diversity, hence the effect of genetic variation in the
dominant plant species is the best studied. Since
plant genotypes can exploit slightly different re-
sources, analogous to population-level effects of
within-species genetic diversity, the prediction is
that genetic diversity enhances productivity or re-
source efficiency of the plant, thereby increasing the
total availability of resources in the community.
The most straightforward experimental approach
is to manipulate genotypic diversity in the commu-
nity and compare indicators of community perfor-
mance, such as plant productivity, carbon storage,
nutrient acquisition or decomposition rate. A flurry
of experimental work has now shown that
increased plant genotypic diversity can explain a
significant proportion of the community properties,
including an increased net primary productivity
(Crutsingeret al. 2006), higher fruit production
(Johnsonet al. 2006), higher resistance to invasion
(De Meesteret al. 2007) and accelerated decomposi-
tion rate (Madritchet al. 2006). Particularly in spe-
cies-poor communities the effects of within-species
diversity have close parallels to benefits at popula-
tion level.
It is unlikely that the positive effects of plant genet-
ic diversity on community functioning are solely due
to sampling or selection effects. The sampling effect
can be observed if diverse mixtures have a higher
chance of containing highly productive genotypes
(Huston 1997). The positive effect of genotypic diver-
sity was not due to greater chances of obtaining
mixtures with more productive genotypes in diverse
communities (Crutsingeret al. 2006). Similar to the
benefits of genetic diversity at population level, the
complementarity principle seems to be the major
determinant of the increased fitness. Complementar-
ity may indicate that mixed genotypes facilitate one
another (Hectoret al. 1999; Mulderet al. 2001), or
niche differentiation among different genotypes
causes the available resources to be used more
completely (Loreau and Hector 2001).
A major limitation of the former studies is that
nearly all have focused on plant genetic diversity in
aboveground communities. A badly needed next
step is to test the generality of the effect of within-
species genetic diversity, by including higher tro-
phic levels such as herbivores. Also, little is known
about how genetic diversity of soil organisms af-
fects rates of decomposition and nutrient availabili-
ty. Soil processes, in particular, appear to be
influenced primarily by the functional characteris-
tics of dominant species rather than by the number
of species present (Heemsbergenet al. 2004). As a
final remark, genetic diversity is mostly defined as
the number of genotypes included in the communi-
ty. The actual extent of genetic differentiation be-
tween genotypes, however, is not known, nor is the
level of phenotypic differentiation or functional dis-
similarity among genotypes included. Given the
weakness of the relation between genetic diversity
and variation in quantitative traits, the present con-
clusion that genotypic diversity has community
benefits through complementarity still greatly
lacks detailed understanding.11.4.2 Diversity begets diversity?
Can within-species variation also enhance species
diversity of the associated animal communities?
Different genotypes of the dominant plant species
may favour different species in competitive and
trophic interactions, leading to a mosaic of spatially
varying selection pressures with a distinct set of
associated arthropod species. This is known as the
‘diversity begets diversity’ hypothesis (Whittaker
1975; Vellend and Geber 2005). On the other hand,
genetic diversity enhances productivity and re-
source efficiency, such that the competitive strength
of genetically diverse species increases and may
lead to competitive exclusion of other species from
the community (Vellend and Geber 2005). Al-
though such a hypothesis would predict that genet-
ic diversity lowers community species diversity, it
does not account for the positive effect of the in-
crease in available energy on the number of herbi-
vores and predator species that can be sustained,
the so-called ‘more individuals hypothesis’ (Srivas-
tava and Lawton 1998). The relative strength of
species exclusion versus energy availability deter-
mines the sign of the relationship between genetic
diversity and species richness.
In fact, experimental studies present overwhelm-
ing evidence that plant genotypic diversity is posi-
tively correlated with arthropod abundance
(Reuschet al. 2005; Crawfordet al2007) and arthro-
pod diversity (Dungeyet al. 2000; Johnson andEVOLUTIONARY PROCESSES IN COMMUNITY ECOLOGY 157