Context-dependent defence in terrestrial plants: the effects of light and nutrient availability on plant resistance against herbivory

The context-dependent defence (CDD) hypothesis predicts that defence levels of plant species against herbivory are not fixed but vary with environmental conditions, in a way that is specific for plant species that share evolutionary adaptations to resource conditions exemplified by similar maximum relative growth rates. More specifically, we expected plants from resource-poor environments to display high defence levels but not when grown under resource-rich conditions, whereas the reverse - plants from resource-rich conditions displaying low defence levels but not when grown under resource-poor conditions - is not necessarily the case. In this study, we used multiple-choice bioassays in which leaf discs were fed to larvae of Spodoptera exigua (Hubner) (Lepidoptera: Noctuidae) as an efficient and effective way of indicating plant defence levels. This generalist herbivore was capable of detecting both inter- and intraspecific differences in defence among plant species. The CDD was tested by exploring the effects of various experimental resource conditions (light, nutrients) upon the herbivore preferences and by comparing these preferences with the maximum relative growth rate of plant species. The experimental results provide general support for the CDD hypothesis with respect to nutrient-level variation but the effects were not related to the origin of the plant species tested. Variation in light conditions did not result in consistent effects upon herbivore preferences. The CDD therefore can be formulated more precisely as: defence levels of plant species vary under different environmental conditions but in a way that is specific for plant species that share evolutionary adaptations to similar nutrient conditions. This more precise CDD hypothesis is a useful addition to existing optimal-defence theory because of its focus on the possible plastic effects of resource conditions upon plant defence levels. This is relevant when designing experimental plant-herbivore studies.