| Summary: | Electronic Thesis or Dissertation Climate change is affecting freshwater ecosystems worldwide. Warming temperatures have had profound impacts on the distribution of organisms, the timing of biological events, and the overall biodiversity of freshwater ecosystems. Much of the recent research investigating the effects of climate warming on freshwater ecosystems has focused on individual fitness and population dynamics. Much less focus has been on higher levels of organization, such as whole communities and food webs. The primary goal of this dissertation was to determine how warming affects community assemblages and the flow of energy through natural food webs. To address this, I used a whole-stream warming experiment in which I experimentally warmed an entire stream reach in Iceland by approximately 3.5°C using a geothermal heat exchanger, and examined the response of invertebrate communities in the warmed stream and a nearby reference stream for 1 year prior to warming and 2 years during warming. Overall, warming altered invertebrate community structure and significantly reduced total community abundance, while total biomass and production remained unchanged. Warming also resulted in increased energy demand by the invertebrate community but no concomitant increase in resource consumption. Neutral effects of warming on total community biomass, production, and resource consumption were largely driven by declines in several small-bodied, cold-adapted taxa and concurrent increases in many larger, warm-adapted taxa, including newly invading species. Overall, our experimental results demonstrate that warming affects stream communities via exchange and relative success of species according to thermal physiology, and that shifts towards smaller body size are not a universal response to warming. Generalizing these results to global anthropogenic warming has important consequences for the future of aquatic communities and food webs.
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