Community Ecology Processes, Models, and Applications

1962) and fauna (Dilly and Irmler 1998). As a result,

successive organic horizons yield different local

connectedness food webs (Fig. 6.5). Small-sized

organisms, such as microbes and Protozoa, are

highly abundant, but their occurrence is vertically

localized because of adaptation to particular organ-

ic matter qualities, in combination with low dis-

persal ability. Fungivores, like springtails and

mites, and small predaceous mites may couple

local food webs by vertical migration (Seta ̈la ̈and

Aarnio 2002), often induced by fluctuations in

temperature or soil moisture content (Brioneset al.

1997), or spatial dimensions in resource use. From a

food web model perspective the variance in local

vertical food web structure and the structural

differences between local food webs versus one

aggregated food web is as challenging as for

horizontal space.

Berget al. (2001) have shown, in one of the few

food web studies in which vertical space is explicit-

ly taken into account, that the rates of C and N

mineralization may strongly depend on vertical

stratification of functional groups and are related

to differences in food web composition between soil

compartments. The observed significant decrease

in C and N losses from litter to humus (50 mg

C/g/year in L to 1.5 mg C/g/year in H; 0.9 mg

N/g/year in L to 0.05 mg N/g/year in H) corre-

lates strongly with model simulations of C and

N mineralization rates for the three successive

a b

c

Figure 6.5A vertical spatially explicit connectedness food web of three trophic groups (1, microbes; 2, microbivores; 3,
predators) with 10 species (indicated with a letter). (a) Distribution of the 10 species (boxes) in vertical space. The length
of the boxes indicates the amount of vertical space occupied by a species. The dashed lines separate the organic
horizons. (b) The potential connectedness food web showing ecologically possible feeding interactions (lines) between
the 10 species (boxes). (c) Three samples from subsequent organic horizons (L, litter; F, fragmented litter; H, humus)
result in three local or actual food webs. The actual food web contains a subset of the species and feeding interactions of
the potentially possible food web.

78 SPACE AND TIME