Dramatic biodiversity changes occurring globally from species loss and invasion have altered native food webs and ecosystem processes. My research objectives are to understand the consequences of fish diversity to freshwater systems by (1) examining the food web consequences of multiple top predators, (2) determining how biodiversity influences fish production, (3) testing fish diversity as a determinant of ecosystem properties, and (4) evaluating the ability of native fish diversity to alter the impact of an invader. I examined multiple predator interactions by comparing treatments of a cruising predator (largemouth bass, "Micropterus salmoides") alone, ambush predator (muskellunge, "Esox masquinongy") alone, and combined predators foraging on bluegill ("Lepomis macrochirus") prey in a replicated pond experiment. I found that multiple predator interactions facilitated growth of the cruising predator and had a substitutable effect on the ambush predator. The combined predator treatments created an emergent effect that was risk-enhanced for prey fish and weakly extended to lower trophic levels. These results highlight the need to consider species traits, interactions between predators, and emergent effects on prey to understand food webs. Experiments were conducted at multiple spatial scales (mesocosms: 1325 L polyethylene tanks and ponds: 0.04 ha) to explore the effect of species richness and composition on fish production. Both fish richness and all species combinations within each richness level were replicated. Resource complementarity through niche partitioning was the primary mechanism driving fish production in the mesocosms, whereas sampling effect was the critical mechanism in the ponds. Scale dependent mechanisms are likely due to differences in resource levels and heterogeneity. Providing empirical evidence on the mechanisms of consumer diversity, as well as the importance of scale, will improve predictions for the effects of changes in biodiversity that have important implications for conservation. Experiments were conducted at multiple spatial scales (mesocosms and ponds) to explore the effect of fish richness and composition on ecosystem processes by measuring changes in food web structure and nutrient cycling. The influence of fish diversity and governing mechanisms differed across trophic levels and between spatial scales. Zooplankton richness decreased with increasing fish richness in mesocosms, whereas in the ponds a dominant fish species determined zooplankton richness. Zooplankton density was driven by the composition at both spatial scales. No effect from fish treatments were found on primary production, whereas increasing levels of phosphorus occurred with increasing fish richness in the mesocosms. I examined the effect of common carp across a gradient of fish richness in mesocosms. The invader reduced native fish production and the effect was reduced with increasing fish richness. The invertebrate community was modified by a top-down effect from the richness treatment, whereas the invader had a bottom up effect increasing primary production and altering the zooplankton community. Higher native diversity mitigated the impact of the invader confirming the need to consider biodiversity when predicting the impact of an invasive species. (Abstract shortened by UMI.) [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]