Community Ecology Processes, Models, and Applications

tremendous variety of microbes and fauna in soil,
with much variation in body size, feeding speciali-
zation, life history strategies, spatial distribution
and responses to abiotic factors. Although their
coexistence is still an enigma, patterns and determi-
nants in soil biodiversity are emerging (Wardle
2002; Bardgettet al. 2005). Factors that seem to
regulate the temporal and spatial patterning of
soil biota relate primarily to the heterogeneity of
their environment, especially variability in resource
quantity and quality and microclimate. This hetero-
geneity provides possibilities for resource and hab-
itat specialization, or niche partitioning. The
importance of detritus for species composition and
abundance is derived from the observation that soil
communities are bottom-up or donor controlled,
which implies that resource supply, i.e. detritus,
determines the structure of the community (Pimm
1982; Wardle 2002). Detritus is the main resource
for fungi and bacteria and detritivores, such as
earthworms, isopods, millipedes and enchytraeids.

Microbes are fed upon by a number of fungivores,

such as springtails, mites and some types of nema-

todes, whereas bacteria are fed upon by various

protozoan groups, such as amoebae, flagellates

and ciliates and other types of nematodes. Fungi-

vores and bacterivores, in turn, are predated by

predaceous mites, spiders and carabid and staphy-

linid beetles (Fig. 6.1). When detritus is added to the

soil there is an increase in microbial biomass, which

is often associated with an increase in soil fauna

biomass (Wardle 2002 and references therein).

The coexistence of so many species also raises

the question of whether all these species are of

importance for the functioning of soil ecosystems.

Many experiments show that ecosystem processes

depend greatly on the diversity of the community

in terms of functional characteristics of the species

present and the abundance and distribution of

these species over time and across space (Loreau

et al.2002;Ha ̈ttenschwileret al. 2005; Hooperet al.

2005). Collectively, decomposer organisms are

Springtails

Oribatid mites

(g)

Oribatid mites

(b)

Prostigmatid

mites

Nematodes (f)

Flagellates

Amoebae

Nematodes (o)

Tardigrades

Predaceous

mites

Spiders

Beetles (o)

Ciliates

Nematodes (b)

Bacteria

Fungi

Detritus Enchytraeids

Figure 6.1The connectedness food web of a coniferous forest soil. The boxes symbolize the functional groups of
organisms and the vectors indicate the direction of the flow of C and nutrients through the web. The white boxes
indicate the bacteria-dominated energy and nutrient flow and the light grey boxes indicate the fungi-dominated flow.
Both flows merge on the level of intermediate and top predators, here in dark grey. The high acidity of the soil prevents
the occurrence of earthworms. g, grazers; b, browsers; f, fungivore; b, bacterivore; o, omnivore. After Berget al. (2001).

70 SPACE AND TIME