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Community Assembly, Stability and Food Web Structure

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Community Assembly, Stability and Food Web Structure The Dissertation Committee for Samraat Shashikant Pawar certifies that this is the approved version of the following dissertation: Community assembly, stability and food web structure Committee: Sahotra Sarkar, Supervisor Daniel Bolnick Timothy Keitt Matthew Leibold Lauren Meyers John Scalo Community assembly, stability and food web structure by Samraat Shashikant Pawar, B.S.; M.S. Dissertation Presented to the Faculty of the Graduate School of The University of Texas at Austin in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy The University of Texas at Austin May 2009 Acknowledgments What a journey it has been. In august 2002, I flew from the old world to the new, landing directly in the lone star state. As a popular bumper sticker says, “I wasn’t born in Texas, but I got here as fast as I could”. Well, I did get here, not necessarily as fast as I could, but more or less intentionally. For this, I must thank Ayşegül Birand, who pestered me into applying to Graduate school, and Eric Pianka, who undertook to mentor me, apparently overlooking my many inadequacies and a rather impracticable (but enthusiastic) statement of purpose. Anyway, upon being set free in the wonderfully unfettered milieu of the “Section of Integrative Biology” at UT, I promptly performed a sailor’s midnight walk over the next few years; through coursework, ideas, prelims, more ideas, PhD proposals, still more ideas, and finally a dissertation that was originally a byproduct of what started off as the main topic of my PhD research. But that is a story for another time and place. At the early stages of my graduate “career”, Eric helped me strengthen my interest in field ecology and prevented me from becoming a “rabid experimentalist” (aka succumbing to the dark side). Larry Gilbert must also share the blame for keeping my interest in community ecology, tropical biology and field ecology alive and kicking. The trip to Sirena (Costa Rica) with Larry was a stimulating one, and strongly influenced the orientation of my PhD research. The need to make sense of observed biological patterns in nature inexorably led me to pursue mathematical modeling. This change of direction resulted in my shift from Eric’s lab to Sahotra Sarkar’s. Here again I must thank Eric for accepting my decision with equanimity. I also thank Sahotra for taking me on, showing great patience and faith in my various schemes, and deftly guiding my training in theoretical biology. To a large extent I also attribute my subsequent progress in grad school to John Scalo. John granted me a series of research assistantships in the department of astronomy that afforded me the luxury to immerse myself in my research. We also sought extraterrestrial life. Well not quite, though that is fun too, as any good astrobiologist will tell you. John and I pursued some exciting themes in population genetics, during which I learnt heaps. I am in awe of the range of issues across physics, astronomy and biology that John is interested in, and understands deeply. I also owe a big thanks to my prelim and dissertation committees. The latter in particular- Lauren Meyers, Tim Keitt, Dan Bolnick, Matthew Leibold and John Scalo, showed great patience and tolerance in the face of a rather mutable PhD research project. For the successful completion of my dissertation, I also thank Jordan Scaff who despite his million hours of classes, volunteered to assist me in data compilation and analysis. On top of this, he had to listen to me blabber endlessly about networks, food webs, and such. iii A large portion of my financial support also came in the form of teaching assistantships. Here I thank K. Sathasivan, who taught me to teach efficiently, creatively, patiently, and effectively. Sata’s influence will always be evident in my future pedagogical undertakings. To the choppy seas of grad school and life, which at times I felt I was navigating with leaky boat, a measure of placidity was brought by Mike Singer and Camille Parmesan. They were and endless source of advice and support throughout. So were my my fellow graduate students, especially the extended cohort: Rob, Barrett, Ron, Naiara, Natalia, Sasha, Krushnamegh, Jelena, Juan, Deepa, Evan, Christian, Simone, Juanita, and Trevon. They provided or shared to various degrees, stimulating drinks, stimulating conversation, support, optimism, pessimism, multi- cultural food, bbq evenings, humor, buck-u-uppos and advice; I cannot visualize how I would have gotten through without them. I am also grateful to the cheerful and efficient staff of the section of Integrative Biology. In particular, Sandy Monahan, who was always there to smooth the bureaucracy for we graduate students. I am indebted to a number of people outside UT too. Ever since we were acquainted and bought counterfeit paintings in Beijing during the Santa Fe Institute’s 2005 summer school in China, Oskar Burger invested a lot of time thinking with me about species interaction networks and metabolic allometric scaling, helping me resolve many issues that arose at the early stages of my research. Let’s finish that paper man! I must also thank him for making me realize that a couple of bottles of fine cognac is all a tropical person needs to roll around in the snow wearing bare essentials. I also thank Jennifer Dunne, Van Savage, and Nicolas Loeuille for discussions that facilitated my progress and greatly improved my research. Finally, I would like to thank my family – Aai, Daddy, Soni and Sheffi. It is partly due to their endless support and belief that led me to embark on the pursuit of a PhD degree. And of course, my wife, friend, philosopher and guide Catalina. She made my quest beautiful, all the more fulfilling and filled it with (sometimes bilingual) puns. Finally finally, Maya. Her I thank for arriving at an opportune time. She made my dissertation writing infinitely more enjoyable, and dutifully entertained me day and night (as any good daughter should). This PhD dissertation research was funded by the Section of Integrative Biology, the Private Lands Research Grants-Environmental Science Institute, and the Department of Astronomy (through a grant from NASA’s exobiology program), University of Texas at Austin. iv Community assembly, stability and food web structure Abstract Samraat Shashikant Pawar, Ph.D. The University of Texas at Austin, 2009 Supervisor: Sahotra Sarkar Natural communities of species embody complex interrelationships between the structure of the interspecific interaction network, dynamics of species’ populations, and the stability of the system as a whole. Studying these interrelationships is crucial for understanding the survival of species in nature. In this context, studying the food web (the network of who-eats-whom) embedded in each interaction network is particularly important because trophic interactions are the main channels of energy flow in all ecosystems. Using a combination of mathematical modeling and empirical data analyses, this study explores the interrelationship between food web structure and multi-species coexistence in local communities. Chapter 1 of this thesis places the overall dissertation study in context of the history of research on species interaction networks and food webs. In Chapter 2, I use a population dynamical model to show how the requirements of stable multi-species coexistence results in the emergence of specific, nonrandom configurations of food web structure during community assembly. These structural “signatures” can be used to empirically gauge the importance of interaction-driven dynamical stability constraints in natural communities. In Chapter 3, I extend the model analyzed in Chapter 2 by imposing biologically feasible constraints on its parameters. This is made possible by the allometric scaling between individual metabolism and body size, and the constraints on interspecific trophic interactions due to body size differences between pairs of interacting species. I show that, using this approach, it is possible to interlink three aspects of local communities that have typically been studied in isolation: the species’ body mass distribution, the distribution of ratios of body sizes of consumer and resource species (e.g., predator and prey), and certain food web structural features. Some of these features have previously lacked explanatory models. Finally in Chapter 4, using empirical data from nine communities across a range of habitats, I test some theoretical predictions of the previous chapter. The results provide strong evidence that the food web structure of natural communities do indeed exhibit signatures of dynamical stability constraints, and that the model developed in Chapters 2 and 3 is successfully able to predict a number of empirically observed food web structural features. v Table of Contents Acknowledgments ............................................................................................................ iii Abstract .............................................................................................................................. v Chapter 1: Introduction ................................................................................................... 1 1.1 Population interaction networks and Darwin’s Gedankenexperiment .............. 1 1.1.1 Food webs as population interaction networks ........................................................4 1.2 Interrelating population dynamics and food web structure ............................... 4 1.2.1 The importance of body size ...................................................................................7
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