Community Ecology Processes, Models, and Applications

is therefore undesirable. Environmental conditions
triggering phenotypic plasticity explicitly include
the biotic environment as well. Well-known exam-
ples of phenotypic plasticity induced by species
interactions are the production of defensive plant
compounds in response to herbivory and the induc-
tion of anti-predator defences.
Genotypes may differ in their response to envi-
ronmental change, i.e. have different levels of phe-
notypic plasticity. In other words, genotypes that
have similar phenotypes under one environmental
condition may diverge in their performance when
the environment changes, or vice versa. A common
phenomenon causing seemingly monomorphic po-
pulations is canalization: genotypes have all
evolved to identical optimal trait values under the
most common environmental condition (Schlicht-
ing and Pigliucci 1998). Exposure to more extreme

conditions, however, often reveals hidden variation

for such traits (Fig. 11.2b). For example, now that

global change causes extreme temperatures to be-

come more common, genetic variation hitherto un-

exposed to selection will become apparent and

important for survival. Overwhelming evidence

now exists that the degree of phenotypic plasticity

is genetically determined (Scheiner and Lyman

1989; Loeschkeet al. 1999) and that natural selection

can lead to local adaptation in plasticity levels

(Liefting and Ellers 2008). There is a growing ap-

preciation of the potential of phenotypic plasticity

to modify species interactions.

11.3 Proof of principle: community

properties result from genetic identity

and selection at the level of individual

organisms

How do population genetic processes extend to the

community level? Population genetic theory could

equally well apply to communities if genes have

extended phenotypes (Dawkins 1982; Whitham

et al. 2003). Genes with extended phenotypes affect

not only the individual carrying the genes but also

the performance of associated species in the com-

munity. For example, we can think of genetic dif-

ferences in secondary chemical compounds that

affect plant defensive capability against foliar her-

bivores (Havill and Raffa 2000; Harveyet al2003).

Althoughinterspecificdifferences in allelochemical

composition of host plants have long been recog-

nized as an important factor structuring commu-

nities (Dungeyet al. 2000; Sznajder and Harvey

2003; Wimpet al. 2005), the potential effect ofintra-

specificdifferences on community composition and

functioning has been acknowledged only relatively

recently. The performance and abundance of herbi-

vores can be expected to be influenced in a similar

way by intraspecific variation in host plants,

provided that the magnitude of genetic differences

is large enough to detect extended phenotype ef-

fects. Other examples of traits with extended phe-

notypes are induction of morphological anti-

predator defences in many amphibians, or varia-

tion in thermal tolerance of algae in coral commu-

nities which is related to bleaching mortality

(Fabriciuset al. 2004).

(a)

Performance(b)

Environment

Figure 11.2(a) Schematic representation of different
reaction norms, showing continuous reaction norms that
differ in the degree of sensitivity of the trait (slope of the
reaction norm) and in the maximum performance (height
of the peaks), but not in optimal value of the
environmental variable. (b) Canalized reaction norms
show the same phenotype under average environmental
conditions but deviate in performance at either extreme
of the range of environmental conditions.

EVOLUTIONARY PROCESSES IN COMMUNITY ECOLOGY 155