PARENT SESSION
Oral Session #78: Herbivory: Effects on Plants.
Presiding: C. Ivey
Thursday, August 8. 9:00 AM to 11:30 AM. Apache Meeting Room, TCC.

Herbivory and resource availability influence plant resistance and response to nectar robbers and seed predators.

Irwin, Rebecca^*,1,3, Brody, Alison^2,3, ¹ University of Georgia, Athens, GA³ Rocky Mountain Biological Laboratory, Crested Butte, CO² University of Vermont, Burlington, VT

ABSTRACT- Plants in natural environments experience direct and indirect interactions with antagonistic and mutualistic visitors. The net effect of multiple antagonists on plant-pollinator mutualisms and subsequent plant fitness are often inferred from the results of pairwise studies, the outcomes of which may be non-additive. It is relatively unknown how multiple antagonists and mutualists interact to ultimately shape the ecology of plants and the evolution of traits in variable environments. Using the focal plant species Ipomopsis aggregata, we examined how early-season ungulate herbivory by Odocoileus hemionus and differential resource availability affect plant risk to a nectar-robbing bumblebee, Bombus occidentalis, a pre-dispersal seed predator, Hylemya sp., and their interaction. We exposed plants to six treatments representing a factorial cross of two levels of herbivory (clipped vs. control) by three levels of resource availability (ambient vs. water addition vs. nutrient addition) and measured subsequent levels of nectar robbing, seed predation, pollination, and female plant reproduction. We found that early-season herbivory delayed flowering and, in so doing, increased plant risk to nectar robbing while reducing plant risk to pre-dispersal seed predation. Nutrient addition augmented flowering time and nectar production, increasing plant risk to nectar robbing. High levels of nectar robbing, which reduced fruit and seed production by deterring hummingbird pollinators, were inversely correlated with high levels of oviposition by the pre-dispersal seed predator in low nutrient environments. However, the effects of robbing on seed-predator attack were less pronounced in high-nutrient environments, potentially because plants could produce enough nectar to feed both mutualistic and antagonistic floral visitors. Our results support the view that herbivory and resource availability can have direct effects on plant fitness as well as indirect effects through changes in plant interactions with other antagonists, such as nectar robbers and seed predators.

KEY WORDS: herbivory, nectar robbing, seed predation, pollination