Abiotic factors are inextricably linked with organismal traits and interaction strategies, and global change threatens to fundamentally alter the abiotic environment. The links between plant chemistry and the abiotic environment have long been of ecological interest, and work has also been done to link these changes to plants' direct biotic interactions, such as herbivory. An outstanding question is to what extent changes in the abiotic environment affect the nature and fitness effects of plants' indirect biotic interactions, including plant mutualisms. In this talk, I will evaluate evidence for the hypothesis that nitrogen addition and drought, two avenues of global change, affect plant metabolism, with indirect effects on plant protective mutualisms with hemipteran insects and ants. We conducted nitrogen-addition and water-stress experiments with several species of milkweed plants (Asclepias spp.) to evaluate the effects of these changes in the abiotic environment on plant chemistry, aphid performance, honeydew chemistry, and the defensive behavior of aphid-tending, honeydew-feeding ants.
In nitrogen-limited fields, Asclepias incarnata plants grew bigger with higher nitrogen, but there were no effects on plant carbon:nitrogen stoichiometry, the chemical composition of aphid honeydew, or the density of associated insects. Nitrogen thus increased plant fitness in a manner apparently independent of plants' biotic interactions. Water stress, in contrast, appeared to alter milkweed chemistry and aboveground interactions with aphids, ants, and folivorous insects, but these effects depended on plant species. Ongoing analyses will determine how and why these effects depend on species, as well as how indirect mutualisms affect milkweed fitness under water stress. We conclude that the effects of these two avenues of global change differ in part because they involve addition or subtraction, respectively, of the plant's limiting nutrient in different habitats. Where plants are nutrient-limited, indirect protective mutualisms may partially compensate for the fitness cost of nutrient limitation.