PARENT SESSION
Oral Session #46: Disturbance Ecology.
Presiding: M. Slocum
Tuesday, August 6. 1:00 PM to 4:45 PM. Grand Ballroom East, Radisson.
Omnivory and food web stability: exploring underlying mechanisms.
HARPER, STEVEN*,1, VANNI, MICHAEL2, HORGAN, MARTIN2, 1 Savannah River Ecology Laboratory, Aiken, SC2 Miami University, Oxford, OH
ABSTRACT- Theory suggests that omnivores may confer stability to food webs by increasing the number of trophic interactions. In aquatic systems, omnivorous fish that consume sedimented detritus may also confer stability to the grazer-based food web by transporting a novel, steady supply of nutrients. We observed previously that phytoplankton biomass was more resistant and resilient to nutrient perturbations in experimental food webs with gizzard shad Dorosoma cepedianum than in those without this omnivorous fish. To distinguish between trophic interaction and nutrient transport mechanisms, we assembled food webs in experimental ponds to resemble those found in reservoirs, systems that receive regular perturbations of allochthonous sediments following storm events. Using a factorial design, we manipulated omnivorous gizzard shad (present or absent) and nutrient additions that simulated transport by gizzard shad (present or absent). We predicted that food webs with gizzard shad, regardless of supplemental nutrient treatment, would be more resistant and resilient to perturbations if the trophic interaction mechanism predominates. If nutrient transport is an important mechanism, food webs with only gizzard shad or only supplemental nutrients should have similar stability. Following a 5-wk pre-manipulation period, we perturbed ponds by adding sediment (25 mg clay/L) consecutively for 10 d, and then monitored responses of food webs over a 5-wk post-manipulation period. In contrast to predictions, phytoplankton biomass responses were erratic and not strongly influenced by experimental treatments. Biomass in most ponds declined in response to the perturbation, but actually increased in some cases due to competitive shifts towards species tolerant of low-light conditions. Thus, responses were primarily a function of community composition before the perturbation. Because stability responses to sediment additions and to previous nutrient additions were not equivalent, additional studies are needed to better understand how the effects of omnivores on food web stability are influenced by the type of perturbation.
KEY WORDS: food web stability, omnivorous fish, trophic interactions, nutrient transport