Food webs of the Cosumnes River, California

The broad purpose of my research was to examine how conditions arising from longitudinal gradients in river habitat structure affect energy flow in food webs. The Cosumnes River Basin is a model site to examine patterns in food webs across fluvial landscapes because it remains the largest river network flowing west out of the Sierra Nevadas into the Sacramento-San Joaquin River system that is unhampered by dam and reservoir effects. In Chapter 1, I used stable isotope-derived values of trophic position to enumerate differences in food chain length in biotic communities along the length of the watershed, and examined relationships between food chain length, productivity, ecosystem size, and other food web properties to test hypotheses of food chain length. Food chain length increased with increasing ecosystem size and fish species richness. Food chain lengths were longest at intermediate levels of primary productivity. In Chapter 2, I examine life history characteristics and feeding ecology of the alien invasive redeye bass (Micropterus coosae). Redeye bass were unexpectedly discovered in the Cosumnes River in year 2000, during the course of this study. Redeye bass are small in size, consume a wide-variety of aquatic insects and crayfish, have spread higher in elevation than other introduced species, and dominate food webs where they exist. Trophic position of redeye bass increased with increasing size, and growth rate remained constant even after the onset of sexual maturity. Redeye bass are thought to eliminate native fishes from their habitats through predation, especially on early life history stages, or by competitive interactions with other size classes. In chapter 3, I use stable carbon isotopes to quantify the contribution of terrestrial and aquatic energy sources to aquatic consumers, in order to test whether fish dependence on carbon sources varied predictably with location in the river network. Terrestrial carbon sources contributed more to fish diet at high elevation in the watershed and at confluences of main tributaries.