My message for end-users or stakeholders is that
your current problem of interest may very well
originate from historical events, or that the cause
can be found at your neighbours, or your neigh-
bours’ neighbour’s yard. My message for scientists
is that a local focus or a snapshot in time will not be
sufficient to fully understand community interac-
tions that take place here and now. I will argue that
applications of community ecology approaches in
terrestrial ecosystems may require a spatio-tempo-
ral approach in order to predict, anticipate or solve
ecological problems as a result of natural, or
human-induced, changes in the environment.
7.1.2 Top-down and bottom-up go hand in hand
The long-standing question of what controls the
abundance of species was strongly fuelled by the
debate between Hairstonet al. (1960) and Ehrlich
and Raven (1964). They disagreed about whether
species are controlled by top-down (predator con-
trolled) or bottom-up (resource controlled) forces.
Nowadays, however, we realize that top-down
forces may become bottom-up (Mooreet al. 2003)
and that evolution takes place in a multitrophic
selection arena where selection pressures change
from time to time (van der Puttenet al. 2001). Com-
munities are dynamic, and alternating species
abundances may be controlled by a mix of resource
and predator controls which, each in their turn, can
exert selection pressure on local individuals. Clear-
ly, this is not the end, but only the start of a dyna-
mical perspective of communities and community
interactions.
Unlike many simple experimental settings, com-
munity composition is not constant; but rather spe-
cies move in and out of communities all the time.
This is observed in lake food webs (Jonssonet al.
2005), and in diversity experiments where species-
diverse plant communities created by sowing seed
were much more stable in composition than non-
sown species-diverse plant communities (Bezemer
and van der Putten 2007). These examples illustrate
that local community interactions can be changed
by incoming or outgoing species and that immigra-
tion and emigration are quite common in natural
communities. That awareness has major implica-
tions for the interpretation of well-controlled plant
biodiversity experiments, where the intentional
sowing of plants promotes spatial and temporal
stability when compared with naturally colonized
communities (Bezemer and van der Putten 2007).
Temporal instability may involve immigration of
species that introduce novel properties. Immigra-
tion of species with different traits is a natural pro-
cess in succession sequences, when grasses and
forbs are replaced by shrubs and trees, producing
litter that differs in rates of decomposition. While
such processes are normal in successional gradi-
ents, artificial introduction of species from different
continents may change the functioning of entire
ecosystems (Vitouseket al. 1987). If such invaders
also reach disproportionate abundance, for whatev-
er reasons, incoming species may cause switches in
community structure and ecosystem processes.
Top-down and bottom-up processes play an impor-
tant role in causing invasiveness and in the conse-
quences of invasions. Invasions are among the
major application issues worldwide and under-
standing why species become invasive, how these
invasions can be prevented or how they should be
managed is an important challenge for community
ecology.
I will first work out how local community inter-
actions may be altered by species that move within
or across ecosystems. I will argue that local com-
munity interactions are influenced by processes
and interactions between adjacent or distant sys-
tems. I use the case of plant interactions with
above- and belowground organisms and discuss
how these organisms may influence each other,
through the shared plant resource. Then, I explain
how aboveground–belowground interactions can
be influenced by legacy effects in soil, and how
these delayed influences may affect succession. I
will focus on ecosystem restoration on post-agri-
cultural land, which is one of the main application
issues in the industrialized world. Subsequently, I
will focus on alien exotic plant species, how they
may influence local community interactions and
how altering above and belowground bottom-up
and top-down processes may contribute to inva-
siveness. Finally, I will speculate about the conse-
quences of (human induced) global warming
beyond the climate envelope approach, and I will84 APPLICATIONS