indigenous biodiversity (Macket al. 2000). Pheno-
typic plasticity may increase invasive success if an
individual’s plastic response to environmental
change allows it to expand ecological niche breadth
(Baker 1965; Richardset al. 2006). Invasive species
are therefore predicted to be more plastic than their
native community members. Richardset al. (2006)
describe three ways in which phenotypic plasticity
may contribute to invasive success: a jack of all
trades, able to maintain fitness in unfavourable en-
vironments; a master of some, able to profit in fa-
vourable environments; and a jack-and-master,
which combines both. Several studies have demon-
strated that the shape of the reaction norm differs
between native and invasive species; however, the
form of changes and the type of traits showing
differences in plasticity levels seem to be idiosyn-
cratic (Chownet al. 2007; Muth and Pigliucci 2007).
11.5 Effect of community composition on the genetic and phenotypic diversity of single species
So far, I have discussed only the possible conse-
quences of individual diversity on higher level pro-
cesses, and the benefits of adopting a more
evolutionary perspective for community ecologists.
The opposite side of this integrative approach is
obviously that evolutionary biologists should be-
come aware of the community context in which
evolution takes place (Fig. 11.5). Performance of
individual species is typically mediated by inter-
specific interactions, and the identity of interacting
species can determine individual response to
changing conditions (Daviset al. 1998; Jiang and
Morin 2004). Natural selection experiments often
leave a large proportion of evolutionary change
unexplained, perhaps because fine-scale environ-
mental variation due to identity of competitors is
not considered. Several experiments have now re-
vealed that plant response to competition is
mediated by interspecific and intraspecific varia-
tion in community composition (e.g. Vavrek 1998;
Callaway and Aschehoug 2000, Fridleyet al. 2007).
To predict how community diversity may affect
genetic diversity of single species, we can basically
just reverse the causation in the two hypotheses
discussed in the previous section. In this context
the ‘diversity begets diversity’ hypothesis proposes
that species diversity can act as a source of diversi-
fying selection on single species, because each in-
teracting species represents a different competitive
environment. If different genotypes of the focal
species have differential competitive responses,
a positive effect of community diversity on within-
species genetic diversity is predicted. In fact, such
fine-scale environmental variation in selection pres-
sures suggests the occurrence of local co-adaptation
between neighbours. Empirical evidence of such
interactions is found forTaraxacum officinalegeno-
types, which exhibited differential response in root
biomass, total biomass and leaf area depending on
the species identity of the competitor (Plantago
major,Poa pratensisorTrifolium pratense). Some gen-
otypes performed best with a specific competitor,
but others behaved like generalists with nearly
equal performance with all competitors (Vavrek
1998). Similar results, at an even more detailed
level, demonstrate that performance depends on
the genetic identity of both interacting species
(Fridleyet al. 2007). These studies provide strong
support for the diversity begets diversity hypothe-
sis, and give good reason for more empirical em-
phasis on this topic. Although it is obviously not
feasible to include such detailed genetic analysis as
a standard protocol in community ecology, it will
increase our appreciation of the relevance of genetic
diversity in multispecies assemblages and the im-
portance of spatially fluctuating biotic environ-
ments for the maintenance of genetic variation.
The alternative hypothesis on the effect of com-
munity composition on genetic diversity of single
species proposes that a diverse community con-
strains the ability of the focal species to exploit
different niches in the environment. Therefore,
community diversity should act as a source of sta-
bilizing selection, reducing the variation within
species. To my knowledge, no studies have explic-
itly measured the degree of genetic variation of a
focal species in communities with manipulated
community composition. We may tentatively reject
this hypothesis because in experimental commu-
nities with a fixed number of species with either
high or low genotypic diversity, each species re-
tains more genotypes when reared with highly di-
verse heterospecifics (Whitlocket al. 2007). This is160 FUTURE DIRECTIONS