Food Web Structure of a Subtropical South American Stream with Particular Focus on the Co-evolution of Form and Function in an Endemic Species Flock

iv FOREWORD Chapters 2, 3, 4, and 5 are either in review, in revision or will be submitted, and thus are formatted according to specific journal formats: Ecology (Chapter 1), Hydrobiologia Food webs and aquatic ecology of subtropical South American streams are poorly understood. I investigated the trophic structure of the upper Rio Cuareim (Uruguay, South America). I analyzed the stable isotope ratios (δ 13 C and δ 15 N) of 23 fish and 8 invertebrate taxa in March and November 2010. Additionally, I analyzed the stomach contents and ecomorphology of six endemic pike cichlids (Crenicichla). Predatory water bugs (Belostomatidae) and caddisflies (Hydropsychidae) displayed the highest trophic positions of invertebrates and bivalves (Neocrobicula limosa and Corbicula fluminea) and crabs (Aegla uruguayana and Trichodactylus panoplus) occupied the lowest trophic positions. Fishes generally occupied higher trophic positions than invertebrates. Prochilodus lineatus displayed the lowest trophic position and Hoplias spp., Serrasalmus spilopleura, and Oligsarcus robustus are the top trophic predators. Trophic guilds were successfully classified using stable isotope ratios and almost half the species sampled were omnivorous. Food web structure shared traits with temperate (e.g., few detritivores) and tropical (e.g., many omnivores) systems and also displayed characteristics that may be unique to subtropical South American streams, such as high proportions of mesopredatory species (e.g., vi Crenicichla). An endemic species flock of pike cichlids strongly partition food-resources and most species display trophic specialization by preferentially exploiting a single suite of prey items. External (skull) and internal (pharyngeal jaw) morphology were associated with dietary patterns. vii ACKNOWLEDGEMENTS Investigations into remote and poorly understood ecosystems present many obstacles. However, I did not face these alone. Indeed, many people contributed in various ways to the completion of this research. First and foremost, I thank my parents, Edward and Robin Burress. Their support has contributed in more ways than they know, from keeping me well fed to passing to me their analytical and strong minds, respectively. With everything they've done, they kept me moving forward. For countless endeavors, including reviewing constant drafts of my work, casual chats about my research, for treating me like family, and above all for her faith and expectations, I thank my major advisor Lynn Siefferman. I am also grateful to my committee members: Michael M. Gangloff and Robert P. Creed, who were ready to discuss my research at a moments notice. Michael D. Madritch also greatly aided in many technical aspects of this …