Effects of Dispersal and Local Dynamics on Spider Diversity (Araneae) in an Old Field System
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Effects of dispersal and local dynamics on spider diversity (Araneae) in an old field system Carol M. Frost Department of Natural Resource Sciences Macdonald Campus McGill University Montréal, Québec, Canada August 2008 A thesis submitted to the Faculty of Graduate Studies and Research in partial fulfillment of the requirements for the degree of Master of Science © Carol M. Frost 2008 Abstract Processes at multiple spatial scales can interact to structure community diversity. I looked at the effects of both dispersal and local dynamics on spider diversity in young buckthorn trees in an old field system in southwestern Québec, Canada. I investigated the rate, mechanism, and timescale of spider re-colonization of emptied trees and found that the rate fluctuated, peaking in mid-June and early August. Preventing cursorial immigration only significantly reduced immigrant abundance of the jumping spider guild (Salticidae), but it reduced species richness and changed taxonomic composition of immigrants. At peak immigration times, four days was enough time for emptied trees to be fully re-colonized such that spider abundance, species richness, and composition matched that in undisturbed trees, suggesting an important role for dispersal in structuring spider diversity in this system. I used a diversity addition experiment to test whether spider diversity is limited only by dispersal and found that local dynamics also limit spider diversity. This work contributes to understanding the forces structuring arboreal spider assemblages. 1 Résumé Certains processus à des échelles spatiales diverses intéragissent avec la structure des communautés. Je me suis intéressée à la fois aux effets de colonisation et de dynamiques locales sur la diversité des araignées dans un champ à l’abandon peuplé d’arbustes situé au sud-ouest du Québec, au Canada. J’ai étudié les mécanismes, variations temporelles et taux de recolonisation d’arbustes préalablement vidés de leurs araignées et j’ai observé que le taux de recolonisation connaissait un pic d’activité entre la mi-Juin et le début Août. Seule l’abondance de la guilde des araignées sauteuses (Salticidae) a été réduite en empêchant la recolonisation terrestre. Cette manipulation a cependant diminué la diversité des espèces collectées et a modifié la composition taxonomique des colonisateurs. Lors des pics de recolonisation, quatres jours ont suffisaient pour permettre la recolonisation totale des arbres vides, de sorte que l’abondance, la richesse et la composition taxonomique correspondent avec celles des arbres témoins. Ceci suggère que la colonisation joue un rôle important dans structuration de la diversité de ce système. Des expériences d’ajout d’espèces ont permis de vérifier si la diversité des araignées est limitée seulement par la colonisation, et ont mis en évidence que des dynamiques locales intervenaient également. Ce travail contribue à une meilleure connaissance des processus structurant les assemblages d’araignées dans les systèmes arbustifs. 2 Table of Contents Page Abstract……………………………………………………………………………………1 Résumé…………………………………………………………………………………….2 Table of Contents………………………………………………………………………….3 Contributions of Co-author………………………………………………………………..4 Acknowledgements………………………………………………………………………..5 Chapter 1: Introduction, literature review, and objectives………………………………...6 1.1 Introduction………………………………………………………………………...6 1.2 Literature Review…………………………………………………………………..7 1.2.1 Community assembly and metacommunity theory…………………………..7 1.2.2 Empirical metacommunity studies…………………………………………...8 1.2.3 Foliage-dwelling spiders as a model metacommunity system……………...11 1.3 Objectives and thesis structure……………………………………………………14 Chapter 2: Effects of dispersal and local dynamics on spider diversity (Araneae) in an old field system………………………………………………………………………………16 2.1 Abstract…………………………………………………………………………...16 2.2 Introduction……………………………………………………………………….17 2.3 Methods…………………………………………………………………………...20 2.3.1 Study site…………………………………………………………………...20 2.3.2 Re-colonization rate, mechanism, and timescale…………………………..20 2.3.3 Diversity addition experiment……………………………………………...22 2.3.4 Data analysis……………………………………………………………….24 2.4 Results…………………………………………………………………………….27 2.4.1 Re-colonization rate and mechanism………………………………………27 2.4.2 Timescale of re-colonization………………………………………………29 2.4.3 Diversity addition experiment……………………………………………..34 2.5 Discussion………………………………………………………………………...37 2.5.1 Re-colonization rate and mechanism………………………………………37 2.5.2 Timescale of re-colonization………………………………………………38 2.5.3 Diversity addition experiment……………………………………………..40 2.5.4 Conclusion…………………………………………………………………41 Chapter 3: Summary and Conclusion……………………………………………………43 References………………………………………………………………………………..45 Appendix 1: Data from colonization rate and mechanism study………………………...51 Appendix 2: Data from timescale of colonization study…………………………………53 3 Contributions of Co-author My supervisor, Dr. Chris Buddle is a co-author on the manuscript in Chapter 2, which is to be submitted for publication. Dr. Buddle helped with the conceptualization and design of the experiments and with the editing of the manuscript. 4 Acknowledgements I would like to thank my supervisor, Dr. Chris Buddle (a.k.a. Rockstar Chris!), for helping to plan my project, for suggestions during my field work, for discussing my results with me, for editing my thesis, for lunchtime discussions about ecology, science, spiders, and other topics, and for his advice, encouragement, and enthusiasm throughout my time in his lab. From him I have learned a great deal about community ecology research, project design, scientific writing, and presentations. Chris Buddle provided financial support for me to attend two conferences, which were great opportunities to present my work for and interact with other students and professors in entomology. I would also like to thank my committee members, Dr. Terry Wheeler and Dr. Sylvie de Blois, for their thoughtful criticism of my proposal and experimental design. I was fully supported during this degree by an NSERC (Natural Sciences and Engineering Research Council of Canada) Canada Graduate Scholarship, and also received a McGill Entrance Scholarship and a McGill Alma Mater Student Travel Grant. I am grateful to Tara Sackett, Jean-François Aublet, Kathleen Aikens, Andrea Déchêne, and my mother for help with field work, to John Watson, Chris Cloutier, and Anne Murphy for the use of and help with the Morgan Arboretum whipper-snippers, and to Raphaёl and Léa Royauté for help with translation. Being part of the Buddle lab has been great fun, and I’d like to thank all the people whom I’ve spent time with there: Kathleen Aikens, Joey Bowden, Maxim Larrivée, Andrea Déchêne, Zach Sylvain, Tara Sackett, Alida Mercado, Charles Stephen, Annie Webb, Raphaёl Royauté, and Agnes Kwasniewska. Finally, I’d like to thank my family, friends, and room mates for encouragement, support, and discussion about my project; in particular, thanks to Mom, Dad, Michael and Peter Frost, Michael Pedruski, Glenna McGregor, Megan Wark, Natasha Prepas- Strobeck, Kristen Whitbeck, Audrey Wachter, Jean Lacasse, Jane Lee, Mylène L’Espèrance, and Vincci Tsui. 5 Chapter 1: Introduction, literature review, and objectives 1.1 Introduction Understanding processes that structure community diversity will be essential to predicting change due to anthropogenic habitat destruction, and in planning conservation initiatives in a wide range of systems (Holyoak et al. 2005; Ricklefs 1987), and is also of relevance in maintaining diverse assemblages of generalist predators for biological control (e.g., Samu et al. 1999). Processes structuring community diversity are currently viewed as a dichotomy of regional and local factors (Ricklefs 1987), and metacommunity theory is working toward a synthesis of how these factors interact (Leibold et al. 2004). There are currently four metacommunity paradigms, each of which fits certain natural systems, but for which a synthesis is lacking (Leibold et al. 2004). A fundamental distinction amongst the different paradigms is the different relative importance of processes operating at regional and local scales in structuring diversity. Many empirical studies have addressed the question of the relative importance of dispersal and local dynamics in structuring community diversity (see meta-analysis by Cadotte, 2006a). However, few studies have addressed this question in taxonomic assemblages other than plants and microorganisms (Cadotte 2006a). Studies in a wider range of taxonomic groups are needed in order to make general conclusions from this body of empirical work. In this thesis I examined the relative importance of dispersal and local dynamics in structuring spider diversity in an old field system. I had three objectives, the first and second addressing spider re-colonization of ‘empty’ local habitat patches, and the third addressing whether dispersal is the only factor structuring spider diversity in this system. My objectives were 1) to determine the rate and mechanism of spider re-colonization of emptied trees, and whether this differed by hunting guild; 2) to examine the time-scale of the re-colonization process by comparing spider diversity in re-colonized trees to that of undisturbed trees; and 3) to determine whether dispersal was the only factor limiting diversity, or whether local dynamics were also important. 6 1.2 Literature Review 1.2.1 Community Assembly and Metacommunity Theory A central question in community ecology is what determines