AN ABSTRACT OF THE DISSERTATION OF Michael T. Bogan for the degree of Doctor of Philosophy in Zoology presented on July 10, 2012. Title: Drought, Dispersal, and Community Dynamics in Arid-land Streams Abstract approved: _____________________________________ David A. Lytle Understanding the mechanisms that regulate local species diversity and community structure is a perennial goal of ecology. Local community structure can be viewed as the result of numerous local and regional processes; these processes act as filters that reduce the regional species pool down to the observed local community. In stream ecosystems, the natural flow regime (including the timing, magnitude, and duration of high and low flow events) is widely recognized as a primary regulator of local diversity and community composition. This is especially true in arid- land streams, where low- and zero-flow events can occur frequently and for extended periods of time (months to years). Additionally, wetted habitat patches in arid-land stream networks are often fragmented within and among stream networks. Thus dispersal between isolated aquatic patches may also play a large role in regulating local communities. In my dissertation, I explored the roles that drought, dispersal, and local habitat factors play in structuring arid-land stream communities. I examined the impact of flow permanence and seasonal variation in flow and other abiotic factors on aquatic communities at both fine spatial scales over a long time period (8 years; Chapter 2) and at a broad spatial scale over a shorter time period (1-2 years; Chapter 4). Additionally, I quantified aquatic invertebrate aerial dispersal over moderate spatial scales (≤ 0.5 km) by conducting a colonization experiment using artificial stream pools placed along and inland from two arid-land streams (Chapter 4). Finally, I examined the roles of spatial isolation, microhabitat type, and local abiotic and biotic factors in structuring aquatic communities in freshwater oases scattered across one of the most arid regions of North America, the southern Sonoran Desert (Chapter 5). In Chapter 2, I found that severe drought caused an unprecedented drying event in isolated perennial stream pools, and that several additional drying events occurred over the following four years. This transition to intermittent flow caused the extirpation of several large, long-lived species with low dispersal abilities (including the top predator) and drove the local community into an alternative state. In the colonization experiment described in Chapter 3, I found that several arid-land stream invertebrate taxa disperse widely and frequently. The widespread dispersers identified by this experiment included several of the earliest colonist taxa observed following the severe drought described in Chapter 2. Other taxa, though, only dispersed overland after receiving an environmental cue (rainfall) or preferentially dispersed along stream corridors. In Chapter 4, where I examined invertebrate community structure across a large network of well-connected intermittent and perennial reaches, I found low diversity in intermittent reaches, regardless of their connectivity to diverse upstream perennial reaches. These species-poor, intermittent communities were composed of a unique suite of species with life- history adaptations that conferred desiccation resistance, including extended egg and larval diapause stages. The short flow duration of intermittent reaches (<100 days) likely precluded upstream perennial taxa from establishing populations in downstream intermittent reaches before drying occurred, while the relative predictability of flow timing (Dec-Apr) likely allowed for a small number of species to develop appropriate life-history traits (e.g., diapause stage, rapid development time) to exploit these temporally-fleeting habitats. In Chapter 5, I found over 220 species of aquatic animals (including ≥ 5 undescribed species) in the 19 desert oases that were sampled across the southern Sonoran Desert. Local community composition in these oases was strongly driven by microhabitat type. Additionally, native aquatic species richness and abundance in these oases were significantly reduced by the introduction of tilapia, an exotic fish species. The threats to arid-land streams presented by increased drought severity, anthropogenic water withdrawals, and local habitat degradation (e.g., introduced species, unmanaged recreational use) are grave across the southwestern US and northwestern Mexico. I hope that in addition to furthering our understanding of ecological processes in arid-land streams, this dissertation makes a small contribution towards the efforts to preserve these habitats. © Copyright by Michael T. Bogan July 10, 2012 All Rights Reserved Drought, Dispersal, and Community Dynamics in Arid-land Streams by Michael T. Bogan A DISSERTATION submitted to Oregon State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy Presented July 10, 2012 Commencement June 2013 Doctor of Philosophy dissertation of Michael T. Bogan presented on July 10, 2012. APPROVED: ______________________________________________________ Major Professor, representing Zoology ______________________________________________________ Chair of the Department of Zoology ______________________________________________________ Dean of the Graduate School I understand that my dissertation will become part of the permanent collection of Oregon State University libraries. My signature below authorizes release of my dissertation to any reader upon request. ______________________________________________________________________________ Michael T. Bogan, Author ACKNOWLEDGEMENTS Thanks to my advisor, Dave Lytle, for his enthusiasm, encouragement, and support for my dissertation research over the last 5 years. Dave also gave me an enormous amount of freedom in the last couple years of my dissertation, which made it possible for me live and conduct my research in Arizona. From jumping off cliffs into the Colorado River and roasting chiles over mesquite coals on our first field outing in 2003, to stressfully bumbling through the Spanish language on our first Sonora trip, to scrambling down limestone canyons on White Sands Missile Range on our most recent field outing, we’ve had a great time and done some great work. I look forward to all of our future collaborations, field outings, and adventures. My time at Oregon State has been made enjoyable by having a great series of lab mates over my dissertation, including Asako Yamamuro, Deb Finn, Emily Hartfield, Ivan Phillipsen, Kate Boersma, and Laura McMullen. Deb has been a great postdoc and collaborator in our lab off and on since 2006, and I thank her for the opportunity to collaborate on conservation genetics projects. Laura has been my great western Arizona companion because of her field work on the Bill Williams River, near my house. Thanks to her for all the fun we’ve had sampling, kayaking, camping, nerding out, and eating barbeque in western Arizona. Kate has been my phenomenal office mate and partner-in-crime both in the field and at conferences. Thanks for all the time you’ve put into our collaborations, and the fun, laughs, birding adventures and carne asada taco dinners we’ve had at my house, in the field, in Mexico, and in exotic faraway cities like La Paz, Louisville, and Providence. Thanks also to Richard Van Driesche for being a great recent addition to our lab and being endlessly enthusiastic about taxonomy. Emeritus professor Norm Anderson has been an amazing resource, fellow intermittent stream and natural history enthusiast, office neighbor, and friend over my graduate career. My split Arizona-Oregon life would not have been possible without the great help provided by the Zoology office staff, especially Torri Givigliano, Tara Bevandich, and Traci Durrell-Khalife. Thanks so much to them for five years of scanning and faxing forms to me in Arizona that I should have filled out in person in Oregon. Thanks also to a great and supportive dissertation committee, including Bruce Menge, Doug Robinson, Gordon Grant, Judith Li, Joe Beatty, and Stan Gregory. My only regret at working remotely on my dissertation is that I didn’t get to work more closely with most of you. Stan, though, has been a great conference and Mexico travel companion (along with the always wonderful Kathryn Boyer) and has both introduced me to many great researchers at conferences and kindly given me feedback on key dissertation chapters. Thanks to a great group of Zoology graduate students for keeping me sane during my extended stays in Corvallis, far away from my dry desert home. Among all the great friends, I’d especially like to thank Catherine Searle, Chris Friesen, Kaitlin Bonner, Margot Hessing-Lewis, Mark Christie, Paul Bradley, Rocky Parker, and Sarah Eddy. Anna Jolles, Brian Parks, Brock and Karen McLeod, Dave Paoletti, Kathyrn Boyer, Maria Kavanaugh, Rebecka Weinsteiger, and Tiffany Garcia have all been outstanding Corvallis friends. Special thanks to Alison Iles and James Feldmann for being phenomenal friends and sharing their home with me over at multiple times over the last few years. Thanks to Dr. Dave Herbst of UC Santa Barbara’s Sierra Nevada Aquatic Research Lab for providing me with part-time remote work (and great summer field adventures) which helped to smooth my financial transition back to graduate school. Several of my old friends from the Eastern Sierra helped me out on several sampling adventures in Sonora,