The extended consequences ofintraspecific genetic
differences have received ample attention over re-
cent years. Several recent studies show that plant
genotype significantly affects various community
properties, such as disease infection (Roscheret al.
2007), arthropod abundance and diversity (Stiling
and Rossi 1996; Johnson and Agrawal 2005; Craw-
fordet al. 2007) and decomposition rate (Madritch
et al. 2006). For instance, genetic identity of the even-
ing primrose (Oenothera biennis) explained more than
40% of the variation in arthropod diversity (Johnson
and Agrawal 2005). Genotypic differences accounted
for more variation in arthropod community structure
than did environmental variation, indicating the rel-
ative importance of genetic factors. Not only species
interactions are influenced by plant genotype. Ma-
dritchet al. (2006) showed that community properties
such as decomposition and nutrient release from
aspen litter are determined by genotype identity.
Litter from different aspen genotypes differed signif-
icantly in carbon and nitrogen release, most likely
because litter chemistries varied across genotypes
(Madritchet al. 2006).
Most studies have focused on genetic differences
in plants because plant diversity is thought to shape
the composition and dynamics of animal commu-
nities. Only some studies have looked at the effect
of genetic differences at other trophic levels than
primary producers. For instance, plant productivity
itself is to a large extent controlled by association
with arbuscular mycorrhizal fungi (AMF). Experi-
ments with genetically different isolates of the ar-
buscular mycorrhizal fungusGlomus intraradices
show that AMF genotype can affect plant growth
response and the extension of the fungal mycelium
(Munkvoldet al. 2004; Kochet al. 2006). The effect of
genetic identity of Glomus intraradices on plant
growth ranged from enhanced plant growth to no
growth benefit or even a reduced growth, also de-
pending on the environmental conditions (Kochet
al. 2006). Associations with different individuals in
the AMF population will present plants with either
costs or benefits, potentially mediating the outcome
of competition between plants.
Although most studies address the importance of
genotype identity, effects of different phenotypes
may also mediate community processes. Harvey
et al. (2003) examined the effect of differences in a
herbivore’s diet on growth and development of its
primary parasitoid and secondary hyperparasitoid.
Hyperparasitoid body mass and survival were sig-
nificantly larger if the herbivore had fed on plants
containing lower concentrations of glucosinolates, a
chemical defence compound. Hence, phenotypic
plasticity caused by the nutritional differences in
herbivore diet may affect performance across sev-
eral trophic levels.11.4 Effects of genetic and phenotypic diversity on community composition and species diversity
In recent years more and more researchers have
identified the influence of genetic diversity on com-
munity functioning as an emerging frontier in ecol-
ogy (Whithamet al. 2003; Agrawalet al. 2007). Yet it
is not sufficient to merely observe a correlation
between genetic diversity and community proper-
ties. To contribute to the understanding of general
mechanistic principles of community ecology, stud-
ies need to assess the relative importance of ecolog-
ical and genetic factors, and address the mechanism
underlying these correlations.
Individual organisms in communities can con-
tribute in two ways to the genetic diversity of the
community. First, each species represents a discrete
genetic entity, and individuals belonging to differ-
ent species cause an associated rise in genetic diver-
sity within the community. The effect of species
diversity on community stability and function has
been discussed extensively elsewhere (Hooperet al.
2005), so I will touch upon this only briefly. A
consensus is emerging that, if plant species richness
increases, community productivity as well as stabil-
ity increases, although this effect levels off at rela-
tively low species numbers. In the context of this
chapter, I will mainly focus on the other way indi-
vidual organisms may contribute to community
genetic diversity – if individuals represent different
genotypes within species.11.4.1 Effects of genetic diversity on community functioning
Composition and functioning of communities is
thought to be strongly shaped by plant species156 FUTURE DIRECTIONS