Murray State Theses and Dissertations


Omnivores can dampen trophic cascades by feeding at multiple trophic levels, yet few studies have evaluated how intraspecific variation influences the effect of omnivores on community structure. The Speckled Dace (Rhinichthys osculus) is an omnivorous cyprinid that consumes algae and invertebrates and is abundant throughout the western United States. I sought to identify the effects of size and size structure on top-down control by dace and learn how these effects scaled with density. Dace were manipulated in a mesocosm experiment and resulting changes in invertebrate and algal communities and measures of ecosystem function were monitored. Omnivores affected experimental communities via two distinct trophic pathways (benthic and pelagic). In the benthic pathway, dace reduced benthic macroinvertebrate biomass, thereby causing density-mediated indirect effects that led to increased benthic algal biomass. Dace also reduced pelagic predatory macroinvertebrate biomass (notonectidae and corixidae), thereby significantly increasing the abundance of emerging insects. The effect of dace and hemipterans on emerging insects was mediated by a non-linear response to dace density with a pronounced peak at intermediate density. Omnivore size and size structure had minimal effects, indicating that the small and large dace used in this experiment share similar functional roles, and contrasting with recent studies that support the differential effect of intraspecific size structure on communities. My results indicate that the degree to which omnivores dampen trophic cascades depends on their relative effect on multiple trophic levels. Availability of an abundant, high calorie food source in the form of macroinvertebrates, and the absence of top predators, may have shifted dace diets from primary to secondary consumption, strengthening density-dependent trophic cascades. Thus, both omnivore density and dietary shifts are important factors influencing omnivore-mediated communities.

Year manuscript completed


Year degree awarded


Author's Keywords

Omnivore, trophic cascades, density, size structure

Thesis Advisor

Howard H Whiteman

Committee Member

Michael B Flinn

Committee Member

Christopher J Mecklin

Committee Member

Timothy Spier

Document Type