Phenotypic plasticity from a predator perspective: empirical and theoretical investigations

Phenotypic plasticity is common in predator-prey interactions. Prey
use inducible defenses to increase their chances of survival in
periods of high predation risk. Predators, in turn, display
inducible offenses (trophic polyphenisms) and adjust their
phenotypes to the prevailing type of prey. In the past, inducible
defenses have received considerably more attention than inducible
offenses. Here, I point out three areas where taking a predator
perspective can increase our understanding of phenotypic plasticity
in predator-prey systems. In Part 1, I describe an inducible
offense in the predatory ciliate Lembadion bullinum: Mean cell size
in a genetically uniform Lembadion population increases with the
size of the dominant prey species. This size polyphenism can be
explained as the result of a trade-off: Large Lembadion are
superior in feeding on large prey, whereas small Lembadion achieve
higher division rates when small prey is available. Consequently,
inducible predator offenses may evolve as adaptations to
environments where important prey characteristics vary over space
or time. In Part 2, I investigate the interplay of Lembadion's
inducible offense with an inducible prey defense. Lembadion
releases a kairomone (i.e. an infochemical) that induces defenses
in several prey species. For example, in the herbivorous ciliate
Euplotes octocarinatus, it triggers the production of protective
lateral "wings". I show that Lembadion can reduce the effect of
this defense by activating its inducible offense. This is one of
the first known examples of reciprocal phenotypic plasticity in a
predator-prey system. While the counter-reaction of Lembadion
decreases the fitness of the prey, it could not be shown to
significantly increase the fitness of Lembadion itself.
Nevertheless, I discuss the hypothesis that phenotypic plasticity
in both species is a result of (diffuse) coevolution. In Part 3, I
further pursue the idea of coevolution and develop a mathematical
model of a coevolving predator-prey pair which displays reciprocal
phenotypic plasticity. In this model, the inducible offense is a
truly effective counter-adaptation to the prey's defense. The model
yields three main conclusions: First, the inducible prey defense
can stabilize predator-prey population dynamics. The effect of the
inducible counter-offense is less clear and depends on the relative
magnitude of its costs and benefits. Second, the maintenance of
phenotypic plasticity requires that both the defense and the
offense are sufficiently strong. Third, preliminary results suggest
that an inducible offense is favored over a constitutive
(permanently expressed) one if and only if the model populations
perform predator-prey cycles. This leads to the hypothesis that
phenotypic plasticity may evolve as an adaptation to temporal
heterogeneity created by the internal dynamics of predator-prey
systems.