Competition and time of damage affect the pattern of selection acting on plant defense against herbivores

In this paper I examine the effects that the competitive environment in which plants are grown and the timing of herbivore damage have on the expression and pattern of selection acting on plant tolerance to herbivory and resistance to herbivores. I conducted a field experiment involving 720 plants from 24 full-sib families of the common morning glory, Ipomoea purpurea. Plants were grown in one of two competitive environments, and the levels of resistance and tolerance to naturally occurring folivory expressed by the 24 families were estimated three times during the growing season: soon after plants emerged, when plants began to flower, and when seed capsules began to mature. Low- and high-competition environments were established using different densities of interspecific competitors. Herbivory had a negative effect on plant fitness in both competitive environments, and consistent with the compensatory continuum hypothesis, herbivory was more detrimental in the high- than low-competition environments. Phenotypic analyses also revealed that early- and late-season damage were more detrimental than mid-season herbivore damage. Although fitness was more negatively affected by herbivore damage in the high- than low-competition environments, competition did not affect the pattern of selection acting on defense traits. Similarly, the patterns of selection acting on either resistance or tolerance to early-, mid-, and late-season herbivory did not differ significantly. Therefore, there was little evidence that environmental or seasonal differences in the pattern of selection acting on defense traits will constrain their evolution.