Document: STE-3-69-4

Effects of omnivorous fish on the stability of aquatic food webs.

VANNI, M.J.* and S.J.HARPER

Miami University, Oxford, OH 45056, U.S.A. ¹

Abstract:
Recent theoretical studies suggest that omnivory confers stability to food webs through the addition of weak- to moderate-strength trophic interactions. We conducted field studies to test predictions generated by this hypothesis. We assembled experimental food webs to resemble those found in reservoirs, systems that receive regular perturbations in the form of large pulses of nutrients following storm events. The omnivorous gizzard shad, Dorosoma cepedianum, is an especially important component of reservoir food webs. By feeding on sedimented detritus and subsequently excreting nutrients, they provide a novel, steady source of nutrients to the grazer-based food web. Theory suggests that a large, steady supply of limiting nutrients can confer stability. Thus, we predicted that food webs with gizzard shad would be more resistant and resilient to nutrient pulses than would those without this omnivorous fish. In experimental ponds we established food webs with phytoplankton, zooplankton, planktivorous bluegill, and various densities of gizzard shad (none, low, moderate, and high). Following a 6-wk pre-manipulation period, we perturbed ponds with a nutrient pulse over a 2-d period and then monitored responses of food webs over a 5-wk post-manipulation period. As predicted, food webs with gizzard shad as a dominant member showed increased resistance. Although all food webs responded to the perturbation with increased phytoplankton biomass and available nutrients, moderate- and high-density shad treatments showed a much smaller increase than control and low-density shad treatments (1.5-fold vs. 4.0-fold increase, respectively). In contrast to increased resistance, shad density did not affect resilience of these factors. Bluegill growth rates, zooplankton species richness, and primary production were largely unaffected by gizzard shad density. Additional studies are required to discern the underlying mechanism by which omnivorous gizzard shad confer resistance to food webs.

Keywords: food web stability, resistance, resilience, omnivory, nutrient transport

This abstract is being presented at: 2:00 PM in session:
Oral Session #11: Trophic Cascades.