PARENT SESSION
Oral Session 108: Trophic Structure I: Hypotheses; Grasslands.
Presiding: D Gruner
Thursday, August 5, 8:00 AM to 11:30 AM, Meeting Room F 152.
Top-down and bottom-up control decoupled via food quality.
Shurin, Jonathan1, Clasen, Jessica2, English, Andrea1, Pelletier, Amy1, Zis, Thalia1, 1 Department of Zoology, Vancouver, BC, Canada2 Department of Earth and Ocean Sciences, Vancouver, BC, Canada
ABSTRACT- Trophic structure varies strongly among ecosystems. Aquatic environments have abundant heterotrophs (relative to autotrophs) that consume a larger fraction of primary productivity than their terrestrial counterparts. Terrestrial herbivores also exhibit greater dietary mismatch in elemental composition from their food. We asked whether plant-herbivore stoichiometry could explain differences in the strength of top-down and bottom-up forces by manipulating light, phosphorus and the presence of Daphnia in a simple laboratory system with phytoplankton. High heterotroph-to-autotroph biomass ratios occurred whenever algae were phosphorus rich. However, strong top-down effects were observed only when algae and grazers were limited by the same resource (phosphorus). At low light, algae stored excess P and thereby became more nutritious to Daphnia, but were unable to respond strongly to Daphnia removal due to light limitation. Our results suggest that top-down and bottom-up control may be decoupled by consumer-resource stoichiometry.
Key words: trophic cascades, energy limitation, stoichiometry, nutrient limitation