocontrol by aboveground insects and patho-
gens was least effective against introduced annual
plant species and most effective against introduced
perennial and clonal plants. They concluded that an
annual lifestyle probably enables rapid evolution of
defences against pathogens or herbivores. There-
fore, managers should consider alternatives, such
as exploring how control agents already present in
the invaded range might be used, or cultivated, to
switch to and control exotic species. In doing so, the
full array of enemies, including viruses, pathogens,
herbivorous invertebrates, both below and above
ground, as well as large herbivores could be con-
sidered. There is often little known about what
controls exotic species in their native range. How-
ever, at the same time it is unclear whether these, or
different, controls are required to reduce the abun-
dance of the exotic species in their new range.7.5 Discussion, conclusions and perspectives
The promise of ecology as a science lies in devel-
oping sufficient knowledge to allow us to under-
stand and predict how individuals, species,
communities and ecosystems will respond to myr-
iad environmental changes. Applied community
ecology should be at the forefront of developing
such predictive power and testing the value of the
predictions. Are there general laws and how well
can we predict? How well do relationships devel-
oped in temperate zones, where most ecological
studies are carried out, apply to tropical or boreal
regions? These and many other applied issues are
open for discussion. I will focus on a subset of
these issues, show opportunities for end-users
and stakeholders, and point out some areas for
future work in ecology.Invasive plant Native plant
Mycorrhizal fungiLocal soil pathogensNovel allelochemicals
- ––
++
+Figure 7.3Three pathways of how invasive plants can
indirectly influence native plants: by reducing, or
suppressing, local symbionts, for example arbuscular
mycorrhizal fungi; by enhancing local pathogens or
parasites; or by excreting allelochemicals that are new to
the local plants or enemies. The consequences of these
three effects are that the invasive plant has an indirect
advantage over native plants, as indicated by the dotted
arrow. Arrow thickness indicates the strength of the
effect.
92 APPLICATIONS