Complementary Assembly Processes Across Biodiversity Gradients
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Zurich Open Repository and Archive University of Zurich Main Library Strickhofstrasse 39 CH-8057 Zurich www.zora.uzh.ch Year: 2014 Complementary assembly processes across biodiversity gradients Fergus, Alexander Jon Posted at the Zurich Open Repository and Archive, University of Zurich ZORA URL: https://doi.org/10.5167/uzh-104357 Dissertation Originally published at: Fergus, Alexander Jon. Complementary assembly processes across biodiversity gradients. 2014, Univer- sity of Zurich, Faculty of Science. COMPLEMENTARY ASSEMBLY PROCESSES ACROSS BIODIVERSITY GRADIENTS Dissertation zur Erlangung der naturwissenschaftlichen Doktorwürde (Dr. sc. nat.) vorgelegt der Mathematisch-naturwissenschaftlichen Fakultät der Universität Zürich von Alexander Jon Fergus aus Neuseeland Promotionskomitee Prof. Dr. Bernhard Schmid (Leitung der Dissertation) Prof. Dr. Andrew Hector Prof. Dr Andreas Prinzing PD Dr. Christiane Roscher Dr. Lindsay Turnbull Zürich 2014 COMPLEMENTARY ASSEMBLY PROCESSES ACROSS BIODIVERSITY GRADIENTS Dissertation zur Erlangung der naturwissenschaftlichen Doktorwürde (Dr. sc. nat.) vorgelegt der Mathematisch-naturwissenschaftlichen Fakultät der Universität Zürich von Alexander Jon Fergus aus Neuseeland Promotionskomitee: Prof. Dr. Bernhard Schmid (Leitung der Dissertation) Prof. Dr. Andrew Hector Prof. Dr Andreas Prinzing PD Dr. Christiane Roscher Dr. Lindsay Turnbull Zürich 2014 For Ma, who would have wanted to have seen this thing finished. Ma and Pa visiting the Jena Experiment in 2008. Table of Contents Summary 8 Zusammenfassung 14 General introduction 20 Chapter 1 Biodiversity experiments: what have we learnt about 38 biodiversity – ecosystem functioning relationships? (Fergus, A.J.F. & Schmid, B. 2010. In Atlas of Biodiversity Risk (eds. J. Settele, R. Grabaum, V. Grobelnick, V. Hammen, S. Klotz, L. Penev, I. Kühn). Pensoft, Sofia, Moscow. Chapter 2: pp. 28-31) Chapter 2 A comparison of the strength of biodiversity effects across 52 multiple functions. (Allan, E., Weisser, W.W., Fischer, M., Schulze, E-D., Weigelt, A., Roscher, C., Baade, J., Barnard, R.L., Beßler, H., Buchmann, N., Ebeling, A., Eisenhauer, N., Engels, C., Fergus, A.J.F. et al. 2013. Oecologia 173: 223-237) Chapter 3 Bryophyte community assembly along a vascular plant 94 Biodiversity gradient. (Fergus, A.J.F., Gerighausen, U., & Roscher, C.; Manuscript) Chapter 4 Janzen-Connell effects are widespread and strong enough to 136 Maintain diversity in grasslands. (Petermann, J.S., Fergus, A.J.F., Turnbull, L.A. & Schmid, B. 2008. Ecology 89: 2399-2406) Chapter 5 Facilitation then negative feedbacks–sequential driving forces of 164 community assembly in experimental grassland. (Fergus, A.J.F., Petermann, J.A., Turnbull, L.A. & Schmid, B.; Manuscript) Chapter 6 Species diversity reduces invasion success in pathogen- 216 regulated communities. (Turnbull, L.A., Levine, J., Fergus, A.J.F. & Petermann, J.S. 2010. Oikos 119: 1040-1046) Chapter 7 Biology, chance or history? The predictable re-assembly of 234 temperate grassland communities (Petermann, J.S., Fergus, A.J.F., Roscher, C., Turnbull, L.A., Weigelt, A. & Schmid, B. 2010. Ecology 91: 408-421) Chapter 8 What happens to the sown species if a biodiversity experiment 274 is not weeded? (Roscher, C., Fergus, A.J.F., Petermann, J.A., Buchmann, N., Schmid, B., & Schulz, E-D. 2013. Basic and Applied Ecology 14: 187-198) Chapter 9 Experimental plant communities become phylogenetically 306 overdispersed during assembly. (Allan, E., Jenkins, T., Fergus, A.J.F., Roscher, C., Fischer, M., Petermann, J.A., Weisser, W. & Schmid, B. 2013. Ecology 94: 465-477) Chapter 10 Non-linear patterns of relatedness in re-assembled grassland. 344 (Fergus, A.J.F., Allan, E., Jenkins, T., Petermann, J. A., Roscher, C., Schmid, B., & Prinzing, A.; Manuscript) . Chapter 11 Phylogenetically poor plant communities receive more alien 382 species, which more easily co-exist with natives. (Gerhold, P., Pärtel, M., Tackenberg, O., Hennekens, S.M., Bartish, I., Schaminée, J.H.J, Fergus, A.J.F., Ozinga, W.A. & Prinzing, A. 2011. The American Naturalist 177: 668-680) Chapter 12 A new index of functional redundancy, and an examination of the 422 trends of redundancy in ecological communities Fergus, A.J.F., Ladley, K.J., Robertshaw, T.S., Brandes, U., Mistral, M. & Wilson, J.B. Submitted to Journal of Ecology. General Discussion 448 Acknowledgements 464 Curriculum vitae 466 Summary 8 Community assembly processes reveal the direction and strength of species interactions. Plant community structure is determined by these interactions and understanding community structure permits a mechanistic insight into the forces driving ecosystem functioning. Community assembly arises from three hierarchical processes: the regional species pool determines which species have viable local populations and can potentially colonise a given site within a region; the local abiotic conditions at a given site select for species from the regional pool with the appropriate range of traits required to grow and reproduce there; biotic interactions – including competition, mutualism and exploitation – govern local community composition. In this thesis I have examined how biotic interactions shape community assembly across biodiversity gradients, considering gradients of species richness, functional group richness, or metrics of evolutionary distance and diversity. The number of species in a community can alter the dynamics of interspecific interactions, and can influence important properties of communities such as their resistance to alien plant invasion or the provisioning of ecosystem functions and services. In reviewing biodiversity–ecosystem functioning relationships we have concluded that biodiversity has significant positive impacts on community production of biomass and associated processes (chapter 1). We have experimentally demonstrated that many ecosystem functions respond to plant species richness, particularly processes involving carbon cycling, and we have also found support for positive bottom- up effects of plant species richness on higher trophic levels (chapter 2). Plant communities are not solely comprised of vascular plant species; consequently we followed the assembly of bryophytes along a vascular plant species richness gradient (chapter 3). Bryophyte richness responded negatively to this gradient and bryophyte community composition varied distinctly along it. By implication, advocating increased richness of one taxonomic group over another potentially impacts collective community diversity. We also considered how other taxonomic groups below ground structure plant communities, which challenged prevailing niche-based dogma for explanations of biodiversity-ecosystem functioning relationships. We demonstrated that negative plant–soil feedbacks depress site re-occupation success by a species belonging to the same functional group, effectively promoting species coexistence and 9 diversity (chapter 4). Effects were driven by soil pathogens and were compounded by interspecific competition. This prompted us to theoretically explore how natural enemy regulation influences establishment. Models were constructed whereby soil pathogens reduced the competitive ability of resident species in sites formerly occupied by the same species (chapter 6). The ability to immigrate and establish was dependent on the competitive ability of the incoming species, regardless of pathogen regulation, as the abundance of the incoming species is initially low. The success, or population density, of an incoming species was however dependent on pathogen-regulation. With increasing community diversity resident species are less likely to occupy a site formerly occupied by the same species, reducing pathogen regulation, and removing any depressed impact on the competitive ability of the resident community. The upshot is that in pathogen regulated communities increasing community diversity increases community resistance to incoming species, or if we consider a scenario involving alien species, more diverse communities have greater invasion resistance. We tested these theoretical results under field conditions in established communities, using two metrics of establishment success to reveal shifts between life history stages (chapter 5). Immigrant seedling abundance was always higher in communities containing the same functional group, probably due to abiotic facilitation or shared mutualists. This pattern was opposite or indistinct after immigrants had established, therefore biotic interactions can sequentially drive assembly in different directions. In a more complex system, we have demonstrated using seed addition experiments that immigrant species complement communities with initially low richness, leading to convergence of species richness, functional group richness and evenness across different communities (chapter 7). Similarly, in the same experimental platform, we showed that spontaneous colonisation by immigrants – after we ceased to enforce a richness gradient – has less impact on the abundance and stability of communities with higher initial species richness (chapter 8). There is an evolutionary backdrop to these feedbacks and patterns of reassembly. Across the initial sown species richness gradient the abundance distributions of species in communities became overdispersed with time (chapter 9). This phylogenetic overdispersion was evident as dominant species in a community 10 being more distantly related than expected based on the performance of individual species in monoculture,