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A Marine Megafaunal Extinction and Its Consequences for Kelp

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A MARINE MEGAFAUNAL EXTINCTION AND ITS CONSEQUENCES FOR KELP FORESTS OF THE NORTH PACIFIC by Cameron David Bullen B.Sc., The University of British Columbia, 2017 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (Resources, Environment and Sustainability) THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) June 2020 © Cameron David Bullen, 2020 The following individuals certify that they have read, and recommend to the Faculty of Graduate and Postdoctoral Studies for acceptance, the thesis entitled: A Marine Megafaunal Extinction and its Consequences for Kelp Forests of the North Pacific submitted by Cameron David Bullen in partial fulfillment of the requirements for the degree of Master of Science in Resources, Environment and Sustainability Examining Committee: Dr. Kai M.A. Chan, Institute for Resources, Environment, and Sustainability (IRES). University of British Columbia Supervisor Dr. Villy Christensen, Institute for Oceans and Fisheries (IOF). University of British Columbia Supervisory Committee Member Dr. Iain McKechnie, Department of Anthropology. University of Victoria Supervisory Committee Member Dr. Edward Gregr, Institute for Resources, Environment, and Sustainability (IRES). University of British Columbia Supervisory Committee Member Dr. Christopher Harley, Department of Zoology. University of British Columbia Additional Examiner ii Abstract Restoration of lost ecosystem functions and species interactions is increasingly seen as central to addressing the extensive degradation of ecosystems and associated losses of biodiversity and ecosystem services. To be effective, such restoration efforts require an understanding of how ecosystems functioned prior to human-caused extinctions and ecological transformations. Global declines of megafauna, such as the extinction of the Steller’s sea cow, are largely a consequence of human action and likely had significant and widespread ecological impacts. Drawing on historical evidence, kelp forest ecology, and extant mammalian herbivore ecology, I first propose six discrete hypotheses about the effects Steller’s sea cows may have had on North Pacific kelp forest dynamics. The natural history observations, historic accounts, and ecological evidence I review offer partial support for these hypotheses. Accordingly, I argue that Steller’s sea cows exerted a significant top-down influence in kelp forests, likely affecting physical ecosystem structure, productivity, nutrient cycling, species interactions, and the export of nutrients to surrounding ecosystems. Next, I build upon these hypotheses to develop an ecosystem model which re-introduces this extinct megaherbivore and highlights its former role in ecosystem dynamics and species interactions in kelp forests. I find that, while not a keystone species, Steller’s sea cows likely had a significant effect on ecosystem dynamics, influencing community composition and increasing the productivity and resilience of kelp forests. The model indicates the presence of Steller’s sea cows may also have enabled the coexistence of sea otters and some large invertebrates, iii suggesting that the often-seen decline of invertebrate populations caused by sea otters may be a phenomenon exacerbated by lost ecosystem functions and species interactions associated with the extinction of the Steller’s sea cow. My findings suggest that kelp forest dynamics and resilience were significantly altered prior to the influence of more recent and well-known stressors, and demonstrate the important ecological roles that can be lost with megafaunal extinction. This work also illustrates the degree to which the loss of species interactions has likely affected North Pacific ecosystems, and how using ecosystem models to consider past ecosystem dynamics can inform management and restoration of current social-ecological systems. iv Lay Summary Understanding how the long history of human influence has changed ecosystems is critical for effectively addressing the widespread declines seen in nature. My research focused on one specific change, the extinction of the Steller’s sea cow (a large coastal grazer), and its consequences for the North Pacific Ocean. I found that Steller’s sea cows were more important than is often thought, in that their extinction may have had a large impact on kelp forest ecosystems. I also found that considering the missing role of the Steller’s sea cow may help us understand current challenges in the management of kelp forests. This research shows that it is important to understand the many ways human actions have changed ecosystems, even if those changes happened hundreds or thousands of years ago. Studying these changes helps us better understand the history of these ecosystems, and hopefully improve their future management and restoration. v Preface The two research chapters presented in this thesis (Chapter 2 and Chapter 3) are intended to be individual manuscripts submitted for publication in academic journals. Each of these chapters is deliberately self-contained, and therefore includes some repetition, primarily in the introductory and concluding remarks. Chapter 2 has been prepared for submission to a peer-reviewed journal. I am the lead author of this manuscript, responsible for concept formation and writing. I was assisted in this research by several coauthors. Alberto Campos contributed to both concept formation and writing for subsection 2.4.6, as well as provided feedback on the manuscript. As members of the supervisory committee, Dr. Edward Gregr and Dr. Iain McKechnie assisted with concept design and provided feedback on the manuscript. As the supervisor and principle investigator of this research, Dr. Kai Chan provided initial project and concept design, assisted in the formation of the supervising committee, and provided substantial feedback on the manuscript. Chapter 3 has also been prepared for submission in a peer-reviewed journal. I am the lead author of this manuscript, responsible for concept formation, research design, data analysis, data visualization, and writing. I was assisted in this research by several coauthors. As members of the supervisory committee, Dr. Edward Gregr and Dr. Villy Christensen assisted with concept formation, research design, data analysis, data visualization, and provided substantial feedback on the manuscript. As the supervisor and principle investigator of this research, Dr. Kai Chan provided initial project and concept design, assisted in the formation of the supervising vi committee, assisted in the analysis and visualization of data, and provided substantial feedback on the manuscript. The Appendix supports the modelling approach employed in Chapter 3, and provides additional detail on specific methods and model parameters. This appendix is intended to be published as supporting material alongside Chapter 3. I am the lead author of the Appendix, responsible for data analysis and writing. However, Table A2 has been adapted from the doctoral dissertation of Dr. Edward Gregr (2016). vii Table of Contents Abstract .................................................................................................................................... iii Lay Summary ........................................................................................................................... v Preface ...................................................................................................................................... vi Table of Contents ................................................................................................................... viii List of Tables............................................................................................................................ xi List of Figures ......................................................................................................................... xii List of Reproduced Images in Figure 2.1 .............................................................................. xiii Acknowledgements ................................................................................................................. xv Chapter 1: Introduction ........................................................................................................... 1 1.1 Approaches for studying historical system states ......................................................... 4 1.2 Case study ................................................................................................................... 5 1.3 Research objectives ..................................................................................................... 7 Chapter 2: The Ghost of a Giant - Six Hypotheses for How an Extinct Megaherbivore Structured Kelp Forests Across the North Pacific Rim ........................................................ 10 2.1 Introduction ............................................................................................................... 10 2.2 Steller’s sea cows and kelp forests ............................................................................. 13 2.3 Lines of evidence ...................................................................................................... 15 2.4 Hypothesized effects on community structure and ecosystem function ...................... 18 2.4.1 Modified physical ecosystem structure .............................................................. 21 2.4.2 Altered primary productivity ............................................................................
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