Agriculture Reference
In-Depth Information
2.4.4.2
Community structure anddynamics
Large scale patterns: microbial successions on decomposing substrates
Primary resources often form a spatial mosaic comprising such structures as the decom-
posing twigs, branches, leaves or inflorescences of the individual species present. As a
result, microbial communities which colonise these substrates have a hierarchical structure
(Swift, 1984). On each fragment of the mosaic, a community develops, generally
consisting of a limited number of species.
Swift (1976) found that small pieces of branches of beech-wood, 10 cm long and
2 to 5 cm in diameter were colonised in the early stages by up to nine species with
a modal value of four. Fifty six species were identified among the 32 branch samples;
most of these species were found on a small number of branches and only five species
were found on more than five branches. The efficiency of the different 'unit-communities',
expressed in terms of decomposition rate, was rather similar.
As predicted by the islands theory of MacArthur and Wilson (1967), the number of
species is proportional to the size of the substrate. According to Swift (1976) two main
processes determine the dynamics of such unit-communities: stress
and
disturbance
(Grime, 1979; Pugh, 1980; Boddy
et al.,
1988). Stress is defined by Grime as the
external constraints which limit the production of plants. Disturbances are unpredictable
events which limit the plant biomass by destroying it, partly or entirely.
On fresh substrates, stress is first represented by the initial quality of the resource
(
e.g.,
high C:N ratio, lignin and polyphenol contents). At later stages, shortage of
substrate, unfavourable microclimatic conditions or antibiosis are common stresses that
micro-organisms are subject to. Disturbances are by definition unpredictable events. This
may be the fall of a dead branch which may occur after it has lost 20 to 80 % of its
weight. Another frequent example of disturbance is the ingestion of the substrate by
a soil invertebrate or the grazing of growing fungi. Microclimatic events such as drought
or frost are other possible disturbances.
Stress leads to successions of communities over time. Species disappear when they can
no longer adapt to the changed conditions of the substrate and new colonisers replace
them. The general pattern is an increase in the number of species to a maximum value
and a decrease when the substrate becomes exhausted or unsuitable for micro-organisms
(Figure III.12). Disturbance generally breaks the dynamic equilibrium between species
by accelerating the extinction of species and the immigration of new colonisers.
Micro-organisms may adapt through two supra-strategies: the 'occupation' and the
'window' strategies. Micro-organisms of the first group are adapted to persist on a
changing resource. In the second group, the strategy is one of rapid colonisation when
conditions are suitable and dispersion when they have changed. Within each of these two
types,
r
and
K
strategies occur, as defined by Pianka (1970) or 'ruderal',' stress-tolerant'
and 'competitive' as defined by Grime (1979) for higher plants.
In such a context, the formation and evolution of unit-communities appears to be
a highly stochastic process. Important differences are expected to occur however,
depending on the nature and size of the substrate; communities certainly behave
differently on large substrates such as decomposing logs, in comparison to those on
leaves or small inflorescences.
