Effects of consumer diversity on prey consumption are not influenced by omnivory

In plant communities, higher levels of taxonomic richness are often shown to be more efficient at utilization of limiting resources due to resource partitioning among taxa. While resource partitioning is also thought to be important in consumer communities, consumers also exhibit more complex interactions like omnivory. Omnivory is generally thought to reduce the effects of consumer richness on the consumption of prey resources; however, empirical tests of this prediction are rare. Here, we report the results of 2 complementary studies to test the hypothesis that omnivory reduces the positive effects of consumer taxonomic richness on prey resource consumption. First, we analyzed data from a dataset consisting of 1,100 freshwater lakes across the continental United States. We show that the relationship between consumer taxonomic richness and the summed biomass of resource prey (phytoplankton) is independent of the proportion of zooplankton (consumers) that are omnivores. However, consumption rates were not explicitly measured in this dataset so that we conducted in situ feeding experiments in 37 lakes near Ann Arbor, MI, USA, to measure omnivorous consumption (Omni) as the amount of smaller microzooplankton (<200 mu m) consumed by larger nonherbivorous mesozooplankton. We also measured the amount of phytoplankton consumption (G) across a gradient of zooplankton taxonomic richness (zpSR). We showed that there was a positive association between zpSR and G, suggesting that G was increased by zooplankton diversity. However, the effects of zooplankton diversity on the G are not altered by the level of Omni among zooplankton. Although omnivory does not influence the effects of consumer diversity on prey consumption, we do not negate the impacts of omnivory on other ecosystem functions in aquatic systems. We attempt to address a question that is of general interest to the field of ecology, especially of aquatic ecology, because omnivory is known to be common in aquatic systems.