Community Ecology Processes, Models, and Applications

in many community studies and food models,
while variability in community composition over
time and space is often ignored (Berg and Bengts-
son 2007). To improve our understanding of how

structural changes in community composition af-

fect ecosystem processes, and vice versa, we should

adopt a more spatio-temporal approach in our

studies.

Depth soil

cm

Actual food webs

Space

Time (log scale)

% remaining mass

L F H

1

2

(^03)
25
50
0
L
F
H
Parent
material
1 2 3 4 5 6
75
0
100
200
300
400
100
Mass loss
Figure 6.6Model for mass loss and rate-regulating factors during degradation of detritus (left; after Berg and Laskowski
2006). Phase I curve (equals litter), mass loss is due to loss of water-soluble substances and non-lignified carbohydrates.
Mass loss is stimulated by high levels of N, P and S. Trophic interactions result in nutrient immobilization. Phase II curve
(equals fragmented litter), mass loss is due to loss of lignified carbohydrates, lignin and lignin-like compounds. Mass loss
is stimulated by Mn and suppressed by high levels of N. Trophic interactions result in nutrient mobilization. Phase 3 curve
(equals humus), mass loss reaches its limit value; concentrations of lignin and lignin-like compounds are about constant.
Trophic interactions result in nutrient mobilization. Over time, which corresponds with across depth, the influence of
climate on decomposition declines, lignin and N concentrations increase and the degradation rate approaches zero.
Particle size strongly declines, resulting in a dominance of bacteria over fungi from litter to humus (bar diagram above
the model curve). White bars, bacterial biomass; black bars, fungal biomass for litter, fragmented litter and humus,
respectively. They-axis shows biomass C in g C/g dry soil/year (after Berget al. 2001). Physico-chemical changes in
detritus over time, down the soil profile, result in local, horizon-specific food webs (right), often with different species
compositions (Fig. 6.3). Species with high trophic position and an extensive vertical resource use may couple local food
webs. The role of soil biota often shifts from the top layer to deeper organic horizons, from immobilization of nutrients in
litter to mobilization of nutrients in fragmented litter and humus. L, litter; F, fragmented litter; H, humus.
80 SPACE AND TIME