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Community ecology in a warmer world: direct and indirect effects of temperature on community dynamics by Rebecca Lee Kordas B.A., The University of Chicago 2002 M.Sc., California State University Northridge 2006 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in The Faculty of Graduate and Postdoctoral Studies (Zoology) THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) September 2014 © Rebecca Lee Kordas 2014 Abstract As the Earth’s climate changes so too do its ecosystems, due to shifts in abundance, biodiversity and interaction strengths among their constituent species. Although warming will simultaneously affect many aspects of ecological communities, disentangling the abiotic and biotic contributions will improve our understanding of how assemblages of interacting species will respond to climate change. My goal was to determine how warming affects community assemblages via direct (mediated by organismal physiology) vs. indirect effects (mediated by species interactions). I addressed this with 12-16 month-long manipulative experiments in the rocky intertidal zone of Salt Spring Island, Canada. I created a novel in situ method for increasing substratum temperature for settling benthic organisms, using black- and white-bordered settlement plates. In the first experiment (Chapter 3), I monitored the response of functional groups and diversity to warmed treatments. Results from this experiment suggest that communities in thermally stressful habitats respond to warming via the interplay between species-specific physiological responses and secondary adaptive strategies such as behavioral microhabitat selection. In Chapter 4, I concentrated on the direct effects of warming. As a case study, I monitored the direct effects of in situ warming on the vital rates of two competing barnacle species. Warming negatively affected both species of barnacles, however the population of the competitive dominant was more severely affected than the subordinate species, leading to a temperature-induced change in space occupancy. In Chapter 5, I focused on the indirect effects of warming on community dynamics by manipulating temperature and herbivore access to communities. Community structure and successional trajectory differed markedly between treatments, due to disturbance from herbivores and high species turnover due to warming. Despite the stochastic nature of development, warmed communities with herbivores ultimately lost the variability created by herbivore-associated disturbances, resulting in highly similar assemblages between warm and cool treatments. These results illustrate how environmental change can alter species-specific thermal responses, complex population dynamics, and interaction strengths, with cascading impacts on community dynamics. They further demonstrate how assemblages of multiple, interacting species will respond to climate change, which is imperative if we hope to effectively prepare for and adapt to its effects. ii Preface Some of the work included in this dissertation has been published or submitted for publication to peer-reviewed scientific journals: A version of Chapter 2 has been published and the appropriate license has been obtained from Elsevier. I was the lead investigator although all authors contributed text to the manuscript: Kordas, RL, CDG Harley, MI O’Connor. 2011. Community ecology in a warming world: the influence of temperature on interspecific interactions in marine systems. Journal of Experimental Marine Biology and Ecology 400(1-2): 218-226. A version of Chapter 3 has been submitted for publication. On March 12, the manuscript was provisionally accepted pending revisions, which were submitted on April 23. Another set of minor revisions were requested on June 7, which will be submitted in early September, 2014. I designed the experiment, conducted all field work, conducted initial data analyses, and wrote the manuscript. Stefan Storey assisted in development of the warming plate design and some field work. Steve Dudgeon analyzed the data using AIC methods. Chris Harley was the supervisory author on this project and was involved throughout the project in concept formation and manuscript edits. Kordas, RL, S Dudgeon, S Storey, CDG Harley. Revision in review. Intertidal community responses to field-based experimental warming. Oikos. Chapters 3 and 4 originated from the same experimental design, although different data and analyses were used for the manuscript. Accordingly, I designed the experiment, collected and analyzed all data, and wrote the manuscript for Chapter 4. Chris Harley was the supervisory author on this project and was involved throughout the project in concept formation and manuscript edits. Chapter 5 uses the same experimental design as Chapters 3-4, though it was conducted at a different time and location. I designed the experiment, conducted field work, collected and analyzed all data, and wrote the manuscript. Chris Harley was the supervisory author on this project and was involved throughout the project in concept formation and manuscript edits. iii Table of Contents Abstract .......................................................................................................................................... ii Preface ........................................................................................................................................... iii Table of Contents ......................................................................................................................... iv List of Tables ............................................................................................................................... vii List of Figures ............................................................................................................................... ix Chapter 1 Introduction .............................................................................................................. 1 1.1 Community ecology: transitioning from a philosophical to an environmental agenda .................. 1 1.2 The rocky intertidal: a laboratory for global warming experiments ............................................... 2 1.3 Structure of this dissertation ............................................................................................................ 4 Chapter 2 The Influence of Temperature on Interspecific Interactions .............................. 6 2.1 Synopsis ........................................................................................................................................... 6 2.2 Introduction ..................................................................................................................................... 6 2.3 The biological importance of temperature ...................................................................................... 9 2.4 Interspecific variation in thermal sensitivity ................................................................................. 13 2.5 Incorporating time and space: phenology and biogeography ........................................................ 17 2.6 The search for generality ............................................................................................................... 21 2.7 Future research priorities ............................................................................................................... 24 Chapter 3 Intertidal Community Responses to Field-Based Warming .............................. 27 3.1 Synopsis ......................................................................................................................................... 27 3.2 Introduction ................................................................................................................................... 27 3.3 Methods ......................................................................................................................................... 30 3.3.1 Study system ...........................................................................................................................................30 3.3.2 Experimental warming treatment ...........................................................................................................30 3.3.3 Statistical analyses ..................................................................................................................................33 3.4 Results ........................................................................................................................................... 35 3.4.1 Temperature ............................................................................................................................................35 3.4.2 Functional group responses ....................................................................................................................38 3.4.3 Community response ..............................................................................................................................43 3.5 Discussion ..................................................................................................................................... 48 Chapter 4 Demographic Responses of Coexisting Species to In Situ Warming ................. 52 4.1 Synopsis ......................................................................................................................................... 52 4.2 Introduction ..................................................................................................................................