Effects of Density and Size Structure on Omnivorous Trophic Cascades

Donald J. Benkendorf, Utah State University


Omnivores generally dampen trophic cascades by feeding at multiple trophic levels. However, there are many factors that may invalidate this broad generalization. For example, intraspecific size variation can affect an omnivorous species’ ability to control lower trophic levels; likewise resource switching and changes in proportional resource utilization may accompany variation in omnivore body size and density. Because omnivory is ubiquitous, understanding how size structure and density interact to affect an omnivore’s functional role is necessary to better predict top-down control in natural food webs. We sought to identify the effects of size and size structure on top-down control by omnivorous Speckled Dace, Rhinichthys osculus, and how these effects scaled with density. Within our study system, R. osculus inhabit small isolated beaver ponds, the conditions of which we replicated in 1000L cattle tanks. Size, size structure, and density of R. osculus were then manipulated within these tanks and resulting changes in invertebrate and algal communities were monitored over 8 weeks. Benthic algal biomass was significantly lower in the fishless control and lowest fish density treatment, indicating that R. osculus may have caused a trophic cascade that varied in intensity by treatment. Invertebrate samples are currently being processed and should provide insight into the specific pathways of this potential trophic cascade. Once completed, this research will contribute to a growing body of knowledge regarding the importance of intraspecific variation in maintaining the full suite of complex interactions that constitute healthy ecosystems.