References

Interactions Between Microorganisms and Soil Micro- and Mesofauna 257
and cannibalism are certainly widespread in decomposer food-webs (Polis
1991; Gunn and Cherrett 1993), which is supported by soil food web analyses
based on stable isotopes (Ponsard and Arditi 2000; Scheu and Falca 2000).
The dominance of generalist predators and the high incidence of IGP and
cannibalism in soil communities add to the intriguing complexity of belowground
food webs. It is likely that the exceptionally high local diversity
ofsoilorganismsatleastinpartresultsfromthefactthatsoilpredatorsare
so closely intermingled. The very large overlap in prey (among predators)
may prevent competitively superior microbi-detritivores from outcompeting
inferior species, despite close resource overlap among them.
The similarity of trophic positions of species in very different taxonomic
groups within microbi-detritivores and predators supports the assumption
that functional redundancy among soil animals is high (Wardle et al.
1997, 1998; Setälä et al. 1998). High redundancy may help to explain why
decomposer populations are extraordinary stable in time (Bengtsson 1994)
and respond little to external perturbations (Scheu and Schaefer 1998; Joergensen
and Scheu 1999). Decomposer diversity–ecosystem functioning
experiments also support the view of high redundancy in detritus communities
(Mikola et al. 2002; Wardle 2002).
2.2
The Detritus vs. Root Exudate-Based Food Web
Plants supply two quite different energy sources to the soil food web, root
exudates and plant debris (Fig. 3). Plant organic matter fuels the detritus
food web, which acts as two distinct, bacterial and fungal-based compartments
(Wardle and Yeates 1993). Carbon is transferred to successively
higher trophic levels and nutrients bound in microbial biomass are liberated
for plant uptake. Root exudates build the resource for a separate short
and fast energy channel. In the rhizosphere, easily degraded carbon sources
become available to microorganisms leading to a biomass often several
times greater than in corresponding nonassociated soil (Wardle 2002). In
contrast to bacterial and fungal channels, the interactions between plants,
microbes and grazers form a loop (‘microbial loop’; Clarholm 1994). The
dominant grazers, protozoa and nematodes may effectively control microbial
species, but themselves appear to be little controlled by higher order
predators (Scheu and Setälä 2002; Wardle 2002), resulting in carbon to be
processed in the loop and not progress to the remainder of the soil food
web.
Theactivityofmicroorganismsinsoilisusuallylimitedbycarbon,
except in the rhizosphere where plants steadily supply easily available carbonsources.Thisfavorsamicrofloratypicallyconsistingoffast-growing