When individuals of two species interact, they can adjust their phenotypes in response to their respective partner, be they antagonists or mutualists. The reciprocal phenotypic change between individuals of interacting species can reflect an evolutionary response to spatial and temporal variation in species interactions and ecologically result in the structuring of food chains. The evolution of adaptive phenotypic plasticity has led to the success of organisms in novel habitats, and potentially contributes to genetic differentiation and speciation. Taken together, phenotypic responses in species interactions represent modifications that can lead to reciprocal change in ecological time, altered community patterns, and expanded evolutionary potential of species.

Costs of defense are central to our understanding of interactions between organisms and their environment, and defensive phenotypes of plants have long been considered to be constrained by trade-offs that reflect the allocation of limiting resources. Recent advances in uncovering signal transduction networks have revealed that defense trade-offs are often the result of regulatory “decisions” by the plant, enabling it to fine-tune its phenotype in response to diverse environmental challenges. We place these results in the context of classic studies in ecology and evolutionary biology, and propose a unifying framework for growth–defense trade-offs as a means to study the plant's allocation of limiting resources. Pervasive physiological costs constrain the upper limit to growth and defense traits, but the diversity of selective pressures on plants often favors negative correlations at intermediate trait levels. Despite the ubiquity of underlying costs of defense, the current challenge is using physiological and molecular approaches to predict the conditions where they manifest as detectable trade-offs.

Plant Anti-Insect Armaments: Because individual plants are unable to relocate, they are subject to extreme selection by the insects feeding upon them. One means by which plants suppress herbivory is to produce toxic compounds to deter feeding (see the Perspective by Hare ). Agrawal et al. (p. 113 ) compared pesticide–treated or untreated evening primroses. Over 5 years of pesticide treatment, the production of defensive chemicals in the fruit reduced and flowering times shifted, and the primrose's competitive ability against dandelions improved. Züst et al. (p. 116 ) examined large-scale geographic patterns in a polymorphic chemical defense locus in the model plant Arabidopsis thaliana and found that it is matched by changes in the relative abundance of two specialist aphids. Thus, herbivory has strong and immediate effects on the local genotypic composition of plants and traits associated with herbivore resistance.