Community Ecology Processes, Models, and Applications

among genotypes. However, these experiments
have yet to be performed.
Some researchers refute a mechanistic approach
to community ecology on the basis of the inherent
complexity of multiple species interactions, and
state that, owing to non-additivity of interactions,
the dynamics of communities cannot be predicted
from the individual characteristics of the compo-
nent species (Werner 1992; Sihet al. 1998). Howev-
er, these early studies did not take the effect of
phenotypic plasticity into account. If interacting
species differ in their response to environmental
conditions, environmental change can alter the
sign and magnitude of interactions among species
(Fig. 11.4), for which Abrams (1995) coined the term
‘trait-mediated interactions’. In addition to trait-
mediated interactions, interacting species may di-
rectly modify each others’ gene expression or phe-
notype by chemical communication, as in the case
of induced defences. Condition dependence of in-

terspecific interactions transforms the standard ge-

notype by environment (G
E) interaction into a

three-way interaction between two species and the

environment (G
G
E; Fig. 11.5). Trait-mediated

interactions appear to be common in nature; for

instance, the presence of predators may influence

foraging behaviour of prey and decrease prey

growth rate (Peacor and Werner 2000; Prasad and

Snyder 2006). Although identifying how each spe-

cies alters its traits in the presence of others may be

a daunting task, Relyea and Yurewicz (2002) show

that this approach yields accurate qualitative pre-

dictions on the outcome of mesocosm experiments.

More simply measured species traits such as body

size do not influence community system function-

ing in the long run, and seem to be merely effects of

initial experimental conditions (Long and Morin

2005). Including the details of such phenotypically

plastic responses in theoretical models can affect

community dynamics and stabilize tritrophic sys-

tems (Bolkeret al. 2003; Verschooret al. 2004).

11.4.4 Phenotypic plasticity and invasive success

Phenotypically plastic responses have also been

implied in the context of invasion success. Invasive

species can dramatically change community com-

position owing to their negative effects on

Nutrient availability Absent

Plant growth rate Prey feeding rate

Present

A

A

(a) (b)

Trait-mediated interactions

B

B

Predator

Figure 11.4The role of phenotypic plasticity in trait-
mediated interactions illustrated by two hypothetical
examples. (a) A switch in competitive dominance under
increased nutrient availability. Plant species A and B have
a differential growth rate response to nutrient availability
indicated by the steepness of the reaction norm. Under
low nutrient availability, species B has the highest growth
rate, but species A is better able to profit from high
nutrient availability. Hence competitive dominance
switches when nutrient availability is high. (b) Predator
presence changes competitive interactions of two
species. Species B has a higher feeding rate when
predators are absent, but is also more sensitive to
predation than species A. Therefore, when predators are
present, species B has to lower its feeding rate to avoid
predation, while species A can continue to feed at the
same rate and now outcompetes species B.

Genetic and phenotypic trait

diversity(species 1)

Genetic and phenotypic trait

diversity(species 2)

Environmental

conditions

Selection

Phenotypic plasticity

Phenotypic plasticity

Selection

Interspecific interactions

(e.g. predation, competition)

Figure 11.5The three-way interaction between two

species and their abiotic environment (G
G
E).

Environmental conditions induce selection or phenotypic

plasticity in traits in component species, but not

necessarily of the same strength. Performance traits of

species may also be mediated by direct interactions such

as competition or physical interactions. For illustrational

purposes, only two species are shown here but any

number can be included.

EVOLUTIONARY PROCESSES IN COMMUNITY ECOLOGY 159