PARENT SESSION
Oral Session # 54: Coastal Ecosystems: Salt Marshes.
Presiding: P Ringold
Wednesday, August 6. 1:30 PM to 5:00 PM, SITCC Meeting Room 105.

Disentangling the effects of host plant quality and quantity on trophic interactions in a salt marsh community.

Stiling, Peter^*,1, Moon, Daniel¹, ¹ Department of Biology, Tampa, FL, USA

ABSTRACT- Productivity has been argued to be one of the most important factors influencing trophic dynamics. A difficulty inherent in the studies that manipulate productivity by changing nutrient availability is that plant quality and plant biomass are influenced simultaneously. In this study we disentangled the potentially confounding effects of plant quality and quantity on trophic dynamics by separately manipulating nutrients and plant biomass, while simultaneously reducing pressure from the most common natural enemies of two focal herbivores, in a fully factorial design. Plant quality of the sea oxeye daisy, Borrichia frutescens, a common coastal species in Florida, was manipulated by adding nitrogen fertilizer and adding sugar to decrease available nitrogen. We manipulated plant biomass by pulling by hand 25% or 50% of Borrichia stems on each plot. Parasitism of our focal herbivores, Pissonotus quadripustulatus and Asphondylia borrichiae was reduced with yellow sticky traps that catch hymenopteran parasitoids. Plant quality significantly affected both herbivores, with fertilization increasing, and sugar decreasing the densities of the two species. Plant quality also significantly affected other common herbivores on Borrichia, as the density of a stem boring lepidopteran and the frequency of leaf chewing varied directly with foliar nitrogen levels. Biomass manipulations had no significant effects on any herbivore species. Parasitoid removal treatments significantly increased the densities of both focal herbivores, and the impact of top-down manipulations was nearly equivalent to the impact of plant quality manipulations on herbivore densities. These top-down manipulations resulted in a trophic cascade, as the density of Borrichia stems decreased significantly on parasitoid removal plots. In this study we found that plant quality and not plant quantity influences trophic interactions. Thus, the utility of predictive trophic theories incorporating productivity may depend upon our ability to distinguish between the effects of plant quality and those of plant quantity and architecture.

Key words: Plant productivity, Herbivores, Trophic cascades, Plant quality