secondary consumers, other studies mention a real “selectivity” of the preys by their predators. Some nematodes, for
example, have a buccal morphology adapted to feed on
either bacteria (bacterial feeders) or fungi (hyphal feeders)
(Yeates et al. 1993). It is also known that some heterotrophic
unicellular eukaryotes are able to detect chemical signals to
differentiate between bacterial and microalgal potential
preys (Verity 1991). This selective consumption of preys
can have an important impact on ecosystem functioning. In
forest systems, for example, the selective feeding of the
fungus Marasmius androsaceus by Onychiurus latus
(Collembola) induces a decrease in abundance of the fungus
and allows an increase in density of another fungus species
Mycena galopus. The latter is not used as food source by this
collenbolan. This change in composition of the fungal community induces a decrease of the degradation rate of conifer
litters resulting from a difference in efficiency between M.
androsaceus and M. galopus, the latter being less efficient
for litter degradation.
Predation of a microorganism species by a consumer may
also stimulate the prey activity without any influence of the
competition processes. Hedlund et al. (1991) have shown
that the specific amylase activity of the fungi Mortierella
isabellina is higher when this fungus is consumed by a
collembolan than when it is protected against predation.
Finally, among the effects of larger organisms on the
communities which are involved in litter degradation as
fungi and heterotrophic prokaryotes, the ingestion of these
microorganisms and their exposure to the digestive activity
of the consumers must be considered. On this subject, literature data are contrasted. Brown (1995) has shown that bacterial community is more abundant after its passage through
the gut of earthworms although Moody et al. (1996) have
observed a decrease in spore germination of fungi following
their ingestion by an earthworm.
13.3.4.3 Role of Other Biotic and Abiotic
Interactions
Some interactions are more based on physical modifications
of the habitat than on trophic relationships. They have a
drastic importance in soil functioning. This is the case for
organisms which ingest phytal detritus and produce fecal
pellets having smaller sizes than the ingested material. The
production of these biological aggregates fundamentally
modifies physical characteristics of soils (e.g., high compaction, improved ability to retain water). This modified litter
becomes a very attractive medium for bacteria resulting in a
considerable increase of the bacteria/fungi ratio. For example, a multiplication by 100 of bacteria and only by 3 for
fungi has been shown in fecal pellets of arthropods, when
compared with the initial material. It is likely that the compaction of the material is the main factor involved in this
differential effect. This effect can be considered as a
mutualistic interaction since (1) the stimulation of the
microbiota in fecal pellets goes with an increase of the OM
mineralization and (2) several ingestions of the modified
pellets by the fauna contribute to improve its diet. This
system functions like an “external rumen” and becomes
very important when the fauna feed on a litter which is still
not modified by microbial communities and has a low nutrient content.
Besides the mutualistic interaction described above, the
engineer organisms of ecosystems and engineer organisms
of soils induce physical modifications of the environment
and create new habitats for microorganisms and microfauna
of soils. This is the case for earthworms, termites, and ants.
As an example, many authors have shown that microbiota
was stimulated in structures created by earthworms both
through an improvement of oxygenation and hydric
conditions of soils and by favoring the accessibility of
resources (e.g., litter burying) (Brown 1995). It should be
noted that in some cases, this stimulating effect is only a
transient effect and that, at long term, these structures can, in
contrast, reduce the degradation of OM.
The input of OM via the phytal litter constitutes a fundament of the functioning of these webs including a very high
diversity of telluric organisms which are mainly
heterotrophs. This is close to the “detritus food chain” of
aquatic ecosystems, mentioned in the introduction. Under a
considered climactic and edaphic context, both the amount
and the quality of litters strongly control the composition of
food webs. Studies that focus on understanding how this
structure is regulated in the soil by the environment were
rare a few years ago (Wardle 2002), but their numbers are
currently increasing. This is particularly due to the new
methodological developments such as the use of the natural
variations of abundance of C and N stable isotopes (
13 C/
12 C
and
15 N/
14 N) (Schmidt et al. 2004).
13.4 Factors Controlling Microbial Food
Webs
13.4.1 Factors Controlling Pelagic Ocean
and Lake Microbial Food Webs
In pelagic environments, different food web structures can
coexist. The predominance of a food web relative to another
one is controlled by environmental forcing factors such as
the nature and concentration of nutrients, temperature
(responsible for stratification or vertical mixing), and pollution. Thus, the difference in structure of the pelagic food
webs between the offshore and coastal marine environments
is linked to different types of environmental forcing factors
exerted in these ecosystems.
500
B. Mostajir et al.
example, have a buccal morphology adapted to feed on
either bacteria (bacterial feeders) or fungi (hyphal feeders)
(Yeates et al. 1993). It is also known that some heterotrophic
unicellular eukaryotes are able to detect chemical signals to
differentiate between bacterial and microalgal potential
preys (Verity 1991). This selective consumption of preys
can have an important impact on ecosystem functioning. In
forest systems, for example, the selective feeding of the
fungus Marasmius androsaceus by Onychiurus latus
(Collembola) induces a decrease in abundance of the fungus
and allows an increase in density of another fungus species
Mycena galopus. The latter is not used as food source by this
collenbolan. This change in composition of the fungal community induces a decrease of the degradation rate of conifer
litters resulting from a difference in efficiency between M.
androsaceus and M. galopus, the latter being less efficient
for litter degradation.
Predation of a microorganism species by a consumer may
also stimulate the prey activity without any influence of the
competition processes. Hedlund et al. (1991) have shown
that the specific amylase activity of the fungi Mortierella
isabellina is higher when this fungus is consumed by a
collembolan than when it is protected against predation.
Finally, among the effects of larger organisms on the
communities which are involved in litter degradation as
fungi and heterotrophic prokaryotes, the ingestion of these
microorganisms and their exposure to the digestive activity
of the consumers must be considered. On this subject, literature data are contrasted. Brown (1995) has shown that bacterial community is more abundant after its passage through
the gut of earthworms although Moody et al. (1996) have
observed a decrease in spore germination of fungi following
their ingestion by an earthworm.
13.3.4.3 Role of Other Biotic and Abiotic
Interactions
Some interactions are more based on physical modifications
of the habitat than on trophic relationships. They have a
drastic importance in soil functioning. This is the case for
organisms which ingest phytal detritus and produce fecal
pellets having smaller sizes than the ingested material. The
production of these biological aggregates fundamentally
modifies physical characteristics of soils (e.g., high compaction, improved ability to retain water). This modified litter
becomes a very attractive medium for bacteria resulting in a
considerable increase of the bacteria/fungi ratio. For example, a multiplication by 100 of bacteria and only by 3 for
fungi has been shown in fecal pellets of arthropods, when
compared with the initial material. It is likely that the compaction of the material is the main factor involved in this
differential effect. This effect can be considered as a
mutualistic interaction since (1) the stimulation of the
microbiota in fecal pellets goes with an increase of the OM
mineralization and (2) several ingestions of the modified
pellets by the fauna contribute to improve its diet. This
system functions like an “external rumen” and becomes
very important when the fauna feed on a litter which is still
not modified by microbial communities and has a low nutrient content.
Besides the mutualistic interaction described above, the
engineer organisms of ecosystems and engineer organisms
of soils induce physical modifications of the environment
and create new habitats for microorganisms and microfauna
of soils. This is the case for earthworms, termites, and ants.
As an example, many authors have shown that microbiota
was stimulated in structures created by earthworms both
through an improvement of oxygenation and hydric
conditions of soils and by favoring the accessibility of
resources (e.g., litter burying) (Brown 1995). It should be
noted that in some cases, this stimulating effect is only a
transient effect and that, at long term, these structures can, in
contrast, reduce the degradation of OM.
The input of OM via the phytal litter constitutes a fundament of the functioning of these webs including a very high
diversity of telluric organisms which are mainly
heterotrophs. This is close to the “detritus food chain” of
aquatic ecosystems, mentioned in the introduction. Under a
considered climactic and edaphic context, both the amount
and the quality of litters strongly control the composition of
food webs. Studies that focus on understanding how this
structure is regulated in the soil by the environment were
rare a few years ago (Wardle 2002), but their numbers are
currently increasing. This is particularly due to the new
methodological developments such as the use of the natural
variations of abundance of C and N stable isotopes (
13 C/
12 C
and
15 N/
14 N) (Schmidt et al. 2004).
13.4 Factors Controlling Microbial Food
Webs
13.4.1 Factors Controlling Pelagic Ocean
and Lake Microbial Food Webs
In pelagic environments, different food web structures can
coexist. The predominance of a food web relative to another
one is controlled by environmental forcing factors such as
the nature and concentration of nutrients, temperature
(responsible for stratification or vertical mixing), and pollution. Thus, the difference in structure of the pelagic food
webs between the offshore and coastal marine environments
is linked to different types of environmental forcing factors
exerted in these ecosystems.
500
B. Mostajir et al.