et al. 2008). Plants play a special role in above-
ground–belowground interactions, as they spend
most of their life above as well as below the soil
surface. Plants influence aboveground and below-
ground communities by modifying the abiotic en-
vironment, providing structure and resources, and
by antagonistic interactions involving direct and
indirect defences (Wardleet al. 2004).
Traditionally, plant community interactions have
been explained primarily by variation in the abiotic
environment. Interspecific competition and above-
ground biotic interactions with vertebrate and in-
vertebrate herbivores, pathogens, pollinators and
other mutualists more recently have been recog-
nized as influencing plant community structure.
The awareness that plant community interactions
are influenced by interactions between plants and
soil organisms is even more recent, whereas interest
in interactions between plants and below- and
aboveground multitrophic communities is less
than a decade old (van der Puttenet al. 2001).
Plants interact with soil organisms through exu-
dates, dead organic material (litter) and through
living tissues. Soil organisms can reduce plant
growth (via nutrient immobilization, herbivory,
pathogenesis) or enhance plant growth (via symbi-
osis, nutrient availability, enhancement through
mineralization) and these processes, and the organ-
isms responsible for them, interact (Wardleet al.
2004). Besides primary metabolites, plants also con-
tain secondary metabolites, which are involved in
plant defence against natural enemies. In fact, a full
understanding of the ecology of plants cannot be
attained without including their interactions with
both aboveground and belowground organisms.
Abiotic drivers, such as climate and soil type,
will have strong predictive power about the com-
position and dynamics of plant communities on
large spatial and temporal scales. However, at
smaller spatial and temporal scales, plant commu-
nity processes will be much more strongly influ-
enced by aboveground–belowground community
interactions. For example, aboveground vertebrate
herbivores enhance root exudation, which fuels soil
decomposer systems resulting in enhanced miner-
alization and increased plant nutrient availability
(Bardgettet al. 1998). Therefore, vertebrate grazers
may enhance primary production to their own
benefit. Aboveground–belowground interactions
also influence plant defence. For example, indirect
defence of plant shoots against caterpillars by para-
sitoid wasps is reduced when cabbage root flies
(Delia radicum) are present in the soil. The root
flies reduce the amount of volatiles which normally
are released by the shoots and attract parasitoids.
The presence of root flies enhances the production
of repelling compounds, which distract parasitoid
wasps (Soleret al. 2007). Aboveground interactions
can also be influenced by changing the entire soil
community composition. For example, in multi-
species grassland mesocosms, soil nematodes en-
hance the survival and quality of aboveground
mites, resulting in enhanced aphid control by their
natural enemies, aphid parasitoids. The result is
that in the presence of nematodes, but not in the
presence of soil microorganisms, both bottom-up
and top-down control of aboveground aphids is
intensified (Bezemeret al. 2005). These examples
show that soil communities can play a key role in
the management of production ecosystems; the
question now is how to translate these findings
into applications.7.2.3 Consequences for application: find the remote cause of local effects
The existence of linkages across ecosystem or sub-
system boundaries has major consequences for
management of agricultural ecosystems, as well as
for understanding and predicting changes in natu-
ral ecosystems. The presence and diversity of spe-
cies, the abundance of populations, the interactions
and organization of communities and the function-
ing of ecosystems all may depend on events that
take place in adjacent populations, communities or
ecosystems. Although conservation tends to be fo-
cused on red list species or species with iconic
value, conservation requires a more integrated con-
sideration of community interactions. As many ap-
plied problems have remote causes, local solutions
will not be effective in the longer term. This applies
to many global human-induced changes, such as
land use change, climate change, as well as to
biological invasions. Solutions for local problems
require insight in processes that exceed local spatial
and short-term temporal scales. For example,86 APPLICATIONS