question is: are changes in microbial activity levels,
observed between treatments or in response to a disturbance
related to concomitant changes in diversity of these microbial communities (Cavigelli and Robertson 2000)? Very
often, the results show that changes in activity of a microbial
community are not or poorly related to changes in composition/diversity of this community. Thus, the operation of
methanogenic bioreactors is maintained over time despite
significant temporal variations in the composition of the
microbial community in the bioreactor (Fernandez et al.
1999). Similarly, the impact of different types of organic
and mineral fertilization on the activity of the denitrifying
community cannot be systematically linked to simultaneous
changes in the diversity of the community (Enwall et al.
2005). However, some studies have reported significant
correlations between microbial diversity and functioning.
For instance, intake of mercury can induce a reduction in
the diversity of the bacterial community of soils concomitant
with a reduction in respiration and in resistance of this
function against disruption by heat (Muller et al. 2002).
It has recently been shown that changes in the activity of
microbial communities in situ are more correlated with some
aspects of the diversity of these communities (such as abundance of some dominant species) than to their diversity
taken very broadly (Patra et al. 2006). These authors showed
that changes in the activity of the soil nitrifying, denitrifying
or N-fixing communities observed between differently managed grassland ecosystems are weakly correlated with
changes in overall diversity of this community but strongly
correlated to changes in relative abundance of some major
populations of nitrifiers, denitrifiers, or N-fixers, respectively. Moreover, the response of the activity of the nitrifying community of grassland soil after a nitrogen input
strongly depended on the type of dominant nitrifying
populations, only some of them being able to ensure a high
level of nitrification in the presence of high levels of urea
(Webster et al. 2005). However, these studies do not really
target the causal relationships between microbial diversity
and ecosystem functioning. Indeed, the impact of diversity
changes on ecosystem functioning can be confused with the
impact of changes in environmental factors, because some of
these factors covary with the microbial diversity in situ. In
addition, the abundance of microorganisms can also vary
between the situations studied, which can largely explain
changes in the level of activity of these communities rather
than community diversity (Attard et al. 2011).
Another way to assess the importance of the diversity of
microbial communities for their functioning in situ, while
avoiding some of the biases presented above, is to “displace”
the microbial communities from one environment to another,
or to compare the functioning of microbial communities
retrieved from different environments displaced into a unique
environment. It was found that after transplantation of soil
cores between forest and grassland sites, the denitrification
activity varied while the diversity of denitrifiers did not change
(Boyle et al. 2006). This type of study allows one in some cases
to show that the functioning of microbial communities is more
linked with environmental conditions than to their diversity.
In all the field studies described above, the experimenter
does not choose the diversity of the communities to be
studied and can only use the natural microbial communities
with their preexisting diversity. However, experimental
approaches permit to manipulate microbial diversity under
conditions as controlled as possible and with equal population levels are necessary to analyze the causal relationships
(or lack thereof) between diversity and functioning of microbial communities.
An approach to study more thoroughly the relationship
between microbial diversity and ecosystem functioning is
species removal. This approach consists in reducing, by
following a predefined scenario, the diversity of microbial
communities present in situ. The possible changes induced
by this diversity erosion on ecosystem functioning are then
characterized. In a study on soil microbial communities
(Griffiths et al. 2001), it was shown that the decomposition
of organic matter (mostly done by soil microorganisms) was
not affected by erosion of microbial diversity. It has even
been shown that the functioning and the resistance and
resilience capabilities are largely insensitive to a marked
reduction of diversity even for microbial communities with
reduced diversity and providing specialized functions, such
as the denitrifying and nitrifying communities (Wertz et al.
2007). In this last study, it was estimated that more than
99 % of the soil bacterial taxa had been removed without
affecting the functioning of microbial communities.
Another approach to study the diversity-functioning
relationships is to proceed by assembling species. Synthetic
microbial communities with different levels of diversity are
made in order to study their functioning. The approach
allows one to analyze the interaction mechanisms and the
complementarity between species that explain the observed
biodiversity-functioning relationships. Such studies have
been conducted on different fungal microorganisms or bacteria (Bell et al. 2005; van der Heijden et al. 1998). Some of
these studies have shown a positive relationship between
level of functioning and richness of the communities studied. However, the maximum number of species used in these
approaches never did exceed a few tens, which is not commensurate with the richness of natural microbial
communities. Using this approach, Salles and collaborators
(2009) demonstrated the paramount importance of functional
diversity among bacteria for diversity-functioning
relationships and proposed a simple index, called community
niche, to predict the functioning of bacterial assemblages.
8 Biodiversity and Microbial Ecosystems Functioning
279
observed between treatments or in response to a disturbance
related to concomitant changes in diversity of these microbial communities (Cavigelli and Robertson 2000)? Very
often, the results show that changes in activity of a microbial
community are not or poorly related to changes in composition/diversity of this community. Thus, the operation of
methanogenic bioreactors is maintained over time despite
significant temporal variations in the composition of the
microbial community in the bioreactor (Fernandez et al.
1999). Similarly, the impact of different types of organic
and mineral fertilization on the activity of the denitrifying
community cannot be systematically linked to simultaneous
changes in the diversity of the community (Enwall et al.
2005). However, some studies have reported significant
correlations between microbial diversity and functioning.
For instance, intake of mercury can induce a reduction in
the diversity of the bacterial community of soils concomitant
with a reduction in respiration and in resistance of this
function against disruption by heat (Muller et al. 2002).
It has recently been shown that changes in the activity of
microbial communities in situ are more correlated with some
aspects of the diversity of these communities (such as abundance of some dominant species) than to their diversity
taken very broadly (Patra et al. 2006). These authors showed
that changes in the activity of the soil nitrifying, denitrifying
or N-fixing communities observed between differently managed grassland ecosystems are weakly correlated with
changes in overall diversity of this community but strongly
correlated to changes in relative abundance of some major
populations of nitrifiers, denitrifiers, or N-fixers, respectively. Moreover, the response of the activity of the nitrifying community of grassland soil after a nitrogen input
strongly depended on the type of dominant nitrifying
populations, only some of them being able to ensure a high
level of nitrification in the presence of high levels of urea
(Webster et al. 2005). However, these studies do not really
target the causal relationships between microbial diversity
and ecosystem functioning. Indeed, the impact of diversity
changes on ecosystem functioning can be confused with the
impact of changes in environmental factors, because some of
these factors covary with the microbial diversity in situ. In
addition, the abundance of microorganisms can also vary
between the situations studied, which can largely explain
changes in the level of activity of these communities rather
than community diversity (Attard et al. 2011).
Another way to assess the importance of the diversity of
microbial communities for their functioning in situ, while
avoiding some of the biases presented above, is to “displace”
the microbial communities from one environment to another,
or to compare the functioning of microbial communities
retrieved from different environments displaced into a unique
environment. It was found that after transplantation of soil
cores between forest and grassland sites, the denitrification
activity varied while the diversity of denitrifiers did not change
(Boyle et al. 2006). This type of study allows one in some cases
to show that the functioning of microbial communities is more
linked with environmental conditions than to their diversity.
In all the field studies described above, the experimenter
does not choose the diversity of the communities to be
studied and can only use the natural microbial communities
with their preexisting diversity. However, experimental
approaches permit to manipulate microbial diversity under
conditions as controlled as possible and with equal population levels are necessary to analyze the causal relationships
(or lack thereof) between diversity and functioning of microbial communities.
An approach to study more thoroughly the relationship
between microbial diversity and ecosystem functioning is
species removal. This approach consists in reducing, by
following a predefined scenario, the diversity of microbial
communities present in situ. The possible changes induced
by this diversity erosion on ecosystem functioning are then
characterized. In a study on soil microbial communities
(Griffiths et al. 2001), it was shown that the decomposition
of organic matter (mostly done by soil microorganisms) was
not affected by erosion of microbial diversity. It has even
been shown that the functioning and the resistance and
resilience capabilities are largely insensitive to a marked
reduction of diversity even for microbial communities with
reduced diversity and providing specialized functions, such
as the denitrifying and nitrifying communities (Wertz et al.
2007). In this last study, it was estimated that more than
99 % of the soil bacterial taxa had been removed without
affecting the functioning of microbial communities.
Another approach to study the diversity-functioning
relationships is to proceed by assembling species. Synthetic
microbial communities with different levels of diversity are
made in order to study their functioning. The approach
allows one to analyze the interaction mechanisms and the
complementarity between species that explain the observed
biodiversity-functioning relationships. Such studies have
been conducted on different fungal microorganisms or bacteria (Bell et al. 2005; van der Heijden et al. 1998). Some of
these studies have shown a positive relationship between
level of functioning and richness of the communities studied. However, the maximum number of species used in these
approaches never did exceed a few tens, which is not commensurate with the richness of natural microbial
communities. Using this approach, Salles and collaborators
(2009) demonstrated the paramount importance of functional
diversity among bacteria for diversity-functioning
relationships and proposed a simple index, called community
niche, to predict the functioning of bacterial assemblages.
8 Biodiversity and Microbial Ecosystems Functioning
279