PARENT SESSION
Oral Session # 84: Trophic Structure II: Terrestrial and Aquatic Systems.
Presiding: MD Moran
Thursday, August 7. 1:30 PM to 5:00 PM, SITCC Meeting Room 202.

Resources and intermediate-consumer identity alters an omnivore food web.

Kneitel, Jamie^*,1, ¹ Washington University, St. Louis, MO

ABSTRACT- Omnivory has been increasingly incorporated into our understanding of food webs and community structure. Models that address these food webs make several predictions about what conditions facilitate coexistence between an omnivore and an intermediate consumer. These predictions include (1) the intermediate consumer must be a better competitor than the omnivore, (2) the intermediate consumers can exclude the omnivore at low resources, but will be excluded by the omnivore at high resources, and (3) intermediate consumer abundance is lower in the presence of an omnivore. Few empirical studies have taken into account the trait variation (competitive ability and predator tolerance) of intermediate consumers and how this variation may alter community dynamics. I conducted a short-term laboratory experiment using species that inhabit the modified leaves of the pitcher plant, Sarracenia purpurea. These species included bacteria (basal trophic level), five protozoa and a rotifer (intermediate consumers), and larvae of the mosquito, Wyeomyia smithii (omnivore). Four replicates were established for each of 32 communities which included 3 trophic levels, 2 resource levels, 5 intermediate consumers, and 2 densities of the omnivore. Resources increased the growth rate and abundance of all species. The intermediate consumers differentially affected both bacterial abundance and Wyeomyia smithii growth. Three of the intermediate consumers significantly reduced bacterial abundance more than the omnivore. The presence of two protozoan species resulted in an increase in omnivore growth rates. The identity of intermediate consumers did not coincide with expected outcomes: depending on the conditions, community dynamics converged with different intermediate consumers or varied according to intermediate consumer differences. The results of this study indicate the need to consider trait variation (competitive ability and predator resistance) within a trophic level because of their differential effects on the basal trophic level, the omnivore, and the emergent community dynamics.

Key words: omnivory, resources, food web, protozoa