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.
The Speckled Dace (Rhinichthys osculus) is an omnivorous cyprinid that consumes algae and invertebrates and is abundant throughout the western United States. We sought to identify the effects of size and size structure on top-down control by R. 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 manipulated within these tanks and resulting changes in invertebrate and algal communities, and various parameters of ecosystem functionality (respiration, decomposition, water quality) were monitored over eight weeks.
Results/Conclusions
Benthic algal biomass was significantly lower in the fishless control and lowest fish density treatment than in several of the highest fish density treatments, indicating that R. osculus may have caused a trophic cascade that varied in intensity by treatment. By week eight of the experiment, the benthic algal concentrations in the control and lowest density treatment were less than half the concentrations in the higher density treatments. Invertebrate samples are currently being processed and should provide insight into the specific pathways of this potential trophic cascade. Interestingly, the benthic algae results were best explained by variation in density among treatments and not by the size and size structure variation. This indicates that the small and large R. osculus used in this experiment share similar functional roles.
These results contrast many recent studies that support the importance of intraspecific size structure in maintaining lower trophic levels and healthy ecosystem functioning. It is possible that the size variation of omnivores used in our experiment was not large enough to trigger an observable difference in top-down control. However, our results do suggest that density is important for determining an omnivore’s trophic impact.