| The Arctic charr Salvelinus alpinus is extremely diverse and exhibits resource polymorphism, as indicated by the presence of discrete groups that differ in size, morphology, diet, and reproductive characteristics. As this differentiation may indicate ecological speciation, most past work has focused on specific case studies. Therefore, this dissertation aims to compare trends in this ecological diversity across broad geographical regions and place it within an ecosystem context by comparing study systems in Iceland and Alaska. In the first chapter, gut contents of Arctic charr across ∼50 lakes in Iceland were analyzed to form 6 habitat-associated prey categories, dominated by fish, snails, pea clams, tadpole shrimp, chironomid pupae, and zooplankton. Consumption of zooplankton was related to high silicon dioxide and low nutrient concentrations in the lake. Snail, tadpole shrimp, pea clam, and fish consumption were related to lake altitude, depth, and brown trout abundance. In the second chapter, the relationship between morphology and consumption within each prey category is analyzed. Evidence is found for previously observed associations between diet and morphology, as well as associations between morphology and diet not commonly described (e.g., with pea clams or tadpole shrimp). In the third chapter, methods are developed to detect polymorphism through the presence of multiple growth curves or multiple morphological distributions within populations using mixture models. After evaluating polymorphim across lakes in Iceland, random forest models indicated that polymorphism was more likely to occur in lakes with low brown trout abundance, high altitude, and conditions with high zooplankton and fish consumption. The fourth chapter analyzes morphological variation in 4 lakes in southwestern Alaska. Two forms were found to coexist in Lower Tazimina Lake, and ecological differences in gill raker and pyloric caeca counts, diet, and body shape are described. Finally, food webs are analyzed in the fifth chapter using stable isotope ratios of fish fauna across 11 lakes in Iceland and 4 lakes in Alaska. Breadth of carbon consumed from benthic and limnetic food chains appears dependent on the presence of morphological differentiation. Piscivory and limnetic resource use appear dependent on the number and type of competitors present. |
